Coupled Thermal-Mechanical Behavior of Slate and its Anisotropic Failure Criterion
Authors: Weng M. C. , Lin S. S. , Lee C. S. , Pham M. T. , Le H. K. 
Published: August 2024

ABSTRACT:
This study investigates the thermal–mechanical behavior of slates in geothermal reservoirs and establishes an anisotropic thermal–mechanical coupling failure criterion. Slate samples obtained from a geothermal site were subjected to a series of direct-shear and triaxial-compression tests at various confining pressures and temperatures. The results showed that the orientation angle within a specific range caused shear sliding failure of the slate, primarily due to foliation strength; beyond this range, intrinsic rock properties led to different failure modes. Elevated temperatures significantly diminished the intrinsic strength of the slate in a specific range of orientation angles. These findings provided a foundation for the development of an anisotropic thermal–mechanical coupling failure criterion. The validity of the proposed criterion, which incorporates both the orientation angle and thermal degradation effects, was demonstrated through experimental results. This study enhances the understanding of the thermal– mechanical behavior of these materials in geothermal reservoirs.

1. INTRODUCTION
Geothermal energy is widely regarded as a sustainable and environmentally friendly energy source, particularly in regions with high tectonic activity. Taiwan, situated at the convergence of two active tectonic plates, has abundant geothermal energy reserves. However, unlike many geothermal fields globally situated in igneous- and sedimentary-rock formations, Taiwan's prospective geothermal resources are predominantly found in areas with slate, which is a type of metamorphic rock (Song and Lu 2018; Lu et al. 2020; Weng et al. 2023). Slate differs from igneous and sedimentary rocks because it exhibits significant anisotropy and heterogeneity. These distinctive characteristics have a substantial influence on the thermal–mechanical behavior and propagation of fractures under hydraulic stimulation within the slate geothermal reservoir.
The presence of a sheet-like planar structure, known as foliation, in slate significantly influences the mechanical characteristics of the rock and the manner in which fractures propagate within it. Researchers have conducted a series of tests, including tensile, uniaxial compression, and triaxial-compression tests, to evaluate the mechanical characteristics of slate (Niandou et al. 1997; Tien and Kuo 2001; Bagheripour et al. 2011; Debecker and Vervoort 2009; McLamore and Gray 1967; Nasseri et al. 2003; Saeidi et al. 2013; Stoeckhert et al. 2015; Tan et al. 2015; Vervoort et al. 2014; Weng et al. 2020; Weng et al. 2022). The collective findings from these studies reveal that the strength and deformation properties of slate are strongly dependent on the orientation angle, which represents the angle between the foliation and major loading direction. Although the anisotropic behavior of slate has been extensively studied, the impact of high temperatures on its mechanical properties remains uncertain, and an adequate thermal–mechanical coupling failure criterion for slate is required.