The completion begins when the drill bit first penetrates the pay. Drilling the pay zone is one of the most important parts of the drilling procedure, thus drilling mud that is adequate for drilling the rest of the well may not be acceptable in the pay. Whereas formation damage created by the mud is acceptable in a nonproductive interval, it cannot be tolerated in the pay zone. What is needed is a mud that can control leakoff without creating permanent damage. The mud may require special treatment and occasionally, a changeout of the mud to a nondamaging fluid. There are several goals in drilling besides well control that are of interest to the completions engineer.
1. Drill a usable hole - A hole through the pay that will not accept the design size of casing limits the possibilities of the well and may impair the productivity.
2. Minimize formation permeability damage - High drilling mud overbalance pressure, uncontrolled particle size, mud filtrate that swells clays and poor leakoff control may mask the response of a productive formation to a drill stem test (DST) and may lead to bypassing a producing zone.
3. Control washouts - Hole stability problems may cause hole enlargements that make perforation and formation breakdown much more difficult.
From a drillers viewpoint, there are five main functions of a drilling mud:’ pressure control, bit lubrication, shale stability, fluid loss control and cuttings retrieval. The most important aspects of a drilling mud from a formation damage standpoint are to prevent loss of the drilling mud filtrate and to make sure that the filtrate that is lost will not react with the formation to reduce permeability. Fluid reactivity is usually controlled by using potassium chloride or other salts to stabilize the clay in the formation.2
Potassium chloride may not always control clay reactions or may require as much as 4% or more salt where smectite clay is present in the larger pore passages. Fluid loss control is accomplished by rapidly sealing off the permeable sections of the formation^.^^^ The mud accomplishes this fluid loss control by creating an almost impermeable mud cake of particles on the surface of the formation where leakoff occurs. The mud cake is produced by simple dehydration; as the liquid penetrates into the formation (the mud filtrite), the solid particles are stranded on the surface of the formation. In a properly formulated mud, there are a wide range of particle sizes that, on dehydration, fit together into a tightly compacted, very low permeability seal. By carefully controlling the size range of particles and minimizing
the clay size particles that could invade the pores of the formation, invasion damage from particles can be stopped. 4-7 In some drilling and workover fluids, fine particles and at least parts of the solids in the fluids will be designed to be acid soluble.8
The time required to form the mud cake will depend upon the mud characteristics, the permeability and the pressure differential, (Must be toward the formation for well control!) A higher permeability formation will generate a mud cake very rapidly than a low permeability formation since the rate of initial fluid loss (spurt) is higher. After the mud cake is formed, further liquid losses depend on the permeability of the cake. Formation of a cake does not insure that leakoff stops. In cases where the formation matrix permeability is between approximately 0.5 md and 100 md and the pressure differential toward the formation is small (APc1 00 psi), the filtrate of even a damaging mud will not likely extend into the formation beyond a depth of a few inches provided that the filter cake is successful in controlling leakoff. To build a successful mud cake, there must be leakoff. If the permeability is very low (e.g., kc0.05 md), the filter cake may be only poorly formed and fluid loss could be much higher than expected. This is especially true when the pay is an upper formation in a
deep well where a high density mud is used and the formation is exposed to the mud for a long period of time. Fortunately, most very low permeability formations require fracture stimulation, so the zone of damage is easily bypassed. The occurrence of the damage is important, however, since a productive interval might be missed on a test of an unstimulated well. The higher permeability formations pose special problems if the mud cake cannot be formed quickly. Since every trip out of the hole scrapes off much of the protective mud cake, the cake must reform easily to prevent the loss of large volumes of
mud filtrate into the formation. Tell-tail identifiers of a permeable formation are deflections on the SP log, bit drag and where the caliper log shows a narrow spot of slightly less than the bit diameter. This sticking point should not be confused with borehole deformation; a plastic flow of the rock in response to bore hole deformation, active faulting, folding, salt domes, etc.l5 The depth of damage created by the filtrate of the mud is directly related to the amount of driving pressure that the mud exerts on the formation. Even with a high quality mud, damage can be very deep if there is high mud overpressure. When high pressure zones elsewhere in the hole require the use of high pressure on the mud system, lower pressure zones are forced to take fluid by the pressure differential. This situation becomes critical when a zone that may be pay is broken down and fractured with the mud. Several hundred barrels of mud can be lost when the well is fractured. Some wells damaged
in this way never produce as expected. The only safe way to prevent this type of fluid loss from occurring is to case through the zones requiring high mud weights before the pay zones are drilled. Improving the filter cake and making the mud filtrate more compatible with the formation is one of the best methods of controlling formation damage. The use of inhibited filtrate prepared with potassium chloride (such as 2% KCI) will often minimize the formation damage in pays with even water sensitive sandstones.
