Hot Dry Rocks (HDRs), with very low permeability and insignificant fluid porosity, are the most common geothermal resource type all over the world. Enhanced Geothermal Systems (EGSs) are the accepted method for extracting heat from HDRs. This study develops a two-dimensional Finite Element Method (FEM) Thermo-Hydro-Mechanical (THM) program to evaluate the output fluid temperature and EGS reservoir response. A discrete fracture inside a deformable rock matrix is the base model. Conservation of energy, mass and momentum is established, and a mechanical constitutive law, Fourier's law, and the cubic law for fluid transport are stipulated. The equations are solved for displacement, temperature, and fluid domain, respectively, inside the rock matrix and the discrete fracture. Advection-diffusion and diffusion heat transfer processes are assumed in the fluid and rock matrix, respectively, and in addition to a deformable fracture aperture, constant and variable convective heat transfer coefficient (h) and velocity are considered in different scenarios in the coupled THM process. The simulation overestimates the output fluid temperature over time if h is considered to be constant in the model. Overestimation of the output fluid temperature is higher in early times of heat production, especially if the fracture aperture and velocity changes are ignored in the THM model.
A massive amount of heat energy, known as geothermal energy, is slowly transmitted from the earth’s depths to the surface. Geothermal energy is not strictly renewable in the engineering time scale (25-50 years); however, considering the useful heat in the upper 10 km of the earth’s crust and slow conductive replenishment from depth, it can be viewed as “forever and renewable”. During 2008-2018, rapid growth of geothermal energy use in countries such as the United States, Philippines, Indonesia, Turkey, New Zealand, and Iceland (Lu 2018), was based on dry and wet steam reservoir exploitation.
Until recently, factors including the existence of high-temperature sources at a drillable depth (<2-3 km), the presence of porous and permeable rock, and the availability of sufficient volumes of hot fluid have been the three intertwined and necessary basic parameters of commercializable geothermal power resources. Although high thermal gradient anomalies are widely found in many Hot Dry Rock (HDR) regions, great depth, the absence of sufficient natural fluids, and low permeability rock present challenges (Duchane and Brown 2002) for the vast majority of the Earth’s land area. Technology developments have led to a relatively new concept known as an Enhanced Geothermal System (EGS), where the rock mass properties are usually enhanced by double well hydraulic stimulation (hydrofracture or hydroshearing) and a circulating fluid provided to access the heat, or the heat exchange volume and area are enhanced by longer multiple wellbores. Although no full-scale commercial projects greater than 10 MW are yet built, the EGS approach may be able to overcome challenges presented in HDR regions.