The role of water in the change of the fracture toughness during the ScCO2 interactions with shale samples

Fracture toughness is a fundamental material property that quantifies the critical loading stress necessary to overcome the resistance to mechanical fracture propagation. Herein, exposure of minerals to certain fluids can cause alterations in the fracture toughness, for example, during underground CO2 storage, influencing the integrity and sealing performance of the reservoirs. These changes would require an extensive examination of various properties of the layers, particularly those related to mechanical ones like fracture toughness. However, the role of water in changing the fracture toughness in different shale samples during the ScCO2 interactions is still unknown. In this study, we collected three samples with different lithologies and compared the fracture toughness under three different states (before the interactions, under the interactions without water, and the interactions with water) to further quantify the role of water in the changes of the fracture toughness. The Mann-Whitney U test shows that the interactions with ScCO2 could change the fracture toughness and the role of water involvement in the interactions could further change the fracture toughness. In addition, the role of water in the changes of the same sample parallel and perpendicular to the bedding lines is also different. In the directions parallel to the bedding, the fracture toughness of samples 2 (carbonate-rich felsic shale) and 3 (clayey carbonate-rich shale) increased by 39.2 % and 10.8 %, respectively. Conversely, the fracture toughness of sample 1 (clayey felsic shale) decreased by 21.3 %. In the directions perpendicular to the bedding, the fracture toughness of samples 1, 2 and 3 increased by 23 %, 55 %, and 8.58 %, respectively. Considering the samples that have been exposed to ScCO2 with water, the fracture toughness of Sample 1 decreased by 5.8 %, while the fracture toughness of Sample 2 and Sample 3 increased by 65.8 % and 11.4 % respectively. This study shows that when considering the storage site for the CO2, we should both consider the role of the lithologies and water in the interactions. •The role of the lithologies and water in the interactions should be considered when considering the storage site for the CO2.•The role of water in the changes of the same sample parallel and perpendicular to the bedding lines is different.•The mineral changes and microstructure alterations can jointly impact the fracture toughness of the samples.