Gas Shutoff Evaluation and Implementation, North Slope, Alaska

Oil production on Alaska's North Slope has been limited as a result of gas production and the ensuing gas handling-facility constraints. Diagnosis of gas production problems is complicated by the fact that many of the wells are high-angle or horizontal. This paper presents case studies in the diagnosis and treatment of excess gas production in three horizontal oil wells located on the North Slope of Alaska. Candidate wells for gas shutoff (GSO) were identified based on gas flow rates and the gas/oil ratio (GOR). Wells that were not commercially producible were selected for evaluation as potential GSO wells. These wells were assessed with new-technology logging tools conveyed on coiled tubing or with a downhole tractor. The new production-logging technology provided direct gas and oil holdup measurements, allowing for interpretation of three-phase production in the horizontal well environments. The goal was to identify dry-gas production (i.e., gas production without associated oil production). After dry-gas production was identified, GSO procedures were designed based on the location of the gas entry into the wellbore. Two of the case studies involved GSO from the toe of the horizontal section of the wells, which was accomplished by placing a cast-iron bridge plug on coiled tubing above the gas-producing perforations. The third case study identified dry-gas production from the top perforated interval. This GSO involved setting a plug below the gas-producing perforations and squeezing them off. The plug was then drilled out with coiled tubing and pushed to total depth (TD). These techniques proved to be economical GSO solutions in difficult well environments, and each case resulted in significant reduction of free gas and increased oil production.