Drilling Engineering III PET 534 PDF

DRILLING ENGINEERING III PET 534 ENGR.DIKE, H. N. Ph.D 1  www.covenantuniversity.edu.n Raising a new Generation of Leaders DRILLINGING ENGINEERING III PET 534 MODULE1: Reviews; dril...

DRILLING ENGINEERING III PET 534 ENGR.DIKE, H. N. Ph.D 1  www.covenantuniversity.edu.n Raising a new Generation of Leaders DRILLINGING ENGINEERING III PET 534 MODULE1: Reviews; drilling problems, kicks & blow outs, under-balanced & overbalanced drilling, casing, tubing, cementing, and Perforations(WK1-2) ENGR. DR. DIKE, HUMHREY N.  LECTURE OUTLINE REVIEW:(WK1-2) Drilling problems: Casing, tubing, Cementing, and Perforations Well control Kick & Blow-outs;over-balance Over-balance and Under-balance drilling 3  MODULE BENEFITS (OUTPUT) Student should be able to: Recall and Describe major problems associated with drilling operation Describe the importance of casing and cementing operation in well engineering. Describe the types of casing, cementing and associated accessories and equipment during drilling operation. Identify the major differences between under-balance and over-balance drilling, application, condtions and challenges. Describe tubing, packers and perforations as completion operation and their importance in well operation. Identify drilling condition for kick occurance, causes, method and prevention mechanism Describe the relevance of the above to PET534 MODULE 4 1  2 Introduction Problems associated with the drilling of oil and gas wells are largely due to the disturbances of earth stresses around the borehole. These are caused by the creation of the hole itself and by drilling mud/formation interaction. Thus, a hole is kept open (or stable) by maintaining a balance between earth stresses and pore pressure on one side and well bore mud pressure and chemical composition on the other side. Whenever this balance is disturbed, well bore problems 6  2 Introduction Problems associated with the drilling of oil and gas wells are largely due to the disturbances of earth stresses around the borehole. These are caused by the creation of the hole itself and by drilling mud/formation interaction. Thus, a hole is kept open (or stable) by maintaining a balance between earth stresses and pore pressure on one side and well bore mud pressure and chemical composition on the other side. Whenever this balance is disturbed, well bore problems occur. 7  3 The major Downhole problems are: Pipe sticking Pipe failure Dog legs and telescopic holes (Crookedness of hole) Key seats in holes Shale problems Lost circulation problems  4 Pipe Sticking One of the most common problems, encountered while drilling a well is pipe sticking. The problem is even more serious and expensive when drilling deep high pressure wells in geologically complex areas. 9  5 Basic terms related with stuck ups Tight pull: It is the amount of extra pull required to pull the string through a tight spot. It is experienced while pulling out and can be directly read on Drill meter or recording charts. Hold up: It is the amount of weight that has to be released to overcome a bridge. It is experienced while running in and can be directly read on Drill meter or recording charts. 10  6 Basic terms related with stuck ups Stuck string: A string is said to be stuck when no free movement is possible either in terms of reciprocation or rotation. Free point: The part of string which is glued to the hole wall is called stuck part and other part which is free is called free part. The junction point at which stuck part ends and free part of string begins is called free point. 11  7 Mechanical Sticking There are mainly two types of pipe sticking. They are as follows: Mechanical sticking Differential sticking A pipe is said to be mechanically stuck, when the stuck up is due to mechanical reasons. 12  8 Mechanical stuck up can be associated with anyone of the following drilling conditions. Distorted well bore geometry Under gauge hole Poor hole cleaning Key seating Junk in the hole Cement related problems Collapsed casing  9 Distorted Well Bore Geometry Doglegs and ledges can stick the drill string, particularly when pulling out because while running in, due to less tension the flexible string can easily by pass the obstacles. 