A series of hydrofractures were performed on the 4100 ft. level of the Sanford Underground Research Facility (SURF) to quantify the minimum principal stress and stress orientation. The motivation for this work was to determine the suitability of the site as a second testbed for the EGS Collab project and to inform the testbed design. EGS Collab is a meso-scale project where experiments are being performed to increase permeability in low-permeability rock and improve our understanding of appropriate techniques and models required for developing enhanced geothermal systems. In order to design the second testbed, a ∼50 m vertical HQ (96 mm) pilot borehole was drilled in June, 2019, to perform a series of mini-frac tests to determine the rock stress state. Utilizing an elastic model based on the ISIP (Instantaneous Shut In Pressure), testing indicates that the minimum principal stress is 20.4 MPa oriented NNE (24°) and plunges at an angle of approximately 28°.

1. INTRODUCTION

As part of determining a location for Experiment 2 of the EGS Collab project (Kneafsey et al. 2018), a 50 m vertical HQ (96 mm) borehole was drilled on the 4100 level of the Sanford Underground Research Facility (SURF), the borehole is located in an alcove near the Yates shaft (Heise, 2015). This borehole was to be used for a series of tests to determine the feasibility of the location for shear fracture stimulation as part of the Experiment 2 test protocol. As part of the testing, a series of hydraulic fractures were performed throughout the length of the borehole. These were used to determine the minimum principal stress and infer stress direction from the comparison of pre- and post-test borehole logs.

2. ROCK STRUCTURE

The rock in question is part of the Yates unit, a heavily folded and metamorphosized amphibolite (Caddey et al., 1991, Hart et al., 2014). This is contrary to the rock type which was used for Experiment one which was the Poorman formation (Oldenburg et al., 2017, Vigilante et al., 2017, Wang et al., 2017), a layer of schist which overlays and the Yates amphibolite. Both formations are steeply dipping, so that even though the tests described here occur at a shallower depth than Experiment one, they are in an underlying formation.

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