Impact of CO2/Water/Rock Reactions on Rock Property Evolution in High-Salinity, Low-Permeability Sandstone Reservoirs

Summary Carbon dioxide (CO2) utilization for enhanced oil recovery is an important segment in the carbon capture, utilization, and storage technique. It makes geological CO2 sequestration economically viable. In particular, CO2 flooding has great potential for low-permeability oil reservoirs with poor water injectivity. Nevertheless, the performance of CO2 flooding may be severely affected by the precipitation and dissolution processes generated by CO2/water/rock reactions in high-salinity formation water reservoirs. In this study, we investigate the processes and impacts of inorganic salt precipitation and mineral dissolution during CO2 injection in sandstone cores, aided by a high-temperature, high-pressure visible reactor. The sandstone cores used are mainly composed of quartz, feldspar, and clay minerals. The effect of CO2 interaction with minerals, including possible dissolution and particle transport, on changes in porosity and permeability was investigated using nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), and coreflooding experiments. CO2 flooding in dual cores was conducted to analyze heterogeneous changes in rock petrophysical properties during the flooding process. The results showed that under density-driven advection during CO2 injection, the bulk phase exhibited an early alkaline-to-acidic pH change at the bottom, whereas the porous medium showed a progressive pH decrease from top to bottom. The inorganic salt precipitation was adsorbed on the surface of the pore throat and resulted in an overall decrease in the radius of the pore throat. The CO2/rock reactions had different effects on permeability change across cores with different initial permeabilities. Mineral dissolution via CO2/rock reactions primarily occurred in macropores, effectively increasing core porosity by up to 1.9%. With increased initial core permeability, the effect of CO2 dissolution strengthened, while the effects of inorganic salt precipitation and particle transport weakened. After multiple rounds of CO2 injection in dual cores, the permeability of the higher-permeability core varied from 1.509 md to 2.238 md, while the permeability of the lower-permeability core changed from 0.232 md to 0.256 md. This enhancement in permeability ratio from 6.6 to 8.6 increases the risk of gas channeling into these formations. This work highlights changes in porosity and permeability of sandstone cores with initial brine permeabilities ranging from 0.1 md to 16 md, resulting from interactions among salt precipitation, mineral dissolution, and particle transport during CO2 injection. The results provide data support for the time-varying permeability calculation, which could be used to more accurately predict changes in formation permeability during the CO2 flooding process in sandstone reservoirs.