Heat Propagation Through Fractures During Hydraulic Stimulation in Crystalline Rock

Abstract

ABSTRACT: The Bedretto Underground Laboratory for Geosciences and Geoenergies (BULGG) is located in central Switzerland and serves as a test bed for geothermal energy research. Several boreholes were drilled from the laboratory section (ca. 1.1 km overburden) to serve as injection boreholes for stimulation and geophysical monitoring boreholes. During a hydraulic stimulation injection in winter 2020 into injection borehole ST2 interval ranging from 313 to 320 m, we observe a thermal perturbation using distributed fiber optic temperature sensing in a neighboring open borehole (MB1) at a depth of 275 m to 295 m. Prior to injection, there is a thermal anomaly in MB1 at about 289 m due to natural fracture fluid flow. Below this depth the temperature is approximately 1.5 °C higher than above. During injection there is a gradual upward movement of the thermal anomaly to ca. 278 m depth. After injection is stopped, the thermal signal gradually recovers to the original depth. The cause for such a temperature change is potentially due to increased warm water flow reaching the base of MB1 from deeper ST2 or poro-elastic fracture closure of the cold-water conducting fractures at 278 and 289 m depth in MB1 during stimulation. 1. INTRODUCTION Geothermal energy is considered a sustainable form of power generation. While the earth generates over three times the total yearly human energy consumption (Pollack et al., 1993; Davies and Davies, 2010), the use of geothermal energy for power generation at the industrial scale has been limited, largely in part due to the need to engineer and stimulate reservoirs at significant depth to tap into the heat sources. Such ‘enhanced’ or ‘engineered’ geothermal systems (EGS) require hydraulic stimulation or fracturing to achieve economic heat extraction, adding to economic impact of drilling and completing a well, as well as creating additional difficulties like induced seismicity. To date, there are just a few examples from across the world which have had success in producing heat from EGS reservoirs (Brown, 1995; Albright and Pearson, 1982; Moore et al., 2018; Link et al., 2020; Genter et al., 2010).