Effect of the fracture aperture distribution on the heat extraction performance from the fractured geothermal systems

Fractures are main flow paths for heat extraction from fractured geothermal systems. The process of injecting cold water to extract hot water from a fractured reservoir results in thermal and poroelastic stresses in the rock matrix. Therefore, these thermo-hydro-mechanical (THM) mechanisms govern the efficiency of an enhanced geothermal system (EGS) operation. Fractures’ aperture is a controlling factor for the heat extraction efficiency. Due to the lack of field and experimental works, a constant aperture is considered for all fractures in previous works. However, insights from outcrop or wellbore shows that there is a possibility of some relationships between fracture length and its aperture. To shed light on the effect of this relationship on the heat extraction efficiency, numerical simulations are conducted on a fully coupled THM manner in which the fracture aperture is controlled by the thermo-poroelastic stress. 100 fractures from a Discrete Fracture Network (DFN) are taken as a basis during simulations. For the sake of the computational efficiency, a two‐dimensional planar model (1000 m × 600 m) is selected. Three types of relationships between fracture length and fracture aperture as constant aperture, linear and power law relationships are considered here. To have a better comparison between different cases, a constant value is used for the summation of the ''fracture length multiplied by fracture aperture'' for these three cases. Fluid and rock properties are selected from the literature in a way to be a good representative of actual cases. Furthermore, fluid properties dependency on pressure and temperature of the system is implemented through the well-known correlations. Constant pressure is assumed as the boundary condition for the injection and production wells. All fractures within the domain are regarded as internal boundaries, implicitly considering the mass and energy exchange between porous media and fractures. We have constrained the displacement in all normal directions. All boundaries of the modeled domain are no flow for both fluid and heat transmission. The local thermal non-equilibrium theory is adopted to simulate the heat exchange between the rock matrix and the flowing fluid. For rock matrix, the energy transfer process is mainly dominated by the heat conduction and the heat exchange between pore fluid. Simulation results reveals that fracture aperture dependency on fracture length is an important factor for heat extraction efficiency from the fractured geothermal systems and requires future attention to this missing factor in the literature. Considering constant aperture results in the later thermal breakthrough which would affect the techno-economic analysis in comparison to the real field data. Possibility of a linear relationship would eventuate the lowest performance between the examined cases.