This paper presents the results and implications of a research project* to study, under controlled laboratory conditions, the 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.
Environmental protection is an important issue for the drilling industry world-wide. A major environmental concern is the use of oil based drilling fluids (OBM), which have a proven record of minimising drilling problems. The most contentious issue is the disposal of low toxicity oil contaminated cuttings, which at present, are normally discharged overboard to the marine present, are normally discharged overboard to the marine environment. This has led many governments to legislate on the maximum oil content of cuttings discharged in order to reduce the environmental effects of oily cuttings settling on the sea bed.
Limits of 100 grams of oil per kilogram of 'dry' cuttings are imposed by many North Sea bordering countries. This has stimulated diversified research into alternative methods of reducing oil effluent levels, such as:
Development of high performance water based drilling fluids, though no 'panacea' oil-free drilling mud currently exists which matches the performance of oil based drilling fluids.
Improvements in oil based mud design. The introduction of low oil: water ratio oil based drilling fluids is a recent development which demonstrably reduces oil on cuttings (OOC).