Drilling Fluids PDF

Drilling fluids Prepared by Dhyaa Hussein Ass. Lec. Dhyaa Hussein Al Maaqal University  Content  Introduction  Functions of a Drilling Fluid  Types of Drilling Fluid Water Based Mu...

Drilling fluids Prepared by Dhyaa Hussein Ass. Lec. Dhyaa Hussein Al Maaqal University  Content  Introduction  Functions of a Drilling Fluid  Types of Drilling Fluid Water Based Mud Oil Based Mud  Engineering Basic Engineering Calculation Material balance The Drilling Mud Report  Drilling Fluid Test  Solid Control Introduction  Drilling fluid or drilling mud is a critical component in the rotary drilling process. Its primary functions are to remove the drilled cuttings from the borehole whilst drilling and to prevent fluids from flowing from the formations being drilled, into the borehole.  The cost of the mud can be as high as 10-15% of the total cost of the well  the consequences of not maintaining good mud properties may result in drilling problems which will take a great deal of time and therefore cost to resolve.  In view of the high cost of not maintaining good mud properties an operating company will usually hire a service company to provide a drilling fluid specialist (mud engineer) on the rig to formulate, continuously monitor and, if necessary, treat the mud. Why is Drilling Fluids so important?  Drilling fluid circulating in the well is the same as blood circulating in a body.  Functions of a Drilling Fluid Major Functions Drilling fluids are designed and formulated to perform three major functions:  Control Subsurface Pressure  Transport Cuttings  Support and Stabilize the Wellbore Minor Functions Minor functions of a drilling fluid include:  Support Weight of Tubulars  Cool and Lubricate the Bit and Drill String  Transmit Hydraulic Horsepower to Bit  Provide Medium for Wireline Logging  Assist in the Gathering of Subsurface Geological Data and Formation Evaluation  Cool and Lubricate the Bit Control Subsurface Pressure A drilling fluid controls the subsurface pressure by its hydrostatic pressure. Hydrostatic pressure is the force exerted by a fluid column and depends on the mud density and true vertical depth (TVD). Hydrostatic Pressure: The fluid must maintain control of these pressure through hydrostatic pressure of the fluid column HSP = 0.052 X Mw X TVD HSP = Hydrostatic pressure (psi) Mw = Mud weight (ppg - pound per gallon) TVD = True vertical depth (ft) According to the equations above, Hydrostatic Pressure is not a function of hole geometry. Only mud weight and True Vertical Depth (TVD) affect on Hydrostatic Pressure. For example, well A and well B have the same vertical depth. With the same mud density in hole, the bottom hole pressure due to hydrostatic pressure is the same. The only different between Well A and Well B is mud volume. Example Mud Weight = 12.0 ppg, True Vertical Depth (TVD) = 10,000 ft Hydrostatic Pressure (HP) = 0.052 × 12.0 × 10,000 Hydrostatic Pressure (HP) = 6,240 psi 2. Calculate hydrostatic pressure in psi by using pressure gradient in psi/ft and feet as the units of True Vertical Depth. Hydrostatic Pressure (HP) = Pressure gradient in psi/ft × True Vertical Depth (TVD) Where Pressure gradient in psi/ft True Vertical Depth in ft Example Pressure Gradient = 0.5 psi/ft True Vertical Depth (TVD) = 10,000 ft Hydrostatic Pressure (HP) = 0.5 psi/ft × 10,000 ft Hydrostatic Pressure (HP) = 5,000 psi Formations are composed of solids of varying porosity and permeability They contain liquids such as water, gas or oil.  These liquids may be under pressure due to the overburden pressure and tectonic forces. Formation Pressure is the pressure exerted by naturally occurring fluids trapped in the pore spaces of formation.  Formation Pressure divided into three categories:  Subnormal pressure (Gradient < 0.433 psi/ft)  Normal Pressure ( 0.433 < Gradient < 0.465 psi/ft)  Abnormal pressure (0.465 < Gradient < 1 psi/ft)  Overburden Pressure is the pressure caused by the weight of overlaying formations above the point of interest. Transport Cuttings  Fluid flowing from the bit nozzles exerts a jetting action to clear cuttings from the bottom of the hole and the bit, and carries these cuttings to the surface. Factors Effect on Transport Cuttings  Velocity - Increasing annular velocity generally improves cuttings transport. Variables include pump output, borehole size and drill string size.  Density - Increasing mud density increases the carrying capacity through the buoyant effect on cuttings.  Viscosity - Increasing viscosity often improves cuttings removal.  Pipe Rotation - Rotation tends to throw cuttings into areas of high fluid velocity from low velocity areas next to the borehole wall and drill string. The drilling fluid must also be capable to transporting cuttings out of the hole, the failure to adequately clean the hole or suspend drilled solids are contributing factors in such hole problems as fill on bottom after a trip, hole pack-off, lost returns, differentially stuck pipe. Support and Stabilize Wellbore  Most permeable formations have pore space openings too small to allow the passage of whole mud into the formation; however, filtrate from the drilling fluid can enter the pore spaces. Formation  The rate at which the filtrate enters the formation is Drilling Fluid dependent on the pressure differential between the formation and the column of drilling fluid, and the quality of the filter cake deposited on the formation face.  