Changes

# Injection or production of fluids at rates exceeding the critical velocity beyond which mobilization of rock fines by mechanical (drag) forces is initiated

# Injection or production of fluids at rates exceeding the critical velocity beyond which mobilization of rock fines by mechanical (drag) forces is initiated

Both the critical salinity (Khilar and<ref name=Khilar_etal_1983>Khilar, K. C., and H. S. Fogler, 1983, Water sensitivity of sandstones: Society of Petroleum Engineers Journal, Feb., p. 55-64.</ref> and critical velocity (Gruesbeck and<ref name=Gruesbeck_etal_1982>Gruesbeck, C., and R. E. Collins, 1982, Entrainment and deposition of fine particles in porous media: Journal of Petroleum Technology, v. 34, 847-856.</ref> can be determined from laboratory tests. These two damage mechanisms can exist independently or in combination (Gabriel and<ref name=Gabriel_etal_1983>Gabriel, G. A., and G. R. Inamdar, 1983, An experimental investigation of fines migration in porous media: Society of Petroleum Engineers Paper No. 12168.</ref>. Therefore, any system of laboratory tests to evaluate only the influence of salinity on flow impairment must be conducted at rates below the critical velocity. Conversely, the critical velocity will vary depending on whether fluids are chemically compatible with the rock.

Both the critical salinity<ref name=Khilar_etal_1983>Khilar, K. C., and H. S. Fogler, 1983, Water sensitivity of sandstones: Society of Petroleum Engineers Journal, Feb., p. 55-64.</ref> and critical velocity<ref name=Gruesbeck_etal_1982>Gruesbeck, C., and R. E. Collins, 1982, Entrainment and deposition of fine particles in porous media: Journal of Petroleum Technology, v. 34, 847-856.</ref> can be determined from laboratory tests. These two damage mechanisms can exist independently or in combination<ref name=Gabriel_etal_1983>Gabriel, G. A., and G. R. Inamdar, 1983, An experimental investigation of fines migration in porous media: Society of Petroleum Engineers Paper No. 12168.</ref>. Therefore, any system of laboratory tests to evaluate only the influence of salinity on flow impairment must be conducted at rates below the critical velocity. Conversely, the critical velocity will vary depending on whether fluids are chemically compatible with the rock.

==Factors influencing rock-water reactions==

==Factors influencing rock-water reactions==

Mineral fines that contribute to [[permeability]] reduction can be composed of clay, quartz, feldspar, or carbonate. Nonclay fines have no significant surface charge, and commercial clay stabilizers will not prevent their migration. Detrital clays forming the rock framework normally have little impact on rock-fluid reactions. Authigenic clays, however, line, fill, or bridge pores. They are exposed to extraneous pore fluids with which they may react (Eslinger and<ref name=Eslinger_etal_1988>Eslinger, E., and D. Pevear, 1988, Clay minerals for petroleum geologists and engineers: Society fo Economic Paleontologists and Mineralogists, Short Course Notes No. 22.</ref>. Expanding clays (smectites and mixed layer clays containing smectites) can reduce cross-sectional areas of pore throats and thus [[permeability]]. More important, as they expand, they often contribute mobile fines. These fines along with illite and kaolinite (which have lesser to no swelling tendency) are the primary maerials that disperse, migrate, bridge, and impair [[permeability]].

Mineral fines that contribute to [[permeability]] reduction can be composed of clay, quartz, feldspar, or carbonate. Nonclay fines have no significant surface charge, and commercial clay stabilizers will not prevent their migration. Detrital clays forming the rock framework normally have little impact on rock-fluid reactions. Authigenic clays, however, line, fill, or bridge pores. They are exposed to extraneous pore fluids with which they may react (Eslinger and<ref name=Eslinger_etal_1988>Eslinger, E., and D. Pevear, 1988, Clay minerals for petroleum geologists and engineers: Society fo Economic Paleontologists and Mineralogists, Short Course Notes No. 22.</ref>. Expanding clays (smectites and mixed layer clays containing smectites) can reduce cross-sectional areas of pore throats and thus [[permeability]]. More important, as they expand, they often contribute mobile fines. These fines along with illite and kaolinite (which have lesser to no swelling tendency) are the primary maerials that disperse, migrate, bridge, and impair [[permeability]].

===Problem prevention and correction===

==Problem prevention and correction==

Table 1 summarizes potential rock fluid reactions based on knowledge of clays present, damage prevention, and corrective procedures (Kersey, 1986)<ref name=Kersey_1986>Kersey, D. G., 1986, The role of petrographic analyses in the design of non-damaging drilling, completion, and [[stimulation]] programs: Society of Petroleum Engineers Paper No. 14089.</ref>. Prevention is preferred and, when possible, is likely to cost less than correction.

Table 1 summarizes potential rock fluid reactions based on knowledge of clays present, damage prevention, and corrective procedures (Kersey, 1986)<ref name=Kersey_1986>Kersey, D. G., 1986, The role of petrographic analyses in the design of non-damaging drilling, completion, and [[stimulation]] programs: Society of Petroleum Engineers Paper No. 14089.</ref>. Prevention is preferred and, when possible, is likely to cost less than correction.

