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Drilling fluids serve to balance formation pressures while drilling to ensure wellbore stability. They also carry cuttings to the surface and cool the bit. The drilling engineer traditionally designs drilling fluids with two primary goals in mind:  
Drilling fluids serve to balance formation pressures while drilling to ensure wellbore stability. They also carry cuttings to the surface and cool the bit. The drilling engineer traditionally designs drilling fluids with two primary goals in mind:
 
*To ensure safe, stable boreholes, which is accomplished by operating within an acceptable mud-weight window
*To ensure safe, stable boreholes, which is accomplished by operating within an acceptable mud-weight window
*To achieve high rates of penetration so that rig time and well cost can be minimized
*To achieve high rates of penetration so that rig time and well cost can be minimized
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These primary considerations do not include well productivity concerns. A growing recognition of the importance of drilling-induced formation damage has led to the use of drill-in fluids (fluids used to drill through the pay zone) that minimize formation damage. This page discusses the formation damage that may be associated with various types of drilling fluids.
These primary considerations do not include well productivity concerns. A growing recognition of the importance of drilling-induced formation damage has led to the use of drill-in fluids (fluids used to drill through the pay zone) that minimize formation damage. This page discusses the formation damage that may be associated with various types of drilling fluids.


==Drill-in fluids to reduce formation damage==
== Drill-in fluids to reduce formation damage ==
Drilling and well productivity concerns are addressed in the design of drill-in fluids. To meet well productivity objectives (i.e., to minimize formation damage), the drill-in fluid must meet the following additional objectives:  
 
Drilling and well productivity concerns are addressed in the design of drill-in fluids. To meet well productivity objectives (i.e., to minimize formation damage), the drill-in fluid must meet the following additional objectives:
 
*Minimize the extent of solids invasion into the formation by bridging across the pores and forming a thin, low-permeability, filter cake
*Minimize the extent of solids invasion into the formation by bridging across the pores and forming a thin, low-permeability, filter cake
*Minimize the extent of filtrate and polymer invasion into the formation through the formation of an external filter cake
*Minimize the extent of filtrate and polymer invasion into the formation through the formation of an external filter cake
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To achieve these goals, various strategies have been adopted. Traditional water-based muds, oil-based muds, and some special formulations of drill-in fluids for fractured formations and unconsolidated sands are discussed below.
To achieve these goals, various strategies have been adopted. Traditional water-based muds, oil-based muds, and some special formulations of drill-in fluids for fractured formations and unconsolidated sands are discussed below.


==Water based muds==
== Water based muds ==
 
The vast majority of drilling fluids consist of:
The vast majority of drilling fluids consist of:
*Bentonite mixed with polymers to enhance the [[Glossary:Rheology|rheology]] (or, more specifically, the cuttings-carrying capacity of the fluid)
*Bentonite mixed with polymers to enhance the [[Glossary:Rheology|rheology]] (or, more specifically, the cuttings-carrying capacity of the fluid)
*Starches to control fluid loss
*Starches to control fluid loss
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*Perhaps a pH buffer to maintain the pH of the mud to the desired level
*Perhaps a pH buffer to maintain the pH of the mud to the desired level


A great deal of work has been done in the last three decades on evaluating the formation damage potential of water based drilling fluids.<ref name="r1" /><ref name="r2" /><ref name="r3" /><ref name="r4" /><ref name="r5" />The following factors have been observed to have an impact on the depth of invasion of solids and filtrate and therefore on the extent and depth of formation damage or permeability impairment:  
A great deal of work has been done in the last three decades on evaluating the formation damage potential of water based drilling fluids.<ref name="r1">Abrams, A. 1977. Mud Design To Minimize Rock Impairment Due To Particle Invasion. J Pet Technol 29 (5): 586-592. SPE-5713-PA. http://dx.doi.org/10.2118/5713-PA</ref><ref name="r2">Nowak, T.J. and Krueger, R.F. 1951. The Effect of Mud Filtrates and Mud Particles upon the Permeabilities of Cores. API Drill. Prod. Prac., 164–181.</ref><ref name="r3">Glenn, E.E. and Slusser, M.L. 1957. Factors Affecting Well Productivity, II: Drilling Fluid Particle Invasion Into Porous Media. Petroleum Transactions Vol. 210, 132-139. Richardson, Texas: AIME.</ref><ref name="r4">Suri, A. and Sharma, M.M. 2001. Strategies for Sizing Particles in Drilling and Completion Fluids. Presented at the SPE European Formation Damage Conference, The Hague, Holland, 21–22 May. SPE-68964-MS. http://dx.doi.org/10.2118/68964-MS</ref><ref name="r5">Ladva, H.K.J., Tardy, P., Howard, P.R. et al. 2000. Multiphase Flow and Drilling Fluid Filtrate Effects on the Onset of Production. Presented at the SPE International Symposium on Formation Damage Control, Lafayette, Louisiana, 23-24 February 2000. SPE-58795-MS. http://dx.doi.org/10.2118/58795-MSfckLR↑ 6.0 6.1 Klotz, J.A., Krueger, R.F., and Pye, D.S. 1974. Effect of Perforation Damage on</ref>The following factors have been observed to have an impact on the depth of invasion of solids and filtrate and therefore on the extent and depth of formation damage or permeability impairment:
 
*State of dispersion of solids in the mud
*State of dispersion of solids in the mud
*Size and concentration of solids and polymers in the mud
*Size and concentration of solids and polymers in the mud
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*Water sensitivity of the formation
*Water sensitivity of the formation


In most instances, the invasion of solids into the formation is limited to 2 or 3 in. from the wellbore wall, which implies that the productivity of perforated wells with relatively shallow depth of damage will not be significantly affected. '''Fig. 1''' shows the productivity index (PI) of a well for different depths of damage assuming an 8-in.-long perforation. It is evident that as long as the depth of damage is smaller than the perforation length, the well PI is not significantly affected. Wells that are completed openhole without stimulation are particularly susceptible to this kind of damage.  
In most instances, the invasion of solids into the formation is limited to 2 or 3 in. from the wellbore wall, which implies that the productivity of perforated wells with relatively shallow depth of damage will not be significantly affected. '''Fig. 1<ref name="r6">Klotz, J.A., Krueger, R.F., and Pye, D.S. 1974. Effect of Perforation Damage on Well Productivity. J Pet Technol 26 (11): 1303-1314. SPE-4654-PA. http://dx.doi.org/10.2118/4654-PA</ref>''' shows the productivity index (PI) of a well for different depths of damage assuming an 8-in.-long perforation. It is evident that as long as the depth of damage is smaller than the perforation length, the well PI is not significantly affected. Wells that are completed openhole without stimulation are particularly susceptible to this kind of damage.


<gallery widths=300px heights=200px>
<gallery widths="300px" heights="200px">
File:Vol4 Page 248 Image 0001.png|'''Fig. 1—(a) Schematic of a gun perforation showing zone of crushed rock around tunnel.'''<ref name="r6" /> '''(b) Effect of damage by mud while drilling on well productivity when perforated with a nondamaging fluid is permeability of crushed zone around perforated tunnel as a percent of initial permeability.'''<ref name="r6" />
File:Vol4 Page 248 Image 0001.png|'''Fig. 1—(a) Schematic of a gun perforation showing zone of crushed rock around tunnel.'''<ref name="r6" /> '''(b) Effect of damage by mud while drilling on well productivity when perforated with a nondamaging fluid is permeability of crushed zone around perforated tunnel as a percent of initial permeability.'''<ref name="r6" />
</gallery>
</gallery>


