Effect of supercritical CO2 on alteration of tensile strength of shale

Abstract

In the process of CO2 geological storage, the tensile strength is an important parameter for evaluating the rock fracture pressure and crack propagation after CO2 injection. The evolution of tensile strength, splitting modulus, and failure characteristics of dry and wet shales under supercritical CO2 (60 ℃, 20 MPa) at different times (10, 20, 30 d) was conducted by the Brazilian splitting test. The results show that: ① Supercritical CO2 exposure weakens the tensile strength and splitting modulus of shale, the weakening increases with the increase of treatment time, and the weakening of wet shale is greater than that of dry shale. ② The raw shale samples only produce linear cracks with single path after splitting failure, while the shale samples treated by supercritical CO2 have diversified crack shapes and produce multiple sub-cracks. ③ The nuclear magnetic resonance (NMR) analysis shows that the changes in porosity and shale tensile properties of shale have a good consistency. As the increase of supercritical CO2 treatment time, the shale porosity gradually increases, resulting in lower shale fracture pressure, and more obvious weakening of shale tensile strength and splitting modulus. The change in pore size distribution also has a significant effect on the tensile strength of shale. The tensile strength and splitting modulus of shale are positively correlated with the decrease of micropore porosity, and negatively correlated with the increase of mesopore and macropore porosity. The increase of mesopore and macropore porosity plays a leading role in weakening the tensile strength of shale. In the process of long-term CO2 storage, the decrease of tensile strength may lead to the damage of shale caprock more easily under the action of fluid pressure, and increase the risk of CO2 leakage. Therefore, it is necessary to consider the change of shale tensile strength for establishing failure criteria and to control the formation pressure for ensuring the safety of CO2 storage.

Keywords

• shale
• supercritical co2
• co2 storage
• tensile strength
• porosity