==Laboratory tests==

===Laboratory tests===

Laboratory tests used to assess rock-water reactions can be grouped into three categories:

Laboratory tests used to assess rock-water reactions can be grouped into three categories:

# Petrographic analysis

# Petrographic analysis

* [[Wettability]] of fines

* [[Wettability]] of fines

===Petrographic analysis===

====Petrographic analysis====

Petrographic analysis indicates the potential for [[permeability]] reduction by identifying types, amount, and location of clays and other minerals. (For details on petrographic methods, see the chapters on [[Thin section analysis]] and "SEM, XRD, CL, and XF Methods" in Part 5.)

Petrographic analysis indicates the potential for [[permeability]] reduction by identifying types, amount, and location of clays and other minerals. (For details on petrographic methods, see the chapters on [[Thin section analysis]] and "SEM, XRD, CL, and XF Methods" in Part 5.)

===Salinity-related tests===

====Salinity-related tests====

Salinity-related tests furnish direct indication of rock-water interaction. They allow evaluation of damage induced by drilling, completion, workover, and injection fluids. Figure 1 illustrates results of a laboratory experiment to evaluate the reaction to reservoir rock with a proposed injection brine. [[Permeability]] was reduced to 20% of its original value after exposure to 20 pore volumes of proposed injected brine. Good reservoir management requires that injected volumes equal produced volumes; therefore, reduced injectivity results in reduced hydrocarbon production rates.

Salinity-related tests furnish direct indication of rock-water interaction. They allow evaluation of damage induced by drilling, completion, workover, and injection fluids. Figure 1 illustrates results of a laboratory experiment to evaluate the reaction to reservoir rock with a proposed injection brine. [[Permeability]] was reduced to 20% of its original value after exposure to 20 pore volumes of proposed injected brine. Good reservoir management requires that injected volumes equal produced volumes; therefore, reduced injectivity results in reduced hydrocarbon production rates.

When the source and composition of brine to be injected is unknown, water shock tests indicate potential rock damage and present a worse-case scenario. [[Permeability]] of the formation rock to a 0.51-M (3 wt. %) NaCl solution is followed by [[permeability]] to freshwater. Sensitive rocks will show [[permeability]] reduction. Rock-water reaction must be evaluated for all aqueous fluids introduced into the reservoir system. Critical salinity (cation concentration) below which damage occurs, as well as schemes to lower brine concentration stepwise so as to avoid clay damage, can be evaluated in the laboratory.

When the source and composition of brine to be injected is unknown, water shock tests indicate potential rock damage and present a worse-case scenario. [[Permeability]] of the formation rock to a 0.51-M (3 wt. %) NaCl solution is followed by [[permeability]] to freshwater. Sensitive rocks will show [[permeability]] reduction. Rock-water reaction must be evaluated for all aqueous fluids introduced into the reservoir system. Critical salinity (cation concentration) below which damage occurs, as well as schemes to lower brine concentration stepwise so as to avoid clay damage, can be evaluated in the laboratory.

===Rate-related tests===

====Rate-related tests====

The critical interstitial velocity at which [[permeability]] reduction due to fines migration is initiated can be determined in laboratory tests. These tests simulate the effect of high flow rates that exist near the wellbores of both injection and production wells. Muecke (1979)<ref name=Muecke_1979>Muecke, T. W., 1979, Formation fines and factors controlling their movement in porous media: Journal of Petroleum Technology, v. 31, p. 144-150.</ref> discusses factors controlling fines movement. These include fluids flowing, fines [[wettability]], and interfacial forces.

The critical interstitial velocity at which [[permeability]] reduction due to fines migration is initiated can be determined in laboratory tests. These tests simulate the effect of high flow rates that exist near the wellbores of both injection and production wells. Muecke (1979)<ref name=Muecke_1979>Muecke, T. W., 1979, Formation fines and factors controlling their movement in porous media: Journal of Petroleum Technology, v. 31, p. 144-150.</ref> discusses factors controlling fines movement. These include fluids flowing, fines [[wettability]], and interfacial forces.

Changes in pH indicate fluid-fluid or rock-fluid reactions; therefore, monitoring of injection and produced water pH should be an integral part of any critical velocity determination. In addition, effectiveness of clay stabilizers should be evaluated as an extension of the critical velocity measurement.

Changes in pH indicate fluid-fluid or rock-fluid reactions; therefore, monitoring of injection and produced water pH should be an integral part of any critical velocity determination. In addition, effectiveness of clay stabilizers should be evaluated as an extension of the critical velocity measurement.

==Chemical (SALINITY-RELATED) fines mobilization==

==Chemical (salinity related) fines mobilization==

Chemically initiated fines migration occurs when authigenic clays are contacted by fluid that (1) has an inadequate total cation (Na^s+, ''K''⁺, NH_4⁺, Ca²⁺, Mg²⁺, Ba²⁺, and Sr²⁺) concentration to prevent dispersion of formation clays or (2) contains an inadequate percentage of divalent cations (C^a2+ and Mg²⁺), even when total cation concentration is high. Clay dispersion is a complex phenomenon dependent on clay type and quantity and the brine composition of both original formation water and extraneous water.

Chemically initiated fines migration occurs when authigenic clays are contacted by fluid that (1) has an inadequate total cation (Na^s+, ''K''⁺, NH_4⁺, Ca²⁺, Mg²⁺, Ba²⁺, and Sr²⁺) concentration to prevent dispersion of formation clays or (2) contains an inadequate percentage of divalent cations (C^a2+ and Mg²⁺), even when total cation concentration is high. Clay dispersion is a complex phenomenon dependent on clay type and quantity and the brine composition of both original formation water and extraneous water.