Changes

==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  

#has an inadequate total cation (Na^s+, ''K''⁺, NH_4⁺, Ca²⁺, Mg²⁺, Ba²⁺, and Sr²⁺) concentration to prevent dispersion of formation clays or  

#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.

===Clay type===

===Clay type===

Clay type influences the brine salinity at which clays start to deflocculate. Higher cation exchange capacity clays require higher salinity to prevent clay deflocculation. Smectite, illite, and kaolinite have cation exchange capacity (CEC) values of approximately 100, 20, and 5, respectively, Thus, total brine cation concentration must be higher to prevent smectite dispersion, followed by lesser concentrations for smectite-illite mixed layer clay, illite, and kaolinite clays, respectively.

Clay type influences the brine salinity at which clays start to [[deflocculate]]. Higher cation exchange capacity clays require higher salinity to prevent clay deflocculation. Smectite, illite, and kaolinite have [[cation exchange capacity (CEC)]] values of approximately 100, 20, and 5, respectively, Thus, total brine cation concentration must be higher to prevent smectite dispersion, followed by lesser concentrations for smectite-illite mixed layer clay, illite, and kaolinite clays, respectively.

Chlorite exhibits essentially no water sensitivity. It contains iron and it can react with spent HCl acid if the pH rises above 1.0. A pore-plugging ferric hydroxide precipitate with a hydrated gel-like structure can form unless an oxygen scavenger and iron-chelating or iron-sequestering agents are used.

Chlorite exhibits essentially no water sensitivity. It contains iron and it can react with spent HCl acid if the pH rises above 1.0. A pore-plugging ferric hydroxide precipitate with a hydrated gel-like structure can form unless an oxygen scavenger and iron-chelating or iron-sequestering agents are used.

Cation species and valence, as well as total cation concentration, influence clay swelling and flocculation. Divalent cations are more effective in promoting flocculation than monovalent cations, and small quantities of Ca²⁺ or Mg²⁺ have stabilizing effects.

Cation species and valence, as well as total cation concentration, influence clay swelling and flocculation. Divalent cations are more effective in promoting flocculation than monovalent cations, and small quantities of Ca²⁺ or Mg²⁺ have stabilizing effects.

Common monovalent ions include ''K''⁺, NH_4⁺, ''H''⁺, and Na⁺. Sodium (Na⁺) is the least effective ion in promoting clay stabilization and will increase clay sensitivity if the clays are subsequently exposed to freshwater. Illite and mica are potassium (K⁺) rich. Their stability requires that ''K''⁺ be present in injected waters to prevent potassium extraction from illite or friable micaceous sands. Ion extraction can result in dispersable particles (Reed, 1977)<ref name=Reed_1977>Reed, M. G., 1977, Formation [[permeability]] damage by mica alteration and carbonate dissolution: Journal of Petroleum Technology, Sept., p. 1056-60.</ref>.

Common monovalent ions include ''K''⁺, NH_4⁺, ''H''⁺, and Na⁺. Sodium (Na⁺) is the least effective ion in promoting clay stabilization and will increase clay sensitivity if the clays are subsequently exposed to freshwater. Illite and mica are potassium (K⁺) rich. Their stability requires that ''K''⁺ be present in injected waters to prevent potassium extraction from illite or friable micaceous sands. Ion extraction can result in dispersible particles.<ref name=Reed_1977>Reed, M. G., 1977, Formation [[permeability]] damage by mica alteration and carbonate dissolution: Journal of Petroleum Technology, Sept., p. 1056-60.</ref>

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.

===Rate of salinity change===

===Rate of salinity change===

Abrupt changes in salinity shock smectite, mixed layer illite-smectite and chlorite-smectite and to a lesser extent illite. Shock results in clay swelling and [[permeability]] decline. Nonswelling kaolinite can also be sensitized by contact with freshwater, which results in fines migration. A stepwise dilution of injection brine reduces total salinity, while maintaining the ratio of divalent to total cations. This gradual change can prevent clay damage (Jones, 1964)<ref name=Jones_1964>Jones, F. O., Jr., 1964, Influence of chemical composition of water on clay blocking of [[permeability]]: Petroleum Transactions, AIME, v. 231, p. 441-446,.</ref>.

Abrupt changes in salinity shock smectite, mixed layer illite-smectite and chlorite-smectite and to a lesser extent illite. Shock results in clay swelling and permeability decline. Nonswelling kaolinite can also be sensitized by contact with freshwater, which results in fines migration. A stepwise dilution of injection brine reduces total salinity, while maintaining the ratio of divalent to total cations. This gradual change can prevent clay damage.<ref name=Jones_1964>Jones, F. O., Jr., 1964, Influence of chemical composition of water on clay blocking of [[permeability]]: Petroleum Transactions, AIME, v. 231, p. 441-446.</ref>

Cation exchange can strip divalent cations from injection waters. If the resultant divalent ion concentration falls below the level required to keep clays flocculated, major [[permeability]] impairment will result. Pretreatment of the near-wellbore region with CaCl₂ (or KCl or NH₄Cl solutions if Ca is not compatible) can prevent subsequent damage. The pretreatment should be of sufficient volume to reach at least 5 ft into the formation.

Cation exchange can strip divalent cations from injection waters. If the resultant divalent ion concentration falls below the level required to keep clays flocculated, major permeability impairment will result. Pretreatment of the near-wellbore region with CaCl₂ (or KCl or NH₄Cl solutions if Ca is not compatible) can prevent subsequent damage. The pretreatment should be of sufficient volume to reach at least 5 ft into the formation.

 

 

===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==