In some instances, deep penetration of drill solids can occur. '''Fig. 2''' shows the depth of invasion of formation damage when a 300-md Berea sandstone core is subjected to dynamic circulation of different water-based drilling fluids across its face. <ref name="r7" /> It is evident that, in overtreated muds (containing too much thinner or dispersant), dispersed bentonite particles can penetrate through > 8 in. of rock and cause severe and irreversible damage. The other extreme, flocculated muds (too little thinner or too much salt), will limit solids invasion but will result in thick, high-permeability filter cakes. Filter cakes can result in such problems as stuck pipe and large filtrate loss. The use of salts and thinners is, therefore, a critical part of the design of drilling fluids for a given application. Appropriately conditioned muds must be used to eliminate the possibility of solids invasion and to minimize filtrate invasion. As discussed later, using sized bridging solids is a powerful tool for reducing solids and polymer invasion.  
In some instances, deep penetration of drill solids can occur. '''Fig. 2''' shows the depth of invasion of formation damage when a 300-md Berea sandstone core is subjected to dynamic circulation of different water-based drilling fluids across its face. <ref name="r7">Jiao, D. and Sharma, M.M. 1992. Formation Damage Due to Static and Dynamic Filtration of Water-Based Muds. Presented at the SPE Formation Damage Control Symposium, Lafayette, Louisiana, 26-27 February 1992. 00023823. http://dx.doi.org/10.2118/23823-MS</ref> It is evident that, in overtreated muds (containing too much thinner or dispersant), dispersed bentonite particles can penetrate through > 8 in. of rock and cause severe and irreversible damage. The other extreme, flocculated muds (too little thinner or too much salt), will limit solids invasion but will result in thick, high-permeability filter cakes. Filter cakes can result in such problems as stuck pipe and large filtrate loss. The use of salts and thinners is, therefore, a critical part of the design of drilling fluids for a given application. Appropriately conditioned muds must be used to eliminate the possibility of solids invasion and to minimize filtrate invasion. As discussed later, using sized bridging solids is a powerful tool for reducing solids and polymer invasion.


<gallery widths=300px heights=200px>
<gallery widths="300px" heights="200px">
File:Vol4 Page 249 Image 0001.png|'''Fig. 2—Depth of permeability damage cause by mud invasion for muds with different degrees of bentonite dispersion. All muds contain 4% bentonite by weight.'''<ref name="r7" />
File:Vol4 Page 249 Image 0001.png|'''Fig. 2—Depth of permeability damage cause by mud invasion for muds with different degrees of bentonite dispersion. All muds contain 4% bentonite by weight.'''<ref name="r7" />
</gallery>
</gallery>


Although solids invasion clearly is detrimental to well productivity, filtrate invasion can also lead to substantial formation damage and to greater depths in some instances. It has been shown, for example, <ref name="r7" /><ref name="r8" /> that the use of freshwater muds can result in filtrates that can be damaging to water-sensitive sandstones. In such instances, the simple process of increasing the salinity of the filtrate can prevent fines migration induced by filtrate leakoff. The loss of aqueous filtrates also results in a reduction in the relative permeability to the hydrocarbon phases. <ref name="r8" /> Such relative permeability effects are referred to as water-blocks and are discussed in [[Formation damage resulting from emulsion and sludge formation]].  
Although solids invasion clearly is detrimental to well productivity, filtrate invasion can also lead to substantial formation damage and to greater depths in some instances. It has been shown, for example, <ref name="r7">Jiao, D. and Sharma, M.M. 1992. Formation Damage Due to Static and Dynamic Filtration of Water-Based Muds. Presented at the SPE Formation Damage Control Symposium, Lafayette, Louisiana, 26-27 February 1992. 00023823. http://dx.doi.org/10.2118/23823-MS</ref><ref name="r8">Roy, R.S. and Sharma, M.M. 2001. The Relative Importance of Solids and Filtrate Invasion on the Flow Initiation Pressure. Presented at the SPE European Formation Damage Conference, The Hague, Netherlands, 21-22 May 2001. SPE-68949-MS. http://dx.doi.org/10.2118/68949-MS</ref> that the use of freshwater muds can result in filtrates that can be damaging to water-sensitive sandstones. In such instances, the simple process of increasing the salinity of the filtrate can prevent fines migration induced by filtrate leakoff. The loss of aqueous filtrates also results in a reduction in the relative permeability to the hydrocarbon phases. <ref name="r8">Roy, R.S. and Sharma, M.M. 2001. The Relative Importance of Solids and Filtrate Invasion on the Flow Initiation Pressure. Presented at the SPE European Formation Damage Conference, The Hague, Netherlands, 21-22 May 2001. SPE-68949-MS. http://dx.doi.org/10.2118/68949-MS</ref> Such relative permeability effects are referred to as water-blocks and are discussed in [[Formation_damage_resulting_from_emulsion_and_sludge_formation|Formation damage resulting from emulsion and sludge formation]].
 
Similarly, the use of [[Polymers|polymers]] is widespread but can, in some instances, lead to formation damage. Its been shown that the use of improper mixing producers in dissolving polymers into brines can result in the formation of "fish eyes," or unhydrated aggregates of polymer that can be several microns in diameter. These particulate gels are very effective as plugging agents and can lead to irreversible damage if not broken up and completely hydrated in the mud. Proper conditioning and dispersal of polymers is of critical importance in the field. <ref name="r9">Hodge, R.M., Augustine, B.G., Burton, R.C. et al. 1997. Evaluation and Selection of Drill-In-Fluid Candidates To Minimize Formation Damage. SPE Drill & Compl 12 (3): 174-179. SPE-31082-PA. http://dx.doi.org/10.2118/31082-PA</ref><ref name="r10">Browne, S.V. and Smith, P.S. 1994. Mudcake Cleanup To Enhance Productivity of High-Angle Wells. Presented at the SPE Formation Damage Control Symposium, Lafayette, Louisiana, 7-10 February 1994. SPE-27350-MS. http://dx.doi.org/10.2118/27350-MS</ref><ref name="r11">Ryan, D.F., Browne, S.V., and Burnham, M.P. 1995. Mud Clean-up in Horizontal Wells: A Major Joint Industry Study. Presented at the SPE Annual Technical Conference and Exhibition, Dallas, Texas, 22–25 October. SPE-30528-MS. http://dx.doi.org/10.2118/30528-MS</ref><ref name="r12">Francis, P. 1997. Dominating Effects Controlling the Extent of Drilling-Induced Formation Damage. Presented at the SPE European Formation Damage Conference, The Hague, The Netherlands, 2–3 June. SPE-38182-MS. http://dx.doi.org/10.2118/38182-MS</ref>
 
There is a limited database on the formation damage caused by starches and other polymers such as xanthan or carboxymethylcellulose. These data indicate that the flow of such polymers can induce a substantial reduction in permeability as a result of constriction of pore throats, particularly in low-permeability formations.


Similarly, the use of [[polymers]] is widespread but can, in some instances, lead to formation damage. Its been shown that the use of improper mixing producers in dissolving polymers into brines can result in the formation of "fish eyes," or unhydrated aggregates of polymer that can be several microns in diameter. These particulate gels are very effective as plugging agents and can lead to irreversible damage if not broken up and completely hydrated in the mud. Proper conditioning and dispersal of polymers is of critical importance in the field. <ref name="r9" /><ref name="r10" /><ref name="r11" /><ref name="r12" />
== Oil based muds ==


There is a limited database on the formation damage caused by starches and other polymers such as xanthan or carboxymethylcellulose. These data indicate that the flow of such polymers can induce a substantial reduction in permeability as a result of constriction of pore throats, particularly in low-permeability formations.  
Oil based muds consist of water droplets dispersed in a continuous oil phase. The water droplets are stabilized by emulsifiers and organophilic clays. Standard API fluid loss tests show that the fluid leakoff rate in oil based muds is substantially lower than for water based muds. However, as shown elsewhere, <ref name="r13">Jiao, D. and Sharma, M.M. 1993. Dynamic Filtration of Invert-Emulsion Muds. SPE Drill & Compl 8 (3): 165-169. SPE-24759-PA. http://dx.doi.org/10.2118/24759-PA</ref> when tests are conducted on oil-saturated cores (not filter paper), leak-off rates for oil-based muds can be combrble to those for water-based muds. One important conclusion of this study is that API fluid leakoff tests should not be used to determine filtration rates in oil-based muds. Instead, dynamic filtration tests conducted on oil-saturated cores are much more representative. The relative permeability to oil in oil-saturated zones is high, leading to large leakoff rates in the productive zone. <ref name="r13">Jiao, D. and Sharma, M.M. 1993. Dynamic Filtration of Invert-Emulsion Muds. SPE Drill & Compl 8 (3): 165-169. SPE-24759-PA. http://dx.doi.org/10.2118/24759-PA</ref>


