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Drilling Engineering PE 311

Chapter 2: Drilling Fluids

Introduction to Drilling FluidsDrilling Engineering Fall 2012Prepared by: Tan NguyenThe principal functions of the drilling fluid are1. Subsurface pressure control 2. Cuttings removal and transport3. Suspension of solid particles4. Sealing of permeable formations5. Stabilizing the wellbore6. Preventing formation damage7. Cooling and lubricating the bit and drill stringTransmitting hydraulic horsepower to the bitFacilitating the collection of formation data10. Partial support of drill string and casing weights11. Controlling corrosion12. Assisting in cementing and completion

Principal Functions of Drilling FluidsDrilling Engineering Fall 2012Prepared by: Tan NguyenA column of drilling fluid exerts a hydrostatic pressure that, in field units, is equal toP = 0.052 x r x TVD where P - hydrostatic pressure of fluid column in wellbore, psi; r - mud weight in pounds per gallon (ppg)TVD - True Vertical Depth, ft - during normal drilling operations, this corresponds to the height of the fluid column in the wellbore.

Principal Functions of Drilling FluidsSubsurface pressure controlDrilling Engineering Fall 2012Prepared by: Tan NguyenCirculation of the drilling fluid causes cuttings to rise from the bottom of the hole to the surface. Efficient cuttings removal requires circulating rates that are sufficient to override the force of gravity acting upon the cuttings. Other factors affecting the cuttings removal include drilling fluid density and rheology, annular velocity, hole angle, and cuttings-slip velocity.In most cases, the rig hydraulics program provides for an annular velocity sufficient to result in a net upward movement of the cuttings. Annular velocity is determined by the cross-sectional area of the annulus and the pump output.

Principal Functions of Drilling FluidsCuttings Removal and TransportDrilling Engineering Fall 2012Prepared by: Tan NguyenWhen the rig's mud pumps are shut down and circulation is halted (e.g., during connections, trips or downtime), cuttings that have not been removed from the hole must be held in suspension. Otherwise, they will fall to the bottom (or, in highly deviated wells, to the low side) of the hole. The rate of fall of a particle through a column of drilling fluid depends on the density of the particle and the fluid, the size of the particle, the viscosity of the fluid, and the thixotropic (gel-strength) properties of the fluid. The controlled gelling of the fluid prevents the solid particles from settling, or at least reduces their rate of fall. High gel strengths also require higher pump pressure to break circulation. In some cases, it may be necessary to circulate for several hours before a trip in order to clean the hole of cuttings and to prevent fill in the bottom of the hole from occurring during a round trip.

Principal Functions of Drilling FluidsSuspension of Solid Particles Drilling Engineering Fall 2012Prepared by: Tan NguyenAs the drill bit penetrates a permeable formation, the liquid portion of the drilling fluid filters into the formation and the solids form a relatively impermeable "cake" on the borehole wall. The quality of this filter cake governs the rate of filtrate loss to the formation. Drilling fluid systems should be designed to deposit a thin, low permeability filter cake on the formation to limit the invasion of mud filtrate. This improves wellbore stability and prevents a number of drilling and production problems. Potential problems related to thick filter cake and excessive filtration include tight hole conditions, poor log quality, increased torque and drag, stuck pipe, lost circulation and formation damage.Bentonite is the best base material from which to build a tough, low-permeability filter cake. Polymers are also used for this purpose.

Principal Functions of Drilling FluidsSealing of permeable formationDrilling Engineering Fall 2012Prepared by: Tan NguyenThe borehole walls are normally competent immediately after the bit penetrates a section. Wellbore stability is a complex balance of mechanical and chemical factors. The chemical composition and mud properties must combine to provide a stable wellbore until casing can be run and cemented. Regardless of the chemical composition of the fluid and other factors, the weight of the mud must be within the necessary range to balance the mechanical forces acting on the wellbore. The other cause of borehole instability is a chemical reaction between the drilling fluid and the formations drilled. In most cases, this instability is a result of water absorption by the shale. Inhibitive fluids (calcium, sodium, potassium, and oil-base fluids) aid in preventing formation swelling, but even more important is the placement of a quality filter cake on the walls to keep fluid invasion to a minimum.

