Brittleness index evaluation of gas-bearing sandstone under triaxial compression conditions

Abstract

Brittleness is a critical property in gas-bearing rock excavation and reservoir fracturing. This paper analyzes the three types of brittleness indices in detail, proposes new indices, and conducts relevant conventional triaxial tests for analysis. A stress–strain curve-based brittleness index would be appropriate for the evaluation of brittleness if the parameters represented unique and certain stress–strain curves. Therefore, a new brittleness index is proposed. It consists of two components: the stress variation index and the strain variation index, which reflects the strain ratio of the stress drop and the relationship between elastic and post-peak strains, combining the stress variation and strain variation in the pre- and post-peak phases. Triaxial compression tests of sandstone under different confining pressures and gas pressures were conducted, and specific stress and strain parameters were collected. Based on the comparison and analysis of various brittleness indices, it is found that the newly proposed brittleness index can be a good measure of the brittle evolution of sandstones under different confining pressures and gas pressures, and it can provide a more accurate estimate of rock brittleness than the existing indices. The index is also used to further investigate the relationship between rock brittleness and confining and gas pressures. This has important implications for the assessment of rock brittleness in practical engineering.