==Oil based muds==
The invasion of solids and oil droplets into the formation is determined largely by the effectiveness of the external filter cake formed by organophilic bentonite and water droplets. The structure of the filter cake formed is substantially different from that of water-based muds. Water droplets bridge across the pore throats to form the external filter cake. Because the droplets are deformable, they can form very impermeable filter cakes, leading to good leakoff control. However, if the overbalance pressure exceeds the capillary pressure needed to squeeze the water droplets into the pores, a significant loss in productivity can result. To prevent this from happening, large overbalance pressures should be avoided.
Oil based muds consist of water droplets dispersed in a continuous oil phase. The water droplets are stabilized by emulsifiers and organophilic clays. Standard API fluid loss tests show that the fluid leakoff rate in oil based muds is substantially lower than for water based muds. However, as shown elsewhere, <ref name="r13" /> when tests are conducted on oil-saturated cores (not filter paper), leak-off rates for oil-based muds can be combrble to those for water-based muds. One important conclusion of this study is that API fluid leakoff tests should not be used to determine filtration rates in oil-based muds. Instead, dynamic filtration tests conducted on oil-saturated cores are much more representative. The relative permeability to oil in oil-saturated zones is high, leading to large leakoff rates in the productive zone. <ref name="r13" />


The invasion of solids and oil droplets into the formation is determined largely by the effectiveness of the external filter cake formed by organophilic bentonite and water droplets. The structure of the filter cake formed is substantially different from that of water-based muds. Water droplets bridge across the pore throats to form the external filter cake. Because the droplets are deformable, they can form very impermeable filter cakes, leading to good leakoff control. However, if the overbalance pressure exceeds the capillary pressure needed to squeeze the water droplets into the pores, a significant loss in productivity can result. To prevent this from happening, large overbalance pressures should be avoided.  
Experimental studies have shown that the accumulation of drill solids in the mud results in the introduction of fines that can be much more damaging than clean mud. Drill-solids control, therefore, is an important issue in oil based muds. In general, however, oil based muds prove to be excellent (albeit expensive) candidates for drilling gauge hole and providing high-productivity wells. <ref name="r13">Jiao, D. and Sharma, M.M. 1993. Dynamic Filtration of Invert-Emulsion Muds. SPE Drill & Compl 8 (3): 165-169. SPE-24759-PA. http://dx.doi.org/10.2118/24759-PA</ref><ref name="r14"> McKinney, L.K. and Azar, J.J. 1988. Formation Damage Due to Synthetic Oil Mud Filtrates at Elevated Temperatures and Pressures. Presented at the SPE Formation Damage Control Symposium, Bakersfield, California, 8-9 February 1988. SPE-17162-MS. http://dx.doi.org/10.2118/17162-MS</ref>


Experimental studies have shown that the accumulation of drill solids in the mud results in the introduction of fines that can be much more damaging than clean mud. Drill-solids control, therefore, is an important issue in oil based muds. In general, however, oil based muds prove to be excellent (albeit expensive) candidates for drilling gauge hole and providing high-productivity wells. <ref name="r13" /><ref name="r14" />
It is important to recognize and identify damage caused by oil based muds because the recommended treatment procedures for stimulating wells damaged by oil based muds can be quite different from those for wells damaged with water based muds. [[Matrix_acidizing|Acidizing]] wells with conventional acid formulations may not be successful, and, in fact, may result in additional damage as a result of the presence of [[Oil_emulsions|emulsifiers]] in the filtrate. Solvent preflushes may need to be designed on the basis of compatibility tests between the mud, crude oil, and acid formulation.


It is important to recognize and identify damage caused by oil based muds because the recommended treatment procedures for stimulating wells damaged by oil based muds can be quite different from those for wells damaged with water based muds. [[Matrix acidizing|Acidizing]] wells with conventional acid formulations may not be successful, and, in fact, may result in additional damage as a result of the presence of [[Oil emulsions|emulsifiers]] in the filtrate. Solvent preflushes may need to be designed on the basis of compatibility tests between the mud, crude oil, and acid formulation.
== Minimum underbalance pressure ==


==Minimum underbalance pressure==
It is clear from the preceding discussion that the formation of an external mud cake is important in protecting the formation from solids and filtrate invasion. There are at least two situations in which an external filter cake does not form across of the face of the formation:
It is clear from the preceding discussion that the formation of an external mud cake is important in protecting the formation from solids and filtrate invasion. There are at least two situations in which an external filter cake does not form across of the face of the formation:
*[[Lost circulation]]
 
*[[Lost_circulation|Lost circulation]]
*Drilling overbalanced below the minimum overbalance pressure
*Drilling overbalanced below the minimum overbalance pressure


When drilling through very-high-permeability rocks or fractured formations, solids present in the drilling fluid may not be able to bridge across the face of the pores or fractures, resulting in leakoff of whole mud into the formation. <ref name="r15" /> This leakoff can result in very severe, irreversible damage to the fracture or matrix. In general, bridging solids are added to the drilling fluid to bridge across the pores or fractures. Sizing of these solids is discussed in more detail in Suri<ref name="r4" />.  
When drilling through very-high-permeability rocks or fractured formations, solids present in the drilling fluid may not be able to bridge across the face of the pores or fractures, resulting in leakoff of whole mud into the formation. <ref name="r15">Weeks, S.G. 1974. Formation Damage or Limited Perforating Penetration? Test-Well Shooting May Give a Clue. J Pet Technol 26 (9): 979–984. SPE-4794-PA. http://dx.doi.org/10.2118/4794-PA</ref> This leakoff can result in very severe, irreversible damage to the fracture or matrix. In general, bridging solids are added to the drilling fluid to bridge across the pores or fractures. Sizing of these solids is discussed in more detail in Suri<ref name="r4">Suri, A. and Sharma, M.M. 2001. Strategies for Sizing Particles in Drilling and Completion Fluids. Presented at the SPE European Formation Damage Conference, The Hague, Holland, 21–22 May. SPE-68964-MS. http://dx.doi.org/10.2118/68964-MS</ref>.


The second case in which filter cakes do not form is less intuitively obvious. To form a mud cake, solids in the mud are pushed against the formation by a hydrodynamic force that is proportional to the leakoff velocity. In addition, because of mud circulation, particles are constantly being sheared away from the face of the external cake. This balance between the hydrodynamic shearing action resulting from mud circulation and the fluid leakoff into the formation results in an equilibrium cake thickness. <ref name="r16" /><ref name="r17" /> Because the leakoff is proportional to the overbalance pressure, smaller overbalance pressures will lead to smaller leakoff rates and thinner external filter cakes, resulting in a minimum overbalance pressure below which no external filter cake is formed at all. Alternatively stated, there is a minimum permeability for a fixed overbalance pressure below which no external filter cake will form. This suggests that we must always drill either [[Underbalanced drilling (UBD)|underbalanced]] or above the minimum overbalance pressure to ensure that an external cake is formed and available to protect the formation when drilling through the productive zone. Additional details for calculating the minimum overbalance pressure are provided in Di and Sharma<ref name="r17" />.
The second case in which filter cakes do not form is less intuitively obvious. To form a mud cake, solids in the mud are pushed against the formation by a hydrodynamic force that is proportional to the leakoff velocity. In addition, because of mud circulation, particles are constantly being sheared away from the face of the external cake. This balance between the hydrodynamic shearing action resulting from mud circulation and the fluid leakoff into the formation results in an equilibrium cake thickness. <ref name="r16">Di, J. and Sharma, M.M. 1994. Mechanism of Cake Buildup in Cross-Flow Filtration of Colloidal Suspensions. J. Colloid and Interface Science 162: 454.</ref><ref name="r17">Jiao, D. and Sharma, M.M. 1993. Investigation of Dynamic Mud Cake Formation: The Concept of Minimum Overbalance Pressure. Presented at the SPE Annual Technical Conference and Exhibition, Houston, Texas, 3-6 October 1993. SPE-26323-MS. http://dx.doi.org/10.2118/26323-MS</ref> Because the leakoff is proportional to the overbalance pressure, smaller overbalance pressures will lead to smaller leakoff rates and thinner external filter cakes, resulting in a minimum overbalance pressure below which no external filter cake is formed at all. Alternatively stated, there is a minimum permeability for a fixed overbalance pressure below which no external filter cake will form. This suggests that we must always drill either [[Underbalanced_drilling_(UBD)|underbalanced]] or above the minimum overbalance pressure to ensure that an external cake is formed and available to protect the formation when drilling through the productive zone. Additional details for calculating the minimum overbalance pressure are provided in Di and Sharma<ref name="r17">Jiao, D. and Sharma, M.M. 1993. Investigation of Dynamic Mud Cake Formation: The Concept of Minimum Overbalance Pressure. Presented at the SPE Annual Technical Conference and Exhibition, Houston, Texas, 3-6 October 1993. SPE-26323-MS. http://dx.doi.org/10.2118/26323-MS</ref>.


