Miscible Gas Injection EOR, Carbonate Reservoirs Well Injectivity Field Cases

Abstract

Miscible gas injection EOR is a well-established and proven recovery mechanism. There are a large number of miscible gas injection EOR developments worldwide. Some of these developments achieved relatively high recoveries. ADNOC is a leader in miscible gas injection EOR in carbonate reservoirs - with a number of developments within its portfolio contributing substantially towards ADNCO overall production. For miscible gas injection EOR developments, especially in relatively low permeability carbonate reservoirs, the following are recognised as key field development planning and reservoir monitoring/management areas: Development pattern, number and location of injectors in relation to producersSufficient injection rate and volume, to maintain reservoir pressure and optimise sweepReservoir monitoring and production management, to ensure adequate data acquisition and studies to support field development and production decisions in a timely manner. In relatively tight carbonate reservoirs, injection well injectivity is a critical factor when addressing the above three key areas. Water injection injectivity is fairly well understood, but gas injection injectivity in a miscible setting can be very complex. The complexity can be attributed to a combination of factors, for example: The dependence of injection gas property parameters on downhole injection pressure and temperatureDevelopment of the miscible oil bank via either vaporising or/and condensing mechanismVapour phase being significantly more mobile than reservoir liquid oil and waterWhere applicable, gravity segregation in areas surrounding the injectorReservoir wettability and hysteresis. An assessment into miscible gas EOR injection injectivity for relatively low permeability carbonate reservoirs has been performed. Actual field injection data (both water injection phase and gas injection phase) from 3 reservoirs were analysed. A compositional sector model was also used. Injectivity evolution trends were discussed, and potential influencing factors identified. Recommendations on best practices for WAG injector injectivity monitoring, reservoir voidage replacement calculations are provided.