Heriot-Watt University Drilling Engineering Glossary PDF

1 Well Control Glossary of Terms 1 2  Glossary of Terms DEFINITIONS AND GLOSSARY OF TERMS A Abandon a well v : to stop producing hydrocarbons when the well becomes unprofitable. A wildcat may be abandoned after poor results from a wel...

1 Well Control Glossary of Terms 1 2  Glossary of Terms DEFINITIONS AND GLOSSARY OF TERMS A Abandon a well v : to stop producing hydrocarbons when the well becomes unprofitable. A wildcat may be abandoned after poor results from a well test. Mechanical and cement plugs are placed in the wellbore to prevent fluid migration to surface and between different zones. Abnormal pressure n : a formation pressure which is greater or less than the "normal" formation fluid hydrostatic pressure. Such pressures may be classified as "subnor- mal" (lower than normal) or "overpressured" (higher than normal). Accelerometer n : a surveying instrument which measures components of the Earth's gravitational field. Acidise v : to apply acids to the walls of oil and gas wells to remove any material which may obstruct flow into the wellbore. Adjustable choke n : a choke in which the rate of flow is controlled by adjusting a conical needle and seat. Air drilling n : a method of drilling that uses compressed air as the circulating medium. Angle unit n : the component of a survey instrument used to measure inclination. Annular preventer n : a large BOP valve that forms a seal in the annular space between the wellbore and the drillpipe. It is usually installed above the ram type preventers in the BOP stack. Annulus n : the space between the drillstring and open hole or drillstring and cased hole in the wellbore. Anticline n : a configuration of folded and stratified rock layers in the shape of an arch. Often associated with a trap. A.P.I. abbr : American Petroleum Institute. The leading standardising organisation on oilfield drilling and production equipment. A.P.I. gravity n : a measure of the density of liquid petroleum products, expressed in degrees. It can be derived from the following equation: API Gravity (degrees) = 141.5 - 131.5 Specific Gravity Department of Petroleum Engineering, Heriot-Watt University 3 1 Azimuth n : used in directional drilling as the direction of the trajectory of the wellbore measured in degrees (0-359) clockwise from True North or Magnetic North. B Back off v : to disconnect a section of stuck drillpipe by unscrewing one of the connections above the stuckpoint. Back up : 1. v - to hold one section of pipe while another is being screwed into or out of it (as in back up tongs). 2. n - a piece of equipment held in reserve in case another piece fails. Badger bit n : a specially designed bit with one large nozzle, which can be used as a deflecting tool in soft formations. Bail n : a rounded steel bar which supports the swivel and connects it to the hook. May also apply to the steel bars which connect the elevators to the hook (links). Ball up v : buildup of a mass of sticky material (drill cuttings) on components of drillstring (especially bits and stabilisers) Barge n : a flat decked, shallow draft vessel which may accommodate a drilling rig, or be used to store equipment and materials or for living quarters. Barite (Baryte) n : Barium Sulphate (BaSO4), a mineral used as a weighting material to increase mud weight (specific gravity = 4.2). Barrel n : a measure of volume for fluids. One barrel (bbl) = 42 U.S. gallons = 0.15899 cubic metres. The term bbl is derived from the blue barrels in which oil was originally transported. Bed n : a geological term to specify one particular layer of rock. Bell nipple n : In marine drilling, the uppermost component of the marine riser attached to the telescopic joint. The top of the nipple is expanded to guide drilling tools into the well. Bentonite n : a finely powdered clay material (mainly montmorillonite) which swells when mixed with water. Commonly used as a mud additive, and sometimes referred to as "gel". Bent sub n : a short piece of pipe whose axis is deviated 1˚-3˚ off vertical. Used in directional drilling as a deflecting tool. Bit n : the cutting element at the bottom of the drillstring, used for boring through the rock. 4  Glossary of Terms Bit breaker n : a heavy metal plate which fits into the rotary table and holds the bit while it is being connected to or disconnected from the drillstring. Bit record n : a report containing information relating to the operating parameters and performance of the bits run in a well. Bit sub n : a short length of pipe installed immediately above the bit. The threads on the bit sub accept the pin thread on the bit and the pin thread for the drillcollars. Bit walk n : the tendency for the bit and drillstring to wander off course by following the direction of rotation (usually to the right) in a directionally drilled well. Blind rams n : one of the valves on the BOP stack. It is designed to close off the wellbore when the drillstring is out of the hole. Blocks n : an assembly of pulleys on a common framework. Blooey line n : the discharge pipe from a well being drilled with compressed air. Blow out n : an uncontrolled flow of formation fluids into the atmosphere at surface. BOP abbr : Blow Out Preventer. A valve installed on top of the wellhead to control wellbore pressure in the event of a kick. BOP stack n : an assembly of BOPs consisting of annular preventers and ram type preventers. For land drilling the BOP stack is installed just below the rig floor, while for floating rigs the stack is positioned on the seabed. Borehole n : the hole made by the drill bit. Bottom hole assembly (BHA) n : the part of the drillstring which is just above the bit and below the drillpipe. It usually consists of drill collars, stabilisers and various other components. Bottom hole pressure (bhp) n : the pressure, 1. at the bottom of the borehole, or 2. at a point opposite the producing formation. Box n : the female section of a tool joint or other connection. Brake n: the device operated by the driller to stop the downward motion of the travelling block and therefore the drillstring. Breakout v : to unscrew one section of pipe from another. Bridge n : an obstruction in the borehole usually caused by the borehole wall caving in. BRT abbr : Below Rotary Table. Reference point for measuring depth. Department of Petroleum Engineering, Heriot-Watt University 5 1 Building assembly n : a BHA specially designed to increase the inclination (drift angle) of the wellbore. Build up rate n : the rate at which drift angle is increasing as the wellbore is being deviated from vertical. Usually measured in degrees per 100 ft drilled. Build up section n : that part of the wellbore's trajectory where the drift angle is increasing. Bumper sub n : a drilling tool, placed in the BHA, consisting of a short stroke slip joint which allows a more constant WOB to be applied when drilling from a floating rig. Cable tool drilling n : an earlier method of drilling used before the introduction of modern rotary methods. The bit was not rotated but reciprocated by means of a strong wire rope. Caliper log n : a tool run on electric wireline which measures the diameter of the C wellbore. It may be used for detecting washouts, calculating cement volumes, or detecting internal corrosion of casing. Cap rock n : an impermeable layer of rock overlying an oil or gas reservoir and preventing the migration of fluids. Cased hole n : that part of the hole which is supported by a casing which has been run and cemented in place. Casing n : large diameter steel pipe which is used to line the hole during drilling operations. Casing head Housing n : a large recepticle which is installed on top of the surface casing string. It has an upper flanged connection. Once it is installed it provides: a landing shoulder for the next casing string; and a flanged connection for the BOP stack to be connected to the well. Casing head Spool n : a large recepticle which is installed on top of the casing head housing or a previous spool. It has both an upper and lower flanged connection. Once it is installed it provides: a landing shoulder for the next casing string; access to the annulus between the casing strings and a flanged connection for the BOP stack to be connected to the well. Casing hanger n : a special component which is made up on top of the casing string to suspend the casing from the previous casing housing or spool. Casing shoe n : a short section of steel pipe filled with concrete and rounded at the bottom. This is installed on the bottom of the casing string to guide the casing past any ledges or irregularities in the borehole. Sometimes called a guide shoe. 6  Glossary of Terms Casing string n : the entire length of all the casing joints run into the borehole. Cathead n : a spool shaped attachment on a winch, around which rope is wound. This can be used for hoisting operations on the rig floor. Caving: 1. v: collapse of the walls of the borehole. Also referred to as "sloughing". 2. n: a small part of the borehole wall that has collapsed into the hole. Centraliser n : a device secured around the casing which is designed to support and centralise the casing in deviated wellbores. Centrifugal pump n : a pump consisting of an impellor, shaft and casing which discharges fluid by centrifugal force. Often used for mixing mud. Centrifuge n : a piece of solids control equipment which separates out particles of varying density. Cement Slurry n: A mixture of cement powder, water and additives which harden to form a cement sheath or cement plug in a well. Cementing v : the placement of a liquid slurry of cement and water inside or outside of the casing. Primary cementing is carried out immediately after the casing is run. Secondary cementing is carried out when remedial work is required. Cement channeling v : the irregular displacement of mud by cement, leaving voids in the cement sheath between the casing and the borehole, thereby reducing the effectiveness of the cement sheath. Cement head n : a manifold system installed on the top of the casing which allows the cement slurry to be pumped from the cement unit down the casing string. The cement head is also used for releasing the top and bottom cement plugs. Cement plug n : 1. A specific volume of cement placed at some point in the wellbore to seal off the well. 2.A device used during a primary cement job to separate the cement slurry from contaminating fluids in the casing. A wiper plug is pumped ahead of the slurry and a shut off plug behind the slurry. Chain tongs n : a tool used by roughnecks on the rig floor to tighten or loosen a connection. The tool consists of a long handle and an adjustable chain which will fit a variety of pipe sizes. Check valve n : a valve which permits flow in one direction only. Choke n : an orifice installed in a line to restrict and control the flow rate. Choke line n : a pipe connected to the BOP stack which allows fluids to be circulated Department of Petroleum Engineering, Heriot-Watt University 7 1 out of the annulus and through the choke manifold when a well kiling operation is beimg performed. Choke manifold n : an