A numerical simulator entitled TOUGH-UDEC is introduced for the analysis of coupled thermal-hydraulic-mechanical processes in fractured porous media. Two existing well-established codes, TOUGH2 and UDEC, are coupled to model multiphase fluid flows, heat transfers, and discontinuous deformations in fractured porous media by means of discrete fracture representation. TOUGH2 is widely used for the modeling of heat transfers and multiphase multicomponent fluid flows, and UDEC is a well-known distinct element code for rock mechanics. The two codes are solved sequentially, with coupling parameters passed to each equation at specific intervals. After solving thermal-hydraulic equations within the TOUGH2 code, pressure and temperature information is imported into the UDEC model. After solving the mechanical equation within the UDEC code the calculated fracture apertures are converted to the equivalent permeability and porosity values for a TOUGH2 flow analysis. The solution is calculated by iteratively following an explicit sequence for numerical efficiency. Verifications are presented to demonstrate the capabilities of the coupled TOUGH-UDEC simulator. Three application examples of (1) shear dilation due to increased pore pressure, (2) thermal stress and (3) CO 2 injection, show that the new simulator can be an effective tool for geoengineering applications involving shear activation of fractures and faults.

This study presents comprehensive coupled hydro-mechanical numerical modeling of the first and second hydraulic stimulations at the Pohang enhanced geothermal system (EGS) site in order to improve the understanding on the key stimulation mechanisms in the fractured reservoir. Two models for PX-2 and PX-1 wells were developed including fracture zones of different orientations and permeability. Shear dilation and frictional plastic strain-softening were implemented in a non-linear stress dependent fracture model. A history matching of the wellhead pressure curves was carried out in the early days of each stimulation. The PX-2 model with a fracture jacking mechanism achieved the reproduction of highly reversible non-linear aperture changes observed in situ. The PX-1 model successfully simulated a wellhead pressure peak and drop associated with shear slip and dilation by applying frictional plastic strain softening. Simulated critical wellhead pressure for shear slip near the PX-1 well was greater than the prediction by a simple slip potential analysis due to local stress changes by the poroelastic effect. The numerical modeling confirmed that the combination of shear dilation and jacking was adequate to capture the pressure evolution during the increasing step-rate test at PX-1. Besides, spatio-temporal changes in pressure, total stress, and permeability evaluated in the fracture zones greatly enhanced the understanding on the coupled behavior caused by the hydraulic stimulations. The possibility of zones with lower permeability away from the PX-1 well was suggested by an alternative model. The current numerical study demonstrates that the key hydro-mechanical processes of shear slip and dilation and hydraulic jacking observed in the fractured reservoir can be successfully reproduced.