==Fractured reservoirs==
== Fractured reservoirs ==
When drilling through fractured formations, large quantities of whole mud can be lost to the fracture network, resulting in fracture plugging. Because fractures contribute almost all the productivity of such wells, it is important to keep these fractures open as much as possible. In such cases, [[Underbalanced drilling (UBD)|underbalanced drilling]] is recommended and frequently used. Underbalanced drilling allows fluids from the fracture to flow into the wellbore, keeping the fractures relatively undamaged. If, however, because of safety and regulatory constraints, underbalanced drilling is not possible, bridging additives need to be added to the mud system to ensure that large-enough particles are available to bridge across the fracture face. The bridging additives most commonly used to ensure the formation of a bridge across the fracture face are calcium carbonate and fibrous additives such as cellulosic fibers and acid-soluble fibers. <ref name="r18" /><ref name="r19" /> Sizing of these granular or fibrous additives has been discussed in detail in Di and Sharma<ref name="r18" />and Singh and Sharma<ref name="r19" />.


==Horizontal wells==
When drilling through fractured formations, large quantities of whole mud can be lost to the fracture network, resulting in fracture plugging. Because fractures contribute almost all the productivity of such wells, it is important to keep these fractures open as much as possible. In such cases, [[Underbalanced_drilling_(UBD)|underbalanced drilling]] is recommended and frequently used. Underbalanced drilling allows fluids from the fracture to flow into the wellbore, keeping the fractures relatively undamaged. If, however, because of safety and regulatory constraints, underbalanced drilling is not possible, bridging additives need to be added to the mud system to ensure that large-enough particles are available to bridge across the fracture face. The bridging additives most commonly used to ensure the formation of a bridge across the fracture face are calcium carbonate and fibrous additives such as cellulosic fibers and acid-soluble fibers. <ref name="r18">Jiao, D. and Sharma, M.M. 1996. Mud Induced Formation Damage in Fractured Reservoirs. SPE Drill & Compl 11 (1): 11-16. SPE-30107-PA. http://dx.doi.org/10.2118/30107-PA</ref><ref name="r19">Singh, T. and Sharma, M.M. 1997. Development of an Acid Degradable Drill-In Fluid for Fractured Reservoirs. Presented at the SPE European Formation Damage Conference, The Hague, The Netherlands, 2–3 June. SPE-38153-MS. http://dx.doi.org/10.2118/38153-MS</ref> Sizing of these granular or fibrous additives has been discussed in detail in Di and Sharma<ref name="r18">Jiao, D. and Sharma, M.M. 1996. Mud Induced Formation Damage in Fractured Reservoirs. SPE Drill & Compl 11 (1): 11-16. SPE-30107-PA. http://dx.doi.org/10.2118/30107-PA</ref>and Singh and Sharma<ref name="r19">Singh, T. and Sharma, M.M. 1997. Development of an Acid Degradable Drill-In Fluid for Fractured Reservoirs. Presented at the SPE European Formation Damage Conference, The Hague, The Netherlands, 2–3 June. SPE-38153-MS. http://dx.doi.org/10.2118/38153-MS</ref>.
Horizontal wells are more susceptible to formation damage than vertical wells for the following reasons. <ref name="r20" /><ref name="r21" />
 
*Pay zone in a horizontal wellbore comes into contact with a drilling fluid for a much longer period than a vertical pay zone (days compared with hours)  
== Horizontal wells ==
*Most horizontal wells are openhole completions, which means that even shallow damage that in a cased perforated completion would be bypassed by the perforations becomes significant  
 
Horizontal wells are more susceptible to formation damage than vertical wells for the following reasons. <ref name="r20">Thomas, B. and Sharma, M.M. 1998. Distribution of Mud Induced Damage Around Horizontal Wellbores. Presented at the SPE Formation Damage Control Conference, Lafayette, Louisiana, 18-19 February 1998. SPE-39468-MS. http://dx.doi.org/10.2118/39468-MS</ref><ref name="r21">Frick, T.P. and Economides, M.J. 1993. Horizontal Well Damage Characterization and Removal. SPE Prod & Oper 8 (1): 15-22. SPE-21795-PA. http://dx.doi.org/10.2118/21795-PA</ref>
 
*Pay zone in a horizontal wellbore comes into contact with a drilling fluid for a much longer period than a vertical pay zone (days compared with hours)
*Most horizontal wells are openhole completions, which means that even shallow damage that in a cased perforated completion would be bypassed by the perforations becomes significant
*Because the fluid velocity and pressure gradient during flowback are usually small, cleanup of internal and external cakes is not as effective as in vertical wellbores. Thus, only a fraction of the wellbore contributes to flow when the well is returned to production
*Because the fluid velocity and pressure gradient during flowback are usually small, cleanup of internal and external cakes is not as effective as in vertical wellbores. Thus, only a fraction of the wellbore contributes to flow when the well is returned to production
*Removing mud-induced formation damage by [[Acidizing horizontal wells|acidizing horizontal wells]] is often very difficult and expensive because of the large volumes of acid required and the difficulty in placing the acid in the appropriate wellbore locations  
*Removing mud-induced formation damage by [[Acidizing_horizontal_wells|acidizing horizontal wells]] is often very difficult and expensive because of the large volumes of acid required and the difficulty in placing the acid in the appropriate wellbore locations
 
Studies conducted on a simulated horizontal wellbore indicated that the heel is more damaged than the toe and that the upper part of the well is less damaged than the bottom of the wellbore where the drillpipe rests. <ref name="r20">Thomas, B. and Sharma, M.M. 1998. Distribution of Mud Induced Damage Around Horizontal Wellbores. Presented at the SPE Formation Damage Control Conference, Lafayette, Louisiana, 18-19 February 1998. SPE-39468-MS. http://dx.doi.org/10.2118/39468-MS</ref> The damage zone around the horizontal wellbore can therefore be modeled as an eccentric cone around the wellbore with a significantly larger depth of penetration at the heel and a shallower depth of penetration at the toe. <ref name="r21">Frick, T.P. and Economides, M.J. 1993. Horizontal Well Damage Characterization and Removal. SPE Prod & Oper 8 (1): 15-22. SPE-21795-PA. http://dx.doi.org/10.2118/21795-PA</ref>
 
