Contacting asperities play an important role in stabilizing the existing fracture networks in the geothermal system. In such a system, the rapid heat exchange occurs when cold water is injected into existing fractures with high reservoir temperatures. The deformation mechanisms of the porous asperities in contact under the coupled thermal-mechanical loading are complex and critical to engineering operations. The present study models the deformation process using a probabilistic damage model (PDM) incorporating the porosity effect. The results revealed the coupled deformation mechanisms, which are the contact induced damage and the radial cracking of the asperities. The interaction between two processes was characterized based on both of the analytical and computational models. It was also found that the porosity distribution affects two mechanisms and their interaction significantly. The field data were input to predict the deformations of fracture apertures during the normal range of operation conditions. A critical overburden pressure was discovered, which draws the line between the increasing or decreasing trend of porosity effect as the temperature increases.