Relationship between pore structure and mechanical properties of bituminous coal under sub-critical and super-critical CO2 treatment

Abstract

Geological sequestration of CO2 in unminable coal seams is one of the effective ways to reduce greenhouse gas emissions and increase coalbed methane production. However, changes in the pore structure after CO2 injection may affect the mechanical properties of the coal reservoir, posing a challenge to the long-term safety of CO2 sequestration. Therefore, a proper understanding of the relationship between pore structure and mechanical properties of coal under CO2 treatment is essential. In this paper, Low-field nuclear magnetic resonance tests were carried out on bituminous coal after sub/super-critical CO2 (Sub-CO2/Sc-CO2) treatment, and uniaxial compression tests were used to analyze the mechanical properties and energy evolution of coal samples. The results demonstrate that the CO2 treatment increased the porosity and proportion of macropores, and the effect of Sc-CO2 treatment on the pore structure was more significant. Compared to the Sub-CO2 treated sample, the uniaxial compressive strength and elastic modulus decreased more in the Sc-CO2 treated sample, which may be related to the extraction capacity of Sc-CO2. Analysis of the energy evolution shows that more energy was converted into dissipated energy in the loading process of Sc-CO2 treated coal samples, which was used to form, expand and connect fractures. Moreover, the correlation between the rate of change in porosity and the rate of change in uniaxial compressive strength was 0.9579, indicating that changes in pore structure were the main cause of the change in the mechanical properties of coal. A conceptual model was proposed to explain the mechanism of the effect of the pore structure on the mechanical properties, and pressure selection during CO2 sequestration was discussed. Therefore, the results of this study are expected to provide references for the long-term safety assessment and injection pressure selection of CO2 sequestration.