Changes

==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.<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.<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==

[[:Image:Table_rose_time-value-of-money_1.jpg|Table 1]] summarizes potential rock fluid reactions based on knowledge of clays present, damage prevention, and corrective procedures.<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.<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.''

[[File:Table_rose_time-value-of-money_1.jpg|thumb|Table 1: Rock-fluid potential problems, prevention, and corrective action]]

|-

|+ {{table number|1}}Rock-fluid potential problems, prevention, and corrective action

| Fines movement (rate sensitive) || Kaolinite lllite Chlorite lllite-smectite Chlorite-smectite Clay size particles of [[quartz]] or other minerals || High transient pressure High flow rates || Perforate slightly under-balanced (1000 psi) Increase well rate slowly Maximize perforations per ft Select rate less than critical velocity Use clay stabilizer || Preflush if needed with HCl and NH₄ CI Acidize with HCI/HF Postflush with NH₄ CI and clay stabilizer

===Laboratory tests===

===Laboratory tests===

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

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

[[File:Rock-water-reaction-formation-damage fig1.png |thumbnail|left|Figure 1: Liquid permeability test indicating permeability reduction due to rock-liquid reaction]]

[[File:Rock-water-reaction-formation-damage fig1.png |300px|thumbnail|{{figure number|1}} Liquid permeability test indicating permeability reduction due to rock-liquid reaction]]

Salinity-related tests furnish direct indication of rock-water interaction. They allow evaluation of damage induced by drilling, completion, [[workover]], and [[injection fluids]]. [[:Image:rocks.jpg|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]]. [[:Image:rocks.jpg|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.

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

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

[[File:Rock-water-reaction-formation-damage fig2.png|thumbnail|'''Figure 2.''' Critical velocity determination with pH monitoring]]

[[File:Rock-water-reaction-formation-damage fig2.png|300px|thumbnail|'''Figure 2.''' Critical velocity determination with pH monitoring]]

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.

The critical flow velocity is normally obtained by testing a cylindrical sample, with flow parallel to the linear axis. The linear velocity can be scaled to the radial flow condition existing in the wellbore. The scaled data yield the maximum [[well flow rate]] in barrels per day that can be tolerated before fines bridging and loss of production rate occurs.<ref name=Gorman_etal_1989>Gorman, I., C. Balnaves, J. Amaefule, D. Kersey, and D. Manning, 1989, Gravel packing in poorly lithified reservoirs: Laboratory systems approach to aid decision-making strategies: Society of Petroleum Engineers Paper No. 19477.</ref> These data allow calculation of the radius of the permeability impaired zone and aid in sizing subsequent acid volumes required to clean up the impairment.

The critical flow velocity is normally obtained by testing a cylindrical sample, with flow parallel to the linear axis. The linear velocity can be scaled to the radial flow condition existing in the wellbore. The scaled data yield the maximum [[well flow rate]] in barrels per day that can be tolerated before fines bridging and loss of production rate occurs.<ref name=Gorman_etal_1989>Gorman, I., C. Balnaves, J. Amaefule, D. Kersey, and D. Manning, 1989, Gravel packing in poorly lithified reservoirs: Laboratory systems approach to aid decision-making strategies: Society of Petroleum Engineers Paper No. 19477.</ref> These data allow calculation of the radius of the permeability impaired zone and aid in sizing subsequent acid volumes required to clean up the impairment.

Test conditions should mirror the field condition under study. Thus, water injection tests should be made on reservoir rock specimens in which simulated injection brine is flowed in the presence of residual [[hydrocarbons]]. Oil or gas [[production tests]] should be made by flowing the appropriate hydrocarbon through the rock specimen with interstitial water present. Drag forces are proportional to both rate and viscosity; therefore, flowing fluid viscosities should also model reservoir values.

Test conditions should mirror the field condition under study. Thus, water injection tests should be made on reservoir rock specimens in which simulated injection brine is flowed in the presence of residual [[hydrocarbons]]. Oil or gas [[production tests]] should be made by flowing the appropriate hydrocarbon through the rock specimen with interstitial water present. Drag forces are proportional to both rate and [[viscosity]]; therefore, flowing fluid viscosities should also model reservoir values.