arrangement of pipes, valves and chokes which allows fluids to be circulated through a number of routes. Christmas tree n : an assembly of control valves and fittings installed on top of the wellhead. The Christmas tree is installed after the well has been completed and is used to control the flow of oil and gas. Circulate v : to pump drilling fluid through the drillstring and wellbore, returning to the mud pits. This operation is carried out during drilling and is also used to improve the condition of the mud while drilling is suspended. Clay n : a term used to describe the aluminium silicate minerals which are plastic when wet and have no well-developed parting along bedding planes. Such material is commonly encountered while drilling a well. Clay minerals n : the constituents of a clay which provide its plastic properties. These include kaolinite, illite, montmorillonite and vermiculite. Closure n : the shortest horizontal distance from a particular survey station back to the reference point. Combination string n : a casing string which is made up of various different grades or weights of casing (sometimes referred to as a tapered string when different sizes of casing are used). Company man n : an employee of an operating company whose job is to represent the operator's interests on the drilling rig (sometimes referred to as "drilling supervi- sor" or "company man"). Compass unit n : the component of a survey instrument used to measure azimuth. Completion 1. v : the activities and methods used to prepare a well for the production of oil or gas. 2. n: the tubing and accessories installed in the production casing and through which the produced fluid flows to surface. Conductor line n : a small diameter wireline which carries electric current. This is used for logging tools and steering tools. Conductor pipe n : a short string of casing of large diameter which is normally the first casing string to be run in the hole. Connection v : the joining of a section of drillpipe to the top of the drillstring as drilling proceeds. 8  Glossary of Terms Core n : a cylindrical rock sample taken from the formation for geological analysis. Core barrel n : a special tool which is installed at the bottom of the drillstring to capture and retain a core sample which is then recovered when the string is pulled out of the hole. Core Bit (Core Head) n: A donut shaped drilling bit used just below the core barrel to cut a cylindrical sample of rock. Correction run n : a section of hole which must be directionally drilled to bring the well path back onto the planned course. Crater n : a large hole which develops at the surface of a wellbore caused by the force of escaping gas, oil or water during a blowout. Cross-over n : a sub which is used to connect drill string components which have different types or sizes of threads. Crown block n : an assembly of sheaves or pulleys mounted on beams at the top of the derrick over which the drilling line is reeved. Cuttings n : the fragments of rock dislodged by the bit and carried back to surface by the drilling fluid. D Deadline n : that part of the drilling line between the crown block and the deadline anchor. This line remains stationary as the travelling block is hoisted. Deadline anchor n : a device to which the deadline is attached and securely fastened to the derrick substructure. Defecting tool n : a piece of drilling equipment which will change the inclination and/ or direction of the hole. Degasser n : a piece of equipment used to remove unwanted gas from the drilling mud. Density n : the mass of a substance per unit volume. Drilling fluid density is usually expressed in psi/ft, Kg/m3, g/cc or ppg. Departure n : one of the coordinates used to plot the path of the well on the horizontal plane (along the x axis). Derrick n : a large load-bearing structure from which the hoisting system and therefore the drillstring is suspended. Derrickman n : a member of the drilling crew whose work station is on the monkey board high up in the derrick. From there he handles the upper end of the stands of Department of Petroleum Engineering, Heriot-Watt University 9 1 drillpipe being raised or lowered. He is also responsible for maintaining circulation equipment and carrying out routine checks on the mud. Desander n : a hydrocyclone used to remove sand from the drilling mud. Desilter n : a hydrocyclone used to remove fine material (silt size) from the drilling mud. Development well n : a well drilled in a proven field to exploit known reserves. Usually one of several wells drilled from a central platform. Deviation n : a general term referring to the horizontal displacement of the well. May also be used to describe the change in drift angle from vertical (inclination). Diamond bit n : a bit which has a steel body surfaced with diamonds to increase wear resistance. Directional drilling : n the intentional deviation of a wellbore in order to reach a certain objective some distance from the rig. Directional surveying n : a method of measuring the inclination and direction of the wellbore by using a downhole instrument. The well must be surveyed at regular intervals to accurately plot its course. Discovery well n : the first well drilled in a new field which successfully indicates the presence of oil or gas reserves. Displace v : to move a liquid (e.g. cement slurry) from one position to another by means of pumping another fluid behind it. Displacement fluid n : the fluid used to force cement slurry or some other material into its intended position. (e.g. drilling mud may be used to displace cement out of the casing into the annulus). Dog house n : a small enclosure on the rig floor used as an office by the driller and as a storage place for small items. Dog leg n : a sharp bend in the wellbore which may cause problems tripping in and out of the hole. Dog leg severity n : a parameter used to represent the change in inclination and azimuth in the well path (usually given in degrees per 100 ft). Dope n : a lubricant for the threads of oilfield tubular goods. Double n : a section of drillpipe, casing or tubing consisting of two single lengths screwed together. Downhole motor n : a special tool mounted in the BHA to drive the bit without 10  Glossary of Terms rotating the drill string from surface (see positive displacement motor). Downhole telemetry n : the process whereby signals are transmitted from a downhole sensor to a surface readout instrument. This can be done by a conductor line (as on steering tools) or by mud pulses (as in MWD tools). Drag n : The force required to move the drillstring due to the drillstring being in contact with the wall of the borehole. Drag bit n : a drilling bit which has no cones or bearings but consists of a single unit with a cutting structure and circulation passageways. The fishtail bit was an early example of a drag bit, but is no longer in common use. Diamond bits are also drag bits. Drawworks n : the large winch on the rig which is used to raise or lower the drill string into the well. Drift angle n : the angle which the wellbore makes with the vertical plane (see inclination). Drill collar n : a heavy, thick-walled steel tube which provides weight on the bit to achieve penetration. A number of drill collars may be used between the bit and the drillpipe. Driller n : the employee of the drilling contractor who is in charge of the drilling rig and crew. His main duties are to operate the drilling equipment and direct rig floor activities. Drilling contractor n : an individual or company that owns the drilling rig and employs the crew required to operate it. Drilling crew n : the men required to operate the drilling rig on one shift or tour. This normally comprises a driller, derrickman and 2 or 3 roughnecks. Drilling fluid n : the fluid which is circulated through the drillstring and up the annulus back to surface under normal drilling operations. Usually referred to as mud. Drilling line n : the wire rope used to support the travelling block, swivel, kelly and drillstring. Drill pipe n : a heavy seamless pipe which is used to rotate the bit and circulate the drilling fluid. Lengths of drill pipe 30ft long are coupled together with tool joints to make the drillstring. Drill ship n : a specially designed ship which is used to drill a well at an offshore location. Drill stem n : used in place of drillstring in some locations. Describes all the drilling components from the swivel down to the bit. Department of Petroleum Engineering, Heriot-Watt University 11 1 Drill stem test (DST) n : a test which is carried out on a well to determine whether or not oil or gas is present in commercial quantities. The downhole assembly consists of a packer, valves and a pressure recording device, which are run on the bottom of the drill stem. Drillstring n : the string of drill pipe with tool joints which transmits rotation and circulation to the drill bit. Sometimes used to include both drill collars and drill pipe. Drop off section n : that part of the well's trajectory where the drift angle is decreasing (i.e. returning to vertical). Duplex pump n : a reciprocating positive displacement pump having 2 pistons which are double acting. Used as the circulating pump on some older drilling rigs. Dynamic positioning n : a method by which a floating drilling rig or drill ship is kept on location. A control system of sensors and thrusters is required. Easting n : one of the co-ordinates used to plot a deviated well's position on the horizontal plane (along the x axis). Electric logging v : the measurement of certain electrical characteristics of formations traversed by the borehole. Electric logs are run on conductor line to identify the type E of formations, fluid content and other properties. Elevators n : a lifting collar connected to the travelling block, which is used to raise or lower pipe into the wellbore. The elevators are connected to the travelling block by links or bails. Emulsion n : a mixture in which one liquid (dispersed phase) is uniformly distributed in another liquid (continuous phase). Emulsifying agents may be added to stabilise the mixture. Exploration well n : a well drilled in an unproven area where no oil and gas production exists (sometimes called a "wildcat"). Fault n : a geological term which denotes a break in the subsurface strata. On one side of the fault line the strata has been displaced upwards, downwards or laterally relative to its original position. F Fastline n : the end of the drilling line which is attached to the drum of the drawworks. Field n : a geographical area in which oil or gas wells are producing from a continuous reservoir. 