14  37 Dog leg Dog legging is a severe drilling problem. No hole is perfectly vertical and any hole has a tendency to spiral. An abrupt changein hole deviation (inclination and/or azimuth) results in troublesome situation which is commonly referred to as a dogleg. Can cause pipe sticking and lead to pipe failure 15  10 Under Gauge Hole Under gauge hole occurs when gauge protection on the bit becomes ineffective while drilling long sections of abrasive formations. If care is not taken new bit can be jammed in an under gauge hole. It can also happen if caution is not taken following a change from a diamond or core bit to roller bit. 16  38 Telescopic Holes Some times it is difficult to stick to planned casing policies of the wells especially in exploratory wells. Casings sometimes do not reach the drilled depth and have to be cemented with shoe much above bottom. This leaves a large open hole from which a smaller hole is drilled. In such wells, the annular velocity of mud falls abruptly in reaching over from smaller to larger well bore. This leads to gradual accumulation of cuttings in the transition zone. During 17  39 Preventive measures (1) Efforts should be made to lower a liner into the telescopic part. (2)The zone, ifleft open should be frequently washed to remove the accumulatedcuttings. 18  11 Poor hole cleaning Poor hole cleaning results in accumulation of cuttings in the annulus, resulting sticking of the drill string. This is usually in wash out or in c aved portion where annular velocity decreases and cuttings get accumulated. In deviated wells cuttings from the low side of the hole can migrate up hole like shifting of 19  12 Key Seating Key seats, grooves in the bore hole wall cut by rotating drill pipe can stick larger diameter collars when tripping out. A key seat wiper placed higher in the string can widen the key seat, allowing collars to pass. 20  13 Junk in hole Junk is any object in the hole which is not desired in the well. Roller cones or even PDC cutters are large enough to stick the string. Junk must be fished out with a reverse circulation junk basket / magnet or should be milled out with flat mill. 21  14 Cement related stuck up Cement related sticking occurs when blocks of cement fall into the well bore from casing rat holes or cement plugs jamming drill string. It also occurs when drill string is run in soft or green cement that flash sets when pressure is applied. Drilling of cement without proper pump discharge will lead to 22 To free the Mechanically stuck 15 pipe If the drill string gets stuck while moving up, jar down with maximum trip load and torque can be applied into drill string while jarring down. Be caution while applying torque, do not exceed make up torque. On the other hand, if the drill string gets stuck while moving down, jar up with maximum trip load. DO NOT apply torque in the drill string while jarring up. Flow rate must be reduced while attempting to free the drill string. Do not use high flow rate because it will make the stuck situation became worse and you will not be able to free the pipe forever. 23 To free the Mechanically stuck 15 pipe If the drill string gets stuck while moving up, jar down with maximum trip load and torque can be applied into drill string while jarring down. Be caution while applying torque, do not exceed make up torque. On the other hand, if the drill string gets stuck while moving down, jar up with maximum trip load. DO NOT apply torque in the drill string while jarring up. Flow rate must be reduced while attempting to free the drill string. Do not use high flow rate because it will make the stuck situation became worse and you will not be able to free the pipe forever. 24  16 Differential Sticking Differential Sticking is one of the most common causes of pipe stuck. It can happen when there is differential pressure (overbalance pressure) pushing a drill string into filter cake of a permeable formation. 25  17 Four factors causing the differential sticking are as follows: Permeable formation as sand stone, lime, carbonate, etc. Overbalance – typically mud weight in the well is more than formation pressure. More overbalance in the wellbore, more chance of getting differential sticking. Filter cake – Poor and thick filter cake increases chances of sticking the drill string. Pipe movement – if the drill string is station for a period of time, the filter cake will tend to develop around permeable zones and the drill string. Therefore, potential of getting differentially stuck is increased.  18 Warning signs There are high over balance between wellbore and formation. Especially, when there is highly depleted formation, the chance of getting differentially stuck is so high. Torque, pick up and slack off weight increase when the drill string is being moved. Once it happens, you may not be able to pull or rotate pipe. 27  19 Stuck identification for differential sticking Drill string is in station for a period of time. The differential sticking is happened when there is no pipe movement for long time. Circulation can be established without increasing in pressure. BHA is across the permeable zone 28 20  21 Formation pressure = 3800 psi Hydrostatic pressure =4500 psi Cross area of stuck pipe = 1500 square inch Force = Differential Pressure x Cross Section Area Where Force is in lb. Differential pressure is in psi. Cross section area is in square inch. Force = (4500 – 3800) x 1500 Force = 1,050,000 lb Reme 22 dies Apply torque into drill string and jar down with maximum allowable trip load. Jar up without apply torque in the drill string. Spot light weight pill to decrease hydrostatic pressure. If you want to the light weight pill, you must ensure that the overall hydrostatic pressure is more than formation pressure. Otherwise, you will face with a well control situation. 