Thermal Response During Cold Water Reinjection in Naturally Fractured Geothermal Reservoirs

Abstract

Results are presented for a theoretical study of the heat transfer processes incolved when separated cold water is reinjected into naturally fractured hot geothermal reservoir rock. This study considers the case when cold water in injected into a hot system initially placed at a given uniform temperature. The fractured system is modeled as two interconnected homogeneous systems, one for the matrix and the one for the fractures. Heat and mass balances are established for the interconnected system when the cold injected fluid travels through the fracture system in close contact with a hot matric system modeled as sphere, which act as a uniformly distributed heat source term in the fractured system. Solutions to this problem are presented for two cases: one in which instantaneous thermal equilibrium takes place between the injected cold fluid and the boot rock, and the second one considers a nonthermal equilibrium situation for which solutions are derived for the cases when heat transfer occur under pseudo-steady state and transient conditions: solutions for these cases are compared. Solutions presented also consider heat interchange with underlying impermeable formation. Type curve are presented which shoe the thermal front rate of advance with dimensionless injection time: also a sensitivity analysis was performed on the effect of several factors on the thermal front rate of advance, such as the intensity of rock fracture interaction, ratio of thermal energy contained within the fractures to total system thermal energy, porosity, as well as Peclet and Biot numbers.