Principal Functions of Drilling FluidsStabilizing the WellboreDrilling Engineering Fall 2012Prepared by: Tan NguyenAny reduction in a producing formations natural porosity or permeability is considered to be formation damage. If a large volume of drilling-fluid filtrate invades a formation, it may damage the formation and hinder hydrocarbon production.There are several factors to consider when selecting a drilling fluid: Fluid compatibility with the producing reservoir Presence of hydratable or swelling formation clays Fractured formations The possible reduction of permeability by invasion of nonacid soluble materials into the formation

Principal Functions of Drilling FluidsPreventing Formation DamageDrilling Engineering Fall 2012Prepared by: Tan NguyenFriction at the bit, and between the drillstring and wellbore, generates a considerable amount of heat. The circulating drilling fluid transports the heat away from these frictional sites by absorbing it into the liquid phase of the fluid and carrying it away.The laying down of a thin wall of "mud cake" on the wellbore aids in reducing torque and drag. The amount of lubrication provided by a drilling fluid varies widely and depends on the type and quantity of drill solids and weight material, and also on the chemical composition of the system as expressed in terms of pH, salinity and hardness. Indications of poor lubrication are high torque and drag, abnormal wear, and heat checking of drillstring components.

Principal Functions of Drilling FluidsCooling and Lubricating the BitDrilling Engineering Fall 2012Prepared by: Tan NguyenDuring circulation, the rate of fluid flow should be regulated so that the mud pumps deliver the optimal amount of hydraulic energy to clean the hole ahead of the bit. Hydraulic energy also provides power for mud motors to rotate the bit and for Measurement While Drilling (MWD) and Logging While Drilling (LWD) tools. Hydraulics programs are based on sizing the bit nozzles to maximize the hydraulic horsepower or impact force imparted to the bottom of the well.

Principal Functions of Drilling FluidsTransmitting Hydraulic Horsepower to the BitDrilling Engineering Fall 2012Prepared by: Tan NguyenThe drilling fluid program and formation evaluation program are closely related. As drilling proceeds, for example, mud loggers monitor mud returns and drilled cuttings for signs of oil and gas. They examine the cuttings for mineral composition, paleontology and visual signs of hydrocarbons. This information is recorded on a mud log that shows lithology, penetration rate, gas detection and oil-stained cuttings, plus other important geological and drilling parameters. Measurement-While-Drilling (MWD) and Logging-While-Drilling (LWD) procedures are likewise influenced by the mud program, as is the selection of wireline logging tools for post-drilling evaluation.Principal Functions of Drilling FluidsFacilitating the Collection of Formation DataDrilling Engineering Fall 2012Prepared by: Tan NguyenWith average well depths increasing, the weight supported by the surface wellhead equipment is becoming an increasingly crucial factor in drilling. Both drillpipe and casing are buoyed by a force equal to the weight of the drilling fluid that they displace. When the drilling fluid density is increased, the total weight supported by the surface equipment is reduced considerably.