12  Glossary of Terms Filter cake n : the layer of concentrated solids from the drilling mud that forms during natural filtration on the sides of the borehole. Sometimes called "wall cake" or "mud cake". Filter press n : a device used in the measurement of the mud's filtration properties. Filtrate n : a fluid which has passed through a filter. In drilling it usually refers to the liquid part of the mud which enters the formation. Filtration v : the process by which the liquid part of the drilling fluid is able to enter a permeable formation, leaving a deposit of mud solids on the borehole wall to form a filter cake. Fish n : any object accidentally left in the wellbore during drilling or workover operations, which must be removed before work can proceed. Fishing v : the process by which a fish is removed from the wellbore. It may also be used for describing the recovery of certain pieces of downhole completion equipment when the well is being reconditioned during a workover. Fishing tool n : a specially designed tool which is attached to the drill string in order to recover equipment lost in the hole. Flange up v : to connect various components together (e.g. in wellheads or piping systems). Flare n : an open discharge of fluid or gas to the atmosphere. The flare is often ignited to dispose of unwanted gas around a completed well. Flex joint n : a component of the marine riser system which can accommodate some lateral movement when drilling from a floater. Float collar n : a special device inserted one or two joints above the bottom of a casing string. The float collar contains a check valve which permits fluid flow in a downward direction only. The collar thus prevents the back flow of cement once it has been displaced. Floater n : general term used for a floating drilling rig. Float shoe n : a short cylindrical steel component which is attached to the bottom of a casing string. The float shoe has a check valve and functions in the same manner as the float collar. In addition the float shoe has a rounded bottom which acts as a guide shoe for the casing. Float sub n : a check valve which prevents upward flow through the drill string. Flocculation v : the coagulation of solids in a drilling fluid produced by special additives or contaminants in the mud. Department of Petroleum Engineering, Heriot-Watt University 13 1 Fluid loss v : the transfer of the liquid part of the mud to the pores of the formation. Loss of fluid (water plus soluble chemicals) from the mud to the formation can only occur where the permeability is sufficiently high. If the pores are large enough the first effect is a "spurt loss", followed by the build up of solids (filter cake) as filtration continues. Formation n : a bed or deposit composed throughout of substantially the same kind of rock to form a lithologic unit. Formation fluid n : the gas, oil or water which exists in the pores of the formation. Formation pressure n : the pressure exerted by the formation fluids at a particular point in the formation. Sometimes called "reservoir pressure" or "pore pressure". Formation testing v : the measurement and gathering of data on a formation to determine its potential productivity. Fracture n : a break in the rock structure along a particular direction. Fractures may occur naturally or be induced by applying downhole pressure in order to increase permeability. Fracture gradient n : a measure of how the strength of the rock (i.e. its resistance to break down) varies with depth. Fulcrum assembly n : a bottom hole assembly which is designed to build hole inclination. Gas cap n : the free gas phase which is sometimes found overlying an oil zone and occurs within the same formation as the oil. Gas cut mud n : mud which has been contaminated by formation gas. G Gas show n : the gas that is contained in mud returns, indicating the presence of a gas zone. Gas injector n : a well through which produced gas is forced back into the reservoir to maintain formation pressure and increase the recovery factor. Gel n : a semi-solid, jelly-like state assumed by some colloidal dispersions at rest. When agitated the gel converts to a fluid state. Gel strength n : the shear strength of the mud when at rest. Its ability to hold solids in suspension. Bentonite and other colloidal clays are added to the mud to increase gel strength. Geostatic pressure n : the pressure exerted by a column of rock. Under normal conditions this pressure is approximately 1 psi per foot. This is also known as 14  Glossary of Terms "lithostatic pressure" or "overburden pressure". Guideline tensioner n : a pneumatic or hydraulic device used to provide a constant tension on the wire ropes which run from the subsea guide base back to a floating drilling rig. Guide shoe n : See Float Shoe. Gumbo n : clay formations which contaminate the mud as the hole is being drilled. The clay hydrates rapidly to form a thick plug which cannot pass through a marine riser or mud return line. Gunk n : a term used to describe a mixture of diesel oil, bentonite and sometimes cement which is used to combat lost circulation. Gusher n : an uncontrolled release of oil from the wellbore at surface. Gyro multi-shot n : a surveying device which measures and provides a series of photographic images showing the inclination and direction of the wellbore. It measures direction by means of a gyroscopic compass. Gyro single-shot n : a surveying device which measures the inclination and direction of the borehole at one survey station. It measures direction by means of a gyroscopic compass Gyroscope n : a wheel or disc mounted on an axle and free to spinto spin rapidly about one axis, but free to rotate about one or both of the other two axes. The inertia of the wheel keeps the axis aligned with the reference direction (True North in directional survey tools). H Hole opener n : a special drilling tool which can enlarge an existing hole to a larger diameter. Hook n : the large component attached to the travelling block from which the drill stem is suspended via the swivel. Hopper n : a large funnel shaped device into which dry material (e.g. cement or powdered clay) can be poured. The purpose of the hopper is to mix the dry material with liquids injected at the bottom of the hopper. H.W.D.P. abbr : heavy weight drill pipe. Thick walled drill pipe with thick walled sections used in directional drilling and placed between the drill collars and drill pipe. Hydrostatic pressure n : the load exerted by a column of fluid at rest. Hydrostatic pressure increases uniformly with the density and depth of the fluid. Department of Petroleum Engineering, Heriot-Watt University 15 1 Inclination n : a measure of the angular deviation of the wellbore from vertical. Sometimes referred to as "drift angle". Injection n : usually refers to the process whereby gas, water or some other fluid is forced into the formation under pressure. I Impermeable adj : preventing the passage of fluid through the pores of the rock. Insert bit n : a type of roller cone bit where the cutting structure consists of specially designed tungsten carbide cutters set into the cones. Intermediate casing n : a string of casing set in the borehole to keep the hole from caving and to seal off troublesome formations. Invert oil emulsion mud n : a drilling fluid which contains up to 50% by volume of water, which is distributed as droplets in the continuous oil phase. Emulsifying agents and other additives are also present. Iron roughneck n : an automated piece of rig floor equipment which can be used to make connections. Jack-up rig n : an offshore drilling structure which is supported on steel legs. Jet deflection n : a technique used in directional drilling to deviate the wellbore by washing away the formation in one particular direction. A special bit (badger bit) is used which has one enlarged nozzle which must be orientated towards the intended J direction. Jet sub n : a tool used at the bottom of the drill pipe when the conductor pipe is being jetted into position (this method of running the conductor is only suitable where the surface formations can be washed away by the jetting action). Joint n : a single length of pipe which has threaded connections at either end. Junk n : debris lost in the hole which must be removed to allow normal operations to continue. Junk sub n : a tool run with the BHA, which is designed to recover pieces of debris left in the hole. Kelly n : the heavy square or hexagonal steel pipe which runs through the rotary table and is used to rotate the drillstring. Kelly bushing n : a device which fits into the rotary table and through which the kelly K 16  Glossary of Terms passes. The rotation of the table is transmitted via the kelly bushing to the kelly itself. Sometimes called the “drive bushing”. Kelly cock n : a valve installed between the kelly and the swivel. It is used to control a backflow of fluid up the drillstring and isolate the swivel and hose from high pressure. Kelly spinner n : a pneumatically operated device mounted on top of the kelly which, when actuated, causes the kelly to rotate. It may be used to make connections by spinning up the kelly. Key seat n : a channel or groove cut into the side of the borehole due to the dragging action of the pipe against a sharp bend (or dog leg). Key seat wiper n : a tool made up in the drillstring to ream out any key seats which may have formed and thus prevent the pipe from becoming stuck. Kick n : an entry of formation fluids (oil, gas or water) into the wellbore caused by the formation pressure exceeding the pressure exerted by the mud column. Kill line n : a high pressure line connecting the mud pumps to the BOP stack through which mud can be pumped to control a kick. Killing a well v : the process by which a well which is threatening to blow out is brought under control. It may also mean circulating water or mud into a completed well prior to workover operations. KOP abbr : kick-off point. The depth at which the wellbore is deliberately deviated from the vertical. L Latitude n : one of the co-ordinates used in plotting the wellpath on the horizontal plane (along the y axis). Lead angle n : the direction at which the directional driller aims the well to compensate for bit walk. Lead angle is measured in degrees left or right of the proposed direction. Liner n : 1. A string of casing which is suspended by a liner hanger from the inside of the previous casing string and does not therefore extend back to surface as other casing strings do. 2. A replaceable sleeve which fits inside the cylinder of a mud pump. Liner hanger n : a slip type device which suspends the liner inside the previous casing shoe. Location n : the place at which a well is to be drilled. Log n : a systematic recording of data (e.g. driller’s log, electric log, etc.) Department of Petroleum Engineering, Heriot-Watt University 17 1 Lost circulation n : the loss of quantities of whole mud to a formation due to caverns, fractures or highly permeable beds. Also referred to as “lost returns”. Magnetic declination n : the angle between True North and Magnetic North. This varies with geographical location, and also changes slightly each year. Magnetic multi-shot n : a surveying instrument which provides a series of photo- graphic discs showing the inclination and direction of the wellbore. It measures M direction by means of a magnetic compass and so direction is referenced to Magnetic North. Magnetic North n : the northerly direction in the earth’s magnetic field indicated by the needle of a magnetic compass. Magnetometer n : a surveying device which measures the intensity and direction of the earth’s magnetic field. Make up v : to assemble and