31  23 Lost Circulation Lost circulation is the significant and continuing loss of whole mud to a formation. It is probably the most common and, overall, the most costly hole problem 32  24 Depending on its severity, lost circulation can lead to- Increased costs for drilling mud and associated materials. Formation damage and decreased productivity. Wellbore fluid level drops, resulting in increased potential for stuck pipe, borehole instability and kicks. Lost formation evaluation data, since the information normally obtained from drilled cuttings and mud returns may be unobtainable 33  25 Occurrences Lost circulation primarily occurs in: Unconsolidated or permeable formations Naturally fractured formations Zones containing induced fractures Cavernous or extremely vugular formations Wells experiencing mechanical problems such as shallow casing leaks 34  26 Unconsolidated or permeable formations Unconsolidated or highly permeable formations porous have long been recognized as contributors to lost circulation. They generally experience seepage, although continued drilling may result in a partial or complete 35  27 Naturally fractured formations Natural fractures exist both in carbonate rocks and in hard sandstones or shales. Lost circulation occurs when a well exposes enough fractures of sufficient supported width to cause significant loss of whole fluid. 36  28 Zones containing induced fractures Induced fractures can result from surge effects, problems with the mud system, or a surface or intermediate casing string set too high. Horizontal fractures are often marked by increased pump pressures and tight drill strings, while large, induced vertical fractures usually cause a sudden, complete loss of returns. 37  29 Cavernous or extremely vugular formations Caverns are normally associated with limestone formations that have been leached by water The large size of these void spaces may cause drill strings to drop from several inches to several feet preceding a sudden, complete loss of returns. 38  31 Methods to prevent Lost Circulation Mud Considerations Minimizing Surge Effects Pre-treatment with LCMs 39  32 Shale problems – Hydro pressured Shale Hydro-pressured shale is a common problem in some area and it could cause stuck pipe. This is also called shale sloughing. With mud weight in the wellbore higher than formation pressure, pore pressure of shale is always charged by hydrostatic pressure from drilling mud. 40  33 When the well has been drilled for a period of time, shale formations become unstable due to charged pressure and finally shale breaks apart and falls down into the hole  34 Finally, a drill string gets stuck due to hydro- pressured shale which accumulates in the annulus  36 Shale finally falls into the well and results in stuck pipe incident  35 Shale problems – Geo pressured Shale Pore pressure in shale is more than hydrostatic pressure; however the well does not flow because shale is in permeable. While drilling through pressured shale formation, pressure in shale causes fractures of shale due to stress crack. 44  40 Drill pipe Failure For new drill pipe, its connection (Tooljoint) is stronger than its body. But practical field experience shows that in most of the cases drill pipe fails from its tool joint and drill collar fails from its connection. 45 41  42 Reasons for Drill pipe failure Improper tightening of connection-Use of under torquing or over torquing during connection. Improper initial making-up of connection. Proportionate reduction in make-up torque not applied while diameter of tool joint is reduced. Swelling of connection box. Thinning out of connection thread and still continuing use. 47  43 Improper connection type. Due consideration wasnot paid towards connection size and drill collar size change over while operating in a deviated/directional hole or against a dogleg. Lack of periodical inspection of pipes for evaluation of its present working strength  44 Remed ies Use of proper make up torque. Limits for reduction in diameter of tool joints and swelling of box. Proper making up procedure. Limits on minimum bending strength ratio. Proper precautions pertaining to thinning out of tool joint thread. 