Discharge of diesel oil based drilling fluids to ocean waters is under increasing scrutiny by industry and by regulatory agencies. Industry and agency sponsored research indicates that the presence of diesel oil in drilling muds contributes to toxicity levels which may not be acceptable for discharge. Some proposed National Pollution Discharge Elimination System (NPDES) general permits ban the discharge of diesel oil muds. Because of increasing concern over the discharge of drilling fluids containing diesel oil, there is considerable interest in the industry in developing diesel oil substitutes.

Recent technological advancements in mineral oil invert emulsion drilling fluids offer promise of a substitute for diesel oil based muds. These mineral oil muds have performed as well as diesel oil muds in most instances. Mineral oil based muds have low toxicity to test species. Toxicity of mineral oil muds compared favorably with water based muds which are routinely approved for offshore discharge.

Because mineral oil muds are a relatively new development, very little data are available to the various regulatory agencies (EPA, MMS, and USCG) concerning the use of these fluids. This has resulted in cautious positions by these agencies in approving the discharge of mineral oil muds.

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