CO₂ Adsorption in Shales under CO₂ Dissolution-Induced Acidic Conditions: Experimental and Molecular Insights for Caprock Integrity

Effective long-term CO2 storage in deep saline aquifers is strongly influenced by the adsorption behavior of shale caprock. During CO2 sequestration, the interaction between CO2 and aquifer brine results in the generation of carbonic acid. This process reduces the pH of the system and may influence the geochemical characteristics of the caprock. Despite extensive research on CO2 adsorption onto shale, the impact of acidic conditions within shale–CO2–brine systems has received limited attention. This study experimentally investigates the adsorption of CO2 on three different shales under acidic conditions using a volumetric method. To further validate the experimental findings and gain deeper insight into the molecular mechanisms governing CO2 adsorption, molecular dynamics simulation was employed. The simulation utilized illite as the representative clay mineral due to its substantial presence in shale formations. The results showed that CO2 adsorption on shale increased under acidic conditions, irrespective of the shale composition. Specifically, the level of CO2 adsorption increases from 2.968 to 3.475 mmol/g in Shale-1, from 3.325 to 4.650 mmol/g in Shale-2, and from 3.573 to 4.852 mmol/g in Shale-3 at 1100 psia and 313 K. The observed increase in the level of CO2 adsorption suggests that the shale caprock’s ability to retain CO2 decreases under acidic conditions. This reduction in the trapping capacity heightens the risk of CO2 leakage. Additionally, excessive CO2 adsorption can induce shale swelling, elevating pore pressure and increasing the likelihood of fracture, which could further compromise caprock integrity. The results of the study contribute to a better understanding of the mechanisms of CO2 trapping in deep saline aquifers.