Because the drilling fluid is in contact with the producing zone for an extended period of time, drill-in fluids have been devised to minimize the potential formation damage. Sized calcium carbonate and sized salt fluids are the drill-in fluids used most often in such applications. Oil based muds have also been evaluated for this purpose. A more detailed discussion of their formation damage potential is provided by several sources.<ref name="r22">Longeron, D.G., Alfenore, J., Salehi, N. et al. 2000. Experimental Approach to Characterize Drilling Mud Invasion, Formation Damage and Cleanup Efficiency in Horizontal Wells with Openhole Completions. Presented at the SPE International Symposium on Formation Damage Control, Lafayette, Louisiana, 23-24 February 2000. SPE-58737-MS. http://dx.doi.org/10.2118/58737-MS</ref><ref name="r23">Bailey, L., Meeten, G., Way, P. et al. 1998. Filtercake Integrity and Reservoir Damage. Presented at the SPE Formation Damage Control Conference, Lafayette, Louisiana, 18-19 February 1998. SPE-39429-MS. http://dx.doi.org/10.2118/39429-MS</ref><ref name="r24">Ali, S. et al. 1999. Alternative Methods Clean Up Filter Cake. Oil & Gas J. (1 February): 54.</ref><ref name="r25">Browne, S.V. and Smith, P.S. 1994. Mudcake Cleanup To Enhance Productivity of High-Angle Wells. Presented at the SPE Formation Damage Control Symposium, Lafayette, Louisiana, 7-10 February 1994. SPE-27350-MS. http://dx.doi.org/10.2118/27350-MS</ref><ref name="r26">Browne, S.V., Ryan, D.F., Chambers, B.D. et al. 1995. Simple Approach to the Cleanup of Horizontal Wells With Prepacked Screen Completions. J Pet Technol 47 (9): 794-800. SPE-30116-PA. http://dx.doi.org/10.2118/30116-PA</ref><ref name="r27">Zain, Z.M. and Sharma, M.M. 1999. Cleanup of Wall-Building Filter Cakes. Presented at the SPE Annual Technical Conference and Exhibition, Houston, Texas, 3-6 October 1999. SPE-56635-MS. http://dx.doi.org/10.2118/56635-MS</ref><ref name="r28">Zain, Z. and Sharma, M.M. 1999. A Simple Model for Filter Cake Lift-Off. Oil & Gas J. (1 November): 70–75.</ref><ref name="r29">Burton, B. 1995. Estimate Formation Damage Effects on Horizontal Wells. Pet. Eng. Intl. (August): 29.</ref>


Studies conducted on a simulated horizontal wellbore indicated that the heel is more damaged than the toe and that the upper part of the well is less damaged than the bottom of the wellbore where the drillpipe rests. <ref name="r20" /> The damage zone around the horizontal wellbore can therefore be modeled as an eccentric cone around the wellbore with a significantly larger depth of penetration at the heel and a shallower depth of penetration at the toe. <ref name="r21" />
== References ==


Because the drilling fluid is in contact with the producing zone for an extended period of time, drill-in fluids have been devised to minimize the potential formation damage. Sized calcium carbonate and sized salt fluids are the drill-in fluids used most often in such applications. Oil based muds have also been evaluated for this purpose. A more detailed discussion of their formation damage potential is provided by several sources.<ref name="r22" /><ref name="r23" /><ref name="r24" /><ref name="r25" /><ref name="r26" /><ref name="r27" /><ref name="r28" /><ref name="r29" />
<references />


==References==
== Noteworthy papers in OnePetro ==
<references>
<ref name="r1" >Abrams, A. 1977. Mud Design To Minimize Rock Impairment Due To Particle Invasion. ''J Pet Technol'' '''29''' (5): 586-592. SPE-5713-PA. http://dx.doi.org/10.2118/5713-PA</ref>
<ref name="r2" >Nowak, T.J. and Krueger, R.F. 1951. The Effect of Mud Filtrates and Mud Particles upon the Permeabilities of Cores. API Drill. Prod. Prac., 164–181. </ref>
<ref name="r3" >Glenn, E.E. and Slusser, M.L. 1957. Factors Affecting Well Productivity, II: Drilling Fluid Particle Invasion Into Porous Media. ''Petroleum Transactions'' Vol. 210, 132-139. Richardson, Texas: AIME. </ref>
<ref name="r4" >Suri, A. and Sharma, M.M. 2001. Strategies for Sizing Particles in Drilling and Completion Fluids. Presented at the SPE European Formation Damage Conference, The Hague, Holland, 21–22 May. SPE-68964-MS. http://dx.doi.org/10.2118/68964-MS </ref>
<ref name="r5" >Ladva, H.K.J., Tardy, P., Howard, P.R. et al. 2000. Multiphase Flow and Drilling Fluid Filtrate Effects on the Onset of Production. Presented at the SPE International Symposium on Formation Damage Control, Lafayette, Louisiana, 23-24 February 2000. SPE-58795-MS. http://dx.doi.org/10.2118/58795-MS </ref>
<ref name="r6" >Klotz, J.A., Krueger, R.F., and  Pye, D.S. 1974. Effect of Perforation Damage on Well Productivity. ''J Pet Technol'' '''26''' (11): 1303-1314. SPE-4654-PA. http://dx.doi.org/10.2118/4654-PA</ref>
<ref name="r7" >Jiao, D. and Sharma, M.M. 1992. Formation Damage Due to Static and Dynamic Filtration of Water-Based Muds. Presented at the SPE Formation Damage Control Symposium, Lafayette, Louisiana, 26-27 February 1992. 00023823. http://dx.doi.org/10.2118/23823-MS </ref>
<ref name="r8" >Roy, R.S. and Sharma, M.M. 2001. The Relative Importance of Solids and Filtrate Invasion on the Flow Initiation Pressure. Presented at the SPE European Formation Damage Conference, The Hague, Netherlands, 21-22 May 2001. SPE-68949-MS. http://dx.doi.org/10.2118/68949-MS </ref>
<ref name="r9" >Hodge, R.M., Augustine, B.G., Burton, R.C. et al. 1997. Evaluation and Selection of Drill-In-Fluid Candidates To Minimize Formation Damage. ''SPE Drill & Compl'' '''12''' (3): 174-179. SPE-31082-PA. http://dx.doi.org/10.2118/31082-PA</ref>
<ref name="r10" >Browne, S.V. and Smith, P.S. 1994. Mudcake Cleanup To Enhance Productivity of High-Angle Wells. Presented at the SPE Formation Damage Control Symposium, Lafayette, Louisiana, 7-10 February 1994. SPE-27350-MS. http://dx.doi.org/10.2118/27350-MS </ref>
<ref name="r11" >Ryan, D.F., Browne, S.V., and  Burnham, M.P. 1995. Mud Clean-up in Horizontal Wells: A Major Joint Industry Study. Presented at the SPE Annual Technical Conference and Exhibition, Dallas, Texas, 22–25 October. SPE-30528-MS. http://dx.doi.org/10.2118/30528-MS  </ref>
<ref name="r12" >Francis, P. 1997. Dominating Effects Controlling the Extent of Drilling-Induced Formation Damage. Presented at the SPE European Formation Damage Conference, The Hague, The Netherlands, 2–3 June. SPE-38182-MS. http://dx.doi.org/10.2118/38182-MS </ref>
<ref name="r13" >Jiao, D. and Sharma, M.M. 1993. Dynamic Filtration of Invert-Emulsion Muds. ''SPE Drill & Compl'' '''8''' (3): 165-169. SPE-24759-PA. http://dx.doi.org/10.2118/24759-PA </ref>
<ref name="r14" >McKinney, L.K. and Azar, J.J. 1988. Formation Damage Due to Synthetic Oil Mud Filtrates at Elevated Temperatures and Pressures. Presented at the SPE Formation Damage Control Symposium, Bakersfield, California, 8-9 February 1988. SPE-17162-MS. http://dx.doi.org/10.2118/17162-MS </ref>
<ref name="r15" >Weeks, S.G. 1974. Formation Damage or Limited Perforating Penetration? Test-Well Shooting May Give a Clue. ''J Pet Technol'' '''26''' (9): 979–984. SPE-4794-PA. http://dx.doi.org/10.2118/4794-PA </ref>
<ref name="r16" >Di, J. and Sharma, M.M. 1994. Mechanism of Cake Buildup in Cross-Flow Filtration of Colloidal Suspensions. ''J. Colloid and Interface Science'' '''162''': 454. </ref>
<ref name="r17" >Jiao, D. and Sharma, M.M. 1993. Investigation of Dynamic Mud Cake Formation: The Concept of Minimum Overbalance Pressure. Presented at the SPE Annual Technical Conference and Exhibition, Houston, Texas, 3-6 October 1993. SPE-26323-MS. http://dx.doi.org/10.2118/26323-MS </ref>
<ref name="r18" > Jiao, D. and Sharma, M.M. 1996. Mud Induced Formation Damage in Fractured Reservoirs. ''SPE Drill & Compl'' '''11''' (1): 11-16. SPE-30107-PA. http://dx.doi.org/10.2118/30107-PA </ref>
<ref name="r19" >Singh, T. and Sharma, M.M. 1997. Development of an Acid Degradable Drill-In Fluid for Fractured Reservoirs. Presented at the SPE European Formation Damage Conference, The Hague, The Netherlands, 2–3 June. SPE-38153-MS. http://dx.doi.org/10.2118/38153-MS </ref>
<ref name="r20" >Thomas, B. and Sharma, M.M. 1998. Distribution of Mud Induced Damage Around Horizontal Wellbores. Presented at the SPE Formation Damage Control Conference, Lafayette, Louisiana, 18-19 February 1998. SPE-39468-MS. http://dx.doi.org/10.2118/39468-MS </ref>
<ref name="r21" >Frick, T.P. and Economides, M.J. 1993. Horizontal Well Damage Characterization and Removal. ''SPE Prod & Oper'' '''8''' (1): 15-22. SPE-21795-PA. http://dx.doi.org/10.2118/21795-PA</ref>
<ref name="r22" >Longeron, D.G., Alfenore, J., Salehi, N. et al. 2000. Experimental Approach to Characterize Drilling Mud Invasion, Formation Damage and Cleanup Efficiency in Horizontal Wells with Openhole Completions. Presented at the SPE International Symposium on Formation Damage Control, Lafayette, Louisiana, 23-24 February 2000. SPE-58737-MS. http://dx.doi.org/10.2118/58737-MS </ref>
<ref name="r23" >Bailey, L., Meeten, G., Way, P. et al. 1998. Filtercake Integrity and Reservoir Damage. Presented at the SPE Formation Damage Control Conference, Lafayette, Louisiana, 18-19 February 1998. SPE-39429-MS. http://dx.doi.org/10.2118/39429-MS </ref>
<ref name="r24" >Ali, S. et al. 1999. Alternative Methods Clean Up Filter Cake. ''Oil & Gas J.'' (1 February): 54. </ref>
<ref name="r25" >Browne, S.V. and Smith, P.S. 1994. Mudcake Cleanup To Enhance Productivity of High-Angle Wells. Presented at the SPE Formation Damage Control Symposium, Lafayette, Louisiana, 7-10 February 1994. SPE-27350-MS. http://dx.doi.org/10.2118/27350-MS  </ref>
<ref name="r26" >Browne, S.V., Ryan, D.F., Chambers, B.D. et al. 1995. Simple Approach to the Cleanup of Horizontal Wells With Prepacked Screen Completions. ''J Pet Technol'' '''47''' (9): 794-800. SPE-30116-PA. http://dx.doi.org/10.2118/30116-PA </ref>
<ref name="r27" >Zain, Z.M. and Sharma, M.M. 1999. Cleanup of Wall-Building Filter Cakes. Presented at the SPE Annual Technical Conference and Exhibition, Houston, Texas, 3-6 October 1999. SPE-56635-MS. http://dx.doi.org/10.2118/56635-MS </ref>
<ref name="r28" >Zain, Z. and Sharma, M.M. 1999. A Simple Model for Filter Cake Lift-Off. ''Oil & Gas J.'' (1 November): 70–75. </ref>
<ref name="r29" >Burton, B. 1995. Estimate Formation Damage Effects on Horizontal Wells. ''Pet. Eng. Intl.'' (August): 29. </ref>
</references>