Borehole stability is also maintained or enhanced by controlling the loss of filtrate to permeable formations and by careful control of the chemical composition of the drilling fluid. Large volumes of drilling fluid filtrate, and filtrates that are incompatible with the formation or formation fluids, may destablize the formation through hydration of shale and/or chemical interactions between components of the drilling fluid and the wellbore Drilling Fluid Classifications Water Based Fluids  NON-INHIBITIVE FLUIDS Non-Inhibitive muds are subdivided into the following types:  Clear Water  Native Muds  Bentonite-Water Muds  Lignite/Lignosulfonate (Deflocculated) Muds Non-Inhibitive muds have properties:  Non-inhibitive fluids are simple and inexpensive.  Their composition will vary depending on the requirements of the local lithology, type of makeup water, hole size, and anticipated contaminants.  These muds have definite limitations which become evident when drilling dispersive formations, encountering contaminants and high temperatures, or increasing fluid density. Each of these situations may require converting to another type of fluid which is more adaptable to these conditions. Clear Water  Clear water is a nearly ideal drilling fluid.  Clear water varies in salinity from fresh to saturated brines. Water selection and salinity will depend upon available makeup water or salinity required to drill specific formations.  viscous sweeps (small batches of high viscosity mud) are pumped around to clear the hole of cuttings as needed.  Caustic soda or lime is usually added for corrosion control. Native Muds  In some areas, drilled formations contain mud-making claystones or shales. When water is pumped down the hole during drilling, it returns with the native solids dispersed in it. Viscosity builds with continued drilling and circulation.  On the other hand, small quantities of bentonite may be added to increase viscosity and improve filtration control. Caustic soda or lime is usually added for corrosion control Bentonite-Water Muds  Bentonite dispersed in fresh water produces a mud with good cuttings lifting capacity, good drilling rate, and usually adequate filtration control. These bentonite-water muds are commonly used as spud muds for drilling surface hole.  Water quality (Make up water) is important in formulating a bentonite-water mud,  The concentration of Salinity (Chlorides (Cl- ) Less than 5000 mg/L) and hardness (Calcium ion Ca++ concentration should not exceed 150 mg/L) in make up water  To get rid of Calcium in make up water, Soda ash (Na2CO3) is added to base fluid  To get rid of Magnesium in make up water, Caustic Soda (NaOH) is added to base fluid Bentonite-Water Muds Make up  Fill tank with fresh water (Chloride less than 5000 mg/l.  Add sod ash to reduce total hardness below 150 mg/l.  Add Caustic Soda to adjust PH.  Add Bentonite concentration 25-35 lb/bbl  Allow to hydrate at maximum for 6 hrs  Lignite-Lignosulfonate Muds  Lignite-Lignosulfonate Muds can be used to drill a variety of formations. They can be weighted up to 18 or 19 lb/gal, provided low-gravity solids (bentonite and drill solids) are in the proper range. As mud density is increased, the bentonite content should be decreased.  Calcium ion should be kept below 200 mg/L. Less than 10,000 mg/L chlorides should not hamper fluid performance, but if chlorides exceed 25,000 mg/L, the mud should be diluted with fresh water  Lignite-lignosulfonate muds are thermally stable to approximately 325°F  The additive Lignosulfonate concentration is 0.25-8 lb/bbl  INHIBITIVE FLUIDS  Inhibitive fluids are fluids which do not cause appreciable formation alteration. These fluids are primarily used for drilling shale and clay formations, , they are also used in areas where contamination is a problem such as large quantities salt, anhydrite, and cement Inhibitive fluids are classified as follows:  Calcium-based Muds: use lime (Ca(OH)2) or gyp (CaSO4 2H2O)  Salt-based Muds: use sodium chloride (NaCl)  Potassium-based Muds: use caustic potash (KOH), potassium chloride (KCl), potassium carbonate (K2CO3) Calcium-Based Muds  Calcium-based muds are primarily used to drill intervals of highly reactive shales  They exhibit greater inhibition than sodium-based muds by reducing hydration of clays.  Calcium-based muds are highly resistant to contamination.  A high concentration of low-gravity solids will cause unstable rheological properties. The principal calcium-based muds are:  Lime Muds  Lime/MOR-REX Muds  Gyp Muds  Lime Muds  Lime muds may be used where an inhibitive mud is desired and where temperatures do not exceed 300-325°F.  They are particularly useful because of their high solids tolerance.  Soluble calcium varies between 120-400 mg/L  The upper limit for chlorides in a lime mud is 40,000-50,000 mg/L  The additive Lime concentration is 2-10 lb/bbl Lime/MOR-REX Muds  Lime/MOR-REX Muds are similar to the lime muds  polysaccharide deflocculant (MOR-REX) is used to counteract rheological problems associated with lime muds.  MOR-REX is a polysaccharide polymer that is added to a lime-based mud to control flocculation and increase lime solubility.  The additive MOR-REX concentration is 2-5 lb/bbl  Gyp Muds  Gyp muds were used for drilling massive sections of anhydrite.  The calcium ion level is maintained from 200-1200 mg/L.  A gyp mud is fairly tolerant up to a maximum of 100,000 mg/L chlorides.  