Determination of Production Potentials of Geothermal Resources for Rapid Exploration and Development Decisions

Abstract This paper presents a semi-analytical wellbore heat flow model coupled with a formation heat conduction model for rapidly determining output potentials of thermal energy from the open-loop and closed-loop geothermal systems. Energy conservation equations are used to simulate thermal flow dynamics of fluids inside the wellbore, while heat diffusion equations govern the transient heat transfer in the formation. A semi-analytical technique is used to solve these coupled equations iteratively. Model validation was done by comparing the semi-analytical solution with numerical simulation results on a 3D wellbore model for a coaxial closed-loop geothermal system. Good agreement of the temperature profile along the wellbore was observed between the semi-analytical and numerical results. Production potentials of thermal energy depend on the geothermal systems selected to extract the heat from the reservoir. Closed-loop systems rely on the circulation of a working fluid to absorb and transport heat to the surface. Their heat extract efficiency depends on the mass flow rate and initial injection temperature of the working fluid. The higher flow rate leads to a lower fluid temperature but a higher thermal energy output at the wellhead. As the injection fluid temperature increases, the energy output decreases but the output fluid temperature increases. Open-loop systems produce the hot reservoir fluid directly to the surface, in which an increase in the flow rate would increase the fluid temperature and thermal energy production at the wellhead. As the production continues, thermal energy output decreases faster at the beginning and reaches a plateau in the long run. Compared to numerical models, the semi-analytical requires minimal inputs to generate the compatible outputs with higher computational efficiency, which is preferred in many applications, in particular for energy resource screen analyses of new geothermal prospects or repurposed oil and gas wells.