49 CASING PROGRAM  CASING: INTRODUCTION Casing serves several important functions in drilling and completing a well It prevents collapse of the borehole during drilling and hydraulically isolates the wellbore fluids from the subsurface formations and its fluids It minimizes damage of both the subsurface environment by the drilling process and the well by a hostile subsurface environment It provides a high-strength flow conduit for the drilling fluid to the surface and with the BOP permits the safe control of formation pressure Selective perforation of properly cemented casing also permits isolated communication with a given formation of interest ENGR. DR DIKE, H. N.. 52 ENGR. DR. DIKE, H. N. ENGR. DR. DIKE, H. N  CASING DOWNHOLE ACCESSORIES Guide shoe –They simply guide Centralizers –Placed on the the casing pass the outside of the casing, they hold irregularities in the borehole the casing in the centre of the wall. hole. Scratchers –Placed on the They also allow for entrapment outside of the casing, they are of contaminated mud or used to help remove mudcake cement, which may result from from the borehole walls. the wiping action of the cementing plug Cement baskets –Placed on the outside of the casing, they are used to help support the weight of the cement slurry at points  hinged metal ribs which are installed on the casing string as it is run CASING DOWNHOLE Function: keep casing away from the ACCESSORIES borehole so that there is some CENTRALIZERS annular clearance around the entire PURPOSE circumference of the casing -centralize casing and The proper use of centralisers will provide stand-off from bore- hole help to: -minimize channelling (a) Improve displacement efficiency -enable uniform distribution (i.e. place cement all the way around of cement scratches and the casing) wiper scrape ‘wall-cake’ (b) Prevent differential sticking from bore-hole (c) Keep casing out of keyseats ENGR. DR. DIKE.H. N. ENGR. DR. DIKE. H. N 58 ENGR. DR. DIKE, H. N.. EXAMPLE HOLE AND CASING SIZES ENGR. DR. DIKE, H. N. CONDUCTOR CASING This is the first string set in a well. The setting depth of the conductor casing can vary from 10ft to more than 300ft. The outside diameter ranges from 16inches to 36inches. The functions of the conductor casing among others include: Acting as a conduit for drilling fluid Casing off shallow unconsolidated formations Provision of structural support for the well DR. DIKE, H. N  SURFACE CASING The purpose of the surface casing is to provides blowout protection during drilling. Setting depth is often chosen to isolate:  Troublesome formations,  Loss zones,  Shallow hydrocarbons,  Water sands, ENGR. DR. DIKE, H..N. 62  INTERMEDIATE CASING The intermediate casing is a string which is used to isolate weak formations, FUNCTIONS To case off Loss zones, Sloughing, caving and reservoir formations. Such strings are also set in transition zones ENGR. DR. DIKE, H. N. 63 PRODUCTION CASING The Production casing is the last full string of pipe set in the well. Sometimes liners are used instead of production casing. The production string extends from the surface to the deepest producing formation. It must be small enough to fit through all the previous casing. The most common sizes are 4.5inches, 5.5inches and 7inches casing. It will be cemented, and then perforated in the producing zone DR. DIKE, H. N.  CASING SETTING DEPTHS The choice of setting depths for all the casing strings is a vital part of the well planning process An incorrect decision with the casing setting depths too shallow could have serious consequences An unnecessarily deep setting depth could have adverse economic consequences when considering the extra time needed to drill the hole deeper and the extra amount of casing required to be run and cemented The selection of deeper casing setting depths will use different criteria to those used for shallow casing seats Initial selection of the setting depth is made with reference to the anticipated lithological column, formation pressure and fracture gradient profiles ENGR. DR. DIKE, H.N. 65 CASING SETTING DEPTH 66 CASING SETTING DEPTH 67 If the well looks good on the logs, we run a final string of casing across the production zone, and cement it in place. Then, we run perforating guns in the hole and perforate (shoot holes ) in the casing across the productive zone. Production tubing is run, with a packer to isolate the produced zone from the casing above. tubing Packer  REVIEW: CEMENTING OPERATION  THE MAKING OF AN OIL WELL Cementing Carried out after casing is in place Cement is transported in special equipments Cements, water