Laboratory evaluation of different modes of supercritical carbon dioxide miscible flooding for carbonate rocks

Field application of supercritical carbon dioxide (sc-CO₂) miscible flooding continues to grow. Optimization of sc-CO₂ injection during miscible-flooding modes represents one of the dominant factors affecting its performance in carbonate oil reservoirs. The main objective of this study was to investigate the effect of different modes of sc-CO₂ miscible injection on oil recovery and injectivity in carbonate rocks. Several modes of sc-CO₂ injection were investigated, including continuous CO₂ miscible flooding, water-alternating-gas (WAG), and tapered-WAG injection. Five coreflooding experiments were conducted to evaluate oil recovery for different modes of sc-CO₂ injection under reservoir conditions. Composite cores of 25-cm length from a specific, producing carbonate reservoir were used in the study. Both horizontal- and vertical-coreflooding experiments of continuous-sc-CO₂-injection mode were performed to compare oil recovery and injectivity during sc-CO₂ flooding. Horizontal-coreflooding experiments under WAG mode were performed at pore pressures of 3,200 and 3,800 psi. The experimental results indicate that tertiary oil recovery was influenced significantly by different sc-CO₂-injection modes and the direction of displacement. Original oil in cores was recovered at 18.4 and 26.74% for horizontal and vertical experiments of continuous-sc-CO₂-injection modes, respectively. In horizontal-WAG experiments with different pore pressures, higher oil recovery was observed at a pore pressure of 3,800 psi compared with 3,200 psi. The marginal increase in incremental oil recovery indicates that pressure increase beyond 3,200 psi will not have a significant effect. The injectivity of different sc-CO₂-injection modes is reported in this paper.