Application of digital rock physics using X-ray CT for study on alteration of macropore properties by CO2 EOR in a carbonate oil reservoir

Abstract To study the fluid flow in reservoirs considering chemical and physical reactions, alterations in pore structure and rock properties should be examined. This study aims to investigate the changes in the main flow path (macropores) and rock properties caused by CO2 injection in carbonate rock. Results from pre- and post-CO2 injection have been obtained by core flooding, mercury injection capillary pressure (MICP), X-ray CT, digital rock physics (DRP) techniques, and subsequent analysis. This study proposes combining the methods of X-ray CT imaging with MICP to improve both the realization of and understanding of alterations in pore structure. Following CO2 injection, the capabilities of flow and storage in carbonate rock are significantly altered by chemical and physical reactions among CO2, fluid, and minerals. In realization of a pore network for main flow paths as elucidated using X-ray CT and MICP, those reaction effects develop abundant non-connected pores due to disintegration of rock grains by dissolution and precipitation of mineral particles in contact with the fluid stream. Furthermore, Lattice Boltzmann (LB) simulation results show increased irreducible oil saturation and entry capillary pressure, indicating reduced displacement efficiency of oil by CO2 in main flow paths, particularly around the injection well where rock is usually exposed to CO2 for a long time. Therefore, alteration of pore networks by CO2 injection can lead to poor injectivity and may require a very delicate injection scheme when follow-up injection method applies. This study can be applied to forecast fluid flow in pore structures altered by CO2 injection for CCS and EOR projects. Further study is needed to investigate the effects of small pore size and complex flow phenomena such as multi-phase flow and fluid-fluid interactions.