and special chemicals called “additives” are mixed Cement in slurry form is pumped into hole using special equipment Cement is allowed to set 73  THE MAKING OF AN OIL WELL CEMENTING OPERATIONS Cementing is the process of mixing a slurry of cement and water and pumping it down through casing to critical points in the annulus around the casing or in open hole below the casing string. Two types of cementing operations are usually carried out: ­Primary cementing ­Remedial or secondary cementing Success of well operations depends on this primary operation Two principal functions of primary cementing are : to restrict fluid movements between the formations to bond and support the casing 74  THE MAKING OF AN OIL WELL OTHER FUNCTIONS OF CEMENT To protect casing from corrosion Preventing blowouts by quickly forming a seal Protecting casing from shock loads in deeper drilling Sealing off zones of lost circulation or thief zones 75 REVIEW: Well control Kick & Blow-outs;over-balance Over-balance and Under-balance drilling  Background Targeting deep for hydrocarbon (HC) exploration requires the understanding of narrow window between formation pore pressure, hydrostatic pressure and fracture pressure This is quite challenging when drilling deep because of underground pressure profile of the formation. Therefore, operating within acceptable window as well control measure becomes a concern. Problems associated with narrow pressure windows are;  Well bore stability  Kick  Lost circulation  Differential sticking  Well control ENGR. DR. DIKE.H. N. 77  Mud Circulating system The Figures below shows the hydraulic flow paths of drilling fluid during drilling operations. 78 ENGR. DR. DIKE, H. N.  Pressure control during drilling Pressure under the earth is one of the most dangerous things that drilling personnel need to deal with. If the pressure is not handled properly, lost of the well can occur. Fluids existing in the formation are under pressure. When drilled, this pressure can escape to the surface if it is not controlled. Normally, drilling mud offsets formation pressure, that is the weight or pressure of the drilling mud keeps fluids in the formation from flowing to the surface. For several reasons however, the mud weight can become lighter than is necessary to offset the pressure in the formation. When this situation occurs, formation fluid enters the hole. When formation fluids enter the hole, this is called a “kick”. Pressure control during drilling A blowout preventer stack is used to keep formation fluids from coming to the surface. These are called BOP’s. By closing off a hole in this equipment the rig crew can seal off the hole. Sealing the hole prevents more formation fluid from entering the hole. With the well sealed or shut-in, the well is under control. Rig crews use a surface BOP system on land rigs, jack up rigs, submersible rigs and platform rigs. They use a sub-sea BOP system on offshore floating rigs like semi-submersibles and drill ships. A blowout is dangerous. Formation fluids like gas and oil rise to the surface and burn. Blowouts can injure or destroy the rigs or the environment. Rig crews therefore train and work hard to prevent blowouts. 81 INTRODUCTI ON TO UNDERBALA NCED DRILLING WALT 1. Describe Underbalance Drilling Technique 2. Explain the advantages of Underbalance Drilling Technique over the Convention Drilling Technique 3. Describe the process of Underbalance Drilling Fluid Design 83  UNDERBALANCED DRILLING - INTRODUCTION Conventional drilling practice calls for maintaining the hydrostatic pressure of the drilling fluid between the formation's pore pressure and its fracture pressure The drilling fluid is continuously circulated within the wellbore to control the formation fluids and transport cuttings to the surface It also works as a stabilizing agent within the wellbore, and lubricates and cools the drill bit The conventional practice has long been recognized as the safest method for drilling a well and It does, however, have drawbacks Since the drilling fluid pressure is higher than the formation pressure, fluid invasion frequently occurs, causing permeability damage to the formation This damage is mainly caused by washout or physical blockage by the intrusion of fluids and/or solids into the formation structure 84  VARIOUS TECHNIQUES OF DRILLING Under Balance Drilling (UBD)  It is defined as the practice of drilling a well with the wellbore fluid gradient less than the natural formation gradient  It differs from conventional drilling in that the bottomhole circulating pressure is lower than the formation pressure, thereby permitting the well to flow while drilling proceeds  Besides minimizing lost circulation and increasing