In formations that are sensitive to fluid, the total time that the sensitive zone is exposed to mud may be critical. Once a section of the well that is known to be sensitive is penetrated, operations should continue as quickly as possible until casing can be cemented over the zone. This treatment is usually reserved for sections of caving shale or other unstable formation; however, it may also be used very successfully in drilling pay zones that are water sensitive. If loss of permeability is plotted against accumulative fluid loss from the mud, permeability damage increases very steadily as total fluid loss increases, almost regardless of the type of fluid. This emphasizes the importance of maintaining a
high quality mud and lowering the exposure of the formation to fluid loss.
Most of the solids and cuttings from the mud are halted at the formation face and very little penetration occurs unless a poorly designed mud with a large amount of clay or silt sizes particles are used in a formation with large pore throats. The damage from these solids is most apparent in the form of formation face plugging. Movement of the solids into the formation is dependent on the size of the pores, particle size and quantity of the finest solids in the mud. Although some tests have shown several centimeter penetration of fine mud particles into high permeability ~andstonea,~ p roperly conditioned mud will probably not invade the formation.
If the formation has rubble zones (very poorly sorted grains with sizes that may range from fines to small boulders), very permeable porous sections, fractures or vugs, then severe whole mud penetration may occur and produce lasting formation damage. It is very advantageous to design the mud or completion fluid to bridge off on the face of the formation to prevent the possibility of particle invasion.
When the mud or kill fluid cannot be circulated, the formation has a lost circulation zone that has very high permeability or cannot support the weight of the mud column without fracturing. For these problem cases, special pills of LCM, lost circulation material, are often run to plug off the high perm zonesg Where the formation will not support the mud column, a cement sheath is often tried to reinforce the zone. After setting a cement plug, the hole is redrilled. The cement invades fractures and vugs, adding strength and controlling leakoff. One problem with lost circulation material (LCMs) cases is that drillers use a variety of LCMs, such as paper, sawdust, leather, grain, etc., that are very effective in preventing leakoff but cannot be removed if the zone is a pay zone. Any LCM used in a potential
pay must be easily removable. The decision on whether a mud system should be changed before the pay is drilled depends upon the sensitivity of the pay to the mud filtrate. If the formation contains swellable clays such as smectite, a filtrite sensitivity test on core from an offset well will tell whether the formation is damaged by introduction of the mud filtrate. Where core is not available, a mud with a low damage potential (potassium chloride) should be considered. Smectite clay in the pore throats is usually reactive to fresh fluids, up
to 5% or more KCI is sometimes needed to prevent clay problems in formations that have 3 to 8% smectite. In gas zones, the use of most oil-based muds should be avoided unless the mud has been proven to be of a nondamaging nature in the zone of interest. In oil or gas zones that are to be frac-tured, less emphasis is placed on the mud damage at the wellbore since a fracture will extend beyond the damage.
When natural fractures or vugs are present in the pay, whole mud can be lost. In these situations, it is often necessary to set a casing string above the pay and drill the formation without returns or use a fluid loss control additive capable of sealing fractures at the wellbore. Other methods, such as drilling the well while flowing and diverting the produced fluids, have also been considered but are dangerous in high pressure formations.
Because of damage by both incompatible filtrate and the migration of very small particles in the mud, the completion zone in many wells has been drilled with completion fluid. This practice eliminates much of the damage from mud and mud filtrate. The basic problem with the process is in completely cleaning the hole and pipe of residuals from the mud so that the left-over mud and cuttings do not contaminate the completion fluid. Fluid loss from solids free systems may be very high, especially in high permeability formations.
In very sensitive pay zones, the wells are often drilled with mud to the top of the pay and the pay itself is drilled with air, mist or foam to reduce the amount of water in contact with pay. Another method of reducing formation damage is to drill the pay with reverse circulation. This approach has been used in sensitive formations to limit the contamination of the mud by drill cuttings. Regardless of the formation sensitivity, well control must always be the Number 1 priority. The importance of drilling a usable hole through the pay and its importance on running and cementing pipe cannot be overstated. Failure to get a casing string or a liner to bottom can be very costly in terms of cost of an additional string or liner and the reduction of working space where pumps and other equipment need to be set. Simply drilling a hole with a certain diameter drill bit through a formation does not lead to a hole that will accept a string of pipe of an outside diameter just smaller than the
drill In most instances where casing cannot be run in a freshly-drilled hole, the problem is that a usable hole has not been drilled, i.e., the drift diameter of the hole is not equal to the bit diameter.