Principal Functions of Drilling FluidsPartial support of Drill String and Casing WeightsDrilling Engineering Fall 2012Prepared by: Tan NguyenThe drilling fluid must produce a wellbore into which casing can be run and cemented effectively, and which does not impede completion operations. During casing runs, the mud must remain fluid and minimize pressure surges so that fracture-induced lost circulation does not occur. The mud should have a thin, slick filter cake. To cement casing properly, the mud must be completely displaced by the spacers, flushes and cement. Effective mud displacement requires that the hole be near-gauge and that the mud have low viscosity and low, non-progressive gel strengths. Completion operations such as perforating and gravel packing also require a near-gauge wellbore and may be affected by mud characteristicsPrincipal Functions of Drilling FluidsAssistance in Cementing and CompletionDrilling Engineering Fall 2012Prepared by: Tan NguyenVarious materials may be added at the surface to change or modify the characteristics of the mud. For example: Weighting agents (usually barite) are added to increase the density of the mud, which helps to control subsurface pressures. Viscosifying agents (clays, polymers, and emulsified liquids) are added to thicken the mud and increase its hole-cleaning ability. Dispersants or deflocculants may be added to thin the mud, which helps to reduce surge, swab, and circulating-pressure problems. Mud IngredientsDrilling Engineering Fall 2012Prepared by: Tan NguyenClays, polymers, starches, dispersants, and asphaltic materials may be added to reduce filtration of the mud through the borehole wall. This reduces formation damage, differential sticking, and problems in log interpretation. Salts are sometimes added to protect down-hole formations as well as to increase density. Other mud additives may include lubricants, corrosion inhibitors, chemicals that tie up calcium ions, and flocculants to aid in the removal of cuttings at the surface. Caustic soda is often added to increase the pH of the mud, which improves the performance of dispersants and reduces corrosion. Preservatives, bactericides, emulsifiers, and temperature extenders may all be added to make other additives work better.Mud IngredientsDrilling Engineering Fall 2012Prepared by: Tan NguyenA water-base fluid is one that uses water for the liquid phase and commercial clays for viscosity. The continuous phase may be fresh water, brackish water, seawater, or concentrated brines containing any soluble salt. The commercial clays used may be bentonite, attapulgite, sepiolite, or polymer. The use of other components such as thinners, filtration-control additives, lubricants, or inhibiting salts in formulating a particular drilling fluid is determined by the type of system required to drill the formations safely and economically. Some of the major systems include fresh-water fluids, brackish or seawater fluids, saturated salt fluids, inhibited fluids, gyp fluids, lime fluids, potassium fluids, polymer-based fluids, and brines used in drilling, completion or workover operations (including single-salt, potassium chloride, sodium chloride, calcium chloride, and two and three-salt brines).Drilling Fluid ClassificationsWater-Based Drilling FluidsDrilling Engineering Fall 2012Prepared by: Tan NguyenIn many areas, diesels were used to formulate and maintain OBMs. Crude oils had sometimes been used instead of diesel but posed tougher safety problems. Thus, today, mineral oils and new synthetic fluids replace diesel and crude due to their lower toxicity. Advantages of OBMs: 1. Shale stability: OBMs are most suited for drilling water sensitive shales. The whole mud results non reactive towards shales.2. ROP: allowing to drill faster than WBMs, still providing excellent shale stability3. High Temperature: can drill where bottom hole temperature exceeds WBMs tolerances; can handle up to 550 0F.

Drilling Fluid ClassificationsOil-Based Drilling FluidsDrilling Engineering Fall 2012Prepared by: Tan Nguyen4. Lubricity: OBMs has a thin filter cake and the friction between the pipe and the wellbore is minimized, thus reducing the risk of differential sticking.5. Low pore pressure formation: Mud weight of OBMs can be maintained less than that of water (as low as 7.5 PPG)6. Corrosion control: corrosion of pipe is controlled Since oil is the external phase. 7. Re-use: OBMs are well-suited to be used over and over again. They can be stored for long periods of time since bacterial growth is suppressed.

Drilling Fluid ClassificationsOil-Based Drilling FluidsDrilling Engineering Fall 2012Prepared by: Tan NguyenAn oil-base drilling fluid is one in which the continuous phase is oil. The terms oil-base mud and inverted or invert-emulsion mud sometimes are used to distinguish among the different types of oil-base drilling fluids. Traditionally, an oil-base mud is a fluid with 0 to 5% by volume of water, while an invert-emulsion mud contains more than 5% by volume of water. However, since most oil muds contain some emulsified water, have oil as the liquid phase, and (if properly formulated) have an oil filtrate, we do not distinguish among them in this discussion. Synthetic muds may include esters, olefins, and paraffin.Drilling Fluid ClassificationsOil-Based Drilling FluidsDrilling Engineering Fall 2012Prepared by: Tan NguyenAir drilling is used primarily in hard-rock areas, and in special cases to prevent formation damage while drilling into production zones or to circumvent severe lost-circulation problems. Air drilling includes dry air drilling, mist or foam drilling, and aerated-mud drilling. In dry air drilling, dry air or gas is injected into the standpipe at a volume and rate sufficient to achieve the annular velocities needed to clean the hole of cuttings. Mist drilling is used when water or oil sands are encountered that produce more fluid than can be dried up using dry air drilling. A mixture of foaming agent and water is injected into the air stream, producing a foam that separates the cuttings and helps remove fluid from the hole. In aerated mud drilling, both mud and air are pumped into the standpipe at the same time. Aerated muds are used when it is impossible to drill with air alone because of water sands and/or lost-circulation situations.Drilling Fluid ClassificationsPneumatic Fluids (Air, Gas, Mist, Foams, Gasified Muds)Drilling Engineering Fall 2012Prepared by: Tan NguyenDrilling Fluid ClassificationsPneumatic Fluids (Air, Gas, Mist, Foams, Gasified Muds)

