Microstructure changes as a response to CO2 storage in sedimentary rocks: Recent developments and future challenges

Due to the complexity of sedimentary rocks, especially the fine-grained and highly-heterogeneous nature of some argillaceous caprocks in the subsurface, it is challenging to directly characterise the macroscale changes during CO 2 storage. The investigation of microstructural changes in CO 2-fluid-rock reactions helps provide a fundamental understanding of macroscopic properties as the microstructure constrains their hydraulic, mechanical and geochemical behaviour. Because of distinct petrophysical properties, the effect of geochemical reaction on caprocks and reservoir rocks is dramatically different. In order to provide a comprehensive knowledge of microstructural changes induced by geochemical reactions and facilitate the application to large scale carbon storage research, this review summarizes representative laboratory measurements conducted on typical caprocks (e.g. mudstone and evaporite) and reservoir rocks (e.g. sandstone and carbonates). Key findings include: (1) Water can generally facilitate CO 2-rock reactions. The effect of dissolution and precipitation on the pore structure largely depends on the inherent heterogeneity of rock (including primary structure, mineralogy, thermal maturity), and the environmental conditions (temperature, pressure, fluid). (2) Adsorption-induced swelling of clay minerals and organic matter in mudstone may increase or reduce the pore space, which is primarily controlled by the treatment pressure of CO 2 within the same caprock system. (3) Fines migration and the re-precipitation of minerals are the major factors causing pore throat blockage of sandstone during CO 2 flooding. (4) Highly conductive wormholes formed in carbonates during CO 2 percolation can significantly enhance fluid injectivity, whereas a long-term thermodynamic equilibrium in the subsurface tends to counteract the increase in porosity due to the healing mechanism of mineral precipitation. Further, how to link laboratory work with modelling to realise large-scale and temporal-scale prediction are summarised. Finally, promising developments and research directions are proposed.