Method for generating softened injection water

A method for generating a softened injection water comprising:

• (A) generating the softened injection water by (i) introducing a source water having a total dissolved solids content of up to 15,000 mg/litre and having a multivalent cation content of greater than 40 mg/litre to a vessel that contains a bed of a cation exchange resin in its monovalent cation form, (ii) passing the source water through the cation exchange resin bed so that at least a portion of the multivalent cations in the source water are replaced by monovalent cations from the cation exchange resin, and (iii) withdrawing from the vessel the softened injection water having a multivalent cation content of up to 40 mg/litre;
• (B) regenerating the cation exchange resin by (i) introducing a regenerating brine to the vessel, (ii) passing the regenerating brine through the resin bed and (iii) withdrawing from the vessel a brine containing displaced multivalent cations, wherein the regenerating brine is a naturally occurring high salinity water having a concentration of monovalent cations and of multivalent cations such that the Softening Limit for the source water is up to 40 mg/litre of multivalent cations wherein the Softening Limit for the source water is defined as the Softening Ratio multiplied by the concentration of multivalent cations in the source water (mg/litre) and wherein the Softening Ratio is defined as:
• (molar concentration of monovalent cations in the source water)2/(molar concentration of multivalent cations in the source water):(molar concentration of monovalent cations in the regenerating brine)2/(molar concentration of multivalent cations in the regenerating brine).

A method of increasing the recovery of crude oil from a reservoir comprising at least one porous and permeable subterranean formation wherein the formation comprises sandstone rock and at least one mineral that has a negative zeta potential under the reservoir conditions and wherein crude oil and connate water are present within the pores of the formation, the method comprising:

• (A) generating a softened injection water having a multivalent cation content of up to 40 mg/litre by (i) introducing a source water having a total dissolved solids content of less than 15,000 mg/litre and having a multivalent cation content of greater than 40 mg/litre to a vessel that contains a bed of a cation exchange resin in its monovalent cation form so that at least a portion of the multivalent cations in the source water are replaced by monovalent cations from the cation exchange resin, (ii) passing the source water through the cation exchange resin bed, and (iii) withdrawing from the vessel the softened injection water;
• (B) injecting the softened injection water formed in step (A) into the formation thereby displacing crude oil from the surface of the pores of the formation;
• (C) regenerating the cation exchange resin by (i) introducing a regenerating brine to the vessel, (ii) passing the regenerating brine through the resin bed and (iii) withdrawing from the vessel a brine containing displaced multivalent cations, wherein the regenerating brine is a naturally occurring high salinity water having a concentration of monovalent cations and of multivalent cations such that the Softening Limit for the source water is less than 40 mg/litre of multivalent cations wherein the Softening Limit for the source water is defined as the ratio of {[source water monovalent cation]2/[source water multivalent cation]} (mol/mol) and {[regenerating brine monovalent cation]2/[regenerating brine multivalent cation]} (mol/mol) (hereinafter referred to as Softening Ratio) multiplied by the concentration of multivalent cations in the source water (mg/litre);
• and
• (D) passing the brine containing the displaced multivalent cations that is withdrawn from the vessel in step (C) to a brine disposal means.