## 2 Physical and Mechanical Properties of Rock Mass Under THMC Coupling
### Overview of Multi-Field Coupled Rock-Deformation Studies
This model also includes permeability and porosity changes under acidic conditions, as illustrated in Figure 2. The interaction of temperature, fluid flow, stress, and chemical processes can accelerate fracture propagation and alter the physical and mechanical properties of the rock. To model this thermal hydromechanical damage (THMD)-model process, damage mechanics theory is often employed, using a damage field to quantitatively describe the deterioration of rock materials. This approach introduces damage factors to represent both the initial state of the rock and the impact of various physical fields on its subsequent deformation. The evolution of damage is influenced not only by the multi-physics of the rock but also by its mesoscopic characteristics, such as porosity and internal structure. These factors add a layer of complexity to the study of multi-field coupling, as they significantly affect how a rock mass responds to external forces. Additionally, [PERSON] et al. [103] introduced a fully coupled THMC model to simulate acid fracturing in carbonate geothermal reservoirs with single incongruous fractures, investigating the impacts of fracture length, reservoir temperature, in situ stress, acid concentration, and injection parameters. [PERSON] et al. [124] noted the significant impact of damage on the seepage, temperature, and stress fields in rock porous media, proposing a damage-evolution equation. [PERSON] et al. [125] developed an elastoplastic damage model for hydrate-bearing sediments, incorporating the impact of hydrate cementation and compaction, reflecting volume changes and confining pressure effects. [PERSON] et al. [126] advanced a statistical model capturing strain-softening and hardening behavior in hydrate sediments by merging statistical strength and damage theories. [PERSON] et al. [127; 128] introduced models for hydrate-bearing sediments that factor in hydrate morphology, temperature, and pore pressure, providing insights into damage evolution and hydrate saturation's influence on mechanical properties. Further developments include [PERSON]'s [129] optimization of the Klar A coupling model using energy dissipation theory and [PERSON]'s [130] THMC model application for analyzing Shenhu hydrate reservoir behavior. [PERSON] et al. [131] created a statistical damage model, integrating a three-parameter Weibull distribution and residual strength factors, to study hydrate-bearing sediment stability during dissociation. Finally, [PERSON] et al. [132] developed a THMC model focusing on displacement, saturation, pore pressure, and temperature  
Figure 1: Internal microstructure and chemical damage in oil shale. changes in permafrost mining scenarios, noting that these damage models generally utilize Weibull distribution for damage-statistical relationships in sediments. A mathematical framework was developed based on continuum mechanics to model the complex interplay between these three fields. In this model, each field--thermal, fluid, and mechanical--is affected by the others, creating a dynamic feedback loop that requires iterative methods for solving multi-field coupling equations. For the THM coupling calculations, contemplating the crack propagation, initiation, and intersection through the fracturing of joint elements was utilized to establish a 2D FDEM analysis program. This program was then applied to study the impact of fractures on the THM behavior of rocks [123]. By applying the principles of static equilibrium, the law of mass conservation, and the conservation of energy, researchers derived governing equations for the temperature-flow-stress coupling in porous media. These equations are particularly useful for modeling the behavior of rock masses in deep engineering applications such as tunneling, deep mining, and nuclear waste disposal [134]. The challenges of nuclear waste treatment, geothermal energy exploitation, and hydraulic high-pressure diversion tunnels under extreme geothermal conditions have made the multi-field coupling of rocks a major research focus in recent years.