join components together to complete a unit (e.g. to make up a string of casing). Make hole v : to drill ahead Marine riser n : the pipe which connects the subsea BOP stack with the floating drilling rig. The riser allows mud to be circulated back to surface, and provides guidance for tools being lowered into the wellbore. Mast n : a portable derrick capable of being erected as a unit unlike a standard derrick which has to be built up. Master bushing n : a sleeve which fits into and protects the rotary table and accommodates the slips and drives the kelly bushing. Measured depth (MD) n : the distance measured along the path of the wellbore (i.e. the length of the drillstring). Mill n : a downhole tool with rough, sharp cutting surfaces for removing metal by grinding or cutting. Milled tooth bit n : a roller cone bit whose cutting surface consists of a number of steel teeth projecting from the surface of the cones. Monel n : term used for a non-magnetic drill collar made from specially treated steel alloys so that it does not affect magnetic surveying instruments. Monkey board n : the platform on which the derrickman works when handling stands of pipe. 18  Glossary of Terms Moon pool n : the central slot under the drilling floor on a floating rig. Motion compensator n : a hydraulic or pneumatic device usually installed between the travelling block and hook. Its function is to keep a more constant weight on the drill bit when drilling from a floating vessel. As the rig heaves up and down a piston moves within the device to cancel out this vertical motion. Mousehole n : a small diameter pipe under the derrick floor in which a joint of drill pipe is temporarily stored for later connection to the drillstring. M.S.L. abbr : Mean Sea Level. Mud n : common term for drilling fluid. Mud balance n : a device used for measuring the density of mud or cement slurry. It consists of a cup and a graduated arm which carries a sliding (counterbalanced) weight and balances on a fulcrum. Mud conditioning v : the treatment and control of drilling fluid to ensure that it has the correct properties. This may include the use of additives, removing sand or other solids, adding water and other measures. Conditioning may also involve circulating the mud prior to drilling ahead. Mud engineer n : usually an employee of a mud service company whose main responsibility on the rig is to test and maintain the mud properties specified by the operator. Mudline n : the seabed. Mudlogging n : the recording of information derived from the examination and analysis of drill cuttings. This also includes the detection of oil and gas. This work is usually done by a service company which supplies a portable laboratory on the rig. Mud motor n : a downhole component of the BHA which rotates the bit without having to turn the rotary table. The term is sometimes applied to both positive displacement motors and turbodrills. Mud pits n : a series of open tanks in which the mud is mixed and conditioned. Modern rigs are provided with three or more pits, usually made of steel plate with built-in piping, valves and agitators. Mud pump n : a large reciprocating pump used to circulate the drilling fluid down the well. Both duplex and triplex pumps are used with replaceable liners. Mud pumps are also called “slush pumps”. Mud return line n : a trough or pipe through which the mud being circulated up the annulus is transferred from the top of the wellbore to the shale shakers. Sometimes called a “flowline”. Department of Petroleum Engineering, Heriot-Watt University 19 1 Mud screen n : shale shaker. Mule shoe n : the guide shoe on the lower end of a survey tool which locates into the key way of the orienting sub. The survey tool can then be properly aligned with the bent sub. M.W.D. abbr : Measurement While Drilling. A method of measuring petrophysical properties of formations, drilling parameters (WOB, torque etc.) and environmental parameters downhole and sending the results to surface without interrupting routine drilling operations. A special tool containing sensors, power supply and transmitter is installed as part of the BHA. The information is transmitted to surface by a telemetry system using mud pulses or signals through the pipe. Nipple n : a short length of tubing (generally less than 12") with male threads at both ends. Nipple up v : to assemble the components of the BOP stack on the wellhead. N Normal pressure n : the formation pressure which is due to a normal deposition process where the pore fluids are allowed to escape under compaction. The normal pressure gradient is usually taken as 0.465 psi per foot of depth from surface. Northing n : one of the co-ordinates used in plotting the position of the wellbore in the horizontal plane along the y axis. Offshore drilling n : drilling for oil or gas from a location which may be in an ocean, gulf, sea or lake. The drilling rig may be on a floating vessel (e.g. semi-submersible, drill ship) or mounted on a platform fixed to the seabed (e.g. jack up, steel jacket). Oil based mud n : a drilling fluid which contains oil as its continuous phase with only O a small amount of water dispersed as droplets. Open hole n : any wellbore or part of the wellbore which is not supported by casing. Operator n : the company which carries out an exploration or development pro- gramme on a particular area for which they hold a license. The operator may hire a drilling contractor and various service companies to drill wells, and will provide a representative (company man) on the rig. Orientation v : the process by which a deflection tool is correctly positioned to achieve the intended direction and inclination of the wellbore. Orienting sub n : a special sub which contains a key or slot, which must be aligned with the scribe line of the bent sub. A surveying instrument can then be run into the sub aligning itself with the key to give the orientation of the scribe line, which defines the tool face. 20  Glossary of Terms Overburden n : the layers of rock lying above a particular formation. Overshot n : a fishing tool which is attached to the drill pipe and is lowered over, and engages, the fish externally. P Packed hole assembly n : a BHA which is designed to maintain hole inclination and direction of the wellbore. Packer n : a downhole tool, run on drillpipe, tubing or casing, which can be set hydraulically or mechanically against the wellbore. Packers are used extensively in DSTs, cement squeezes and completions. Pay zone n : the producing formation. Pendulum assembly n : a BHA which is designed to reduce hole inclination by allowing the drill collars to bend towards the low side of the hole. Perforate v : to pierce the casing wall and cement, allowing formation fluids to enter the wellbore and flow to surface. This is a critical stage in the completion of a well. Perforating may also be carried out during workover operations. Perforating gun n : a device fitted with shaped charges which is lowered on wireline to the required depth. When fired electrically from the surface the charges shoot holes in the casing and the tool can then be retrieved. Permeability n : a measure of the fluid conductivity of a porous medium (i.e. the ability of fluid to flow through the interconnected pores of a rock). The units of permeability are darcies or millidarcies. pH value n : a parameter which is used to measure the acidity or alkalinity of a substance. Pilot hole n : a small diameter hole which is later opened up to the required diameter. Sometimes used in directional drilling to control wellbore deviation during kick off. Pin n : the male section of a threaded connection. Pipe ram n : a sealing device in a blowout preventor which closes off the annulus around the drill pipe. The size of ram must fit the drillpipe which is being used. Polycrystalline diamond compact bit (PDC bit) n : a PDC bit is a type of drag bit which uses small discs of man-made diamond as the cutting surface. P.O.H. abbr : Pull Out of Hole. Pore n : an opening within a rock which is often filled with formation fluids. Department of Petroleum Engineering, Heriot-Watt University 21 1 Porosity n : a parameter used to express the pore space within a rock (usually given as a percentage of unit volume). Positive displacement motor (PDM) n : a drilling tool which is located near the bit and is used to rotate the bit without having to turn the entire drillstring. A spiral rotor is forced to rotate within a rubber sleeved stator by pumping mud through the tool. Sometimes called a “Moineau pump” or “screw drill”. Pressure gradient n : the variation of pressure with depth. Commonly used under hydrostatic conditions (e.g. a hydrostatic column of salt water has a pressure gradient of 0.465 psi/ft) Primary cementing n : placing cement around the casing immediately after it has been run into the hole. Prime mover n : an electric motor or internal combination engine which is the source of power on the drilling rig. Production casing n : the casing string through which the production tubing and accessories are run to complete the well. Propping agent n : a granular material carried in suspension by the fracturing fluid which helps to keep the cracks open in the formation after fracture treatment. Protective casing n : an intermediate string of casing which is run to case off any troublesome zones. p.s.i. abbr : pounds per square inch. Commonly used unit for expressing pressure. Pup joint n : a short section of pipe used to space out casing or tubing to reach the correct landing depths. Rathole n : R 1. A hole in the rig floor 30'-60' deep and lined with pipe. It is used for storing the kelly while tripping. 