Analysis of flow and thermal breakthrough in leaky downhole coaxial open loop geothermal system

Abstract

Geothermal energy is crucial to energy transformation; however, the difficulty of reinjection and the low efficiency of heat extraction need to be solved urgently. Coaxial tube heat exchanger has no difficulty of reinjection, but the averaged heat output per unit length is very limited. A new type of leaky downhole coaxial open loop geothermal system using the technique of “no water withdrawn but heat only” is proposed, which makes full use of the convective heat transfer of the aquifer and the leakage characteristics of the aquitard. From the test, heat output is up to 930 kW, which is 3.5 times that of the traditional coaxial casing, which is a promising geothermal heat extraction technique. In this study, a coupled analytical and numerical simulation model of wellhole aquifer flow and heat transfer was established to study the heat extraction performance, as well as the flow and thermal breakthrough of the system. Through the analysis of calculation results and the introduction of critical Rayleigh number, which represents the degree of convection and heat conduction, the optimal correlation between the thickness of the aquitard, injection temperature, flow rate, and permeability ratio is established. Regarding the base case thickness of the aquitard, a value of 19.51 m was suggested, and Kratio ≥ 20 was suggested regarding the reservoir structure, which provides quantitative guidance and is significant for system optimization design.