RIASSUNTO
Abstract
In many locations in the Rocky Mountains, water associated with oil production is very fresh (less than 5000 mg/l 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 i he 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.
Introduction
The Toxic Substances parameter of the Wyoming NPDES monitoring criteria was not considered an enforcement issue until the EPA completed whole effluent testing on several oilfield effluents and discovered toxicity. State officials began to prepare a list of toxic discharge locations in accordance with the Clean Water Act - Section 304(1), and began developing a strategy to eliminate the toxicity at these locations by July, 1992. Only those discharges that flow to live surface waters required additional regulation. The end result was modification of NPDES permits to include the directive for all discharges entering streams that have potential for game fish propagation. For the specified discharges, proof must be submitted that the effluent passed EPA dictated acute or chronic toxicity testing, by December, 1992. This testing is to be performed in addition to the routine NPDES requirements. The only other option was to eliminate the discharge. Of the 575 permitted discharges in Wyoming, only 40 locations are currently required to test for toxicity.
Conoco had 25 NPDES permitted discharges within the State in 1990, with total volumes discharged in excess of 72,000 barrels of water per day (BWPD). Only five of these discharges were subject to the toxicity requirement deadline of 1992. Three of these have subsequently been closed, leaving only Sussex B-2 and Sussex A discharges for consideration. The effluent from these discharges were tested in 1988 using EPA protocol, and found to be acutely toxic to Ceriodaphnia and Fat Head Minnow juveniles. However, in early 1992, the Sussex A discharge passed the toxicity test.
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