2. That part of the wellbore which is below the bottom of the casing or completion zone. Reactive torque n : the tendency of the drillstring to turn in the opposite direction from that of the bit. This effect must be considered when setting the toolface in directional drilling. Ream v : to enlarge the wellbore by drilling it again with a special bit. Reamer n : a tool used in a BHA to stabilise the bit, remove dog legs or enlarge the hole size. Reeve v : to pass the drilling line through the sheaves of the travelling block and crown block and onto the hoisting drum. 22  Glossary of Terms Relief well n : a directionally drilled well whose purpose is to intersect a well which is blowing out, thus enabling the blow out to be controlled. Reservoir n : a subsurface porous permeable formation in which oil or gas is present. Reverse circulate v : to pump fluid down the annulus and up the drillstring or tubing back to surface. Rig n : the derrick, drawworks, rotary table and all associated equipment required to drill a well. R.I.H. abbr : Run In Hole. Riser tensioner n : a pneumatic or hydraulic device used to provide a constant strain in the cables which support the marine riser. R.K.B. abbr : Rotary Kelly Bushing. Term used to indicate the reference point for measuring depths. Roller cone bit n : a drilling bit with 2 or more cones mounted on bearings. The cutters consist of rows of steel teeth or tungsten carbide inserts. Also called a “rock bit”. R.O.P. abbr : rate of penetration, normally measured in feet drilled per hour. Rotary hose n : a reinforced flexible tube which conducts drilling fluid from the standpipe to the swivel. Also called "kelly hose" or “mud hose”. Rotary table n : the main component of the rotating machine which turns the drillstring. It has a bevelled gear mechanism to create the rotation and an opening into which bushings are fitted. Roughneck n : an employee of a drilling contractor who works on the drill floor under the direction of the driller. Round trip v : the process by which the entire drillstring is pulled out the hole and run back in again (usually to change the bit or BHA). Roustabout n : an employee of the drilling contractor who carries out general labouring work on the rig. R.P.M. abbr : revolutions per minute. Term used to measure the speed at which the drillstring is rotating. Department of Petroleum Engineering, Heriot-Watt University 23 1 disengage by turning it from surface. S Safety joint n : a tool which is often run just above a fishing tool. If the fishing tool has gripped the fish but cannot pull it free the safety joint will allow the string to Salt dome n : an anticlinal structure which is caused by an intrusion of rock salt into overlying sediments. This structure is often associated with traps for petroleum accumulations. Sand n : an abrasive material composed of small quartz grains. The particles range in size from 1/16mm to 2mm. The term is also applied to sandstone. Sandline n : small diameter wire on which light-weight tools can be lowered down the hole (e.g. surveying instruments). Scratcher n : a device fastened to the outside of the casing which removes mud cake and thus promote a good cement job. Semi-submersible n : a floating drilling rig which has submerged hulls, but not resting on the seabed. Shale n : a fine-grained sedimentary rock composed of silt and clay sized particles. Shale shaker n : a series of trays with vibrating screens which allow the mud to pass through but retain the cuttings. The mesh must be chosen carefully to match the size of the solids in the mud. Shear ram n : the component of the BOP stack which cuts through the drillpipe and forms a seal across the top of the wellbore. Sheave n : (pronounced “shiv”) a grooved pulley. Sidetrack v : to drill around some permanent obstruction in the hole with some kind of deflecting tool. Single n : one joint of pipe. Slips n : wedge-shaped pieces of metal with a gripping element used to suspend the drillstring in the rotary table. Slug n : a heavy viscous quantity of mud which is pumped into the drillstring prior to pulling out. The slug will cause the level of fluid in the pipe to fall, thus eliminating the loss of mud on the rig floor when connections are broken. Slurry (cement) n : a pumpable mixture of cement and water. Once in position the slurry hardens and provides an impermeable seal in the annulus and supports the casing. Spear n : a fishing tool which engages the fish internally and is used to recover stuck pipe. 24  Glossary of Terms Specific gravity n : the ratio of the weight of a substance to the weight of the same volume of water. S.P.M. abbr : Strokes Per Minute. Rate of reciprocation of a Mud Pump. Spool n : a wellhead component which is used for suspending a string of casing. The spool also has side outlets for allowing access to the annulus between casing strings. Spud v : to commence drilling operations. Squeeze cementing v : the process by which cement slurry is forced into place in order to carry out remedial work (e.g. shut off water producing zones, repair casing leaks). Stab v : to guide the pin end of a pipe into the tool joint or coupling before making up the connection. Stabbing board n : a temporary platform erected in the derrick 20'-40' above the drill floor. While running casing one man stands on this board to guide the joints into the string suspended on the rig floor. Stabiliser n : a component placed in the BHA to control the deviation of the wellbore. One or more stabilisers may be used to achieve the intended well path. Stage collar n : a tool made up in the casing string which is used in the second stage of a primary cement job. The collar has side ports which are opened by dropping a dart from surface. Cement can then be displaced from the casing into the annulus. Also called a “DV collar”. Stand n : three joints of pipe connected together, usually racked in the derrick. Standpipe n : a heavy wall pipe attached to one of the legs of the derrick. It conducts high pressure mud from the pumps to the rotary hose. Standpipe manifold n : a series of lines, gauges and valves used for routing mud from the pumps to the standpipe. Steering tool n : surveying instrument used in conjunction with a mud motor to continuously monitor azimuth, inclination and toolface. - These measurements are relayed to surface via conductor line, and shown on a rig floor display. Stimulation n : a process undertaken to improve the productivity of a formation by fracturing or acidising. Stripping v : movement of pipe through closed BOPs. Stuck pipe n : drillpipe, collars, casing or tubing which cannot be pulled free from the wellbore. Department of Petroleum Engineering, Heriot-Watt University 25 1 Sub n : a short threaded piece of pipe used as a crossover between pipes of different thread or size. Subs may also have special uses (e.g. bent subs, lifting subs, kelly saver sub). Subsea wellhead n : the equipment installed on the seabed for suspending casing strings when drilling from a floater. Suction pit n : the mud pit from which mud is drawn into the mud pumps for circulating down the hole. Surface casing n : a string of casing set in a wellbore to case off any fresh water sands at shallow depths. Surface casing is run below the conductor pipe to depth of 1000'- 4000' depending on particular requirements). Surge pressures n : excess pressure exerted against the formation due to rapid downward movement of the drillstring when tripping. Survey v : to measure the inclination and direction of the wellbore at a particular depth. Survey interval n : the measured depth between survey stations. Survey station n : the point at which a survey is taken. Swabbing n : a temporary lowering of the hydrostatic head due to pulling pipe out of the hole. Swivel n : a component which is suspended from the hook. It allows mud to flow from the rotary hose through the swivel to the kelly while the drillstring is rotating. Syncline n : a trough-shaped, folded structure of stratified rock. Target n : the objective defined by the geologist which the well must reach. Target area n : a specified zone around the target which the well must intersect. T Target bearing n : the direction of the straight line passing through the target and the reference point on the rig. This is used as the reference direction for calculating vertical section. T.D. abbr : Total Depth. Telescopic joint n : a component installed at the top of the marine riser to accommo- date vertical movement of the floating drilling rig. Thread protectors n : a device made of metal or plastic which is screwed onto pipe threads to prevent damage during transport or movement around the rig. 26  Glossary of Terms Tight formation n : a formation which has low porosity and permeability. Tongs n : the large wrenches used to connect and disconnect sections of pipe. The tongs have jaws which grip the pipe and torque is applied by pulling manually or mechanically using the cathead. Power tongs are pneumatically or hydraulically operated tools which spin the pipe. Tool face n : the part of the deflection tool which determines the direction in which deflection will take place. When using a bent sub the tool face is defined by the scribe line. Tool joint n : a heavy coupling device welded onto the ends of drill pipe. Tool joints have coarse tapered threads to withstand the strain of making and breaking connec- tions and to provide a seal. They also have seating shoulders designed to suspend the weight of the drillstring when the slips are set. On the lower end the pin connection is stabbed into the box of the previous joint. Hardfacing is often applied in a band on the outside of the tool joint to resist abrasion. Toolpusher n : an employee of the drilling contractor who is responsible for the drilling rig and the crew. Also called rig superintendent. Torque n : the turning force which is applied to the drillstring causing it to rotate. Torque is usually measured in ft-lbs. Tour n : (pronounced “tower”) an 8 hour or 12 hour shift worked by the drilling crew. Trajectory n : the path of the wellbore. Trap n : the geological structure in which petroleum reserves may have accumulated. Travelling block n : an arrangement of pulleys through which the drilling line is reeved, thereby allowing the drillstring to be raised or lowered. Trip v : to pull the drillstring out of the hole, or to run in back in. Trip gas n : a volume of gas (usually a small amount) which enters the wellbore while making a trip. Triplex pump n : a reciprocating mud pump with three pistons which are single acting. True North n : the direction of a line joining any point with the geographical North pole. Corresponds with an azimuth of 000˚. Tugger line n : a small diameter cable wound on an air operated winch which can be used to pick up small loads around the rig floor. Turbodrill n : a drilling tool located just above the bit which rotatesd the bit without Department of Petroleum Engineering, Heriot-Watt University 27 1 turning the drillstring. The tool consists of a series of steel bladed rotors which are turned by the flow of drilling fluid through the tool. T.V.D. abbr : True Vertical Depth. One of the co-ordinates used to plot the wellpath on the vertical plane. Twist off v : to sever the drillstring due to excessive force being applied at the rotary table. Underground blow out v : this situation arises when lost circulation and a kick occur simultaneously. Formation fluids are therefore able to enter the wellbore at the active zone and escape through an upper zone which has been broken down. (Sometimes called an “internal blow out") U Under ream v : to enlarge the size of the wellbore below casing. Upset n : the section at the ends of tubular goods where the OD is increased to give better strength. Valve n : a device used to control or shut off completely, the rate of fluid flow along a pipe. Various types of valve are used in drilling equipment. V door n : an opening in one side of the derrick opposite the drawworks. This opening is used to bring in pipe and other equipment onto the drill floor. V Vertical section n : the horizontal distance obtained by projecting the closure onto the target bearing. This is one of the co-ordinates used in plotting the wellpath on the vertical plane of the proposed wellpath. Viscometer n : a device used to measure the viscosity of the drilling fluid. Viscosity n : a measure of a fluid’s resistance to flow. The resistance is due to internal friction from the combined effects of cohesion and adhesion. Vug n : geological term for a cavity in a rock (especially limestone). Washout n : 1. Wellbore enlargement due to solvent or erosion action of the drilling fluid. 