North Sea environmental legislation imposes limits on the amount of oil on cuttings (OOC) discharged to the marine environment when using oil-based drilling fluids. This paper presents the laboratory development by International Drilling Fluids (IDF) of a conceptually new type of oil-based drilling fluid designed to reduce OOC, and the results of field trials undertaken by Amerada Hess Limited (AHL) with this fluid.

A recent research study sponsored by the Drilling Engineers Association (Europe), as project DEA(E) 17, identified under controlled laboratory conditions, the critical physical and chemical variables which affect OOC. It was found during this study that the use of conventional oil-wetting surfactants promotes spontaneous imbibition of oil into the internal pores of cuttings, causing an increase in OOC. Minimising or eliminating the use of strongly oil-wetting surfactants in oil-based fluids was identified as a desirable research objective in oil-based mud development, to reduce OOC.

Onward laboratory work has identified polymers, soluble in low-toxicity mineral-oil, such that polymer-based invert oil emulsion drilling fluids can be formulated to provide all the essential properties of 50/50 oil-water ratio oil-based fluids. Two field trials by AHL in Blocks 31/26 and 39/1-2 using this polymeric oil-based fluid resulted in an 18% reduction in OOC to below the critical 100 g/kg limit and a 35% reduction in mud losses per foot on the second trial well. Fluid properties were stable, and the wells completed ahead of schedule. Field trials have validated this new concept in oil-based drilling fluid design to reduce OOC.

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