Mature Oilfields Screening for CO2 EOR Utilization and Storage – A Case Study

The potential use of CO2 as an Enhanced Oil Recovery (EOR) agent (either miscible or immiscible CO2-flooding) to increase oil recovery from five mature oil reservoirs (fields A through E) followed by CO2-storage has been evaluated. Primary screening of the five mature fields’ potential for EOR by CO2-flooding has been carried out by comparing the reservoir description, oil composition/ properties and reservoir conditions with literature proposed criteria. In the binary screening, the same parameters were compared with data from oil reservoirs where miscible CO2-flooding has been carried out. Based on the primary and secondary screenings, three reservoirs (C, D and E) are expected to give miscible CO2-flooding, and due to their relative homogeneous nature CO2 injection for EOR purposes appears to be promising. Of these reservoirs, reservoir D is the most interesting one since the volume of the remaining oil is the highest, similar to reservoir E, and it also has the largest degree of pressure depletion compared to reservoirs C and E. A fourth reservoir (B) is also expected to give miscible CO2-flooding, but CO2-flooding is not recommended in this field because of its heterogeneous nature and wax deposition tendency reported to have caused severe production problems. In the fifth reservoir (A) the pressure is too low to achieve miscibility between the injected CO2 and reservoir crude oil at reservoir conditions; in addition, this reservoir is heterogeneous and the risk of asphaltene deposition is expected to be high. For the three most promising fields, the Sword software was also utilized to identify the two most promising assets with respect to application of existing EOR processes and make preliminary evaluations based on the existing data. Finally, the CO2-storage potential for the two most promising assets, fields C and D, was assessed for CO2 EOR followed by CO2 storage processes in the oil-bearing formation region. This was based on the requirement of a safe CO2 storage with the reservoir unit re-pressurized back to original average field pressure. Enhancement of stored CO2 volumes could be achieved by increasing the final field abandonment pressure to levels higher than the original ones and/or utilizing the pore volume of the underlying aquifers for CO2 storage.