==Noteworthy papers in OnePetro==
Use this section to list papers in OnePetro that a reader who wants to learn more should definitely read
Use this section to list papers in OnePetro that a reader who wants to learn more should definitely read


==External links==
== External links ==
 
Use this section to provide links to relevant material on websites other than PetroWiki and OnePetro
Use this section to provide links to relevant material on websites other than PetroWiki and OnePetro


==See also==
== See also ==
[[Formation damage]]
 
[[Formation_damage|Formation damage]]
 
[[Drilling_problems|Drilling problems]]


[[Drilling problems]]
[[PEH:Formation_Damage]]


[[PEH:Formation Damage]]
== Category ==
[[Category:1.6 Drilling operations]] [[Category:1.8 Formation damage]] [[Category:YR]]

Revision as of 15:03, 26 June 2015

Drilling fluids serve to balance formation pressures while drilling to ensure wellbore stability. They also carry cuttings to the surface and cool the bit. The drilling engineer traditionally designs drilling fluids with two primary goals in mind:

  • To ensure safe, stable boreholes, which is accomplished by operating within an acceptable mud-weight window
  • To achieve high rates of penetration so that rig time and well cost can be minimized

These primary considerations do not include well productivity concerns. A growing recognition of the importance of drilling-induced formation damage has led to the use of drill-in fluids (fluids used to drill through the pay zone) that minimize formation damage. This page discusses the formation damage that may be associated with various types of drilling fluids.

Drill-in fluids to reduce formation damage

Drilling and well productivity concerns are addressed in the design of drill-in fluids. To meet well productivity objectives (i.e., to minimize formation damage), the drill-in fluid must meet the following additional objectives:

  • Minimize the extent of solids invasion into the formation by bridging across the pores and forming a thin, low-permeability, filter cake
  • Minimize the extent of filtrate and polymer invasion into the formation through the formation of an external filter cake
  • Ensure ease of removal of the external filter cake during flowback to maximize the inflow area during production and to avoid plugging gravel packs

To achieve these goals, various strategies have been adopted. Traditional water-based muds, oil-based muds, and some special formulations of drill-in fluids for fractured formations and unconsolidated sands are discussed below.

Water based muds

The vast majority of drilling fluids consist of:

  • Bentonite mixed with polymers to enhance the rheology (or, more specifically, the cuttings-carrying capacity of the fluid)
  • Starches to control fluid loss
  • Dissolved salts such as potassium chloride or sodium chloride
  • Perhaps a pH buffer to maintain the pH of the mud to the desired level

A great deal of work has been done in the last three decades on evaluating the formation damage potential of water based drilling fluids.[1][2][3][4][5]The following factors have been observed to have an impact on the depth of invasion of solids and filtrate and therefore on the extent and depth of formation damage or permeability impairment:

  • State of dispersion of solids in the mud
  • Size and concentration of solids and polymers in the mud
  • Pore throat size or permeability of the formation
  • pH and salinity of the filtrate
  • Water sensitivity of the formation

In most instances, the invasion of solids into the formation is limited to 2 or 3 in. from the wellbore wall, which implies that the productivity of perforated wells with relatively shallow depth of damage will not be significantly affected. Fig. 1[6] shows the productivity index (PI) of a well for different depths of damage assuming an 8-in.-long perforation. It is evident that as long as the depth of damage is smaller than the perforation length, the well PI is not significantly affected. Wells that are completed openhole without stimulation are particularly susceptible to this kind of damage.

In some instances, deep penetration of drill solids can occur. Fig. 2 shows the depth of invasion of formation damage when a 300-md Berea sandstone core is subjected to dynamic circulation of different water-based drilling fluids across its face. [7] It is evident that, in overtreated muds (containing too much thinner or dispersant), dispersed bentonite particles can penetrate through > 8 in. of rock and cause severe and irreversible damage. The other extreme, flocculated muds (too little thinner or too much salt), will limit solids invasion but will result in thick, high-permeability filter cakes. Filter cakes can result in such problems as stuck pipe and large filtrate loss. The use of salts and thinners is, therefore, a critical part of the design of drilling fluids for a given application. Appropriately conditioned muds must be used to eliminate the possibility of solids invasion and to minimize filtrate invasion. As discussed later, using sized bridging solids is a powerful tool for reducing solids and polymer invasion.

