Comparison of oil recovery and carbonate rock's properties alterations by CO₂ miscible flooding

This present study indicates experimental investigation about the impact of CO₂ flooding on oil recovery and rock's properties alteration in carbonate reservoir under the miscible condition. In order to compare the effect to initial pore characteristic, two type of carbonate rock was used; an Edward white represents homogeneous mainly consisted micropore, whereas an Indiana limestone represented heterogeneous mainly consisted macropore in this study. Under the miscible condition (9.65 MPa and 40°C), five pore volume of CO₂ were injected into oil-wet carbonate rock, which was fully saturated with oil and connate water. After CO₂ flooding, several analyses for each sample conducted to investigate oil recovery and rock properties change in porosity, permeability, and pore structure by chemical and physical reaction between CO₂, water, and carbonate mineral before and after CO₂ flooding by using core analysis, MICP, SEM, ICP, and X-ray CT techniques. From the results of oil recovery, it was more effective and larger in Edward white than in Indiana limestone. Because homogeneous characteristic with a large ratio of low permeable micropore in Edward white contributed to occur long reaction time between oil and CO₂ for enough miscibility as well as to displace stably oil by CO₂. Conversely, heterogeneous pore structure mainly consisted of high permeable conduit (macropore) in Indiana limestone has brought ineffective and low oil production. From the analysis of rock's properties alteration, we found that, for the homogeneous sample, dissolution dominantly changed pore structure and became better flow path by improving permeability and reducing tortuosity. While plugging by precipitation of mineral particles was not critically affected rock' properties, despite the sample mainly consisted small pores. In the case of the heterogeneous sample, both dissolution and precipitation critically affected change of rock's properties and pore structure. In particular, superior precipitation in complex pore network seriously damaged flow path and change of rock's properties. The largest porosity change markedly appeared in inlet section because of exposing rock surface from fresh CO₂ during a long time. In conclusion, it shows that CO₂ miscible flooding in carbonate reservoirs significantly affected to alteration of rock's properties such as porosity, permeability, tortuosity, and pore connectivity, in particular in heterogeneous system compared with in homogeneous system. These experimental results can be useful to characterize carbonate rock as well as to study rock properties alteration on CO₂ EOR and CCS processes.