Influence of Specific Heat Capacity Variation with Temperature and Other Important Parameters on the Thermal Reservoir Performance in the Geothermal Doublet System

Doublet system has been widely used in the geothermal system heat extraction, which mainly involved the hot groundwater extraction and geothermal water reinjection into the underground thermal reservoir. Many factors can influence the thermal reservoir performance, such as the production or injection mass flow rate, injection fluid temperature, and lateral well spacing. In this study, taking the specific heat capacity varying with the formation temperature, the rock porosity, and permeability variations into consideration, a three-dimensional thermo-hydraulic coupled numerical model was established to assess the influence of heat capacity variation with formation temperature and other correlation parameters on the sandstone and carbonate thermal reservoir performance over a period of 40 years. Results show that the influence of specific heat capacity variation on the thermal reservoir performance was less than the rock porosity variation and less than the rock permeability variation. On the other hand, the influence of the above three parameter variations is more significant on the sandstone reservoir than the carbonate formation, in which the specific heat capacity variation hardly made any difference for the carbonate thermal reservoir, but for the sandstone reservoir, the specific heat capacity and its temperature variation can yield a 1.3 K decreasing for the average output temperature and a 0.02 × 10⁷ W decreasing for the average energy production rate over the 40 years. Finally, in this case, the temperature and energy production rate from the production well is higher after considering the rock porosity variation no matter for the sandstone or the carbonate. But the result may be opposite after changing the initial and boundary conditions or parameter selection. The influence of rock permeability variation on the thermal reservoir performance was also studied, which can produce a decreasing 10.1 K for the average output temperature and 0.19 × 10⁷ W for the energy production rate over the 40 years.