Abstract
Sulfate-reducing bacteria (SRB) are notorious for causing decreases in the quality of oilfield injection waters due to their ability to increase total suspended solids (TSS). Through dissimilatory respiration, SRB reduce sulfate to sulfide, which subsequently forms hazardous hydrogen sulfide (H2S) gas. H2S is not only an environmental health threat, but it also can indirectly contribute to decreased water injectivity by reacting with soluble iron to form iron sulfides. These very tenacious solids can plug formation faces, thereby reducing the volume of water a well can take at a specified injection pressure. A previously reported study showed that an anthraquinone treatment program was capable of controlling biogenic H2S generation while maintaining TSS within field specifications (SPE 50741). Subsequently, an anthraquinone program was initiated in Chevron's Lost Hills Field (California) produced water injection system. Chevron's water treatment program had been a continuous injection of a quaternary biocide compound plus a weekly slug dose injection of another biocide (acrolein) to control SRB, TSS and H2S. Chevron also maintained a monthly pigging program that required the disposal of classified hazardous wastes due to the continuous injection of the quaternary chemical. The new treatment program included weekly slug injections of acrolein followed by a slug dose of a soluble form of anthraquinone. The monthly pigging program was maintained, but the continuous quaternary compound usage was eliminated. This paper describes the produced water injection system, the treatment program and the parameters monitored, including TSS, SRB, and H2S levels before and after the implementation of the anthraquinone treatment program. These data show that the new program has resulted in injection water quality which is currently better than previously measured during the recent history of the field and which now for the first time consistently meets Chevron's TSS specification.
