Geothermal reservoirs often contain saline brine, while many studies assumes geo-fluid as pure water to lowsalinity brine. Such simplifications neglect important salinity-dependent thermodynamics and transport processes, which can lead to inaccurate prediction of geothermal recovery. This work presents a new geothermal reservoir simulator, Geo-energY Multiphysics Simulator (GYMS). The simulator enables fully implicit simulation of non-isothermal multiphase flow with brine at different salinity levels. It incorporates phase transitions and salt precipitation, and is implemented via automatic differentiation (AD) method that provides accurate and efficient Jacobian calculation.

Through GYMS, the impact of salinity on geothermal recovery is investigated. Comparing to the low-salinity hot sedimentary aquifer (HSA) scenario, our simulations show that the high-salinity HSA exhibits salt precipitation in near-perforation zone, which reduces permeability, limits boiling, and significantly lowers the geothermal energy recovery by 74 %. Further, the simulations about Enhanced Geothermal System (EGS) demonstrate that the fractures dominate the spatial–temporal evolution of pressure and temperature. In this case the pressure-driven boiling and salt precipitation mainly occur in the matrix around the production well. Nevertheless, due to the continuous migration of cold low-salinity injected brine, there is no salt precipitation in the fractures. Hence, the permeability reduction due to salt precipitation in the EGS occurs in the matrix near the production well. This behavior leads to less impact on the geothermal energy recovery where produced energy reduction is around 9% compared to low-salinity scenario. Therefore, it is critical to carefully account salinity into geothermal reservoir simulation, and establish GYMS as a robust modeling framework for such tasks.