In recent years, gas injection, especially CO2 injection, has been acknowledged as a promising approach for enhanced oil recovery (EOR) and CO2 capture and storage (CCS), especially for tight reservoirs. However, when CO2 is injected into the oil reservoirs, it can disturb the equilibrium of the system and lead to chemical reactions between CO2, formation water, and reservoir rocks. The reactions will alter some geochemical and physicochemical characteristics of the target reservoirs. However, the reactions still lack quantitative characterization at the pore scale, especially under reservoir conditions. Herein, we conducted an experimental study of the interactions between CO2, brine, and rocks in the Mahu oilfield at 20 MPa and 70 °C. The low-field nuclear magnetic resonance (LF-NMR) measurements showed that the incremental amplitude for tight cores of CO2-rock-water tests was larger than that for CO2-rock tests, and the amplitude alteration presented significant differences corresponding to different types of minerals and pores. Furthermore, the interplanar spacing of the core samples was increased with the increase of reaction time in the CO2-rock experiments but still lower than that in CO2-rock-water tests. This research demonstrated evident changes in the geochemistry in tight reservoirs caused by CO2, brine, and rock reactions. The results of this study may provide a significant reference for the exploration of similar reservoirs in the field of CO2-EOR and CO2 sequestration.
© 2022 The Authors. Published by American Chemical Society.