Chayvo Field, Sakhalin Island, Russia: Identification of a Significant Oil Leg from 3D Seismic and Execution of a World Class Extended Reach Drilling Program

Abstract

Abstract In 1997, the ExxonMobil operated Sakhalin I Consortium acquired a 3D seismic survey over Chayvo Field, located off the northeast coast of Sakhalin Island, Russia. At the time of the acquisition, Chayvo was thought to contain primarily gas given the results of five pre-existing crestal wells. Interpretation of 3D seismic revealed seismic amplitude anomalies that conformed to structural spill, were down dip of gas accumulations seen in crestal wells, and indicated the potential for very large, previously unrecognized oil rims. Based on the 3D seismic interpretation, an appraisal well was drilled in 2000 that penetrated several large oil legs. This appraisal well and associated sequence stratigraphic studies resulted in a significant oil project in a field previously thought to be mostly gas. When the oil rims were confirmed, the challenge was how to develop the Chayvo field lying 8 to 10 kilometers offshore. Development drilling at the field began in 2003, with the first phase comprising a series of extended reach wells drilled from an onshore location reaching offshore to the western limb of the Chayvo anticline. These wells are technically complex and very expensive, but attractive considering offshore platform costs in a remote, environmentally sensitive location characterized by seismic activity and pack ice in the winter. In 2002, a multi-disciplinary team was assembled to address the challenges associated with designing and executing extended reach wells in stacked deltaic reservoirs comprised of prograding clinoforms. The primary geological concern was the accuracy with which the horizontal well bores could be placed within the oil column, as reservoir simulation indicated the need to be within 10 m of a specified vertical depth. Ongoing drilling has proven to be successful with actual reservoir depths within a few meters of prognosed depths at well reaches of 9–11 kilometers, underscoring the value in the integration of seismic interpretation and extended-reach well planning in a 3D visualization environment. Introduction A 3D seismic survey acquired in 1997 over Chayvo Field, offshore Sakhalin Island (Figure 1), identified the potential for large oil rims in reservoirs which had tested mostly gas in crestal wells. The rims were identified from pairs of amplitude anomalies that conformed to structural contours and were interpreted as gas-oil and oil-water contacts. Several interpretation techniques were used to increase confidence in the postulated contacts and predict contact depths. The oil rims and contact depths were tested and confirmed with an appraisal well drilled in 2000, resulting in a significant oil project in a field thought to contain mostly gas.[1, 2]