The temperature limitation of a gyp mud is 350°F  The additive Gypsum concentration is 4-8 lb/bbl  Salt-Based Muds  salt-based muds are muds containing varying amounts of predominantly sodium chloride ranging from 10,000 mg/L NaCl up to saturation 315,000 mg/L NaCl.  The effect that salt has on a drilling mud is dependent on the amount of salt in the fluid and the type and quantity of solids.  Muds are usually called salt-based muds if the sodium chloride content is greater than 10,000 mg/L NaCl.  Potassium-Based Muds  Potassium-based muds are used in areas where inhibition is required to limit chemical alteration of shales.  Potassium performance is based on cationic exchange of potassium for sodium or calcium ions on smectites and interlayered clays  a sufficient concentration must be maintained at all times to guarantee inhibition due to exchange reactions with cations in the shale effectively reduce the potassium level in the mud.  Potassium interactions with clay surfaces can be traced to two effects: ionic size and hydrational energy, The ionic diameter of potassium is 2.66 Å, very close to the available distance of 2.8 Å in the lattice space of the clay structure.  Basically, there are four types of potassium-based muds:  KCl-Polymer (KCl-PHPA)  KOH-Lignite Muds  KOH-Lime Muds  KCl-Cationic Polymer Muds KCl-Polymer (KCl-PHPA) Muds  KCl-Polymer Muds were developed to provide wellbore stability and minimize cuttings dispersion.  When properly formulated, benefits such as low formation damage and high return permeability encourage their use for drilling water-sensitive formations.  For KCl muds to be economical, drill solids concentrations should be low and efficient solids control practices must be used.  PHPA - Partially hydrolyzed polyacrylamide is primarily added to encapsulate solids and provide inhibition KCl-Polymer system Make up  Treat makeup water with 0.25 lb/bbl soda ash and 0.12 lb/bbl caustic potash to remove calcium and magnesium. However, if the makeup water does not contain magnesium, the caustic potash may be omitted.  Prehydrate bentonite in fresh water.  To mix polymers, start with the viscosifying polymers first. If the mud becomes too thick to efficiently pump, add potassium chloride. The salt will reduce the viscosity. Adjust pH, after the viscosity has been reduced, add the remaining polymers.  Add barite and allow mud to stir until needed POLYMER FLUIDS  These fluids contain polymers to viscosify, polymers to control filtration, polymers to deflocculate, polymers to provide high-temperature stabilization.  Polymer fluids generally contain only minor amounts of bentonite to build viscosity. Primary viscosification is provided by high molecular weight polymers such as PHPA, PAC, XC polymer.  Because these fluids contain only small quantities of bentonite or clay solids, they are less prone to rheological and filtration property fluctuations resulting from the effects of contaminants on the clay structure.  Polymers fluids also reduce cuttings dispersion and stabilize the wellbore through encapsulation. Usually these fluids contain less than 5% by volume total low-gravity solids. Oil-Based Muds  An oil base fluid can be defined as a drilling fluid which has oil as its continuous or external phase and the water, if present, is the dispersed or internal phase.  The solids in an oil base fluid are oil wet, The water, if present, is emulsified in the oil phase. Oil Mud Applications  Shale Stability: oil muds can prevent water movement from the mud into the shale  Penetration Rates: Oil-mud formulations can usually drill faster than water muds and still provide excellent shale stability  High Temperatures: Oil muds have been used at temperatures approaching 550°F  Drilling Salts: Invert oil muds will provide gauge hole and do not leach out salt  Coring Fluids: Oil mud emulsifiers are very strong oil-wetting agents and can cause oil-wetting of the formation. Oil-based coring fluids will not introduce any water into the core  Packer Fluids: Oil muds provide long term stable packer fluids under conditions of high temperature since the additives are extremely temperature stable  Lubricity: The high lubricity offered by oil muds makes them especially suited for highly deviated and horizontal wells  Low Pore Pressure Formations: The ability to drill low pore pressure formations is easily accomplished with oil muds since the mud weight can be maintained at a weight less than that of water  Corrosion Control: Corrosion of pipe is controlled since oil is the external phase and coats the pipe Disadvantages of Oil Muds  The initial cost of oil mud is high  Kick detection is reduced when using oil muds  Kick detection is reduced when using oil muds  Greater emphasis is placed on environmental concerns when using oil muds as related to discharge of cuttings, loss of whole mud and disposal of the oil mud.  Greater emphasis is placed on environmental concerns when using oil muds as related to discharge of cuttings, loss of whole mud and disposal of the oil mud.  Greater emphasis is placed on environmental concerns when using oil muds as related to discharge of cuttings, loss of whole mud and disposal of the oil mud.  Oil muds pose potential fire hazards due to low flash points of vapors coming off the oil mud.  Additional rig equipment and modifications are necessary to minimize the loss of oil muds.  Electric logging must be modified for use in oil-based muds. Oil Mud System Make up