This problem is shown schematically in Figures 1.1 and 1.2. Figure 1.1 illustrates problems with hard ledges or changes in formation, while Figure 1.2 shows an extreme case of bit wobble. The spiral hole illustrated in Figure 1.2 was caused by an under-stabilized bit creating a hole too small to run the planned casing. Normally, casing strings are run with 1-112 to 2 in. minimum clearance between the hole diameter and the outside diameter of the pipe. In a straight hole, this is adequate clearance, but in a hole with an incorrect BHA (bottomhole assembly of drilling bit, collars, and stabilizers), problems will develop during running of the pipe. Drilling “slick” (drill collars in the BHA without stabilizers) usually
leads to a hole with a usable diameter significantly less than the diameter of the drill bit. Estimation
of this usable hole or drift diameter is:
The formula points out that the usable diameter of the hole may be smaller than the bit. If the hole has been drilled with the intention of running a liner, the problem may be even more pronounced. Liners are usually characterized by close tolerances between the pipe and the hole, thus it is essential that good hole diameter stability be maintained.
The type of drilling mud may also make a difference in getting pipe to the bottom. Differential sticking is caused by a pressure differential into a permeable zone that holds the pipe (or logging tool) against the wall and buries the lower side of the pipe in the mud cake.14 Sticking is increased by thick mud cakes because of increased contact area, Figure 1.3. An efficient mud forms a thin, slick mud cake with very low permeability. A thin mudcake keeps the pipe from becoming deeply embedded, resulting in less torque and drag.14 The goal is a high colloidal clay-to-silt (or cuttings) ratio that produces a slick, thin cake.
Diagnostics of stuck casing are often made after examining the drilling record and trying different types of pipe movement and circulation. A simple, stuck pipe diagnostic routine is shown schematically in Figure 1.4.14
Calculating the true vertical depth, TVD, from the measured vertical depth, MD, can be accomplished for consistent deviated wells from simple trigonometry or from tables. When wells use long turn radii, other corrections may be needed.
During drilling of wildcats or field development wells in sparsely drilled areas, mud density is handled as a function of well control, with pore pressures estimated from other data. In this type of environment, high mud overbalance conditions may occur, especially in deep formations. Although fracturing is the most obvious effect of high mud weights, excess formation permeability damage may also occur. In a study of factors influencing stimulation rates, Paccaloni, et al.,16 reports that in formations greater than 100 md, 90% of DST's were dry or doubtful when an overbalance of over 11 00 psi was used during drilling. Excessive mud overbalances should be avoided in pay zones.
1. Drill a usable hole - A hole through the pay that will not accept the design size of casing limits the possibilities of the well and may impair the productivity.
2. Minimize formation permeability damage - High drilling mud overbalance pressure, uncontrolled particle size, mud filtrate that swells clays and poor leakoff control may mask the response of a productive formation to a drill stem test (DST) and may lead to bypassing a producing zone.
3. Control washouts - Hole stability problems may cause hole enlargements that make perforation and formation breakdown much more difficult.
From a drillers viewpoint, there are five main functions of a drilling mud:’ pressure control, bit lubrication, shale stability, fluid loss control and cuttings retrieval. The most important aspects of a drilling mud from a formation damage standpoint are to prevent loss of the drilling mud filtrate and to make sure that the filtrate that is lost will not react with the formation to reduce permeability. Fluid reactivity is usually controlled by using potassium chloride or other salts to stabilize the clay in the formation.2
Potassium chloride may not always control clay reactions or may require as much as 4% or more salt where smectite clay is present in the larger pore passages. Fluid loss control is accomplished by rapidly sealing off the permeable sections of the formation^.^^^ The mud accomplishes this fluid loss control by creating an almost impermeable mud cake of particles on the surface of the formation where leakoff occurs. The mud cake is produced by simple dehydration; as the liquid penetrates into the formation (the mud filtrite), the solid particles are stranded on the surface of the formation. In a properly formulated mud, there are a wide range of particle sizes that, on dehydration, fit together into a tightly compacted, very low permeability seal. By carefully controlling the size range of particles and minimizing
the clay size particles that could invade the pores of the formation, invasion damage from particles can be stopped. 4-7 In some drilling and workover fluids, fine particles and at least parts of the solids in the fluids will be designed to be acid soluble.8
The time required to form the mud cake will depend upon the mud characteristics, the permeability and the pressure differential, (Must be toward the formation for well control!) A higher permeability formation will generate a mud cake very rapidly than a low permeability formation since the rate of initial fluid loss (spurt) is higher. After the mud cake is formed, further liquid losses depend on the permeability of the cake. Formation of a cake does not insure that leakoff stops. In cases where the formation matrix permeability is between approximately 0.5 md and 100 md and the pressure differential toward the formation is small (APc1 00 psi), the filtrate of even a damaging mud will not likely extend into the formation beyond a depth of a few inches provided that the filter cake is successful in controlling leakoff. To build a successful mud cake, there must be leakoff. If the permeability is very low (e.g., kc0.05 md), the filter cake may be only poorly formed and fluid loss could be much higher than expected. This is especially true when the pay is an upper formation in a
