Modeling thermal flux through a normal fault damage zone with enhanced permeability: implications for geothermal energy

Geothermal energy supplies less than 2% of the renewable energy produced in the US. Geothermal resources are limited because traditional systems require natural fluids, high permeability, and high heat flow, or “hot wet rock” reservoirs. However, most heat sources in the US consist of deeper hot dry rock. Fault damage zones, volumes of fractured rock adjacent to faults, are associated with higher geothermal gradients but lack sufficient permeability. Enhanced Geothermal Systems (EGS) extract energy from hot dry rock reservoirs, reducing the geographic limitations on geothermal energy in the US. EGS use directional drilling and hydraulic fracturing to create a reservoir of high permeability. Cold fluid is pumped into the reservoir by an injection well and hot fluid is drawn out by an extraction well to generate energy. We seek to quantify the degree of hydraulic fracturing necessary to make a normal fault damage zone viable for geothermal energy generation.

We investigate fluid flow and thermal flux in a simplified EGS model by establishing a simplified geometry consisting of a hydraulically fractured cylindrical channel. The model consists of nested cylinders with radially increasing thermal conductivity and radially decreasing permeability. We use MATLAB to numerically solve the coupled partial differential equations governing thermal and fluid flux. We assume Darcy’s law governs the fluid flux and the advection-diffusion equation governs the thermal flux. We modify the permeability and ranges of fracture radii in our equations to determine how those parameters affect the total thermal flux from the extraction well.