2. A leak in the drillstring due to abrasive mud or mechanical failure. Water back v : to reduce the weight and solids content of the mud by adding water. This is usually carried out prior to mud treatment. W 28  Glossary of Terms Water based mud n : a drilling fluid in which the continuous phase is water. Various additives will also be present. Water injector n : a well which is used to pump water into the reservoir to promote better recovery of hydrocarbons. Wear bushing n : a piece of equipment installed in the wellhead which is designed to act as a bit guide, casing seat protector and prevent damage to the casing hanger already in place. The wear bushing must be removed before the next casing string is run. Weight indicator n : an instrument mounted on the driller’s console which gives both the weight on bit and the hook load. Wellbore n : a general term to describe both cased hole and open hole. Wellhead n : the equipment installed at the top of the wellbore from which casing and tubing strings are suspended. Whipstock n : a long wedge-shaped pipe that uses an inclined plane to cause the bit to deflect away from its original position. Wildcat n : an exploration well drilled in an area where no oil or gas has been produced. Wiper trip n : the process by which the drill bit is pulled back inside the previous casing shoe and then run back to bottom. This may be necessary to improve the condition of the wellbore (e.g. smooth out any irregularities or dog legs which could cause stuck pipe later). Wireline n : small diameter steel wire which is used to run certain tools down into the wellbore. Also called slick line. Logging tools and perforating guns require conductor line. W.O.B. abbr : Weight On Bit. The load put on the bit by the drill collars to improve penetration rate. W.O.C. abbr : Waiting On Cement. The time during which drilling operations are suspended to allow the cement to harden before drilling out the casing shoe. W.O.W. abbr : Waiting On Weather. The time during which drilling operations must stop due to rough weather conditions. Usually applied to offshore drilling. Workover n : the carrying out of maintenance and remedial work on the wellbore to increase production. Department of Petroleum Engineering, Heriot-Watt University 29  Overview of Drilling Operations CONTENTS 1. INTRODUCTION 1.1 Exploration and Production Licences 1.2 Exploration, Development and Abandonment 2. DRILLING PERSONNEL 3. THE DRILLING PROPOSAL AND DRILLING PROGRAM 4. ROTARY DRILLING EQUIPMENT 5. THE DRILLING PROCESS 6. OFFSHORE DRILLING 7. DRILLING ECONOMICS 7.1 Drilling Costs in Field Development 7.2 Drilling Cost Estimates Institute of Petroleum Engineering, Heriot-Watt University 1 LEARNING OBJECTIVES: Having worked through this chapter the student will be able to: Exploration, Appraisal and Development: Describe the role of drilling in the exploration, appraisal and development of a field. Describe the types of information gathered during the drilling of a well. Define the objectives of an exploration, appraisal and development well. Describe the licensing process for an exploration, appraisal and development well. Personnel: Describe the organisations and people, and their respective responsibilities, involved in drilling a well. Describe the differences between a day-rate and turnkey drilling contract. Drilling and Completing a Well: Describe the steps involved in Drilling and Completing a well, highlighting the reasons behind each step in the operation. Drilling Economics : Identify the major cost elements when drilling a well Identify the major time consuming operations when drilling a well. 2  Overview of Drilling Operations 1. INTRODUCTION 1.1 Exploration and Production Licences : In the United Kingdom, the secretary of State for Energy is empowered, on behalf of the Government, to invite companies to apply for exploration and production licences on the United Kingdom Continental Shelf (UKCS). Exploration licences may be awarded at any time but Production licences are awarded at specific discrete intervals known as licencing ‘Rounds’. Exploration licences do not allow a company to drill any deeper than 350 metres (1148ft.) and are used primarily to enable a company to acquire seismic data from a given area, since a well drilled to 1148 ft on the UKCS would not yield a great deal of information about potential reservoirs. Production licences allow the licencee to drill for, develop and produce hydrocarbons from whatever depth is necessary. The cost of field development in the North Sea are so great that major oil companies have formed partnerships, known as joint ventures, to share these exploration and development costs (e.g. Shell/Esso). 1.2 Exploration, Development and Abandonment: Before drilling an exploration well an oil company will have to obtain a production licence. Prior to applying for a production licence however the exploration geologists will conduct a ‘scouting’ exercise in which they will analyse any seismic data they have acquired, analyse the regional geology of the area and finally take into account any available information on nearby producing fields or well tests performed in the vicinity of the prospect they are considering. The explorationists in the company will also consider the exploration and development costs, the oil price and tax regimes in order to establish whether, if a discovery were made, it would be worth developing. If the prospect is considered worth exploring further the company will try to acquire a production licence and continue exploring the field. This licence will allow the company to drill exploration wells in the area of interest. It will in fact commit the company to drill one or more wells in the area. The licence may be acquired by an oil company directly from the government, during the licence rounds are announced, or at any other time by farming-into an existing licence. A farm-in involves the company taking over all or part of a licence either: by paying a sum of money to the licencee; by drilling the committed wells on behalf of the licencee, at its own expense; or by acquiring the company who owns the licence. Before the exploration wells are drilled the licencee may shoot extra seismic lines, in a closer grid pattern than it had done previously. This will provide more detailed information about the prospect and will assist in the definition of an optimum drilling target. Despite improvements in seismic techniques the only way of confirming the presence of hydrocarbons is to drill an exploration well. Drilling is very expensive, and if hydrocarbons are not found there is no return on the investment, although valuable geological information may be obtained. With only limited information available a large risk is involved. Having decided to go ahead and drill an exploration well proposal is prepared. The objectives of this well will be: Institute of Petroleum Engineering, Heriot-Watt University 3  To determine the presence of hydrocarbons To provide geological data (cores, logs) for evaluation To flow test the well to determine its production potential, and obtain fluid samples. The life of an oil or gas field can be sub-divided into the following phases: Exploration Appraisal Development Maintenance Abandonment SEISMIC SURVEY DRILL EXPLORATION WELL DRILL APPRAISAL WELL MUD LOGGING Lithological and Textural Description of Formation from Drill Cuttings. Hydrocarbon Shows. CORING Lithological and Textural Description from Massive Sample. Samples used for Laboratory Analysis on Porosity, Permeability, Capillary Pressure etc. WELL LOGGING Electrical, Radioactive and Sonic Tools provide Quantitative Assessment of Fluid Type and Distribution. WELL TESTING Flowing from the Well allows large Representative Samples of the Reservoir Fluid to be recovered. Pressure Response of reservoir allows extent, Producibility and Drive Mechanisms of the Reservoir to be evaluated. Evaluate Information gathered above. From Exploration and Appraisal Information compile reservoir Model. Compile Economic Model. DRILL DEVELOPMENT WELLS Figure 1 Role of drilling in field development 4  Overview of Drilling Operations The length of the exploration phase will depend on the success or otherwise of the exploration wells. There may be a single exploration well or many exploration wells drilled on a prospect. If an economically attractive discovery is made on the prospect then the company enters the Appraisal phase of the life of the field. During this phase more seismic lines may be shot and more wells will be drilled to establish the lateral and vertical extent of (to delineate) the reservoir. These appraisal wells will yield further information, on the basis of which future plans will be based. The information provided by the appraisal wells will be combined with all of the previously collected data and engineers will investigate the most cost effective manner in which to develop the field. If the prospect is deemed to be economically attractive a Field Development Plan will be submitted for approval to the Secretary of State for Energy. It must be noted that the oil company is only a licencee and that the oilfield is the property of the state. The state must therefore approve any plans for development of the field. If approval for the development is received then the company will commence drilling Development wells and constructing the production facilities according to the Development Plan. Once the field is ‘on-stream’ the companies’ commitment continues in the form of maintenance of both the wells and all of the production facilities. After many years of production it may be found that the field is yielding more or possibly less hydrocarbons than initially anticipated at the Development Planning stage and the company may undertake further appraisal and subsequent drilling in the field. At some point in the life of the field the costs of production will exceed the revenue from the field and the field will be abandoned. All of the wells will be plugged and the surface facilities will have to be removed in a safe and environmentally acceptable fashion. 