[A-HW25] Near Surface Investigation and Modeling for Groundwater Resources Assessment and Conservation

[AHW25-P08] Development of a Coupled Hydro-Mechanical Model for Fracture Aperture and Permeability Evolution in Dual-Porosity Media.

*Kai-Lin Hsu¹, Shih-Jung Wang^1,2 (1.Graduate Institute of Applied Geology, National Central University, Taoyuan, Taiwan, 2.Department of Earth Sciences, National Central University, Taoyuan, Taiwan)

Keywords:Fracture aperture, Pressure solution, Free surface dissolution, Permeability, Coupled hydro-mechanical model, Equivalent hydraulic conductivity

For the design of high-level radioactive waste (HLW) disposal sites, most countries currently adopt the deep geological disposal concept to prevent the leakage of radioactive nuclides, which could pose risks to both biological systems and the environment. However, fractures in the bedrock caused by stress are main pathways for radionuclide migration. These fractures may be influenced by pressure solution and free surface dissolution, leading to changes in fracture aperture and permeability, thereby affecting radionuclide transport characteristics. This study employs the fracture geometry simulation method proposed by Liu et al. (2006) and utilizes the multiphysics coupling analysis software COMSOL Multiphysics® to establish a coupled two-dimensional hydro-mechanical model in dual-porosity media. Numerical simulation results for hydro-mechanical interactions are imported into the commercial mathematical software Matlab to calculate the geometric changes in fractures under stress. The results of fracture evolution are then iteratively fed back into the numerical model to simulate the temporal changes in fracture aperture under the combined effects of pressure solution and free surface dissolution. The equivalent hydraulic conductivity K_eq is calculated to evaluate the fluid flow characteristics of the rock mass. In the simulation conducted over 1,500 hours with hourly intervals under the setting of models, the results indicate that: 1) When considering only the effects of pressure solution, the equivalent hydraulic conductivity decreases by approximately 11%. 2) When considering only free surface dissolution, the continuous widening of the fracture aperture during the dissolution eliminates contacting asperity, forming fluid channels that result in an increase in equivalent hydraulic conductivity by approximately seven orders of magnitude. 3) When both effects are considered simultaneously, free surface dissolution is more dominant than pressure solution, the fracture aperture continues to widen and the equivalent hydraulic conductivity increases by approximately 47%. Future research can involve conducting sensitivity analyses of rock masses under different environmental conditions to further understand the underlying mechanisms. This study aims to develop geometric models of fracture evolution and hopes to contribute to the safety assessment of in-situ applications.