deep well where a high density mud is used and the formation is exposed to the mud for a long period of time. Fortunately, most very low permeability formations require fracture stimulation, so the zone of damage is easily bypassed. The occurrence of the damage is important, however, since a productive interval might be missed on a test of an unstimulated well. The higher permeability formations pose special problems if the mud cake cannot be formed quickly. Since every trip out of the hole scrapes off much of the protective mud cake, the cake must reform easily to prevent the loss of large volumes of
mud filtrate into the formation. Tell-tail identifiers of a permeable formation are deflections on the SP log, bit drag and where the caliper log shows a narrow spot of slightly less than the bit diameter. This sticking point should not be confused with borehole deformation; a plastic flow of the rock in response to bore hole deformation, active faulting, folding, salt domes, etc.l5 The depth of damage created by the filtrate of the mud is directly related to the amount of driving pressure that the mud exerts on the formation. Even with a high quality mud, damage can be very deep if there is high mud overpressure. When high pressure zones elsewhere in the hole require the use of high pressure on the mud system, lower pressure zones are forced to take fluid by the pressure differential. This situation becomes critical when a zone that may be pay is broken down and fractured with the mud. Several hundred barrels of mud can be lost when the well is fractured. Some wells damaged
in this way never produce as expected. The only safe way to prevent this type of fluid loss from occurring is to case through the zones requiring high mud weights before the pay zones are drilled. Improving the filter cake and making the mud filtrate more compatible with the formation is one of the best methods of controlling formation damage. The use of inhibited filtrate prepared with potassium chloride (such as 2% KCI) will often minimize the formation damage in pays with even water sensitive sandstones.
In formations that are sensitive to fluid, the total time that the sensitive zone is exposed to mud may be critical. Once a section of the well that is known to be sensitive is penetrated, operations should continue as quickly as possible until casing can be cemented over the zone. This treatment is usually reserved for sections of caving shale or other unstable formation; however, it may also be used very successfully in drilling pay zones that are water sensitive. If loss of permeability is plotted against accumulative fluid loss from the mud, permeability damage increases very steadily as total fluid loss increases, almost regardless of the type of fluid. This emphasizes the importance of maintaining a
high quality mud and lowering the exposure of the formation to fluid loss.
Most of the solids and cuttings from the mud are halted at the formation face and very little penetration occurs unless a poorly designed mud with a large amount of clay or silt sizes particles are used in a formation with large pore throats. The damage from these solids is most apparent in the form of formation face plugging. Movement of the solids into the formation is dependent on the size of the pores, particle size and quantity of the finest solids in the mud. Although some tests have shown several centimeter penetration of fine mud particles into high permeability ~andstonea,~ p roperly conditioned mud will probably not invade the formation.
If the formation has rubble zones (very poorly sorted grains with sizes that may range from fines to small boulders), very permeable porous sections, fractures or vugs, then severe whole mud penetration may occur and produce lasting formation damage. It is very advantageous to design the mud or completion fluid to bridge off on the face of the formation to prevent the possibility of particle invasion.
When the mud or kill fluid cannot be circulated, the formation has a lost circulation zone that has very high permeability or cannot support the weight of the mud column without fracturing. For these problem cases, special pills of LCM, lost circulation material, are often run to plug off the high perm zonesg Where the formation will not support the mud column, a cement sheath is often tried to reinforce the zone. After setting a cement plug, the hole is redrilled. The cement invades fractures and vugs, adding strength and controlling leakoff. One problem with lost circulation material (LCMs) cases is that drillers use a variety of LCMs, such as paper, sawdust, leather, grain, etc., that are very effective in preventing leakoff but cannot be removed if the zone is a pay zone. Any LCM used in a potential
pay must be easily removable. The decision on whether a mud system should be changed before the pay is drilled depends upon the sensitivity of the pay to the mud filtrate. If the formation contains swellable clays such as smectite, a filtrite sensitivity test on core from an offset well will tell whether the formation is damaged by introduction of the mud filtrate. Where core is not available, a mud with a low damage potential (potassium chloride) should be considered. Smectite clay in the pore throats is usually reactive to fresh fluids, up
to 5% or more KCI is sometimes needed to prevent clay problems in formations that have 3 to 8% smectite. In gas zones, the use of most oil-based muds should be avoided unless the mud has been proven to be of a nondamaging nature in the zone of interest. In oil or gas zones that are to be frac-tured, less emphasis is placed on the mud damage at the wellbore since a fracture will extend beyond the damage.
