In many locations in the Rocky Mountains, water associated with oil production is very fresh (less than 5000 mg/1 Total Dissolved Solids (TDS)). Many operators in Wyoming discharge excess produced water to the surface, where it is depended upon by local livestock and wildlife. Much of Wyoming is semi-arid high plains, where the evaporation rate is 4 to 5 times the yearly precipitation total, so water is a precious commodity. When the EPA issued the Clean Water Act and the National Pollutant Discharge Elimination System (NPDES) in the 1970's, regulations were imposed affecting the quality of produced water discharges, restricting the TDS, specific chemical constituent concentrations and the dissolved oil and grease concentration. Operators must comply to the restrictions or face fines and eventual cancellation of the permit.

In 1988, the State of Wyoming issued a toxicity reduction regulation that required compliance by year's end 1992, to comply with national EPA requirements. Produced water that is discharged to streams has to fulfill whole-effluent Acute Toxicity testing protocol, in which immature fathead minnows and water fleas are exposed to the discharge fluid. If the water fails this testing, the toxicity has to be removed or reduced or the discharge discontinued. Many locations cannot economically tolerate the expense of some of the options for compliance, which could include sub-surface disposal. Furthermore, if all the water is injected, many streams will be dry most of the year, and the livestock and wildlife would suffer.

This presentation will recount the procedure that Conoco used to resolve the toxicity problem at the Sussex Field discharge. The process included 1) selection of toxicity reduction over sub-surface discharge, including economic considerations, 2) an attempt to identify the toxic components of the discharge, 3) selection of a toxicity remediation process, 4) design of a workable treatment facility and 5) installation and operation, at comparatively low capital expense, of a workable system.

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