New Water-Based Muds for Tertiary Shale Control

Abstract

Summary This paper describes laboratory and field assessments of a series of inhibitive water muds developed by BP. RCS 1 contains cationic starch to control fluid loss and to inhibit shale expansion further. RCS 2 is a highly inhibitive version of RCS 1 containing polyglycerol. RCS 3, the most inhibitive mud of the series, contains a phosphate salt and polyglycerol. Levels of inhibition appear close to those of oil-based mud. Introduction Oil-based drilling fluids are used widely throughout the U.K. North Sea and confer operational and economic benefits to exploration, appraisal, and development drilling activities. These benefits are well-quantified and, without other (environmental) considerations, the use of oil-based drilling muds (particularly those prepared with low-toxicity paraffinic oils) would be even more extensive. Unfortunately, the use of these muds results in the discharge of contaminated cuttings overboard, which consequently damages the benthic community. Addy et al. quantified the extent of the affected zone around development platforms, and restrictions on the discharge of oily cuttings are progressively being imposed on the basis of this and subsequent data. Because of legislative changes, operators have reappraised their drilling fluid programs and have developed a variety of responses, varying from a reversion to water-based drilling fluids to novel methods of oily-cuttings disposal. It is against this background that the present development of inhibitive water-based drilling fluids is taking place. The prime focus is control of the Tertiary shale formations found across much of the central and northern North Sea and Norwegian continental shelf. One of the more successful water-based drilling fluid formulations used to control Tertiary shales to date is the KCl/polymer system. These muds were used extensively for Forties field development drilling in the mid-1970's and early 1980's. Although these wells could be drilled, shale control clearly still was inadequate and mud dilution was excessive. This was both environmentally and economically undesirable. This paper details the development of a series of fluids modeled on the basic KCl/PHPA system at BP Research Centre, Sunbury-on-Thames. Three fluids, RCS 1, RCS 2, and RCS 3, have been developed, and RCS 1 and RCS 2 have been field tested. We will describe the laboratory testing of these fluids and discuss experience gained from the field trials. Laboratory Development of RCS Mud Systems The RCS muds were developed after a detailed study of many commercial muds; they are based on the widely used potassium chloride/partially hydrolyzed polyacrylamide (KCl/PHPA) systems. They arose from consideration of the mechanisms by which KCl/PHPA muds are understood to function. Potassium ions retard the expansion of swelling clays (smectites) and, hence, of shales containing these minerals. The high salt levels used in many of these muds also promote clay flocculation by collapsing extended electrical double layers. This helps limit shale dispersion. High-molecular-weight linear polymers, such as PHPA, adsorb on mineral surfaces to form a slick, robust coating that provides a degree of mechanical integrity to shale softened by the ingress of mud filtrate. Bailey et al. suggested that adsorbed polyacrylamide does not reduce either the amount of water taken up by the shale or the degree of swelling but simply provides resistance to erosion by circulating mud. As discussed earlier, it is commonly held that the KCl/PHPA system is the most inhibitive water-based mud in widespread use. This system is adequate for drilling in many areas but still causes severe hole problems when used in very reactive shales. The RCS mud systems demonstrate improvements over the performance of KCl/PHPA muds. Series RCS 1, RCS 2, and RCS 3 each successively increased inhibition of shale expansion. The key components and properties of the fluids follow. properties of the fluids follow. RCS 1 RCS 1 features an additive that increases the tenacity with which PHPA adheres to shale surfaces. PHPA adheres to shale surfaces. In an alkaline mud environment, the edges and faces of exposed clay mineral particles are negatively charged, and, therefore, it is difficult to envisage that significant quantities of anionic PHPA molecules will be strongly adsorbed. The adsorption that does PHPA molecules will be strongly adsorbed. The adsorption that does occur is ascribed to one of several mechanisms, including calcium ion bridges and screening of negative charges on the mineral surfaces and polymers by highly saline fluids. We reasoned that shale inhibition could improve if the charge on the polymer were reduced or, better, reversed. This prompted a study of cationic polyacrylamides and bridging agents. Laboratory tests showed that the greatest improvement in shale inhibition was obtained, not from cationic polyacrylamide, but from a mixture of conventional PHPA and a cationic starch. [Cationic starch is prepared by reacting potato starch with either 2-dimethylaminoethyl chloride or n-(2,3-epoxypropyl)-trimethyl ammonium chloride.] This starch functions to control fluid loss as well as to inhibition. Although RCS 1 contains a cationic polymer, we have not observed incompatibility with the anionic components of the mud (xanthan gum and PHPA). The fluid's response to salt, cement, and drilled solids contamination is similar to that of PHPA muds. We assessed shale inhibition with a series of routine tests: a modified dispersion, an unconfined swelling, and a penetrometer test (see Ref. 12). Shales used in the tests are London (Eocene, swelling, and dispersive), Kimmeridge (Jurassic, nonswelling, but very dispersive), and Oxford (Jurassic, swelling, and slightly dispersive) clays. All shales are preserved specimens taken from working U.K. quarries. Figs. 1 through 3 compare RCS 1 inhibition results with those for several other systems. These results confirm that the combination of cationic starch and PHPA improves control of shale dispersion considerably but does not affect swelling or softening significantly. This is consistent with the enhanced performance of the encapsulating polymer in the RCS 1 system. While RCS 1 significantly but incrementally improves inhibition compared with conventional KCl/PHPA muds, it is still inadequate for drilling the most highly reactive shales. Further improvements in dispersion control and at least some management of the swelling and softening reactions clearly are necessary. These requirements are addressed by RCS 2. RCS 2 Pericone et al. and Hale et al. extensively discussed the ability Pericone et al. and Hale et al. extensively discussed the ability of water-soluble glycol and glycerol derivatives to control reactive shales. Laboratory work and recent BP field trials of a KCl mud containing one such product, polyglycerol, confirmed that, if used correctly, these materials could improve inhibition significantly. SPEDE P. 237

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