Abstract:

As China progresses toward its dual-carbon goals, saline aquifers are being prioritized as sites for CO₂ sequestration. The saline reservoir in the southwestern region of our country, in particular, boasts significant potential for CO₂ storage due to its substantial storage capacity. However, existing studies on CO₂-water-rock reactions in saline reservoirs have predominantly focused on the macroscopic scale, with a notable lack of detailed characterization of microscopic-scale changes in rock pore structures before and after these reactions. This study examines the carbonate rock saline aquifer of the second member of Jialingjiang Formation in the Moxi area of the central Sichuan Basin, conducting 20 sets of CO₂-water-rock interaction experiments under simulated in-situ formation conditions of 69 MPa and 97 ℃. Employing analytical methods such as X-ray diffraction, nuclear magnetic resonance, scanning electron microscopy, and computed tomography, the research explores the evolution characteristics of the rock's physical properties and pore-throat structures under the influence of CO₂. The results show that: as the reaction progresses, there is a noticeable increase in the proportion of clay minerals and quartz, while the content of feldspar minerals decreases substantially. The content of calcite initially drops before it begins to increase, and the percentage of dolomite initially rises then falls. These mineral dissolution processes significantly alter the rock's pore throat structure, enhancing pore connectivity, enlarging the pore-throat radius, and increasing both porosity and permeability of the carbonate rock. This results in an expansion of the pore space within the reservoir. Moreover, the higher the concentration of CO₂, the more pronounced are the changes in the physical properties and pore-throat structures of the carbonate rock. Under conditions of pure CO₂, the experiments reveal that after 50 days of reaction, the porosity and permeability of the carbonate rock increased by 18.64% and 522.03%, respectively. These findings provide crucial data support for the geological sequestration of CO₂ in saline aquifers by revealing the significant effects of CO₂-water-rock reaction on porosity, permeability, and mineral composition of carbonate rocks.

Key words: CO₂ sequestration, saline aquifer, carbonate rock, water-rock reaction, microscopic analysis of rock