Although solids invasion clearly is detrimental to well productivity, filtrate invasion can also lead to substantial formation damage and to greater depths in some instances. It has been shown, for example, [7][8] that the use of freshwater muds can result in filtrates that can be damaging to water-sensitive sandstones. In such instances, the simple process of increasing the salinity of the filtrate can prevent fines migration induced by filtrate leakoff. The loss of aqueous filtrates also results in a reduction in the relative permeability to the hydrocarbon phases. [8] Such relative permeability effects are referred to as water-blocks and are discussed in Formation damage resulting from emulsion and sludge formation.

Similarly, the use of polymers is widespread but can, in some instances, lead to formation damage. Its been shown that the use of improper mixing producers in dissolving polymers into brines can result in the formation of "fish eyes," or unhydrated aggregates of polymer that can be several microns in diameter. These particulate gels are very effective as plugging agents and can lead to irreversible damage if not broken up and completely hydrated in the mud. Proper conditioning and dispersal of polymers is of critical importance in the field. [9][10][11][12]

There is a limited database on the formation damage caused by starches and other polymers such as xanthan or carboxymethylcellulose. These data indicate that the flow of such polymers can induce a substantial reduction in permeability as a result of constriction of pore throats, particularly in low-permeability formations.

Oil based muds

Oil based muds consist of water droplets dispersed in a continuous oil phase. The water droplets are stabilized by emulsifiers and organophilic clays. Standard API fluid loss tests show that the fluid leakoff rate in oil based muds is substantially lower than for water based muds. However, as shown elsewhere, [13] when tests are conducted on oil-saturated cores (not filter paper), leak-off rates for oil-based muds can be combrble to those for water-based muds. One important conclusion of this study is that API fluid leakoff tests should not be used to determine filtration rates in oil-based muds. Instead, dynamic filtration tests conducted on oil-saturated cores are much more representative. The relative permeability to oil in oil-saturated zones is high, leading to large leakoff rates in the productive zone. [13]

The invasion of solids and oil droplets into the formation is determined largely by the effectiveness of the external filter cake formed by organophilic bentonite and water droplets. The structure of the filter cake formed is substantially different from that of water-based muds. Water droplets bridge across the pore throats to form the external filter cake. Because the droplets are deformable, they can form very impermeable filter cakes, leading to good leakoff control. However, if the overbalance pressure exceeds the capillary pressure needed to squeeze the water droplets into the pores, a significant loss in productivity can result. To prevent this from happening, large overbalance pressures should be avoided.

Experimental studies have shown that the accumulation of drill solids in the mud results in the introduction of fines that can be much more damaging than clean mud. Drill-solids control, therefore, is an important issue in oil based muds. In general, however, oil based muds prove to be excellent (albeit expensive) candidates for drilling gauge hole and providing high-productivity wells. [13][14]

It is important to recognize and identify damage caused by oil based muds because the recommended treatment procedures for stimulating wells damaged by oil based muds can be quite different from those for wells damaged with water based muds. Acidizing wells with conventional acid formulations may not be successful, and, in fact, may result in additional damage as a result of the presence of emulsifiers in the filtrate. Solvent preflushes may need to be designed on the basis of compatibility tests between the mud, crude oil, and acid formulation.

Minimum underbalance pressure

It is clear from the preceding discussion that the formation of an external mud cake is important in protecting the formation from solids and filtrate invasion. There are at least two situations in which an external filter cake does not form across of the face of the formation:

When drilling through very-high-permeability rocks or fractured formations, solids present in the drilling fluid may not be able to bridge across the face of the pores or fractures, resulting in leakoff of whole mud into the formation. [15] This leakoff can result in very severe, irreversible damage to the fracture or matrix. In general, bridging solids are added to the drilling fluid to bridge across the pores or fractures. Sizing of these solids is discussed in more detail in Suri[4].

The second case in which filter cakes do not form is less intuitively obvious. To form a mud cake, solids in the mud are pushed against the formation by a hydrodynamic force that is proportional to the leakoff velocity. In addition, because of mud circulation, particles are constantly being sheared away from the face of the external cake. This balance between the hydrodynamic shearing action resulting from mud circulation and the fluid leakoff into the formation results in an equilibrium cake thickness. [16][17] Because the leakoff is proportional to the overbalance pressure, smaller overbalance pressures will lead to smaller leakoff rates and thinner external filter cakes, resulting in a minimum overbalance pressure below which no external filter cake is formed at all. Alternatively stated, there is a minimum permeability for a fixed overbalance pressure below which no external filter cake will form. This suggests that we must always drill either underbalanced or above the minimum overbalance pressure to ensure that an external cake is formed and available to protect the formation when drilling through the productive zone. Additional details for calculating the minimum overbalance pressure are provided in Di and Sharma[17].

Fractured reservoirs

When drilling through fractured formations, large quantities of whole mud can be lost to the fracture network, resulting in fracture plugging. Because fractures contribute almost all the productivity of such wells, it is important to keep these fractures open as much as possible. In such cases, underbalanced drilling is recommended and frequently used. Underbalanced drilling allows fluids from the fracture to flow into the wellbore, keeping the fractures relatively undamaged. If, however, because of safety and regulatory constraints, underbalanced drilling is not possible, bridging additives need to be added to the mud system to ensure that large-enough particles are available to bridge across the fracture face. The bridging additives most commonly used to ensure the formation of a bridge across the fracture face are calcium carbonate and fibrous additives such as cellulosic fibers and acid-soluble fibers. [18][19] Sizing of these granular or fibrous additives has been discussed in detail in Di and Sharma[18]and Singh and Sharma[19].

Horizontal wells

Horizontal wells are more susceptible to formation damage than vertical wells for the following reasons. [20][21]

  • Pay zone in a horizontal wellbore comes into contact with a drilling fluid for a much longer period than a vertical pay zone (days compared with hours)
  • Most horizontal wells are openhole completions, which means that even shallow damage that in a cased perforated completion would be bypassed by the perforations becomes significant
  • Because the fluid velocity and pressure gradient during flowback are usually small, cleanup of internal and external cakes is not as effective as in vertical wellbores. Thus, only a fraction of the wellbore contributes to flow when the well is returned to production
  • Removing mud-induced formation damage by acidizing horizontal wells is often very difficult and expensive because of the large volumes of acid required and the difficulty in placing the acid in the appropriate wellbore locations

Studies conducted on a simulated horizontal wellbore indicated that the heel is more damaged than the toe and that the upper part of the well is less damaged than the bottom of the wellbore where the drillpipe rests. [20] The damage zone around the horizontal wellbore can therefore be modeled as an eccentric cone around the wellbore with a significantly larger depth of penetration at the heel and a shallower depth of penetration at the toe. [21]

Because the drilling fluid is in contact with the producing zone for an extended period of time, drill-in fluids have been devised to minimize the potential formation damage. Sized calcium carbonate and sized salt fluids are the drill-in fluids used most often in such applications. Oil based muds have also been evaluated for this purpose. A more detailed discussion of their formation damage potential is provided by several sources.[22][23][24][25][26][27][28][29]

