3D Modeling of Thermal Shearing of Fractures at the Asperity Scale

Abstract

ABSTRACT Understanding the shearing behavior of fractures under time-varying thermal conditions (i.e., thermal shearing of fractures) is critical to the long-term viability assessment of geothermal reservoirs and nuclear waste repositories, but it remains uncertain what physical properties of fracture and/or the host rock control thermal shearing. In this study, a three-dimensional thermo-mechanically (TM) coupled numerical simulation of a single granite fracture was carried out to identify key thermal and mechanical parameters that control thermal shearing, namely shear and normal displacements in the fracture and the corresponding stress changes in the host rock. The coupled TM simulation was carried out with the commercial software, FLAC3D, where the geometry of a rough fracture obtained from a laboratory experiment on a tensile-split granite sample was incorporated as an explicit representation of the fracture asperity. It was found that satisfactory agreement between the numerical and laboratory experimental results could not be obtained with the present 3D fracture model where the simple isotropic linear elasticity and linear interface stiffness with the classic Coulomb friction failure criterion were assumed for the host rock and fracture interface, respectively. This suggests that thermal shearing is controlled by physics with greater complexities than these simple assumptions. INTRODUCTION Thermal shearing of rock fractures may occur when an increase in temperature in the host rock generates thermal gradient along with uneven thermal expansion of the rock asperities, leading to shearing of the fractures. Such shearing of fractures can result in potentially large changes in the fracture permeability via changing the connectivity of a fracture network and/or apertures of single fractures. When the fluid and/or fluid chemistry is present, thermal shearing is further complicated by thermal pressurization, changes in fracture friction, dissolution, precipitation, and pressure solution (Hu et al., 2022), which result from temperature gradient. Thus, thermal shearing is a very important process to be considered in a wide range of geoscience applications such as geothermal reservoirs and geological nuclear waste repositories to evaluate the long-term feasibility of these projects. Currently, research on thermal shearing has mainly been laboratory experiments. Fig. 1 shows one of such experiments where a cubic granite sample with a single fracture in its diagonal (Fig. 2a) is subjected to an increase in temperature under mechanically constrained conditions, thereby inducing thermal shearing (Fig. 2b) (Sun et al., 2021). Such experiments offer valuable insights into the physics of thermal shearing via measurements of fracture displacements, acoustic emissions, stress changes, etc. However, such a laboratory experiment can be time-consuming and hence cannot be readily repeated to examine different granite samples with varied thermo-mechanical properties, which is necessary to assess the physics of thermal shearing. In addition, due to the difficulties in controlling some of the aspects in laboratory experiments, it becomes very challenging to provide predictive understanding of thermal shearing if only using the experimental approach alone. One example is that as it is difficult to control the asperity distribution of tensile-split fractures for a laboratory test, it is difficult to investigate how fracture geometry affects the thermal shearing– which is the key for thermal shearing.