Modeling of Thermo-Hydro-Mechanical coupled processes in fractured media by Discrete Fracture Network

Non-isothermal fluid flow in fractured media is a complex process because both pressure and temperature variations deform the rock, thus modifying the aperture of the fractures. This in turn affects the pressure diffusion and the heat transfer, creating the so-called Thermo-Hydro-Mechanical (THM) coupling. The complexity is particularly great because the hydraulic process and the thermal process occur at different time scales and the fracture heterogeneity happen at different length scales. This means that strains are different if observed at the small scale. The numerical simulation of such coupled problems is challenging. An additional difficulty resides in relating these models to the large scale response, to get a characterization of the aquifer. In this paper, we simulate the THM coupling by adopting a Discrete Fracture Network (DFN) approach with a Lagrangian method for the transport. We assume that the heat transfer occurs by means of advection within the fractures and diffusion toward the matrix. Thus, we adopt a particle tracking method to model heat transfer along streamlines with steady state flow, and we introduce a temperature decay to mimic the heat loss due to diffusion to the matrix. The methodology has been implemented within the DFN.lab platform, which allows for the generation and management of three-dimensional fracture networks with millions of fractures. The software allows for including heterogeneity within each fracture, which makes it possible to represent the presence of heterogeneity at small scale. At each fracture patch we calculate the variation of the fracture aperture in response to the thermal perturbation by means of simple elasticity laws. This small scale THM behavior is translated into a large scale behavior by observing at the variation of the heat transfer regime due to the mechanical response. In fact, the aperture variation is related to the hydraulic conductivity, which affects the flow regime and thus the heat transfer. Future developments will include the comparison of numerical results with field experimental data of non-isothermal injection into a fractured aquifer. This would allow us to calibrate the model parameters, in order to get a more physically-based model. Another future extension is aimed at including thermal effects in the prediction of fracture growth