In 1997, the ExxonMobil operated Sakhalin I Consortium acquired a 3-D 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 3-D 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 3-D seismic interpretation, an appraisal well was drilled in 2000 and 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 icebergs.
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 to delay water/gas breakthrough. 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 integrating seismic interpretation and extended-reach well planning in a 3-D visualization environment.