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.

Calcium chloride brine is incompatible with formation or injection brines containing CO_3^2-, HCO_3^-, or SO_4^2-. Consequently, scale inhibitor must be used or a suitable (normally higher concentration and more costly) substitution of KCl or NH₄Cl brines will suffice. Another alternative is to displace the noncompatible water with a slug of KCl or NH₄Cl and follow this with CaCl₂.

Calcium chloride brine is incompatible with formation or injection brines containing CO_3^2-, HCO_3^-, or SO_4^2-. Consequently, scale inhibitor must be used or a suitable (normally higher concentration and more costly) substitution of KCl or NH₄Cl brines will suffice. Another alternative is to displace the noncompatible water with a slug of KCl or NH₄Cl and follow this with CaCl₂.

The contact of calcium or potassium fluids with low concentration, spent HF acid used for [[stimulation]] of sandstones will also form damaging precipitates. NH₄Cl can be used to displace spent HF acid several feet into the formation prior to CaCl₂ or KCl treatment, thereby preventing acid reaction with Ca²⁺ or ''K''⁺. Conversely, HF acid should not be allowed to contact calcium-bearing minerals such as calcite or dolomite. It is necessary to use HCl as a preflush or cation exchange Ca with an NH₄Cl preflush. Only underreaming or fracturing will correct calcium fluoride precipitate damage.

The contact of calcium or potassium fluids with low concentration, spent HF acid used for [[stimulation]] of sandstones will also form damaging precipitates. NH₄Cl can be used to displace spent HF acid several feet into the formation prior to CaCl₂ or KCl treatment, thereby preventing acid reaction with Ca²⁺ or ''K''⁺. Conversely, HF acid should not be allowed to contact calcium-bearing minerals such as calcite or [[dolomite]]. It is necessary to use HCl as a preflush or cation exchange Ca with an NH₄Cl preflush. Only underreaming or fracturing will correct calcium fluoride precipitate damage.

===Rate of salinity change===

===Rate of salinity change===

===Water pH===

===Water pH===

Negative charges exist on clay surfaces, and kaolinite is weakly cemented. Consequently, a repulsive force between quartz and clay promotes clay dispersion and reduced permeability at normal to high pH values. Effects of pH are intensified in low salinity solutions and are less important in high ionic strength solutions. Values of pH greater than 9.0 also result in silica dissolution, with resultant fines release. High pH also promotes formation of oil-water emulsions that reduce flow rate. Thus, it is best to avoid high pH systems.

Negative charges exist on clay surfaces, and kaolinite is weakly cemented. Consequently, a repulsive force between [[quartz]] and clay promotes clay dispersion and reduced permeability at normal to high pH values. Effects of pH are intensified in low salinity solutions and are less important in high ionic strength solutions. Values of pH greater than 9.0 also result in silica dissolution, with resultant fines release. High pH also promotes formation of oil-water emulsions that reduce flow rate. Thus, it is best to avoid high pH systems.

===Temperature effects===

===Temperature effects===

The rate of permeability impairment has been summarized and shown to decrease with increasing temperatures up to [[temperature::200&deg;F]] when brine flows in the presence of oil. It has also been shown that higher temperatures require higher salt content to stabilize clays when only brine flows. Solubility of quartz increases with temperature, and additional fines can be released and mobilized. It is prudent to make evaluation measurements at temperatures expected to exist at operating conditions.

The rate of permeability impairment has been summarized and shown to decrease with increasing temperatures up to [[temperature::200&deg;F]] when brine flows in the presence of oil. It has also been shown that higher temperatures require higher salt content to stabilize clays when only brine flows. Solubility of [[quartz]] increases with temperature, and additional fines can be released and mobilized. It is prudent to make evaluation measurements at temperatures expected to exist at operating conditions.

==See also==

==See also==

[[Category:Laboratory methods]] [[Category:Test content]][[Category:Pages with unformatted tables]]

[[Category:Laboratory methods]] [[Category:Test content]][[Category:Pages with unformatted tables]]