Subsurface issues for CO2 flooding of UKCS reservoirs

The injection of CO2 is a successful IOR technique applied in onshore fields, where it is sourced from natural CO2 reservoirs. Concern over greenhouse gas emissions has renewed interest in the potential for CO2 flooding on the UKCS. CO2 would be captured onshore and injected into UKCS reservoirs to improve recovery and leave CO2 trapped in the reservoirs at the end of project life. Experience from onshore CO2 injection projections is reviewed. Reservoir parameters for onshore projects are compared with typical UKCS conditions, and the key similarities and differences highlighted. Incremental oil production in onshore CO, WAG projects expressed in terms of reservoir barrels is related to the volume of CO2 retained, and is consistent with a rule of thumb developed for offshore hydrocarbon gas injection projects. The impact of compositional effects and the operating temperature and pressure on offshore CO2 injection performance is assessed using sector models of WAG and crestal (gravity stable) injection schemes. Reducing the operating pressure reduces the incremental recovery but decreases the volume of CO2 required. Temperature has a significant impact on WAG recovery, and needs to be considered if CO2 is to be injected in parts of a reservoir that have been cooled by seawater injection.