A complete and comprehensive mud plan must be included in the well planning process. History has proven that an incomplete mud plan will cost the operator many hours of rig time (check also oil well drilling well cost per foot) and may mean the difference between a productive or a nonproductive well. Here, we shall discuss the functions of the drilling fluids or mud in oil and gas wells.
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The use of drilling fluid to remove cuttings from the borehole was first conceived by Fauvelle, a French mud engineer, in 1845. “Drilling fluids” describes a broad range of fluids, both liquids and gases, used in drilling operations to achieve specific purposes. The fluids may be air, natural gas, water, oil, or a combination of liquids used with special drilling chemicals and additives. Drilling fluids are designed to solve or minimize many drilling problems and, as such, an understanding of these purposes will help the drilling supervisor successfully prepare a mud program, use proper additives, and diagnose trouble areas.
On a drilling rig, drilling fluid is pumped from the mud tanks through the Drill String, where it sprays out of nozzles on the Oilfield Drilling Bit, thus cleaning and cooling the drill bit in the process. The mud then carries the crushed or cut rock (“cuttings”) up the annular space (“annulus”) between the drill string and the sides of the hole being drilled, up through the surface casing, where it emerges back at the surface. Cuttings are then filtered out with either a shale shaker or the newer shale conveyor technology, and the mud returns to the mud pits. The mud pits let the drilled “fines” settle; the pits are also where the fluid is treated by adding chemicals and other substances.
The returning mud can contain natural gases or other flammable materials which will collect in and around the shale shaker/conveyor area or in other work areas. Because of the risk of a fire or an explosion, if they ignite, special monitoring sensors and explosion-proof certified equipment are commonly installed, and workers are trained in safety precautions. The mud is then pumped back down the hole and further recirculated. After testing, the drilling fluid is treated periodically in the mud pits to ensure there are the desired mud properties that optimize and improve drilling efficiency, borehole stability, and other requirements, as listed below.
Drilling fluids serve many functions. The major functions of drilling mud include the following:
- Cool and lubricate the bit and Drill String
- Clean the hole bottom
- Carry cuttings to the surface
- Remove cuttings from mud at the surface
- Minimize formation damage
- Control formation pressures
- Maintain hole integrity
- Assist in well logging operations
- Minimize corrosion of the Drill String, casing, and tubing
- Minimize contamination problems
- Minimize torque, drag, and pipe sticking
- Improve drilling rate
Due to various drilling conditions encountered, all functions of drilling fluids 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.
The drilling engineer must attempt to select a system that will achieve the following goals:
- Satisfy the crucial items such as pressure control
- If possible, satisfy all lower-priority requirements
- Avoid using systems that are completely unsatisfactory
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.
Drilling Mud Function Is To Cool and Lubricate the Bit and Drill string.
A considerable amount of heat due to friction is generated during the drilling operation. Muds can function as a coolant and 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 are 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.
One Function of Drilling Mud Is To Clean the Hole Bottom.
The removal of cuttings from below the bit is one of the most important functions of 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, the density of the fluid, and fluid velocity (Fig.1).
The chip hold-down phenomenon occurs when the Drilling 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.
This feature has been accomplished successfully with drilling fluids that exhibit a high “spurt loss, initial filtration” by helping equalize pressures above and below the chip.
The Task of Drilling Fluid Is To Carry Drilling Cuttings to the Surface.
Transporting the cuttings that are removed from below the bit is a huge and important function of drilling fluid or mud. The fluid velocity in the annulus must exceed the downward falling rate, or slip velocity, of the cuttings (Fig.2). Mud weight, fluid viscosity, suspension, and gelation properties of the mud affect its carrying capacity. Laminar and turbulent flow regimes exhibit different lifting capabilities.
When circulation is stopped, the cuttings that have not been removed must be suspended or they will fall downward. If mud does not exhibit the proper characteristics to suspend the cuttings, reentry into the hole and reaching the 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 and this is considered one of the functions of drilling fluid. Mud pits usually do not allow sufficient time for solids to settle out. Mechanical solids removal equipment such as shale shakers, desilters, 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 the largest solids should occur near the flow line; removal of finer solids should occur prior to entering the suction pit.
Drilling Fluid Is Able To Minimize Formation Damage.
The protection of potential pay zones is important for drilling fluid. The deposition of a filter cake that allows the drilling operation to continue and protects a productive zone is often a forgotten consideration of a mud system. The formation damage is generally a reduction in permeability near the wellbore with perhaps a slight porosity reduction. The problem can be severe in low-permeability reservoirs or reservoirs with high clay content.
Several mechanisms can cause formation damage during drilling (Fig.3). Filtrate loss from the mud can enter the producing zone and cause interstitial clay 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
How Controlling Formation Pressures is a Function Of Drilling Fluid.
Drilling intervals that have abnormally high formation pressures require that the the function of drilling mud system is 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.
Maintaining Hole Integrity Is One Of The Functions Of Drilling Mud.
Wellbores often exhibit stability problems resulting from geological phenomena such as fractured zones, unconsolidated sections, hydratable clays, and pressured sections. The function of 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 drilling 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 occurrance.
How Drilling Fluid Function In Well Logging.
The physical and chemical properties of drilling fluid may affect the well logging program. As an example, a high-salt-content mud may prevent the 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.
The Task Of Drilling Fluid For Corrosion of Drill string, Casing, and Tubing.
An increasing number of wells are being drilled in formations containing toxic gases such as hydrogen sulfide (H2S). 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 drilling mud system can use additives such as scavengers to facilitate its function to remove these contaminants. In addition, oil muds will minimize corrosion problems, although they do not necessarily reduce the health hazards.
The Related Contamination Problems To Drilling Fluid Functions.
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 lignosulfonate 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.
The Duty Of Drilling Fluids To 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 Drill String. Drag is the incremental force above the string weight required to move the pipe vertically. Excess torque can cause drill string twist-off, while high drag forces can cause pipe sticking and pipe parting.
One of the functions of drilling mud system is to 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 Stuck Pipe 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.4). Low water loss muds can reduce the frequency and severity of the occurrences. In many cases, oil muds will virtually eliminate the problem.
The Last Drilling Fluid Function Is To Improve Drilling Rate.
There are a lot of factors affecting the drilling rate & it is also affected by various properties of the mud system. High-viscosity muds reduce the cross-flow velocity beneath the Drilling 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 function to optimize drill rates can reduce the drilling time. However, caution must be exercised so other problems do not occur, the formation damage, hole stability, or Stuck Pipe.