the penetration rate, this technique has a widely recognized benefit of minimizing the damage caused by invasion of drilling fluid into the formation  In many UBD applications, additional benefits are seen due to reduction in drilling time, increased bit life, and early detection  Because the majority of hydrocarbons today are found in existing fields with depleting pressures, or in complex and low quality reservoirs, the economical use of UBD becomes more and more popular 85  UNDERBALANCED DRILLING (UBD) UBD is defined as the practice of drilling a well with the wellbore fluid gradient less than the natural formation gradient In this type of drilling, the hydrostatic pressure of drilling fluid is intentionally designed to be lower than the pressure of the formation drilled. This is usually done naturally during design or induced by adding different substances to the liquid phase of the mud (N/G; Nitrogen and Air). In any case, the target is to achieve an influx of formation fluid into the well, circulated to the surface where it is been controlled. It differs from conventional drilling in that the bottomhole circulating pressure is lower than the formation pressure, thereby permitting the well to flow while drilling proceeds 86  UNDERBALANCED DRILLING (UBD) Besides minimizing lost circulation and increasing the penetration rate, this technique has a widely recognized benefit of minimizing the damage caused by invasion of drilling fluid into the formation In many UBD applications, additional benefits are seen due to reduction in drilling time, increased bit life, and early detection and dynamic testing of productive intervals while drilling Because the majority of hydrocarbons today are found in existing fields with depleting pressures, or in complex and low quality reservoirs, the economical use of UBD becomes more and more popular 87  CHARACTERISTICS OF UBD During drilling operation, the effective down-hole circulating pressure (the pressure required to circulate the fluid in and out the hole) is function of hydrostatic pressure (Head) of the drilling fluid plus pressure due frictional losses plus any pressure applied at the surface For conventional OBD: The sum of the pressures must be greater than the formation pressure= Primary well control mechanism; fluid still in place (No Kick). The column of fluid with certain density in hole provides the primary mechanism OBD= Pres< P(Bh.)= Hp + Pf.+ Pc. (where; Pc= choke pressure; Pf =frictional pressure; Bh= Bottom hole flowing Pressure. UBD= Pres> Bhp= Hp + Pf+ Pc 88 UB DRILLING - JOBS  UNDERBALANCED DRILLING TECHNIQUES Underbalanced drilling techniques are classified according to density of the fluids used in the process Typical fluid densities range from near 0 to 7 pounds per gallon In fresh-water applications, the density of the circulating fluid can be reduced by nitrogen gas injection This reduced density helps to achieve a bottom hole circulating pressure that is less than that of the formation pressure Even conventional liquids can provide underbalanced conditions with proper density control of the drilling fluid On the other hand, it is also possible for a low-density fluid to cause overbalance due to the frictional pressure drop 91  UNDERBALANCED DRILLING TECHNIQUES Underbalanced drilling has proved to be an economical method for drilling in depleted/low pressure reservoirs Since it is possible to record production during drilling, operators can easily and accurately identify inflow mechanisms and pay intervals, and cease drilling operation as soon as the target zones are identified One method of controlling the bottomhole pressure (BHP) is to use a choke at the surface BHP is controlled by opening or closing the choke to lower or raise the standpipe pressure This technique essentially creates an increasing fluid density gradient between the surface and the bottomhole Since ECD is a function of flow, underbalanced conditions should be preserved by controlling the hydrostatic head when flow stops during connections 92  ADVANTAGES OF UNDERBALANCED DRILLING Faster Drilling Rates: One of the primary advantages of UBD is the potential for faster drilling rates. By maintaining a lower pressure in the wellbore, there is less risk of wellbore instability issues such as stuck pipe or differential sticking. This allows for more efficient drilling operations. Reduced Formation Damage: Underbalanced drilling can help minimize formation damage by preventing invasion of drilling fluids into the reservoir. This is particularly important in reservoirs with low permeability or sensitive formations where invasion of drilling fluids could impair production. Improved Wellbore Stability: Underbalanced conditions reduce the likelihood of wellbore collapse or instability. This is especially beneficial in areas with weak or unconsolidated formations. Enhanced Recovery Rates: UBD can contribute to improved hydrocarbon recovery rates. By minimizing formation damage and optimizing drilling rates, reservoir productivity may be enhanced. 