A Review on the Influence of CO2/Shale Interaction on Shale Properties: Implications of CCS in Shales

Carbon capture and storage (CCS) is a developed technology to minimize CO(2)emissions and reduce global climate change. Currently, shale gas formations are considered as a suitable target for CO(2)sequestration projects predominantly due to their wide availability. Compared to conventional geological formations including saline aquifers and coal seams, depleted shale formations provide larger storage potential due to the high adsorption capacity of CO(2)compared to methane in the shale formation. However, the injected CO(2)causes possible geochemical interactions with the shale formation during storage applications and CO(2)enhanced shale gas recovery (ESGR) processes. The CO2/shale interaction is a key factor for the efficiency of CO(2)storage in shale formations, as it can significantly alter the shale properties. The formation of carbonic acid from CO(2)dissolution is the main cause for the alterations in the physical, chemical and mechanical properties of the shale, which in return affects the storage capacity, pore properties, and fluid transport. Therefore, in this paper, the effect of CO(2)exposure on shale properties is comprehensively reviewed, to gain an in-depth understanding of the impact of CO2/shale interaction on shale properties. This paper reviews the current knowledge of the CO2/shale interactions and describes the results achieved to date. The pore structure is one of the most affected properties by CO2/shale interactions; several scholars indicated that the differences in mineral composition for shales would result in wide variations in pore structure system. A noticeable reduction in specific surface area of shales was observed after CO(2)treatment, which in the long-term could decrease CO(2)adsorption capacity, affecting the CO(2)storage efficiency. Other factors including shale sedimentary, pressure and temperature can also alter the pore system and decrease the shale caprock seal efficiency. Similarly, the alteration in shales' surface chemistry and functional species after CO(2)treatment may increase the adsorption capacity of CO2, impacting the overall storage potential in shales. Furthermore, the injection of CO(2)into shales may also influence the wetting behavior. Surface wettability is mainly affected by the presented minerals in shale, and less affected by brine salinity, temperature, organic content, and thermal maturity. Mainly, shales have strong water-wetting behavior in the presence of hydrocarbons, however, the alteration in shale's wettability towards CO2-wet will significantly minimize CO(2)storage capacities, and affect the sealing efficiency of caprock. The CO2/shale interactions were also found to cause noticeable degradation in shales' mechanical properties. CO(2)injection can weaken shale, decrease its brittleness and increases its plasticity and toughness. Various reductions in tri-axial compressive strength, tensile strength, and the elastic modulus of shales were observed after CO(2)injection, due to the dissolution effect and adsorption strain within the pores. Based on this review, we conclude that CO2/shale interaction is a significant factor for the efficiency of CCS. However, due to the heterogeneity of shales, further studies are needed to include various shale formations and identify how different shales' mineralogy could affect the CO(2)storage capacity in the long-term.