Differences in CO2-Water-Rock Chemical Reactions among 'Sweet Spot' Reservoirs: Implications for Carbon Sequestration

The Lucaogou Formation, located in the Jimsar Sag, Junggar Basin, NW China, has great potential for shale oil resources. In the process of CO2-EOR(CO2enhance oil recovery), mineral dissolution, precipitation and transformation, leading to the local corrosion or blockage of reservoirs, have a significant influence on recovery. In this study, a combination of high-temperature and high-pressure laboratory experiments and coupled temperature/fluid-chemistry multifield numerical simulations are used to investigate CO2-water-rock reactions under various reservoir conditions in the upper and lower ’sweet spots’, to reveal the mechanisms underlying CO2-induced mineral dissolution, precipitation and transformation. In addition, we quantitatively calculated the evolution of porosity over geological timescales; compared and analyzed the variability of CO2transformation in the reservoir under a variety of temperature, lithology and solution conditions; and identified the main factors controlling CO2-water-rock reactions, the types of mineral transformation occurring during long-term CO2sequestration and effective carbon sequestration minerals. The results demonstrate that the main minerals undergoing dissolution under the influence of supercritical CO2are feldspars, while the main minerals undergoing precipitation include carbonate rock minerals, clay minerals and quartz. Feldspar minerals, especially the initially abundant plagioclase in the formation, directly affects total carbon sequestration, feldspar-rich clastic rocks therefore having considerable sequestration potential.