New Low Viscosity Ester Is Suitable for Drilling Fluids in Deepwater Applications

Abstract

Abstract High biodegradability and relatively low toxicity have long made esters universally recognized as the best base fluids for synthetic-based muds in regards to environmental performance. A major limiting factor in the use of ester-based fluids, particularly in deep water, is the inherently high kinematic viscosity, a condition that is magnified in the cold temperatures encountered in deepwater risers. These higher viscosities are believed to be especially critical in deepwater wells where lack of overburden causes a severely narrowed window between pore pressures and fracture gradients. Other implications of these higher viscosities include limitations on oil/water ratios, mud weights, and drill solids tolerance. A new low viscosity (LV) ester has been developed which overcomes these limitations while maintaining the significant environmental advantages of the original esters. The new LV ester has a kinematic viscosity nearly equal to that of currently used internal olefins. It allows formulations of drilling fluids that can be used effectively in virtually all drilling applications in the Gulf of Mexico (GOM), including deep water. The GOM test protocol was developed to prove the viability of the LV ester as a base fluid. This protocol was designed to consider all conditions drilling fluids are expected to encounter during the course of a well. It evaluates the temperature stability, low temperature rheological properties, and the contamination tolerance of drilling fluids formulated with an LV ester base fluid. Three LV ester fluids were subjected to this protocol: an 11.0 ppg mud; a 14.0 ppg mud; and a 16.0 ppg mud. Results from this extensive laboratory testing indicate new LV ester-based fluids have overcome previous limitations, exhibiting exceptional cold temperature rheological properties, the ability to use a wide range of mud weights and oil/water ratios, and a high tolerance to contamination. In July 2000, an LV ester-based drilling fluid was used to drill a 3651 ft. interval of a GOM well in 3,669 ft. of water. The LV ester fluid performed well in comparison to the same interval drilled on an offset well with an internal olefin fluid. The initial results from that field trial are included in this paper. Development of Esters In 1985, development began on a fully biodegradable base fluid at the request of operators facing restrictions on the use of and discharges from conventional oil base fluids. Esters were found to be the most suitable naturally derived base fluids in terms of potential for use in drilling fluids, being exceptional lubricants, and showing low toxicity and a high degree of both aerobic and anaerobic biodegradability. Ester fluid provides similar shale stabilization and superior lubricity to mineral oil-based mud, and yet also satisfies environmental parameters. The first trial of the conventional ester-based fluid in February 1990, took place in Norwegian waters and was a technical and economic success. Since then, nearly 400 wells have been drilled worldwide using this C12-C14 ester-based system (Table 1). The release of ester-based fluids into the global drilling fluids market initiated the era of synthetic-based invert drilling fluids. Following the success of esters, other drilling fluids classed as synthetics were formulated, but these synthetics have not matched the environmental performance of ester-based drilling fluids. One of the most important criteria that they have failed to meet is that of biodegradability; additionally, some have been rejected as a result of poor eco-toxicological or technical performance. Drivers for Low Viscosity Ester A low viscosity base fluid was required to give operators the choice of using a system that fulfils more technically demanding requirements than existing ester-based systems, with a particular value in the deeper, colder environments increasingly explored today. The conventional ester is a C12-C14 ester manufactured from palm oil. Research demonstrates that C8 esters have lower viscosities than their conventional equivalents over a wide temperature range. This led to the development of a lower viscosity C8 ester base fluid.