Analysis and Treatment of Formation Damage at Prudhoe Bay, Alaska

Abstract

Summary Formation damage, which can occur at any time duringa well's history, is recognized by lower-than-expected productivity and high decline rate. At Prudhoe Bay, productivity and high decline rate. At Prudhoe Bay, investigations have shown that formation damage can beinduced as a result of incompatibility of certain drilling. completion, workover, and stimulation fluids with formation water, or can occur naturally as a result of the scaling tendency of connate water with a decrease inpressure and migration of fines. In general, damage is pressure and migration of fines. In general, damage is the result of adverse fluid/fluid or fluid/formation interactions. Because of the complex nature of these interactions, effective resolution of damage problems requires the combined efforts of engineers, geologists, and laboratory personnel. A comprehensive damage mitigation program personnel. A comprehensive damage mitigation program includes both preventative and corrective actions. Theseefforts are directed toward optimizing the drilling investment by ensuring that a well's full capacity can berealized while using cost-effective methods. Introduction The Prudhoe Bay field is located on the coast of Alaska'sNorth Slope, just south of the Beaufort Sea, and about250 miles [400 km] north of the Arctic Circle. Theprinciple producing interval within the Prudhoe Bay field is principle producing interval within the Prudhoe Bay field is the Ivishak formation of the Sadlerochit group. The field, which was unitized by 16 working interest owners beforebeing placed on production in June 1977, has twooperators-Sohio Alaska Petroleum Co. in the west and ARCO Alaska Inc. in the east. After the first few months of field production, it becameevident that the majority of wells could be placed into two categories: those with low decline rates (10 to 20% peryear) and those with high decline rates (50 to 70% peryear). No clear pattern emerged that could account forthe high decline rates. Therefore, a concerted effort to resolve this problem was initiated in the form of a UnitWell Damage Study Group, which included engineering, geological, and laboratory representatives from the threemajor owners-Sohio Alaska Petroleum Co., ARCOAlaska Inc. and Exxon U.S.A.Historically, the approach to the formation damageproblem at Prudhoe Bay can be divided into three phases. problem at Prudhoe Bay can be divided into three phases. Phase I was directed toward categorizing the types of Phase I was directed toward categorizing the types of damage, defining formation damage mechanisms, andevaluating various remedial treatments. During Phase 2, a fundamental understanding of the damage mechanisms was achieved, diagnostic techniques were developed toverify the kind of damage present in producing wells, andviable methods to mitigate formation damage were identified. Currently, Phase 3 includes improving operationalprocedures. analyzing ways to increase treatment procedures. analyzing ways to increase treatment success/failure ratios, and systematically optimizing treatmentprocesses, thereby improving the cost-effectiveness of the processes, thereby improving the cost-effectiveness of the overall capacity restoration/sustainment programs. A geologic description of the Prudhoe Bay field hasbeen presented previously by Wadman et al., and ahistory of the well damage problem was summarizedearlier by Braunston. In addition, laboratory studieshave been reported by Shaughnessy and Kline andMeyers, et. al. This paper describes the interdisciplinaryefforts that have been applied in developing acomprehensive field program to resolve formation damage problems, with emphasis on operational practices and field results. Reservoir Geology The Prudhoe Bay field structure was formed when thesandstones and conglomerates of the Permo-Triassic Ivishak formation, the main producing interval, were truncated near the southern flank of the Barrow Arch by theLower Cretaceous unconformity. This unconformable surface was overlain by Lower Cretaceous marine shales. These marine shales. in conjunction with major normalfaults, provide the trapping mechanism for the presenthydrocarbon accumulation. The exact source of hydrocarbons remains open to interpretation. However. the latestdata indicate that the hydrocarbons were resourced by the Cretaceous and Jurassic shales. The Ivishak formation has been divided into four zoneson the basis of log response as a function of lithology andinferred environment of deposition. A generalized stratigraphic section, type log, and average zonal properties are illustrated in Fig. 1. properties are illustrated in Fig. 1. Zone 4, the uppermost interval within the Ivishak formation, is the most texturally uniform of the four. It isbest described as a gradual fining-upward sequence of fineto lower-medium-sized sandstone with thin, interbedded, tight siltstones and shales. Bedding is typically massive, although occasional cross and ripple laminations are present, as are minor pebble-lag conglomerates. The upper present, as are minor pebble-lag conglomerates. The upper contact with the Shublik formation is unconformable and marked by a distinctive conductivity spike on the dualinduction log, the effect of a large concentration of modular and disseminated pyrite. Composition is dominated byquartz, dense chert, and microporous chert. Vermicular kaolinite and subordinate illite clays occur as porefillings and as a replacement of microporous chert. JPT p. 1010