Reducing Synthetic Oil-Based Drilling Mud Contamination in Geologic Materials

Harry Dembicki¹, Robert K. Olson², and N. Hung^2
1 Anadarko Petroleum Corporation, Houston, TX
² Baseline Resolution, Inc, Shenandoah, TX

Synthetic oil-based drilling muds, such as Nova Plus, are widely used in the offshore Gulf of Mexico and their use is rapidly spreading to other regions of the world. The reason for the growing popularity of these drilling fluids is that the synthetic oil is an environmentally friendly material that provides stable borehole characteristics. The problem with this material, and all hydrocarbon-based mud systems, is that it nearly always contaminates well cuttings, sidewall and conventional core samples, and formation fluids recovered during testing. This contamination often overwhelms the signal from source rock extracts and crude oils preventing basic information about the characteristics and origin of these materials from being obtained.

The synthetic oils used in drilling muds are predominantly composed of olefins (unsaturated hydrocarbons) not normally found in crude oils. Using Nova Plus as a model material, a procedure was developed that effectively remove approximately 95% of the Nova Plus contaminants from the saturate, aromatic and resin fractions of samples containing this material. Laboratory blends composed of 50:50, 75:25, 95:5, and 99:1 mixtures of Nova Plus and a Gulf of Mexico crude oil were decontaminated using this procedure. The resulting saturate and aromatic fractions of all but the most severely contaminated sample were suitable for all routine geochemical characterization procedures including biomarkers and carbon isotope analyses. In the most severely contaminated samples, the carbon isotope composition was altered by that portion of the Nova Plus which could not be removed. The procedure should work equally well with other olefin-based synthetic oils.