This paper presents the results and implications of a research project* to study, under controlled laboratory conditions, the influence of oil based mud physical properties and chemical composition on the quantity of oil retained on cuttings. Substrates, representing common rock types drilled, were exposed to different oil mud formulations in which one variable was altered as far as possible independently of other properties. The most influential parameters were found to include the use of strong oil wetting agents such as amidoamines and imidazolines (which promote oil imbibition), the percent oil in the formulation and the water activity imbalance between formation and oil mud. Less influential, but nevertheless an important consideration for oil mud formulation, is the effect of plastic viscosity and intermediate oil wetting surfactants. Mud weight and HTHP fluid loss have a minor effect.

Imbibition of oil into the internal fabric of cuttings is an important process which is largely dependent on the choice and concentration of surfactant, fluid temperature and exposure time. Minimising or eliminating strongly wetting surfactants in oil based mud formulations is thus a desirable objective for oil based mud development.

A new concept in oil based mud design is presented, in which all the essential properties of an oil based mud are achieved utilising oil-soluble polymers. Laboratory tests suggest oil on cuttings can be reduced by up to 30% on current 50:50 oil:water ratio fluids.

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