Environmentally Acceptable Drilling Fluids for Offshore Saudi Arabia

Abstract

Abstract Two environmentally acceptable drilling fluid systems have been developed and successfully used in the field: an oil-based mud and an inhibitive water-based system. Offshore Saudi Arabia, the high angle hole section is primarily unconsolidated sandstone with water sensitive shale stringers. Conventional water-based mud causes drilling problems such as hole cave-in and stuck pipe. To try to avoid some of these problems, oil-based mud has been the preferred system. In order to comply with the environmental protection regulations which specifically address the disposal of drilling fluids and their associated cuttings in the Arabian Gulf, the search for environmentally acceptable and economical alternative drilling fluids was initiated. A locally produced low aromatic oil was chosen and used for formulating the oil-based mud. Laboratory tests indicated that it had an LC50 value of 180,726 ppm which was substantially less toxic than a white mineral oil used worldwide as the diesel oil substitute. All the oil mud additives were found to be compatible with the low aromatic oil and the desired mud characteristics were easily achieved. The laboratory designed formulation was used in the field to drill a trouble-free horizontal well. An inhibitive water-based mud was also developed to stabilize the water sensitive shale and provide the lubricity needed for drilling the high angle and horizontal wells. Several wells were successfully drilled and the toxicity level of this mud was maintained in an acceptable range, allowing the disposal of drilled cuttings and mud into the Gulf. Laboratory data and field applications are presented in this paper detailing the use and the economic benefits of the environmentally safe water-based mud system. Introduction Oil-based or invert emulsion drilling fluids are used to overcome many undesirable characteristics of water-based muds. Conventional water-based muds can dissolve water soluble rocks and promote the disintegration and dispersion of clays. Because hole collapse while drilling interbeded shale and sand sections in Saudi Arabia's offshore fields was attributed to the effect of water, an oil-based mud was used. Oil mud reduced the heaving shale problems when the proper mud densities were maintained. The internal brine phase in the invert emulsion provides osmotic dehydration of clays, thus preventing clay swelling and shale sloughing. Oil muds provide excellent lubricity, which aids in drilling high angle holes and reduces occurrences of "stuck pipe". It causes little or no hole washout in soft claystone, salt, gypsum and anhydrite. Oil-based muds are thermally stable for drilling deep hot holes and can provide corrosion protection against acid gases (CO2 and H2S). The initial cost of oil-based muds is two to four times greater than most water-based muds. However, oil muds can be justified if drilling problems can be eliminated and the mud can be recovered and reused. Today, the major concern in the use of oil-based drilling fluids revolves around their potential for adverse environmental impact, especially in ecologically sensitive areas in the Arabian Gulf. In February 1990, the Regional Organization for the Protection of the Marine Environment for the Arabian Gulf Countries (ROPME) issued a protocol and voluntary regulations. These regulations specifically address the disposal of drilling fluids and their associated cuttings in the Arabian Gulf. The protocol states that "Drilling fluids discharged from offshore operations must not contain persistent systemic toxins". To comply with these regulations, the search for environmentally acceptable and economical alternative drilling fluids was initiated. P. 353^