Numerical Modeling Assessment of CO2-EOR and Sequestration Potential in a Light-Oil Carbonate Reservoir

Abstract

Abstract This numerical study investigates the potential of mutually fulfilling EOR and sequestration objectives of carbon dioxide (CO2) injection into a hypothetical light-oil, carbonate reservoir. Formation and fluid properties were selected considering the ranges published in the literature for light-oil, carbonate reservoirs. The potential sources of CO2 were assumed to be the upstream and downstream activities in the reservoir geographic area, such as atmospheric emissions from refineries and steam boilers operating in nearby fields. Therefore, the example case is intended to explore sequestration and recovery potential of enhanced oil recovery (EOR) by CO2 injection (CO2-EOR) — not in isolation but as part of regular oil-field operations. The light oil carbonate reservoir meets the EOR screening criteria for miscible CO2 flood. An equation of state was used to characterize the reservoir hydrocarbon system to generate phase envelope and minimum miscibility pressure (MMP) via a ‘mixing cell method’ and benchmarked against published correlations. Compositional numerical simulations of waterflooding, CO2-flooding, water-alternating-gas with CO2 (WAG-CO2), and methane (CH4) were performed under the assumptions of anticipated reservoir management and operational conditions. The EOR and sequestration potentials of CO2 flooding and WAG-CO2 were quantified by CO2 gross and net utilization ratios (GUR and NUR, respectively) and retention capacity (RC). Simulation results yielded the highest incremental oil recovery for the WAG-CO2 flooding surpassing that of the continuous CO2 flooding by a factor of 1.75. Typical GURs and NURs for continuous CO2 flooding were 29,000 SCF/STB and 9,600 SCF/STB, respectively. For WAG-CO2, the gross CO2 utilization ratio was 8,300 SCF/STB and the net CO2 utilization ratio was 3,100 SCF/STB, which indicated considerably lower CO2 requirements per barrel of incremental oil than those for the continuous CO2 injection case. Although the CO2 utilization ratios indicated a higher EOR efficiency of WAG-CO2, the RCs of the two methods were similar: 33% and 36% for the continuous CO2 and WAG-CO2 injection cases, respectively. This amounted to 83% more CO2 being sequestered for the continuous CO2 injection compared to WAG-CO2. Adding 2,000 ppm H2S impurity and 10 – 20 % CH4 to the injected CO2 mixture caused little effect on the efficiency of CO2-EOR and sequestration. Therefore, recycling produced CO2, contaminated by the reservoir gases, does not impair the EOR and sequestration objectives of CO2 injection into the light-oil, carbonate reservoir considered in this study. The results of this work corroborate the conflicting effectiveness of continuous-CO2 and WAG-CO2 methods in meeting the EOR and CO2 sequestration objectives. Operational restrictions, such as to accommodate continuous injection of CO2, the WAG-CO2 method requires at least two parallel patterns with alternating CO2 time periods, further exacerbate the optimization considerations.