Summary Annular reinjection offers a cost-effective disposal mechanism for the oily cuttings and associated wastes generated when oil-based muds are used during drilling. This disposal method eliminates overboard cuttings discharge and hence removes any environmental impact. Continued use of oil-based muds is therefore feasible. A 12-member Drilling Engineering Assn. in Europe (DEAE) project was initiated in 1990 to study the engineering aspects of this approach in the North Sea. This paper reports on the first trial off a fixed platform in the North Sea and discusses the preliminary engineering studies, injection, and analysis of fracture propagation. It demonstrates that, with proper regard to the engineering of the injection well, the process is a safe, efficient, and effective disposal technique. Introduction Low-toxicity oil-based muds (LTOBM's), used widely in the North Sea operating area, offer significant operational benefits, especially for exploration and appraisal drilling and, more importantly, for development drilling. However, the overboard discharge of the oil-contaminated cuttings leads to environmental damage that, although localized, is unacceptable. To resolve this conundrum, novel water-based muds were evaluated and a range of cleaning and disposal options for the cuttings was reviewed. From these options, cuttings slurrying and reinjection appeared to offer the most cost-effective solution because this approach would also address the problem of oily liquids from the drilling unit. Cuttings slurrying and reinjection is established practice in Alaska and the Gulf of Mexico, but the technique had not been used previously in the North Sea. We knew that the legislative authorities would have some concerns. Therefore, 12 operating companies sponsored a joint project, through the auspices of the DEAE, to evaluate the engineering issues associated with this approach. The project was established with three objectives in mind:to demonstrate that cuttings slurrying and reinjection was technically viable for the North Sea,to gain acceptance of this approach to waste management from North Sea legislative authorities, andto evaluate a novel approach to cuttings grinding.

The ability to predict accurately the circulating system pressure drop whilst drilling is a prerequisite of drilling optimisation. The quantification of the annular pressure drop is also necessary to calculate the equivalent circulating density of the drilling fluid. With invert emulsion drilling fluids, using 'low toxicity' paraffinic oils, these calculations are complicated by the viscosity changes under downhole conditions. This paper presents data from three separate hole sections where annular and drill string pressures were continuously monitored using Measurement While Drilling (MWD) instrumentation. Readings were taken between 2625 and 14764 ft (800 and 4500m) with mud densities ranging from 9.33 to 11.67 ppg (1120 to 1400 kg/m3). The data are summarised and tabulated with respect to the hole geometry, circulating rates and drilling fluid properties. The derivation of correction factors for the estimation of the drilling fluid rheology at elevated temperatures and pressures is detailed with results of laboratory studies using. HP/HT viscometry. The theoretical pressure drops through the annular and drill string sections, with and without application of the rheology correction factors, are then presented. Two rheological models are employed, the power law with yield stress (Herschel-Bulkley) and the Bingham model. The theoretical pressure drops are compared with the actual values. The application of the rheology correction factors is discussed and the comparative merits of the two models reviewed. Finally the paper discusses the implications of the research to hydraulics planning and identifies critical areas of development required to enhance the predictions. Introduction The accurate prediction of the circulating system pressure drop, and the distribution of this within the various sections of the system, is required in the planning and monitoring of the drilling operation. Drilling optimisation demands an understanding of these pressures to maximise the energy transfer to the bit. The drilling fluid specification requires a knowledge of the systems pressure drop when considering annular flow rates and consequential hole cleaning performance. There is also a need to understand annular pressure drops, especially in weak formations, to minimise excessive overbalance within the wellbore. Historically, the drilling industry has relied upon the application of non-Newtonian fluid flow models for these calculations using surface measured flow properties. However, particularly for invert emulsion systems, doubts have been raised as to the accuracy of these models, and of the distribution of the pressure drops within the system. The use of the low toxicity, paraffinic, invert oil emulsion drilling fluids has developed within the North Sea operating area during the last five years. This is primarily due to the improved inhibition these systems impart to the soft Tertiary shales of the area. Additionally, enhanced rates of penetration have been documented when employing these invert emulsion fluids through the Cretaceous shales (Ref. 1). Therefore, despite the constraints imposed by environmental considerations, most development wells, many appraisal and some exploration wells now rely on the use of these fluids. P. 519^

Laboratory tests showed that tetra-potassium pyrophosphate (TKPP or K4P207) gave levels of shale inhibition well beyond that obtained with equivalent concentrations of potassium chloride. A detailed evaluation indicated that stable mud formulations could be prepared which were considerably more inhibitive than other mud systems currently available. Inhibition even approached the level of that observed with oil based mud with some shale types.

Introduction Concern over the environmental impact of drilling operations - in particular the disposal of whole mud and contaminated cuttings - is leading to a major reappraisal of acceptable mud types and the way in which they are used. Strict control over the use and discharge of fluids is exercised in, for example, the USA, the North Sea, the Adriatic and the CIS. Government agencies in almost every area of the world are conscious of the potential damage caused by drilling mud and cuttings discharges and are legislating accordingly. In addition to these legislative pressures, increasing demands are placed on fluids as the industry drills more extended reach, horizontal and high temperature/high pressure wells. These new challenges are in addition to existing ones - such as the need to control reactive shales, provide good fluid loss control and minimise formation damage - which remain critically important to the success of a well. Given these requirements asked of the fluid, many operators are convinced of the benefits of continuing to use oil based mud (OBM). Hence the industry, including companies such as BP, is committing considerable resources to developing ways of using OBM which comply with good environmental practices. These methods, such as cuttings cleaning and cuttings injection, are discussed by Minton, 1992 and Minton, Begby et. al., 1991. Another approach is to develop water based muds (WBM) which have little environmental impact but offer performance closer to OBM than existing WBM. The search for such fluids is being aggressively carried out by service companies and many operators. The final outcome of these studies is likely to be that a mix of options - water based muds, cuttings injection and cuttings cleaning - will be practicable and the final choice will be governed by a cost and risk analysis, local conditions and the preference of individual operators or their contractors. In any event, the use of cuttings cleaning or injection is unlikely to be practical for large hole sections in exploration and appraisal wells where a suitable annulus does not exist for injection and where the large volume of cuttings precludes the use of cleaning technology. In these situations the effective use of water based muds may be the best technical and economic solution. If water based muds are to be used successfully they must satisfy a number of criteria: From a technical standpoint: They must provide good shale inhibition. This is an essential requirement in many areas, including the Gulf of Mexico and much of the North Sea, Far East and CIS. If these muds are to be used in extended reach and horizontal wells they must be highly lubricating. P. 453^

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