Application of a geothermal wellbore simulator in evaluating an enhanced geothermal system

Generally, the exploitation of a high-temperature geothermal resource mainly includes heat extraction from the thermal reservoir, geothermal fluid transportation in the wellbore and energy conversion in the power generation system. As the intermediate link, the flow behaviour of geothermal fluid inside the wellbore plays a significant role in ensuring the efficient operation of an enhanced geothermal system (EGS) for the development of hot dry rock geothermal resource. In the present work, to explore the effect of the wellbore flow behaviour on the wellhead performance, a transient two-phase flow wellbore simulator is developed based on the finite element method. In the mathematical model, the fluid pressure, velocity, and enthalpy are selected as the primary variables. Shi's drift velocity model is employed to represent the velocity slip between the liquid and vapour phases, and heat transfer between wellbore and formation is described by an analytical approach. The feasibility and reliability of the presented simulator is validated with an analytical solution, numerical solutions and logging data. The novelty of present work lies in addressing the shortcomings of current studies that use single-phase flow models to estimate the wellbore productivity. The application of deliverability equation can link the wellbore with the heat extraction system of the hot dry rocks, thereby achieving integrated and efficient operation strategy management of EGS reservoir and wellbore. According to the geological conditions of Qiabuqia geothermal field in China, a comprehensive analysis is conducted on the discharge test and sensitive parameters. The results demonstrate that the decrease of fluid pressure is the root cause of flashing of the high-temperature geothermal fluid. Wellhead pressure, bottom-hole temperature and wellbore inner diameter have a significant effect on the flow behaviour of the geothermal fluid. The predicted power generation of Qiabuqia geothermal field is about 4.8 MW.