Numerical Simulation of Effects of Multilateral Wells on Thermal Characteristics of a Fractured EGS Based on a Thermal-hydraulicmechanical Coupling Model

摘要

A novel enhanced geothermal system (EGS)with multilateral wells is presented to exploitgeothermal energy. The EGS includes one mainwellbore with an inner insulated tubing, severalupper injection lateral wells and lower productionlateral wells. The working fluid is injected into thereservoir through the annulus, and produced fromthe insulated inner tubing. Hence, this EGS canachieve the injection and production through onemain wellbore instead of double wells. A thermalhydraulic-mechanical (THM) coupling modelwith discrete fractures is established to investigatethermal characteristics of multilateral-well EGS.The model is verified by an analytical solution ofa thermal consolidation problem. Based on themodel, the effects of rock mechanical behavior on the heat extraction performance are analyzed. Theinfluences of geometrical parameters of multilateralwells, including well length, number, spacing anddiameter, on the EGS performance are studied.The results indicate that the effective stress of rockinduced by variations of temperature and porepressure can increase the fracture permeabilityand promote the thermal breakthrough. Besides,the longer lateral well and larger well spacing arebeneficial for enhancing the EGS performance.The well number has a complicated effect onthe heat extraction process, which needs to befurther optimized. The influence of the lateral-welldiameter on the EGS performance can be ignored.The results of this study provide good guidance forthe lateral wells design of the multilateral-well EGS.