Due to various drilling conditions encountered, all functions will not be addressed on each well.
The drilling fluid program must be designed to satisfy the highest-priority requirements for drilling the prospect well. Unfortunately, these requirements may often be conflicting and/or may place demanding constraints on the system. For example, a low-solids system may be desirable for improved drilling rates and minimum formation damage. However, if high pressure, high-activity shales are drilled in an extreme temperature range, oil muds or dispersed lignosulfonate systems may be easier to control.
An'example of an unsatisfactory system might be using oil muds in formations that have historically proven nonproductive due to emulsion blockage when oil muds are used.
Cool and Lubricate the Bit and Drillstring. A considerable amount of heat due to friction is generated during the drilling operation. Muds can help transmit this heat to the surface as well as lubricate the wellbore. Air bits, which are not used usually with liquid muds, have special ports that use air circulation within the bit bearings for heat dissipation.
Various additives arc available to help lubricate the wellbore. Deep holes or highly deviated wells may require oil or invert emulsion muds in order to provide the necessary reduction of torque and drag resulting from friction.
Clean the Hole Bottom. The removal of cuttings from below the bit is one of the most important functions of a drilling fluid. Cuttings removal is controlled by factors such as the chip hold-down effect of the mud, cross flow of the fluid, fluid viscosity, density of the cuttings, size of the cuttings, density of the fluid, and fluid velocity (Fig. 8-1).
The chip hold-down phenomenon occurs when the bit breaks a chip from the formation. This chip will resist removal and movement from below the bit due to the difference in the hydrostatic pressure of the mud and the formation pressure. In order to remove this chip, fluids must penetrate beneath the chip.
Fig. 8-1 Chip removal from below the hit
This feature has been accomplished successfully with muds that exhibit a high "spurt loss, initial filtration" by helping equalize pressures above and below the chip.
Carry Cuttings to the Surface. Transporting the cuttings that are removed from below the bit is essential for a mud system. The fluid velocity in the annul us must exceed the downward falling rate, or slip velocity, of the cuttings (Fig. 8-2). Mud weight, fluid viscosity, suspension, and gellation properties of the mud affect its carrying capacity. Laminar and turbulent flow regimes exhibit different lifting capabilities. (See Chapter 18 for a further discussion on hydraulics.)
When circulation is stopped, the cuttings that have not been removed must be suspended or they will fall downward. If a mud does not exhibit the proper characteristics to suspend the cuttings, reentry into the hole and reaching bottom through the settled cuttings may become very time consuming and costly. However, excessive gellation to suspend the cuttings may require high pump pressures to break circulation, thus increasing the possibility of lost circulation.
Removal of Cuttings from Mud at the Surface. Drilled rock cuttings must be removed from the mud system at the surface to prevent a high solids concentration buildup. Mud pits usually do not allow sufficient time for solids to settle out. Mechanical solids removal equipment such as shale shakers, de-silters, mud cleaners, and centrifuges has proven its worth in effective drilling. Placement of the solids control equipment in conjunction with the surface circulation system is also very important. Removal of largest solids should occur near the flow line; removal of finer solids should occur prior to entering the suction pit.
Minimize Formation Damage. The protection of potential pay zones is important for a drilling fluid. The deposition of a filter cake that allows the drilling operation to continue and protects a productive zone is often a forgotten
■ VA Mud annular velocity
Fig, 8-2 The upward annular velocity of the mud must exceed the slip velocity of the cuttings consideration of a mud system. The formation damage is generally a reduction in permeability near the wellborc with perhaps a slight porosity reduction. The problem can be severe in low-permeability reservoirs or reservoirs with a high clay content.
Several mechanisms can cause formation damage during drilling (Fig. 8-3). Filtrate loss from the mud can enter the producing zone and cause interstitial ciay swelling, resulting in permeability reductions. Colloidal solids, barite, or clay can be lost into the formation and cause a plugging effect. Oil mud filtrates containing emulsifying agents can cause emulsion blockage. These effects can often be reduced in a remedial manner by using acid, mutual solvents, or fracture jobs. Many reservoirs, however, do not respond effectively to remedial methods, which emphasizes the importance of minimizing the original damage.
