The dissolution mechanism of calcite and its impact on CO2 sequestration in deep-water sandstone during CO2 flooding: A case study in the Chang 7 member, Ordos Basin, China

Robust and innovative strategies to reduce carbon emissions to address the growing threat of global climate change. Deep-water tight sandstone, with calcite as its primary cementing material, can be an essential reservoir for CO₂ sequestration. However, the microscopic dissolution mechanism of calcite is still poorly understood and can significantly impact the effectiveness of geological CO₂ storage. We investigated the dissolution behaviours of calcite in deep-water tight sandstone and its volume changes on CO₂ during CO₂ flooding. We used a novel 3D, micrometric-scale reactive-transport model, where the distribution and geometric characteristics of calcite were informed by section casting and cathode luminescence images. We found that the calcite particles adjacent to each other in deep-water tight sandstone form numerous micrometer-scale calcite aggregates. During CO₂ flooding, the dissolution rates of calcite particles vary since solute migration limits the overall dissolution rate. Calcite aggregates react as single particles, with the external surface continuously dissolving and the internal surface maintaining balance. The dissolution products of the calcite aggregates accumulate in the formation water, inhibiting the dissolution of the surrounding calcite and thereby reducing the overall dissolution rate of the deep-water tight sandstone calcite. Large volumes of calcite initially dissolve rapidly, then gradually slow down. This affects the CO₂ sequestration efficiency and migration patterns. These findings provide vital insights into the physical properties and geomechanical integrity during CO₂ storage in deep-water tight sandstone and have long-term storage security and environmental protection implications.