Evaluating the importance of different coupled thermal, hydraulic, mechanical and chemical process simulations during fluid flow experiments in fractured novaculite and fractured granite

Abstract

Fluid migration in the subsurface has the potential to induce changes in fluid pressure distribution, temperature distribution, mechanical stresses and the chemistry of both the fluid and the natural geological material it is flowing through. In many situations, the change in all of these processes gives a coupled response, in that one process feeds back to another. When trying to understand fluid flow through naturally and artificially fractured systems, it is important to be able to identify the relative importance of the processes occurring and the degree of interactions between them. Modelling of such highly nonlinear coupled flow is complex. Current and predicted computational ability is not able to simulate discretely all the known and physically described processes operating. One approach to coping with this complexity is to identify the relative importance and impact of relevant processes, dependent on the application of interest. Addressing such complexity can be particularly important when the characteristics of natural and disturbed geological materials are being evaluated in the context of disposal of radioactive waste or other geo-engineering systems where an understanding of the long-term evolution is required. Based on a series of coupled (THMC—Thermal, Hydraulic, Mechanical and Chemical) experimental investigations on the flow of fluid through fractured novaculite and granite crystalline rock samples, several couplings are examined where there is both a significant kinetic chemical control as well as mechanical and temperature control on the fluid flow behaviour. These interactions can be shown both in the literature and experimentally to have a significant effect on the rate of fluid flow through fractures. A new discrete numerical approach and a new homogenous approach are used to model the experimental results of coupled flow through fractures. The results of these modelling approaches are benchmarked both against one another and against the experimental results, and then the processes included in the approaches are ranked in order of impact.