When natural fractures or vugs are present in the pay, whole mud can be lost. In these situations, it is often necessary to set a casing string above the pay and drill the formation without returns or use a fluid loss control additive capable of sealing fractures at the wellbore. Other methods, such as drilling the well while flowing and diverting the produced fluids, have also been considered but are dangerous in high pressure formations.
Because of damage by both incompatible filtrate and the migration of very small particles in the mud, the completion zone in many wells has been drilled with completion fluid. This practice eliminates much of the damage from mud and mud filtrate. The basic problem with the process is in completely cleaning the hole and pipe of residuals from the mud so that the left-over mud and cuttings do not contaminate the completion fluid. Fluid loss from solids free systems may be very high, especially in high permeability formations.
In very sensitive pay zones, the wells are often drilled with mud to the top of the pay and the pay itself is drilled with air, mist or foam to reduce the amount of water in contact with pay. Another method of reducing formation damage is to drill the pay with reverse circulation. This approach has been used in sensitive formations to limit the contamination of the mud by drill cuttings. Regardless of the formation sensitivity, well control must always be the Number 1 priority. The importance of drilling a usable hole through the pay and its importance on running and cementing pipe cannot be overstated. Failure to get a casing string or a liner to bottom can be very costly in terms of cost of an additional string or liner and the reduction of working space where pumps and other equipment need to be set. Simply drilling a hole with a certain diameter drill bit through a formation does not lead to a hole that will accept a string of pipe of an outside diameter just smaller than the
drill In most instances where casing cannot be run in a freshly-drilled hole, the problem is that a usable hole has not been drilled, i.e., the drift diameter of the hole is not equal to the bit diameter.
This problem is shown schematically in Figures 1.1 and 1.2. Figure 1.1 illustrates problems with hard ledges or changes in formation, while Figure 1.2 shows an extreme case of bit wobble. The spiral hole illustrated in Figure 1.2 was caused by an under-stabilized bit creating a hole too small to run the planned casing. Normally, casing strings are run with 1-112 to 2 in. minimum clearance between the hole diameter and the outside diameter of the pipe. In a straight hole, this is adequate clearance, but in a hole with an incorrect BHA (bottomhole assembly of drilling bit, collars, and stabilizers), problems will develop during running of the pipe. Drilling “slick” (drill collars in the BHA without stabilizers) usually
leads to a hole with a usable diameter significantly less than the diameter of the drill bit. Estimation
of this usable hole or drift diameter is:
The formula points out that the usable diameter of the hole may be smaller than the bit. If the hole has been drilled with the intention of running a liner, the problem may be even more pronounced. Liners are usually characterized by close tolerances between the pipe and the hole, thus it is essential that good hole diameter stability be maintained.
The type of drilling mud may also make a difference in getting pipe to the bottom. Differential sticking is caused by a pressure differential into a permeable zone that holds the pipe (or logging tool) against the wall and buries the lower side of the pipe in the mud cake.14 Sticking is increased by thick mud cakes because of increased contact area, Figure 1.3. An efficient mud forms a thin, slick mud cake with very low permeability. A thin mudcake keeps the pipe from becoming deeply embedded, resulting in less torque and drag.14 The goal is a high colloidal clay-to-silt (or cuttings) ratio that produces a slick, thin cake.
Diagnostics of stuck casing are often made after examining the drilling record and trying different types of pipe movement and circulation. A simple, stuck pipe diagnostic routine is shown schematically in Figure 1.4.14
Calculating the true vertical depth, TVD, from the measured vertical depth, MD, can be accomplished for consistent deviated wells from simple trigonometry or from tables. When wells use long turn radii, other corrections may be needed.
During drilling of wildcats or field development wells in sparsely drilled areas, mud density is handled as a function of well control, with pore pressures estimated from other data. In this type of environment, high mud overbalance conditions may occur, especially in deep formations. Although fracturing is the most obvious effect of high mud weights, excess formation permeability damage may also occur. In a study of factors influencing stimulation rates, Paccaloni, et al.,16 reports that in formations greater than 100 md, 90% of DST's were dry or doubtful when an overbalance of over 11 00 psi was used during drilling. Excessive mud overbalances should be avoided in pay zones.
