Drilling of Arun Gas Field

Abstract

Summary The Arun gas field was discovered in late 1971 when the discovery Well Arun A-1 penetrated the thick Arun limestone reef. During the following 3 years, 12 delineation wells were drilled. Three of these delineation wells are used for observation wells, five for dry gas injection, one for condensate water disposal, and three are abandoned. Clustered development well drilling started in Sept. 1976. At this writing 40 wells have been drilled to delineate and develop the field. Drilling continues so that the growing demand from the expanding liquefied natural gas (LNG) plant is met. The problems of high temperatures, abnormally high-pressured shales, and saltwater sands overlying the lower-pressured Arun limestone have been conquered by numerous technique changes. The current techniques include the use of inverted oil emulsion muds, cements containing 35% silica flour, high-strength heavyweight tubulars, and clear packer fluids. The evolution of drilling and completion practices are discussed in the paper. Introduction The Arun gas field is located in the northern portion of Sumatra, Aceh Province, which lies within Mobil Oil Indonesia Inc. Block B production sharing contract area (Fig. 1). It was discovered in 1971 by the drilling of Arun A-1 in the thickest part of the Arun limestone reef close to the present Cluster III location. Cluster development drilling (Fig. 2) started in Sept. 1976. Currently two clusters (Clusters II and III) have been developed with the third cluster (Cluster I) development started in 1981. The fourth cluster development (Cluster IV) started in 1983 to meet the growing demand for the expanding LNG plant. Cluster drilling is used to centralize and consolidate the surface facilities for safety and security reasons and to reduce the disruption to rice growing in the area of the gas field. Provision is made on each cluster for a maximum of 16 producing wells located in two lines of eight locations on a 262-ft [80-m] spacing along the eastern boundary of the cluster with about one-half of the cluster area allocated to the locations. Initially up to eight wells are drilled in each cluster, the remaining locations to be drilled when additional producing capacity is required. The gas processing and compression equipment occupies the western half of the cluster. The entire cluster is surrounded by a dike and antipollution ditch and completely fenced with night lighting and a staffed security building at the cluster entrance. The development wells are deviated out from the clusters (Fig. 2) to drain the reservoirs effectively along the axis of its structural high. Gas injection wells are generally drilled vertically on the western flank of the reservoir to recycle the excess dry gas after the condensate has been recovered. Drilling Problems Associated With Geology Drilling problems are associated with the high temperatures and abnormally high-pressured shales and saltwater sands overlying the high-pressure Arun limestone gas reservoir. The Arun field gas reservoir is a and of Early and Middle Miocene age located near the base of the tertiary sedimentary section. The gas accumulation is mainly stratigraphic, having been trapped in a porous reefoid facies overlapped by the Baong shales (Fig. 3). The Baong shales are abnormally high pressured and are overlain by the Keutapang formation, which exhibits a pressure transition zone grading into an abnormally high pressure. The Keutapang formation contains a number of high-pressure gassy saltwater sands, and above the Keutapang pressure transition zone the pore pressures of the formation are normal (Fig. 4). The Arun limestone reservoir pressure, although lower than the Baong shale, is abnormally high, being originally 7,100 psi [49 mPa] at 10,000 ft [3048 m] subsea. The gas contains 13.75% CO2 and 0.005 to 0.01% H2S. The static bottomhole temperature (BHT) is 352 deg. F [178 deg. C]. This high BHT is also reflected in the overlying Baong shale and Keutapang formation, which exhibit high temperature gradients. The high temperatures and pressures require weighted mud capable of retaining its properties to allow successful drilling and casing operations. The cement slurries must be capable of allowing the casing to be single-stage cemented to surface and to exhibit a high compressive strength. Surface and intermediate casing setting depths must be selected to ensure that loss or flow problems do not occur. The changes in temperature during production require casing tension landing techniques to avoid buckling. The CO2 and H2S content of the Arun gas requires special high-grade production tubulars and wellhead equipment to resist corrosion and erosion at high production rates. Discovery and Delineation Drilling JPT P. 771^