References

  1. Abrams, A. 1977. Mud Design To Minimize Rock Impairment Due To Particle Invasion. J Pet Technol 29 (5): 586-592. SPE-5713-PA. http://dx.doi.org/10.2118/5713-PA
  2. Nowak, T.J. and Krueger, R.F. 1951. The Effect of Mud Filtrates and Mud Particles upon the Permeabilities of Cores. API Drill. Prod. Prac., 164–181.
  3. Glenn, E.E. and Slusser, M.L. 1957. Factors Affecting Well Productivity, II: Drilling Fluid Particle Invasion Into Porous Media. Petroleum Transactions Vol. 210, 132-139. Richardson, Texas: AIME.
  4. 4.0 4.1 Suri, A. and Sharma, M.M. 2001. Strategies for Sizing Particles in Drilling and Completion Fluids. Presented at the SPE European Formation Damage Conference, The Hague, Holland, 21–22 May. SPE-68964-MS. http://dx.doi.org/10.2118/68964-MS
  5. Ladva, H.K.J., Tardy, P., Howard, P.R. et al. 2000. Multiphase Flow and Drilling Fluid Filtrate Effects on the Onset of Production. Presented at the SPE International Symposium on Formation Damage Control, Lafayette, Louisiana, 23-24 February 2000. SPE-58795-MS. http://dx.doi.org/10.2118/58795-MSfckLR↑ 6.0 6.1 Klotz, J.A., Krueger, R.F., and Pye, D.S. 1974. Effect of Perforation Damage on
  6. 6.0 6.1 6.2 Klotz, J.A., Krueger, R.F., and Pye, D.S. 1974. Effect of Perforation Damage on Well Productivity. J Pet Technol 26 (11): 1303-1314. SPE-4654-PA. http://dx.doi.org/10.2118/4654-PA
  7. 7.0 7.1 7.2 Jiao, D. and Sharma, M.M. 1992. Formation Damage Due to Static and Dynamic Filtration of Water-Based Muds. Presented at the SPE Formation Damage Control Symposium, Lafayette, Louisiana, 26-27 February 1992. 00023823. http://dx.doi.org/10.2118/23823-MS
  8. 8.0 8.1 Roy, R.S. and Sharma, M.M. 2001. The Relative Importance of Solids and Filtrate Invasion on the Flow Initiation Pressure. Presented at the SPE European Formation Damage Conference, The Hague, Netherlands, 21-22 May 2001. SPE-68949-MS. http://dx.doi.org/10.2118/68949-MS
  9. Hodge, R.M., Augustine, B.G., Burton, R.C. et al. 1997. Evaluation and Selection of Drill-In-Fluid Candidates To Minimize Formation Damage. SPE Drill & Compl 12 (3): 174-179. SPE-31082-PA. http://dx.doi.org/10.2118/31082-PA
  10. Browne, S.V. and Smith, P.S. 1994. Mudcake Cleanup To Enhance Productivity of High-Angle Wells. Presented at the SPE Formation Damage Control Symposium, Lafayette, Louisiana, 7-10 February 1994. SPE-27350-MS. http://dx.doi.org/10.2118/27350-MS
  11. Ryan, D.F., Browne, S.V., and Burnham, M.P. 1995. Mud Clean-up in Horizontal Wells: A Major Joint Industry Study. Presented at the SPE Annual Technical Conference and Exhibition, Dallas, Texas, 22–25 October. SPE-30528-MS. http://dx.doi.org/10.2118/30528-MS
  12. Francis, P. 1997. Dominating Effects Controlling the Extent of Drilling-Induced Formation Damage. Presented at the SPE European Formation Damage Conference, The Hague, The Netherlands, 2–3 June. SPE-38182-MS. http://dx.doi.org/10.2118/38182-MS
  13. 13.0 13.1 13.2 Jiao, D. and Sharma, M.M. 1993. Dynamic Filtration of Invert-Emulsion Muds. SPE Drill & Compl 8 (3): 165-169. SPE-24759-PA. http://dx.doi.org/10.2118/24759-PA
  14. McKinney, L.K. and Azar, J.J. 1988. Formation Damage Due to Synthetic Oil Mud Filtrates at Elevated Temperatures and Pressures. Presented at the SPE Formation Damage Control Symposium, Bakersfield, California, 8-9 February 1988. SPE-17162-MS. http://dx.doi.org/10.2118/17162-MS
  15. Weeks, S.G. 1974. Formation Damage or Limited Perforating Penetration? Test-Well Shooting May Give a Clue. J Pet Technol 26 (9): 979–984. SPE-4794-PA. http://dx.doi.org/10.2118/4794-PA
  16. Di, J. and Sharma, M.M. 1994. Mechanism of Cake Buildup in Cross-Flow Filtration of Colloidal Suspensions. J. Colloid and Interface Science 162: 454.
  17. 17.0 17.1 Jiao, D. and Sharma, M.M. 1993. Investigation of Dynamic Mud Cake Formation: The Concept of Minimum Overbalance Pressure. Presented at the SPE Annual Technical Conference and Exhibition, Houston, Texas, 3-6 October 1993. SPE-26323-MS. http://dx.doi.org/10.2118/26323-MS
  18. 18.0 18.1 Jiao, D. and Sharma, M.M. 1996. Mud Induced Formation Damage in Fractured Reservoirs. SPE Drill & Compl 11 (1): 11-16. SPE-30107-PA. http://dx.doi.org/10.2118/30107-PA
  19. 19.0 19.1 Singh, T. and Sharma, M.M. 1997. Development of an Acid Degradable Drill-In Fluid for Fractured Reservoirs. Presented at the SPE European Formation Damage Conference, The Hague, The Netherlands, 2–3 June. SPE-38153-MS. http://dx.doi.org/10.2118/38153-MS
  20. 20.0 20.1 Thomas, B. and Sharma, M.M. 1998. Distribution of Mud Induced Damage Around Horizontal Wellbores. Presented at the SPE Formation Damage Control Conference, Lafayette, Louisiana, 18-19 February 1998. SPE-39468-MS. http://dx.doi.org/10.2118/39468-MS
  21. 21.0 21.1 Frick, T.P. and Economides, M.J. 1993. Horizontal Well Damage Characterization and Removal. SPE Prod & Oper 8 (1): 15-22. SPE-21795-PA. http://dx.doi.org/10.2118/21795-PA
  22. Longeron, D.G., Alfenore, J., Salehi, N. et al. 2000. Experimental Approach to Characterize Drilling Mud Invasion, Formation Damage and Cleanup Efficiency in Horizontal Wells with Openhole Completions. Presented at the SPE International Symposium on Formation Damage Control, Lafayette, Louisiana, 23-24 February 2000. SPE-58737-MS. http://dx.doi.org/10.2118/58737-MS
  23. Bailey, L., Meeten, G., Way, P. et al. 1998. Filtercake Integrity and Reservoir Damage. Presented at the SPE Formation Damage Control Conference, Lafayette, Louisiana, 18-19 February 1998. SPE-39429-MS. http://dx.doi.org/10.2118/39429-MS
  24. Ali, S. et al. 1999. Alternative Methods Clean Up Filter Cake. Oil & Gas J. (1 February): 54.
  25. Browne, S.V. and Smith, P.S. 1994. Mudcake Cleanup To Enhance Productivity of High-Angle Wells. Presented at the SPE Formation Damage Control Symposium, Lafayette, Louisiana, 7-10 February 1994. SPE-27350-MS. http://dx.doi.org/10.2118/27350-MS
  26. Browne, S.V., Ryan, D.F., Chambers, B.D. et al. 1995. Simple Approach to the Cleanup of Horizontal Wells With Prepacked Screen Completions. J Pet Technol 47 (9): 794-800. SPE-30116-PA. http://dx.doi.org/10.2118/30116-PA
  27. Zain, Z.M. and Sharma, M.M. 1999. Cleanup of Wall-Building Filter Cakes. Presented at the SPE Annual Technical Conference and Exhibition, Houston, Texas, 3-6 October 1999. SPE-56635-MS. http://dx.doi.org/10.2118/56635-MS
  28. Zain, Z. and Sharma, M.M. 1999. A Simple Model for Filter Cake Lift-Off. Oil & Gas J. (1 November): 70–75.
  29. Burton, B. 1995. Estimate Formation Damage Effects on Horizontal Wells. Pet. Eng. Intl. (August): 29.

Noteworthy papers in OnePetro

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External links

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See also

Formation damage

Drilling problems

PEH:Formation_Damage

Category