Filtrate invaded clay
Filtrate invaded clay
Control Formation Pressures. Drilling intervals that have abnormally high formation pressures require that the mud system be able to provide sufficient pressures to equal or exceed the formation pressure. The hydrostatic pressure of the mud system achieves this purpose. Insufficient pressure control can cause hole heaving, kicks, and blowouts.
Maintain Hole Integrity. Wcllbores often exhibit stability problems resulting from geological phenomena such as fractured zones, unconsolidated sections, hydratable clays, and pressured sections. The drilling fluid must control these problems so a drilled section remains open and deeper drilling can proceed. Designing a mud system to maintain the integrity of the hole after it has been drilled is often the basis for selecting mud types and properties.
Hole stability problems can usually be grouped into either heaving or sloughing shales. Heaving shale is a mechanical problem, whereas sloughing occurs as a result of some chemical reaction between the mud system and the formation. The causes of the hole stability problems must be identified before selecting a remedy since solutions for mechanical problems such as mud weight increases will not solve a chemical reaction occurrence.
Well Logging. The physical and chemical properties of a drilling fluid may affect the well logging program. As an example, a high-salt-content mud may prevent use of a spontaneous potential (SP) tool since the salt concentration of the mud and formation may be approximately equal. In addition, oil muds inhibit the use of resistivity logs since the oil acts as an insulator and prevents current flow. The selection of an adequate suite of logs must be coordinated with the drilling fluids program to allow proper formation evaluation.
Corrosion of Drillstring, Casing, and Tubing. An increasing number of wells are being drilled in formations containing toxic gases such as hydrogen sulfide (H;S). These gases not only pose health and safety problems to personnel but also present dangers to metal components from hydrogen embrittlement, blistering, and stress cracking. The mud system can use additives such as scavengers to remove these contaminants. In addition, oil muds will minimize corrosion problems, although they do not necessarily reduce the health hazards.
Contamination Problems. The mud system must often control various types of contaminants, including toxic gases, high solids contents, hydrocarbon gases, and ionic contamination. Mud types commonly used in the drilling phase have varying abilities to control these contaminants. For example, a dispersed Iignosulfonate mud has a high solids tolerance, whereas some polymer systems function poorly with even small concentrations of solids. Drilling environments must be evaluated when developing the well plan to allow for selecting the proper mud system to control possible contaminants.
Minimize Torque, Drag, and Pipe Sticking. Excessive torque and drag are problems commonly encountered in drilling operations that can be addressed by selecting the proper mud system and additives. Torque is the force required to rotate the drillstring. Drag is the incremental force above the string weight required to move the pipe vertically. Excess torque can cause drillstring twistoff, while high drag forces can cause pipe sticking and pipe parting.
The mud system can reduce the severity of torque and drag problems. If the problems occur as a result of formation hydration and swelling, certain chemicals or mud types will inhibit the formation. As a minimum effort, lubricants can be used to reduce the friction coefficient along the walls of the wellbore.
Pipe sticking is a costly problem that can be avoided in many cases by proper mud system maintenance and selection. Differential sticking occurs when the pipe becomes embedded in the filter cake opposite a permeable zone and is held in place by the difference between hydrostatic and formation pressure (Fig. S—4>. Low water loss muds can reduce the frequency and severity of the occurrences. In many cases, oil muds will virtually eliminate the problem.
Improve Drilling Rate. The drilling rate is affected by various properties of the mud system. High-viscosity muds reduce the cross flow velocity beneath the bit, which inhibits cuttings removal. Lower water losses and high solids
content retard equalization of pressure around the drilled chip, thereby requiring regrinding prior to removal. Mud selection to optimize drill rates can reduce the drilling time. However, caution must be exercised so other problems do not occur, i.e., formation damage, hole stability, or stuck pipe.
Many types of drilling Huids are used in the industry. Major categories include air-, water-, and oil-based fluids. Each has many subcategories based on purpose, additives, or clay states. A brief description of major types of mud systems will be presented. Due to the large variety of muds presently in use, it is impossible to describe all of the systems. Omission of a mud type in this discussion does not dismiss it as an important mud in some operations.
Water-Based Fluids. The mud system used most frequently throughout the industry is the water-based system. Water is the continuous phase, but it may contain oil (i.e., emulsion muds) or air (i.e., aerated mud) as the discontinuous phase. The oil must remain as segregated droplets and not combine in a distinct phase termed "discontinuous" (Fig. 8-5).
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