The mechanism of mineral dissolution and its impact on pore evolution of CO2 flooding in tight sandstone: A case study from the Chang 7 member of the Triassic Yanchang formation in the Ordos Basin, China

Abstract

CO2 flooding of tight sandstone reservoirs is an effective technique for geological storage of CO2. The CO2-water-rock reactions can lead to structural and property changes in sandstone. Therefore, understanding the characteristics and mechanisms of CO2-water-rock reactions in tight sandstone formations is essential for comprehending the evolution and stability of pore structures following CO2 flooding. This study combines physical experiments on CO2 core displacement and numerical simulations with constraints of present-day mineral and pore water compositions in the Yanchang Formation of the Ordos Basin to investigate the influence of CO2-brine-rock reactions on dynamic changes in minerals. The results show that during the CO2 core displacement process, minerals exhibit selective dissolution characteristics, with samples containing higher calcite contents displaying lower calcite dissolution rates. In the presence of calcite, both K- and Na-feldspar undergo dissolution. Simulation calculations indicate that the dissolution characteristics of minerals are related to the equilibrium constant of the dissolution reactions. Specifically, the equilibrium constant of calcite dissolution is much smaller than that of feldspar dissolution. In the initial stages of the CO2 reactions, only a small amount of calcite is dissolved, while significant feldspar dissolution occurs over an extended period, accompanied by the precipitation of quartz, clay, and some calcite. The secondary precipitation of calcite can further facilitate the dissolution of feldspar. Both the CO2 displacement experiment and simulation results confirm that substantial dissolution occurs primarily in feldspar, while quartz, clay, and some calcite minerals precipitate. The pore evolution mechanism during CO2 injection is reassessed through numerical simulation. With the inhibition of calcite dissolution in tight sandstone, CO2 primarily dissolves feldspar minerals. However, within the confined environment of mudstone-encased tight sandstone, the CO2 dissolution products cannot be effectively transported out of the reservoir or over long distances. CO2 dissolution effectively replaces the intergranular porosity with feldspar dissolution porosity. The change in reservoir porosity after CO2 injection shows little variation among the studied samples. CO2 injection into tight sandstone exhibits stability in long term. These findings provide a geological basis for enhancing oil recovery and mitigating climate change.