2. DRILLING PERSONNEL Drilling a well requires many different skills and involves many companies (Figure 2). The oil company who manages the drilling and/or production operations is known as the operator. In joint ventures one company acts as operator on behalf of the other partners. There are many different management strategies for drilling a well but in virtually all cases the oil company will employ a drilling contractor to actually drill the well. The drilling contractor owns and maintains the drilling rig and employs and trains the personnel required to operate the rig. During the course of drilling the well certain specialised skills or equipment may be required (e.g. logging, surveying). These are provided by service companies. These service companies develop and maintain specialist tools and staff and hire them out to the operator, generally on a day-rate basis. The contracting strategies for drilling a well or wells range from day-rate contracts to turnkey contracts. The most common type of drilling contract is a day-rate contract. In the case of the day-rate contract the operator prepares a detailed well design and program of work for the drilling operation and the drilling contractor Institute of Petroleum Engineering, Heriot-Watt University 5 simply provides the drilling rig and personnel to drill the well. The contractor is paid a fixed sum of money for every day that he spends drilling the well. All consumable items (e.g. drilling bits, cement), transport and support services are provided by the operator. In the case of the turnkey contract the drilling contractor designs the well, contracts the transport and support services and purchases all of the consumables, and charges the oil company a fixed sum of money for whole operation. The role of the operator in the case of a turnkey contract is to specify the drilling targets, the evaluation procedures and to establish the quality controls on the final well. In all cases the drilling contractor is responsible for maintaining the rig and the associated equipment. The operator will generally have a representative on the rig (sometimes called the “company man”) to ensure drilling operations go ahead as planned, make decisions affecting progress of the well, and organise supplies of equipment. He will be in daily contact with his drilling superintendent who will be based in the head office of the operator. There may also be an oil company drilling engineer and/or a geologist on the rig. The drilling contractor will employ a toolpusher to be in overall charge of the rig. He is responsible for all rig floor activities and liaises with the company man to ensure progress is satisfactory. The manual activities associated with drilling the well are conducted by the drilling crew. Since drilling continues 24 hours a day, there are usually 2 drilling crews. Each crew works under the direction of the driller. The crew will generally consist of a derrickman (who also tends the pumps while drilling), 3 roughnecks (working on rig floor), plus a mechanic, an electrician, a crane operator and roustabouts (general labourers). Service company personnel are transported to the rig as and when required. Sometimes they are on the rig for the entire well (e.g. mud engineer) or only for a few days during particular operations (e.g. directional drilling engineer). An overall view of the personnel involved in drilling is shown in Figure 2. DRILLING OPERATING CONTRACTOR COMPANY RIG DESIGN AND DRILLING ACCOUNTING RESERVOIR PRODUCTION DRILLING ACCOUNTING ENGINEERING SUPERINTENDANT MAINTENANCE SUPERINTENDANT ENGINEERING DRILLING OTHER ENGINEERING GEOLOGY RIGS TOOLPUSHER COMPANY MAN OTHER WELLS DRILLER SERVICE RIG CREW COMPANIES MUD DIRECTIONAL ENGINEERING DRILLING SURVEYING / MWD Figure 2 Personnel involved in drilling a well 6  Overview of Drilling Operations 3. THE DRILLING PROPOSAL AND DRILLING PROGRAM The proposal for drilling the well is prepared by the geologists and reservoir engineers in the operating company and provides the information upon which the well will be designed and the drilling program will be prepared. The proposal contains the following information: Objective of the Well Depth (m/ft Subsea), and Location (Longitude and Latitude) of Target Geological Cross section Pore Pressure Profile Prediction The drilling program is prepared by the Drilling Engineer and contains the following: Drilling Rig to be used for the well Proposed Location for the Drilling Rig Hole Sizes and Depths Casing Sizes and Depths Drilling Fluid Specification Directional Drilling Information Well Control Equipment and Procedures Bits and Hydraulics Program 4. ROTARY DRILLING EQUIPMENT The first planned oilwell was drilled in 1859 by Colonel Drake at Titusville, Pennsylvania USA. This well was less than 100 ft deep and produced about 50 bbls/day. The cable-tool drilling method was used to drill this first well. The term cable-tool drilling is used to describe the technique in which a chisel is suspended from the end of a wire cable and is made to impact repeatedly on the bottom of the hole, chipping away at the formation. When the rock at the bottom of the hole has been disintegrated, water is poured down the hole and a long cylindrical bucket (bailer) is run down the hole to collect the chips of rock. Cable-tool drilling was used up until the 1930s to reach depths of 7500 ft. In the 1890s the first rotary drilling rigs (Figure 3) were introduced. Rotary drilling rigs will be described in detail in the next chapter but essentially rotary drilling is the technique whereby the rock cutting tool is suspended on the end of hollow pipe, so that fluid can be continuously circulated across the face of the drillbit cleaning the drilling material from the face of the bit and carrying it to surface. This is a much more efficient process than the cable-tool technique. The cutting tool used in this type of drilling is not a chisel but a relatively complex tool (drillbit) which drills through the rock under the combined effect of axial load and rotation and will be described in detail in the chapter relating to drillbits. The first major success for rotary drilling was at Spindletop, Texas in 1901 where oil was discovered at 1020 ft and produced about 100,000 bbl/day. Institute of Petroleum Engineering, Heriot-Watt University 7  Crown Block Monkey Board Drilling Line Travelling Block Hook Swivel Kelly Hose Standpipe Kelly Weight Indicator Rotary table Drawworks Derrick Floor Mud Pump Blowout Preventer Cellar Shale Shakers Mud Flowline Conductor Drillpipe Drill Collar Drill Bit Figure 3 Drilling rig components 5. THE DRILLING PROCESS The operations involved in drilling a well can be best illustrated by considering the sequence of events involved in drilling the well shown in Figure 4. The dimensions (depths and diameters) used in this example are typical of those found in the North Sea but could be different in other parts of the world. For simplicity the process of drilling a land well will be considered below. The process of drilling a subsea well will be considered in a later chapter. 8  Overview of Drilling Operations The following description is only an overview of the process of drilling a well (the construction process). The design of the well, selection of equipment and operations involved in each step will be dealt with in greater depth in subsequent chapters of this manual. 30 in Casing Shoe 26 in. Hole 20 in Casing Shoe Cement 17 1/2 in. Hole 13 3/8 in Casing Shoe 12 1/4 in. Hole Figure 4 Typical hole and casing sizes Installing the 30” Conductor: The first stage in the operation is to drive a large diameter pipe to a depth of approximately 100ft below ground level using a truck mounted pile-driver. This pipe (usually called casing or, in the case of the first pipe installed, the conductor ) is installed to prevent the unconsolidated surface formations from collapsing whilst drilling deeper. Once this conductor, which typically has an outside diameter (O. D.) of 30” is in place the full sized drilling rig is brought onto the site and set up over the conductor, and preparations are made for the next stage of the operation. Institute of Petroleum Engineering, Heriot-Watt University 9 Drilling and Casing the 26” Hole: The first hole section is drilled with a drillbit, which has a smaller diameter than the inner diameter (I.D.) of the conductor. Since the I.D. of the conductor is approximately 28”, a 26” diameter bit is generally used for this hole section. This 26" hole will be drilled down through the unconsolidated formations, near surface, to approximately 2000'. If possible, the entire well, from surface to the reservoir would be drilled in one hole section. However, this is generally not possible because of geological and formation pressure problems which are encountered whilst drilling. The well is therefore drilled in sections, with casing being used to isolate the problem formations once they have been penetrated. This means however that the wellbore diameter gets smaller and smaller as the well goes deeper and deeper. The drilling engineer must assess the risk of encountering these problems, on the basis of the geological and formation pressure information provided by the geologists and reservoir engineers, and drilling experience in the area. The well will then be designed such that the dimensions of the borehole that penetrates the reservoir, and the casing that is set across the reservoir, will allow the well to be produced in the most efficient manner possible. In the case of an exploration well the final borehole diameter must be large enough to allow the reservoir to be fully evaluated. Whilst drilling the 26” hole, drilling fluid (mud) is circulated down the drillpipe, across the face of the drillbit, and up the annulus between the drillpipe and the borehole, carrying the drilled cuttings from the face of the bit to surface. At surface the cuttings are removed from the mud before it is circulated back down the drillpipe, to collect more cuttings. When the drillbit reaches approximately 2000’ the drillstring is pulled out of the hole and another string of pipe (surface casing) is run into the hole. This casing, which is generally 20" O.D., is delivered to the rig in 40ft lengths (joints) with threaded connections at either end of each joint. The casing is lowered into the hole, joint by joint, until it reaches the bottom of the hole. Cement slurry is then pumped into the annular space between the casing and the borehole. This cement sheath acts as a seal between the casing and the borehole, preventing cavings from falling down through the annular space between the casing and hole, into the subsequent hole and/or fluids flowing from the next hole section up into this annular space. Drilling and Casing the 17 1/2” Hole: Once the cement has set hard, a large spool called a wellhead housing is attached to the top of the 20” casing. This wellhead housing is used to support the weight of subsequent casing strings and the annular valves known as the Blowout prevention (BOP) stack which must be placed on top of the casing before the next hole section is drilled. Since it is possible that formations containing fluids under high pressure will be encountered whilst drilling the next (17 1/2”) hole section a set of valves, known as a Blowout prevention (BOP) stack, is generally fitted to the wellhead before the 17 1/2” hole section is started. If high pressure fluids are encountered they will displace the drilling mud and, if the BOP stack were not in place, would flow in an 10  Overview of Drilling Operations uncontrolled manner to surface. This uncontrolled flow of hydrocarbons is termed a Blowout and hence the title Blowout Preventers (BOP’s). The BOP valves are designed to close around the drillpipe, sealing off the annular space between the drillpipe and the casing. These BOPS have a large I.D. so that all of the necessary drilling tools can be run in hole. When the BOP’s have been installed and pressure tested, a 17 1/2" hole is drilled down to 6000 ft. Once this depth has been reached the troublesome formations in the 17 1/2" hole are isolated behind another string