Preliminary observations from a decameter-scale field test of fracture thermal energy storage: Initial and induced in-situ stress and its impact on fluid flow

We present preliminary data, results, and analysis from the first decameter-scale test of fracture thermal energy storage. The test took place at the Sanford Underground Research Facility 1250 m underground in an amphibolite test bed comprising five boreholes connected by a stimulated fracture network. The system was “charged” by injecting hot water in the central borehole while producing from the others, and subsequently “discharged” by injecting ambient temperature in the peripheral boreholes while producing from the center borehole. During the charging phase more than 60% of the injected water was recovered but during the discharging phase recovery rates were less than 10%. Significant compressive stress heterogeneity on the fracture network (> 4 MPa difference) existed between boreholes before hot water injection and was exacerbated (up to ∼ 9 MPa) during hot water injection. We observed a strong correlation between the compressive stress gradient on the fracture network and the injection impedance between boreholes. Our results suggest that adding thermal energy to our system induced poroelastic and thermoelastic effects that exacerbated existing compressive stress heterogeneity on the fracture network. This, in turn, guided fluid flow away from the areas of highest compressive stress (near the center borehole) towards areas of lower compressive stress.