Two-dimensional numerical modelling of strategies to avoid thermal stress induced flow channeling in fractured geothermal reservoirs

Abstract

Thermal drawdown induced flow channeling in fractured geothermal reservoirs has been found to be inevitable in previous work. This hypothesis is questioned and found not to be true for all scenarios. We investigate flow in a simple two-fracture network to investigate the basic behavior of thermal drawdown induced flow pattern changes, using a Thermo-Hydro-Mechanical (THM) numerical model. In addition, we propose two operational interventions to mitigate the negative effects of uneven flow on geothermal production. We found that uneven flow is not inevitable, but dependent on several factors, including initial fracture aperture, the actual value of the aperture, and the total injection rate. A small difference in aperture, a higher aperture value and a larger total injection rate are all factors improving the chances for even flow. The operation interventions include variable injection rates, and sequential injection in the fractures before normal production. Both are shown to improve the chances for even flow and better production. Overall, changes in stress and pressure as a result of cooling, are found to have significant effects on flow pattern and production values, underlining the need for advanced numerical models capable of THM modeling for accurate geothermal production analysis in low permeability fracture networks.