Numerical study on optimum injection temperature inferred from coupled-hydro-thermal-mechanical behavior of rock

For geothermal energy development, we verify in this paper the existence of an optimum temperature difference for injection, ΔT_c. Injectivity of geothermal wells depends on the temperature difference between the rock and injected fluid, ΔT. If fluid injection into a well is carried out with ΔT larger than ΔT_c, the injectivity of the well will be high, since the low-temperature flow induces sufficient thermal contraction to open fractures and increase permeability. However, such high injectivity is not expected if ΔT is smaller than ΔT_c. We derive an expression for ΔT_c based on a simple elastic model, and the formulation is verified through comparison with the results of 2D-FEM simulations and field tests.