93  ADVANTAGES OF UNDERBALANCED DRILLING Extended Reach and Horizontal Drilling: Underbalanced drilling is often used in extended reach and horizontal drilling scenarios where conventional drilling methods may encounter difficulties. The reduced frictional pressure allows for smoother drilling operations in these challenging well profiles. Early Detection of Formation Fluids: Maintaining underbalanced conditions allows for early detection of formation fluids, which can be crucial for identifying productive zones and optimizing well placement. Reduced Lost Circulation: Underbalanced drilling can help prevent lost circulation issues, where drilling fluids are lost to the formation, by maintaining a pressure lower than the formation pressure. It's important to note that while underbalanced drilling offers these advantages, it also presents challenges and risks, such as well control issues and the potential for kicks or blowouts. Therefore, the decision to use underbalanced drilling should be based on a careful evaluation of the specific reservoir conditions and drilling objectives, and it requires thorough planning and well control measures to ensure safety and success. 94  UNDERBALANCED DRILLING FLUIDS There are three primary types of fluids used in underbalanced drilling operations: Gaseous (Compressible)  The oldest and most basic technique is dry air drilling, which involves pumping air down the drill string and up through the annulus.  Nitrogen is another common drilling fluid and Natural gas is also a drilling fluid option, since it is easily available from pipelines Two-phase  It consist of either foam-type fluids or aerated drilling mud  Liquids are mixed with gas to achieve a required circulating fluid density  The equation of state method is used to predict fluid properties at downhole conditions 95  UNDERBALANCED DRILLING FLUIDS Liquid (Incompressible)  Since formation pressure is usually larger than the hydrostatic pressure of fresh water or saline water, conventional drilling fluids might also provide underbalanced conditions  Even if the drilling fluid density exceeds the formation pore gradient, fluid loss into a formation can cause reduced pressure regions within the wellbore, thus allowing formation fluids to flow in The fluid type is dictated by the boundary conditions of the drilling system Typically, the boundary conditions are defined by bottomhole flowing pressure, formation fracture pressure, borehole collapse pressure and formation pore pressure The density range of various drilling fluids is summarized in the following diagram 96  DRILLING FLUID SELECTION Fluid system design is one of the most overlooked parts of underbalanced projects In designing an UB fluid system the impact on the desired ECD must be considered ECD is a combination of annular fluid density, frictional pressure loss in the annulus and surface chock pressure The carrying capacities of underbalanced fluids range from extremely poor for pure gas systems, to extremely good in foam systems Temperature stability must also be considered in designing an underbalanced fluid system 98  DRILLING FLUID SELECTION Two different measures are used GAS-TO-LIQUID RATIOS to define the type of fluid OF VARIOUS DRILLING system: FLUID SYSTEMS 1. Ratio, the gas-to-liquid volume at standard conditions. 2. Quality, the ratio of gas volume to liquid volume at hole conditions. 99 UBD TECHNIQUES  UNDERBALANCED DRILLING (UBD) APPLICATION Normally Pressured Reservoirs  Applications for normal to above normal pressured reservoirs utilizing fluid systems in a controlled flow drilling technique. Depleted Reservoirs  Where a multi-phase circulating fluid is necessary to achieve required Bottom Hole Circulating Pressure (BHCP) - underbalanced or with minimal overbalance. 