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