Sustainable heat farming: Modeling extraction and recovery in discretely fractured geothermal reservoirs

Although many natural hydrothermal geothermal systems have been shown to be productive over long periods of time, limited field testing of Enhanced or Engineered Geothermal Systems (EGS) has prevented adequate assessment of their sustainability. To estimate how renewable EGS reservoirs might be, an analytical approach employing Green's function was used to model transient thermal conduction in an idealized reservoir containing a single rectangular fracture to evaluate heat transfer effects during alternating periods of extraction and recovery. During recovery, the temperature along the fracture surface approaches the temperature of the bulk rock with the deviation from the surrounding bulk temperature decaying as 1/t where t is the recovery time. Numerical simulations of a multiple parallel fracture reservoir using the TOUGH2 code agreed with the derived analytical solutions over a range of flow rates and interfracture spacings with only small deviation due to multidimensional effects. Multidimensional effects are more pronounced near the inlet and outlet of the fracture and are reduced at higher flow rates. Thermal interactions between sufficiently spaced fractures are negligible for production periods of 10–30 years, suggesting that the single fracture analytical model can be applied to multifracture reservoirs provided that the mass flow used is on a per fracture basis. Simulation results show that multifracture EGS reservoirs have a greater capacity to sustain high outlet temperatures, suggesting that conductively dominated EGS systems can be regarded as renewable over time scales of societal utilization systems (three to five times the heat extraction time).

► Shows that EGS reservoirs can recover thermally following a period of extraction.
► Demonstrates how to model EGS reservoirs analytically and numerically for both extraction and recovery phases.
► Suggests how to operate EGS reservoirs to increase their overall sustainability.
► Points out results that can aide in operating EGS reservoirs sustainably.