Drilling Engineering Fall 2012Prepared by: Tan NguyenThe physical properties of a drilling fluid, particularly its density and rheological properties, are monitored to assist in optimizing the drilling process. These physical properties contribute to several important aspects of successful drilling, including: Providing pressure control to prevent an influx of formation fluid Providing energy at the bit to maximize Rate of Penetration (ROP) Providing wellbore stability through pressured or mechanically stressed zones Suspending cuttings and weight material during static periods Permitting separation of drilled solids and gas at surface Removing cuttings from the wellDrilling Fluid PropertiesDrilling Engineering Fall 2012Prepared by: Tan NguyenThe concepts of shear rate and shear stress apply to all fluid flow, and can be describe in term of two fluid layers (A and B) moving past each other when a force (F) has been applied.Drilling Fluid PropertiesViscosity

Drilling Engineering Fall 2012Prepared by: Tan NguyenWhen a fluid is flowing, a force exists in the fluid that opposes the flow. This force is known as the shear stress. It can be thought of as a frictional force that arises when one layer of fluid slides by another. Since it is easier for shear to occur between layers of fluid than between the outer most layer of fluid and the wall of a pipe, the fluid in contact with the wall does not flow. The rate at which one layer is moving past the next layer is the shear rate. The shear rate is therefore a velocity gradient. The formula for the shear rate is Drilling Fluid PropertiesViscosity

Drilling Engineering Fall 2012Prepared by: Tan NguyenIn the most general sense, viscosity describes a substances resistance to flow. Hence a high-viscosity drilling mud may be characterized as "thick," while a low-viscosity mud may be described as "thin." Viscosity (m), by definition, is the ratio of shear stress (t) to shear rate (g):

Unit: PaS, NS/m2, kg/ms, cp, dyneS/cm2, lbfS/100ft2 Drilling Fluid PropertiesViscosity

Drilling Engineering Fall 2012Prepared by: Tan NguyenThe simplest class of fluids is called Newtonian. The base fluids (freshwater, seawater, diesel oil, mineral oils and synthetics) of most drilling fluids are Newtonian. In these fluids, the shear stress is directly proportional to the shear rate. The points lie on a straight line passing through the origin (0,0) of the graph on rectangular coordinates. The viscosity of a Newtonian fluid is the slope of this shear stress/shear rate line. The yield stress (stress required to initiate flow) of a Newtonian fluid will always be zero. When the shear rate is doubled, the shear stress is also doubled. When the circulation rate for this fluid is doubled, the pressure required to pump the fluid will be squared (e.g. 2 times the circulation rate requires 4 times the pressure).Fluid TypesNewtonian FluidsDrilling Engineering Fall 2012Prepared by: Tan NguyenThe shear stress at various shear rates must be measured in order to characterize the flow properties of a fluid. Only one measurement is necessary since the shear stress is directly proportional to the shear rate for a Newtonian fluid. From this measurement the shear stress at any other shear rate can be calculated from the equation:Fluid TypesNewtonian Fluids

Drilling Engineering Fall 2012Prepared by: Tan NguyenWhen a fluid contains clays or colloidal particles, these particles tend to bump into one another, increasing the shear stress or force necessary to maintain a given flow rate. If these particles are long compared to their thickness, the particle interference will be large when they are randomly oriented in the flow stream. However, as the shear rate is increased, the particles will line up in the flow stream and the effect of particle interaction is decreased. This causes the velocity profile in a pipe to be different from that of water. In the center of the pipe, where the shear rate is low, the particle interference is high and the fluid tends to flow more like a solid mass. The velocity profile is flattened. This flattening of the velocity profile increases the sweep efficiency of a fluid in displacing another fluid and also increases the ability of a fluid to carry larger particles.

Fluid TypesNon-Newtonian FluidsDrilling Engineering Fall 2012Prepared by: Tan NguyenA rheological model is a description of the relationship between the shear stress and shear rate. Newtons law of viscosity is the rheological model describing the flow behavior of Newtonian fluids. It is also called the Newtonian model. However, since most drilling fluids are non-Newtonian fluids, this model does not describe their flow behavior. In fact, since no single rheological model can precisely describe the flow characteristics of all drilling fluids, many models have been developed to describe the flow behavior of non-Newtonian fluids. Bingham Plastic, Power Law and Modified Power Law models are discussed. The use of these models requires measurements of shear stress at two or more shear rates. From these measurements, the shear stress at any other shear rate can be calculated.

