Development of Oil-Water Transitional Zone by Rim Lowering in a Mature GOGD Field in Sultanate of Oman

Abstract

Abstract Field X is one of the largest oilfields in Sultanate of Oman and has highly fracture carbonate reservoirs. The field has been under production since 1967 and has undergone multiple phases of production including: pressure depletion, simultaneous gas and water injection within different units and sectors, in addition to steam injection trial. A redevelopment of field starting 2010 had switched the development from waterflood to GOGD for most parts of the field. The primary reservoir has a long oil-water transition zone exceeding 50 m. Historical development was focused on primary above the oil-water transition zone. Large volume of injected water remains in the reservoir lowering efficiency of GOGD in areas of the field. A new development opportunity was identified to develop the long transitional zone as well as de-water the reservoirs which had historical waterflood, thus reducing the gap to top quartile recovery. A systematic appraisal cum development plan was initiated in the asset to develop the transitional zone. There had been no recent wells drilled through the transition zone to validate the remaining oil saturation. Modelling suggested that oil saturation up to 60% should be present. Two vertical appraisal wells were drilled targeting the transitional zone and provided invaluable data on reservoir quality and remaining oil saturation. The wells were also hooked up to validate productivity of the transition zone and confirmed the feasibility of developing the transition zone. Further analytical modelling was conducted to test the concept of rim lowering to enhance GOGD as well produce incremental volumes. Horizonal rim lowering well opportunities were identified and located 20 to 30 m vertically below the horizontal final rim wells in areas with high water cut. The wells were designed to intentionally cross fracture corridors to enhance productivity of wells as the risk of gas breakthrough was deemed to be small at these depths. A phased development was proposed to validate performance prior to large scale development. The first phase of rim lowering wells was drilled in 2021 and early 2022. Production data for the last 6 months to 8 months indicate that most wells outperform base case forecast. This confirms long term producibility of transition zone. Further, these wells have reduced water cut in up-dip wells enhancing GOGD and have accelerated the tail production. The rim lowering project will lower the oil rim by 10 to 30 m, exposing more rock volume to GOGD process and thus increase in oil production and recovery. Petrophysical logs were also acquired throughout the horizontal section while drilling. The log data has helped to map out areas with poor reservoir quality and will be used to adjust the development plan. The case study describes one of the few implemented concepts of rim lowering in mature GOGD field. The concept can be now used to expand the development of transition zones in similar fields in Sultanate of Oman.