Physicochemical Properties of Synthetic Drilling Fluids

Abstract

ABSTRACT Synthetic-based muds (SBM's) were developed as environmentally friendly alternatives to conventional petroleum-derived oil-based muds (OBM's). Although each synthetic fluid is chemically unique, the physicochemical properties of the SBM's exhibit some common trends. SBM's are safer to work with, more biodegradable and more dispersible in seawater than conventional OSM's. On the other hand, SBM's are more viscous at low temperatures, thin more with increasing temperature, have lower thermal stability and do not dehydrate shales as readily as conventional OSM's. By adjusting the emulsifier package in each mud, these properties may be brought more in line with those of conventional OBM's. INTRODUCTION As environmental regulations governing the disposal of drilling fluids become increasingly restrictive, use of conventional OBM's is rapidly being curtailed.1 The main issue is that the base fluids in conventional OBM's are petroleum products, such as diesel or mineral oil, which tend to persist for many years in marine environments. In the past, cuttings generated in offshore drilling operations were generally discarded into the ocean, where they settled to form a cuttings pile. Seabed surveys found that the oily residue on cuttings generated from drilling with OBM's degrades very slowly. Plant and animal life beneath the cuttings pile suffocates. Perhaps even worse, ocean life in the vicinity of the pile suffers a similar fate, because the slow aerobic biodegradation process that the oil undergoes depletes the surrounding water of oxygen for many years. Today, oily cuttings generated offshore are disposed of by a) shipping them to shore and discarding or treating them there with other waste; b) down hole injection (down the annulus or into another well); or c) cleaning them on site via solvent extraction or distillation. All of these options are expensive. An alternative is to find a drilling fluid that performs as well as conventional OBM's but which can be discarded into the ocean without causing any environmental damage. Synthetic-based muds (SBM's) may be a solution to that quest. Operationally, SBM's are thought to perform as well, in general, as conventional OBM's. Some differences remain, however, which on the one hand give them desirable environmental attributes, but on the other hand may pose some limitations on handling and use. These physicochemical properties are described below. CHEMICAL COMPOSITION Unlike the base oils in OBM's (diesel and mineral oil), which are refined from crude oil, the base fluids in SBM's are synthesized organic compounds that act like petroleum-derived oils with respect to drilling but biodegrade readily - it is claimed - in seawater. Like most OBM's, SBM's are invert emulsions, with the synthetic fluid serving as the external, or continuous, phase and a brine serving as the internal phase. Several synthetic fluids, all in the size range C18-C24, have been introduced in the marketplace during the last few years:Ester2,3 - made by the reaction of a fatty acid with an alcohol.. [the ester may be considered a synthetic vegetable oil]Di-Ether - made by condensation and partial oxidation of alcohols; the di-ether was preceded by a mono-ether [lower molecular weight di-ethers are used as solvents, cosmetics, perfumes and flavoring agents]