Fluid TypesNon-Newtonian FluidsDrilling Engineering Fall 2012Prepared by: Tan NguyenFluid TypesRotational Viscometer

Drilling Engineering Fall 2012Prepared by: Tan NguyenThe Bingham Plastic model has been used most often to describe the flow characteristics of drilling fluids. It is one of the older rheological models currently in use. This model describes a fluid in which a finite force is required to initiate flow (yield point) and which then exhibits a constant viscosity with increasing shear rate (plastic viscosity).

Fluid TypesBingham Plastic Fluids

Drilling Engineering Fall 2012Prepared by: Tan NguyenThe two-speed viscometer was designed to measure the Bingham Plastic rheological values for yield point and plastic viscosity. A flow curve for a typical drilling fluid taken on the two-speed Fann VG meter is illustrated in Figurebelow. The slope of the straight line portion of this consistency curve is plastic viscosity.

Fluid TypesBingham Plastic Fluids

Drilling Engineering Fall 2012Prepared by: Tan NguyenMost drilling fluids are not true Bingham Plastic fluids. For the typical mud, if a consistency curve for a drilling fluid is made with rotational viscometer data, a non-linear curve is formed that does not pass through the origin, as shown in Flow diagram of Newtonian and typical mud. The development of gel strengths causes the y-intercept to occur at a point above the origin due to the minimum force required to break gels and start flow. Plug flow, a condition wherein a gelled fluid flows as a plug with a flat viscosity profile, is initiated as this force is increased. As the shear rate increases, there is a transition from plug to viscous flow. In the viscous flow region, equal increments of shear rate will produce equal increments of shear stress, and the system assumes the flow pattern of a Newtonian fluid.

Fluid TypesBingham Plastic FluidsDrilling Engineering Fall 2012Prepared by: Tan NguyenFluid TypesBingham Plastic Fluids

Drilling Engineering Fall 2012Prepared by: Tan NguyenThe Power Law model attempts to solve the shortcomings of the Bingham Plastic model at low shear rates. The Power Law model is more complicated than the Bingham Plastic model in that it does not assume a linear relationship between shear stress and shear rate. However, like Newtonian fluids, the plots of shear stress vs. shear rate for Power Law fluids go through the origin.

Fluid TypesPower Law Model

Drilling Engineering Fall 2012Prepared by: Tan NguyenThis model describes a fluid in which the shear stress increases as a function of the shear rate mathematically raised to some power. Mathematically, the Power Law model is expressed ast = Kgn Where:t = Shear stressK = Consistency indexg = Shear raten = Power Law index

Fluid TypesPower Law ModelDrilling Engineering Fall 2012Prepared by: Tan NguyenPlotted on a log-log graph, a Power Law fluid shear stress/shear rate relationship forms a straight line in the log-log plot. The slope of this line is n and K is the intercept of this line. The Power Law index n indicates a fluids degree of non-Newtonian behavior over a given shear rate range.Fluid TypesPower Law Model

Drilling Engineering Fall 2012Prepared by: Tan Nguyenn = Power Law index or exponentK = Power Law consistency index or fluid index (dyne secn/cm2)q1 = Mud viscometer reading at lower shear rateq2 = Mud viscometer reading at higher shear ratew1 = Mud viscometer RPM at lower shear ratew2 = Mud viscometer RPM at higher shear rate

Fluid TypesPower Law Model

Drilling Engineering Fall 2012Prepared by: Tan NguyenA rotational viscometer containing a non-Newtonaian fluid gives a dial reading of 12 at a rotor speed of 300 rpm and a dial reading of 20 at a rotor speed of 600 rpm. Determine the rheological model of this fluid in two cases: Bingham model and Power Law model

Fluid TypesExampleDrilling Engineering Fall 2012Prepared by: Tan NguyenA rotational viscometer containing a non-Newtonaian fluid gives a dial reading of 12 at a rotor speed of 300 rpm and a dial reading of 20 at a rotor speed of 600 rpm. Determine the rheological model of this fluid in two cases: Bingham model and Power Law model:Bingham model:

Power Law model:Fluid TypesExample

Drilling Engineering Fall 2012Prepared by: Tan Nguyen