Evaluation and Comparison of Various Miscible CO2-EOR Methods
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Enhanced oil recovery (EOR) by miscible carbon dioxide (CO2) injection is one of the most interesting EOR methods, both because of high oil recovery and the possibility of mitigating CO2 emissions by storing CO2 in the reservoir. Limited by the cost or availability of CO2, the industry has utilized a range of different miscible CO2-EOR methods. Additionally, pure CO2 injection may lead to poor sweep efficiency because of its high mobility. Hence, CO2 is often alternated with water to increase sweep efficiency, while lowering the amount of CO2 injected. Another method to reduce the CO2 mobility is by CO2-foam injection. However, CO2-foam injection could suffer from foam stability issues. Reservoir simulations were conducted for various miscible CO2-EOR methods on a homogeneous model. A simple economic model was constructed to determine the optimum injection parameters of each tertiary injection method. The tertiary injection was initiated after seven years of waterflooding without primary production. The optimums of CO2 injection, water alternating CO2 (WAG), hybrid WAG, simultaneous water alternating CO2 (SWAG), water alternating CO2-foam (WAG-foam) and CO2-foam injection were evaluated and compared in terms of net present value (NPV). Sensitivity analyses were done on parameters of the foam model, the miscible flood model and the economic model, to investigate their significance in terms of NPV. CO2-foam injection gave the greatest NPV of all the methods evaluated. Pure CO2 injection yielded the second best NPV. CO2-foam injection provided good CO2 mobility control, although pure CO2 injection almost contacted the reservoir completely. WAG parameters such as injection locations, cycle time and WAG ratio were found important. An overall poor WAG performance in terms of oil recovery was identified. This was related to low CO2 sweep in the lower portions of the reservoir. For both CO2-foam and CO2 injection, chase water after continuous CO2 injection increased oil recovery and the NPV. Hybrid WAG was found to be the most efficient method of the water alternating CO2 schemes. Foam parameters, which governed the foam stability, were found important. The efficiency of CO2-foam was found to be significantly decreased when the foam stability was lowered.