101  BENEFITS OF UNDERBALANCED DRILLING Maintaining wellbore pressure below the reservoir pressure allows reservoir fluids to enter the wellbore, thus avoiding formation damage During underbalanced drilling there is no physical mechanism to force drilling fluid into the formation drilled (lost circulation is minimum) Drilling underbalanced can help in detecting potential hydrocarbon zones, even identifying zones that would have been bypassed with conventional drilling methods Due to the decreased pressure at the bit head, UBD operations demonstrate superior penetration rates compared to conventional drilling techniques 102  BENEFITS OF UNDERBALANCED DRILLING Since there is no filter cake around the wellbore wall, the chances of differential sticking are also reduced Since conventional drilling fluids are not used in underbalanced drilling applications, there is no need to worry about disposing potentially hazardous drilling mud A combination of all these factors can significantly improve the economics of drilling a well UBD is often preferred if it reduces formation damage and hole problems, and reduces the cost of stimulation in fractured or moderate/high permeability formations 103  LIMITATIONS OF UNDERBALANCED DRILLING There is a higher risk of blowout, fire or explosion Underbalanced drilling is still an expensive technology  Depending on the drilling fluid used, the cost can be significant, particularly for extended reach horizontal wells It is not always possible to maintain a continuously underbalanced condition  Since there is not a filter cake around the wellbore, any instantaneous pulse of overbalance might cause severe damage to the unprotected formation UBD has its own unique damage mechanisms, such as surface damage of the formation due to lack of heat conduction capacity of underbalanced drilling fluids It is more complicated to model and predict the behavior of compressible drilling fluids 104  BARRIERS TO UB DRILLING &COMPLETION Regulatory  Lack of Standards  Lack of knowledge  Little statistical history  Concern about well control  Environmental questions Operators Barriers  Unfamiliar with the system  Lack of experienced people  Economics - Too expensive  Concern- liability  Concern- well bore stability 105  REASONS FOR UBD GROWTH Depleted reservoirs Awareness of skin damage Limits of lost circulation materials Cost of differential sticking Service company competition Trade journal publications Horizontal drilling Consultants available 106  LIMITING TECHNICAL FACTORS Reduced wellbore pressure gradients can cause hole stability problems Formation of mud rings can block air flow, leading to downhole fires Water causes cuttings to accumulate, possibly causing the drill string to stick  If aerated mud is used rather than air, differential underbalance can be reduced HC’s and air often mix to achieve a flammable range  With a small spark, which can be generated by the contact between the drill string and hard minerals, the risk of fire increases Stable foam condition is not easy to achieve 107  TECHNICAL IMPROVEMENTS Compressor Evolution Hammer drills Nitrogen  Increased availability  Reduced cost  On site generation Recyclable foam systems Better gas separators Closed loop circulation Hydraulics models 108  TECHNICAL IMPROVEMENTS Improved rotating heads Wire line wet connect Improved MWD Rig assist snubbing units Coiled tubing equipment Non-damaging drilling fluids Top drive 109  REASONS FOR UB DRILLING IMPROVED LOSS FAST NO REDUCED PRODUCTION CIRCULATI DRILLING DIFFERENTIA RESERVIOR EVALUATION ON L STICKING DAMAGE  PHYSICAL LIMITATION TO UBD Borehole Instability Unconsolidated Sands Weak Formations Geopressured Shales Salt Beds Inadequate Casing 111 PRODUCTION LIMITS TO UBD Permeability is so low the zone needs to be fracked Zones must be isolated 112 TYPES OF FLOW REGIMES CLOSED LOOP CIRCULATION SYSTEM  AIR DRILLING Air/Gas drilling (“dust”) is a technique used in areas where the formations are “Dry” i.e., there is no influx of water or liquid hydrocarbons This medium requires significant compressed gas volumes to clean the well with average velocities of over 3,000 ft per minute Benefits Increased Rate of Penetration Reduced Formation Damage Improves Bit Performance Lost Circulation Control Continuous Drill Stem Test 115 EQUIPMENT - GAS SOURCE  MISTING Addition of 6 to 30 bbl/hr of fluid to the air stream. Clean and lubricates the bit Carries the cuttings to the surface as a mist or more normally in a modified two phase flow. 117  FOAM DRILLING The most versatile of the gas-generated systems. Effective operating range from 0.2 to 0.6 s.g. Mixture of gas phase and foaming solution. Foam flow varies with depth in the hole. Adjustable effective BHP. Enhanced lifting and well bore cleaning. 118  FOAM DRILLING BENEFITS Faster Penetration rate Low Air requirements Low fluid requirements Low Hydrostatic head No damage to formation Best for large holes 119 GASEATED OR AERATED DRILLING  AERATED FLUID Gasification of Primary Drilling Fluid. Initially designed as a technique to lighten mud to reduce lost circulation. Methods: Standpipe injection Jet Sub Parasite String Dual Casing String As an UB fluid, it is easiest to control in small holes. 121 PARASITE STRING JET SUB PARALLEL CASING STRING AERATED DRILLING PROBLEMS ENGR. DR. OKORO E.E.