Low toxic oil- based muds with high oil contents and adequate rheology are difficult to produce under conditions of low shear and low temperature (e.g. mud plant mixing conditions). They also suffer from high cost and lead to large quantities of oil entering the marine environment during off, shore drilling. High water content oil-based muds are shown to minimise these problems while maintaining mud properties at a level consistent with efficient drilling.
The use of oil based muds to minimise drilling problems such as torque/drag, balled bits and bottom hole assemblies, shale dispersion, disintegration and hole washout often experienced In documented in several drilling North Sea wells Is papers 1, 2,3.
These early papers referred to the use of diesel oil based muds with oil/water ratios of between 80/20 and 90/10 and were written based on experiences gained while drilling the thick sedimentary sequences (primarily shales of Tertiary age) found in the central and northern areas of the North Sea.
Diesel oiIs well known to be toxic to marine organisms 4 (the 96 Hr LC50 for Brown Shrimp (Crangon crangon) is 1Sppm) so the introduction of dearomatised, highly refined mineral oils in 1981 was a welcome step. By reducing base oil toxicity (96 Hr LC 50 was raised to at least 400ppm, usually 1000-1200ppm) operators were able to use the new low toxic oil base muds 5, 6 much more widely and were no longer constrained by toxicity considerations or inefficient cuttings cleaning processes. As a result, the use of low toxic oil-based mud (LTDBM) mushroomed in the North Sea from 197B to 1988 so that today 80-85% of all wells drilled in the U.K. sector of the North Sea utilise LTDBM.
With environmental concerns, cost of low toxic base oils and a competitive market, optimization of oil mud formulations and products has become of paramount importance and was a major stimulus in the development of a high water content oil-based mud (HWDBM) system.
Organophillc clays (high cation exchange capacity clays treated with quaternary amines to render them oil dispersible) were and still are the main viscosiflers used in LTOBM. Organophilic clays were originally devised for use in the coatings (paint) and grease industry before their application in oil muds. Work carried out on grease systems 7, 8 showed clearly that for a given oil, shear and type/amount of polar activator affected final rheology production. The mode of action of a polar activator such as acetone on an organophilic clay Is to associate with the clay surface Inducing a larger spacing between the clay platelets. As work (shear) is applied this association of activator and clay allows the clay to be more easily dispersed and greater rheology results.
