Study of the brittleness properties of granites under thermal and mechanical action through a new brittleness evaluation method

Abstract

The brittleness of rock is important for guiding hydraulic fracturing methods for geothermal development. Existing brittleness evaluation methods (BEMs) have been summarized and analyzed. Nevertheless, there is no consensus thus far as to which criteria are the most reliable or appropriate. Energy distribution and plastic deformation are crucial factors in determining the scale of the brittleness from the analysis of the complete stress-strain curve. Moreover, the proposed BEM comprehensively considers the mechanical properties in the pre-peak and post-peak stages while also considering both energy distribution and plastic deformation. The brittleness indices (BIs) of materials with ideal plastic, ideal elastic-plastic, and super brittleness properties are evaluated as 0, 0.5, and 1, respectively, accurately reflecting the range of brittleness properties, from ideal plastic deformation to super brittle fracture. According to the sensitivity analysis of BI, the BIs of granite increase and then decrease for the change of water molecule content and microstructure with increasing temperature. The maximum and minimum BIs are 0.579 at 300 °C and 0.344 at 750 °C, respectively. The BIs of granite decreases substantially from 0.714 to 0.408 as the confining pressure increases from 0 to 40 MPa. These findings indicate that the brittleness of granite is strongly influenced by low confining pressures and high temperatures. Furthermore, the results are compared with those obtained using other widely used BEMs, confirming the superior predictive ability of the new BEM. This attribute is critical to enhancing the evaluation of brittleness in geothermal systems.