Coupled THMC analyses for predicting the permeability evolution of single rock fracture depending on pH condition of seepage water

Abstract:

In order to examining the long-term performance of a geological repository of high-level radioactive waste (HLW) for delaying the transport of radionuclides, it is necessary to estimate the long-term permeability evolution of rock fractures. Previously, it has been revealed that occurrence of geochemical reactions between mineral and seepage water (e.g., pressure dissolution, free-face dissolution/precipitation) depending on stress/temperature conditions change the permeability of rock fractures with time. Moreover, the geochemical reactions also depend on chemical condition of groundwater (e.g. pH condition), and therefore, should be influenced by the inflow of the alkaline cement solution from an artificial barrier during the disposal period of HLW. However, existing numerical simulator cannot well evaluate the effect of pH condition of groundwater on time-dependent permeability alteration of rock fractures enhanced by geochemical reactions.

In this work, a coupled thermal-hydraulic-mechanical-chemical (THMC) simulator incorporating the mineral kinetic dissolution/precipitation depending on pH condition has been proposed. Subsequently, the presented simulator was utilized to replicate the measurements of evolving permeability and the effluent solute concentrations obtained from flow-through experiments on a single rock fracture in granite under various pH conditions of permeable water, under stressed and temperature-elevated conditions. The predicted permeability alterations can trace the experimental data well and exactly expressed the observed difference in the reduction rate depending on the pH condition. The measured solute concentrations were agreement with the predicted results relatively well. Overall, validation of proposed simulator was confirmed.