of casing (13 5/8" intermediate casing). This casing is run into the hole in the same way as the 20” casing and is supported by the 20” wellhead housing whilst it is cemented in place. When the cement has set hard the BOP stack is removed and a wellhead spool is mounted on top of the wellhead housing. The wellhead spool performs the same function as a wellhead housing except that the wellhead spool has a spool connection on its upper and lower end whereas the wellhead housing has a threaded or welded connection on its lower end and a spool connection on its upper end. This wellhead spool supports the weight of the next string of casing and the BOP stack which is required for the next hole section. Drilling and Casing the 12 1/4” Hole: When the BOP has been re-installed and pressure tested a 12 1/4" hole is drilled through the oil bearing reservoir. Whilst drilling through this formation oil will be visible on the cuttings being brought to surface by the drilling fluid. If gas is present in the formation it will also be brought to surface by the drilling fluid and detected by gas detectors placed above the mud flowline connected to the top of the BOP stack. If oil or gas is detected the formation will be evaluated more fully. The drillstring is pulled out and tools which can measure for instance: the electrical resistance of the fluids in the rock (indicating the presence of water or hydrocarbons); the bulk density of the rock (indicating the porosity of the rocks); or the natural radioactive emissions from the rock (indicating the presence of non-porous shales or porous sands) are run in hole. These tools are run on conductive cable called electric wireline, so that the measurements can be transmitted and plotted (against depth) almost immediately at surface. These plots are called Petrophysical logs and the tools are therefore called wireline logging tools. In some cases, it may be desireable to retrieve a large cylindrical sample of the rock known as a core. In order to do this the conventional bit must be pulled from the borehole when the conventional drillbit is about to enter the oil-bearing sand. A donut shaped bit is then attached a special large diameter pipe known as a core barrel is run in hole on the drillpipe. Institute of Petroleum Engineering, Heriot-Watt University 11  Christmas Tree Wellhead Casing Strings Production Tubing Packer Pay Zone Figure 5 Completion schematic 12  Overview of Drilling Operations This coring assembly allows the core to be cut from the rock and retrieved. Porosity and permeability measurements can be conducted on this core sample in the laboratory. In some cases tools will be run in the hole which will allow the hydrocarbons in the sand to flow to surface in a controlled manner. These tools allow the fluid to flow in much the same way as it would when the well is on production. Since the produced fluid is allowed to flow through the drillstring or, as it is sometimes called, the drilling string, this test is termed a drill-stem test or DST. If all the indications from these tests are good then the oil company will decide to complete the well. If the tests are negative or show only slight indications of oil, the well will be abandoned. Completing the Well: If the well is to be used for long term production, equipment which will allow the controlled flow of the hydrocarbons must be installed in the well. In most cases the first step in this operation is to run and cement production casing (9 5/8" O.D.) across the oil producing zone. A string of pipe, known as tubing (4 1/2" O.D.), through which the hydrocarbons will flow is then run inside this casing string. The production tubing, unlike the production casing, can be pulled from the well if it develops a leak or corrodes. The annulus between the production casing and the production tubing is sealed off by a device known as a packer. This device is run on the bottom of the tubing and is set in place by hydraulic pressure or mechanical manipulation of the tubing string. When the packer is positioned just above the pay zone its rubber seals are expanded to seal off the annulus between the tubing and the 9 5/8" casing (Figure 5). The BOP’s are then removed and a set of valves (Christmas Tree) is installed on the top of the wellhead. The Xmas tress is used to control the flow of oil once it reaches the surface. To initiate production, the production casing is “perforated” by explosive charges run down the tubing on wireline and positioned adjacent to the pay zone. Holes are then shot through the casing and cement into the formation. The hydrocarbons flow into the wellbore and up the tubing to the surface. 6. OFFSHORE DRILLING About 25% of the world’s oil and gas is currently being produced from offshore fields (e.g. North Sea, Gulf of Mexico). Although the same principles of rotary drilling used onshore are also used offshore there are certain modifications to procedures and equipment which are necessary to cope with a more hostile environment. In the North Sea, exploration wells are drilled from a jack-up (Figure 6) or a semi- submersible (Figure 7) drilling rig. A jack-up has retractable legs which can be lowered down to the seabed. The legs support the drilling rig and keep the rig in position (Figure 6). Such rigs are generally designed for water depths of up to 350 ft water depth. A semi-submersible rig is not bottom supported but is designed to float (such rigs are commonly called “floaters”). Semi-submersibles can operate in water depths of up to 3500 ft. (Figure 7). In very deep waters (up to 7500 ft) drillships (Figure 8) are used to drill the well. Since the position of floating drilling rigs is constantly changing relative to the seabed special equipment must be used to Institute of Petroleum Engineering, Heriot-Watt University 13 connect the rig to the seabed and to allow drilling to proceed. The equipment used to drill wells from these drilling rigs will be discussed at length in a subsequent chapter. If the exploration wells are successful the field may be developed by installing large fixed platforms from which deviated wells are drilled (Figure 9). There may be up to 40 such wells drilled from one platform to cover an entire oilfield. For the very large fields in the North Sea (e.g. Forties, Brent) several platforms may be required. These deviated wells may have horizontal displacements of 10,000 ft and reach an inclination of 70 degrees or more. For smaller fields a fixed platform may not be economically feasible and alternative methods must be used (e.g. floating production system on the Balmoral field). Once the development wells have been drilled the rig still has a lot of work to do. Some wells may require maintenance (workovers) or sidetracks to intersect another part of the reservoir (re-drill). Some wells may be converted from producers to gas injectors or water injectors. Figure 6 Jack-up rig 14  Overview of Drilling Operations H Figure 7 Semi- submersible rig H Figure 8 Drillship Institute of Petroleum Engineering, Heriot-Watt University 15  Figure 9 Fixed platform (Steel Jacket) A well drilled from an offshore rig is much more expensive than a land well drilled to the same depth. The increased cost can be attributed to several factors, e.g. specially designed rigs, subsea equipment, loss of time due to bad weather, expensive transport costs (e.g. helicopters, supply boats). A typical North Sea well drilled from a fixed platform may cost around $10 million. Since the daily cost of hiring an offshore rig is very high, operating companies are very anxious to reduce the drilling time and thus cut the cost of the well. 7. DRILLING ECONOMICS 7.1 Drilling Costs in Field Development It is quite common for Drilling costs to make up 25-35% of the total development costs for an offshore oilfield (Table 1). The costs of the development will not be recovered for some time since in most cases production is delayed until the first few platform wells are drilled. These delays can have a serious impact on the economic feasibility of the development and operators are anxious to reduce the lead time to a minimum. the development wells are being drilled. 16  Overview of Drilling Operations Cost ($ million) Platform structure 230 Platform equipment 765 Platform installation 210 Development drilling 475 Pipeline 225 Onshore facilities 50 Miscellaneous 120 Total 2075 Table 1 Estimated development costs (Brae field) 7.2 Drilling Cost Estimates Before a drilling programme is approved it must contain an estimate of the overall costs involved. When drilling in a completely new area with no previous drilling data available the well cost can only be a rough approximation. In most cases however, some previous well data is available and a reasonable approximation can be made. A typical cost distribution for a North sea Well is Shown in Table 2. Some costs are related to time and are therefore called time-related costs (e.g. drilling contract, transport, accommodation). Many of the consumable items (e.g. casing, cement) are related to depth and are therefore often called depth-related costs. These costs can be estimated from the drilling programme, which gives the lengths or volumes required. Some of the consumable items such as the wellhead will be a fixed cost. The specialised services (e.g. perforating) will be a charged for on the basis of a service contract which will have been agreed before the service is provided. The pricelist associated with this contract will be a function of both time and depth and the payment for the service will be made when the operation has been completed. For wells drilled from the same rig under similar conditions (e.g. platform drilling) the main factor in determining the cost is the depth, and hence the number of days the well is expected to take. Figure 10 shows a plot of depth against days for wells drilled from a North Sea platform. It is interesting to note that of the total time spent drilling a well less than half is spent actually rotating on bottom (Table 3). Institute of Petroleum Engineering, Heriot-Watt University 17  Breakdown of Well Costs ($1000) ( %) Wellhead 105 1.1 Flowline and surface equipment 161 1.7 Casing and downhole equipment 1465 15.5 Sub- total 1731 18.3 Drilling contractor 2061 21.8 Directional drilling/surveying 319 3.4 Logging/testing/perforating 603 6.4 Mud processing/chemicals 858 9.1 Cementing 288 3.0 Bits 282 3.0 Sub-total 4411 46.7 Transport 1581 16.7 Equipment rental 391 4.1 Communications 120 1.3 Mobilisation 686 7.3 Power and fuel 225 2.4 Supervision 300 3.2 Sub-total 3303 35.0 Total well cost $9,445,000 TABLE 2 Breakdown of well costs Time breakdown for a North Sea well (fixed platform) HOURS % Drill 552.0 41.9 Trips/Lay Down Drill Pipe 195.0 14.8 Directional Surveys 104.0 7.9 Core/Circ. Samples 91.5 6.9 Guide Base/Conductor 60.0 4.6 Wash/Ream/Clean Out Borehole 59.0 4.5 Lost Time 49.5 3.8 Run Casing/Tubing/Packer 37.5 2.8 Nipple down, up/Run Riser 37.0 2.8 Log/Set Packer/Perforate 26.5 2.0 Test Bops/Wellhead 25.0 1.9 Rig Maintenance 20.5 1.6 Circ. & Cond./Displace Mud 20.5 1.5 Fishing/Milling 20.0 1.5 Cement/Squeeze/WOC 18.0 1.4 Rig Down/Move/Rig Up