The EGS Collab hydroshear experiment at the Sanford underground research facility - Siting criteria and evaluation of candidate sites

The objective of the EGS Collab project is to establish a suite of intermediate-scale (~10-20 m) field test beds coupled with stimulation and interwell flow tests to provide a basis to better understand fracture stimulation methods, resulting fracture geometries, and processes that control heat transfer between rock and stimulated fractures. Experiment 1 of the project is being conducted at a depth of ~1.5 km in the Sanford Underground Research Facility (SURF) on the 4850 Level (ft. below the ground surface). The stimulation method planned for Experiment 2 of this project is hydroshearing of an existing natural fracture. In siting this experiment, there are several key geologic criteria that need to be met. These include: 1) the fracture should be least 10 m in length so that it could be intersected by two boreholes that are spaced that far apart; 2) it should be optimally oriented relevant to the stress field so that it is critically stressed; 3) the site should have appropriate stress conditions (not too shallow a depth); 4) the fracture should have sufficient permeability to allow for prestimulation flow testing, but not too high a permeability that would preclude permeability enhancement via shear stimulation; 5) the fracture should not intersect other features (permeable fractures, boreholes, adjacent drifts) that could serve as major leak-off zones; 6) the site should not be complicated by geology (a single lithology would be preferred). In addition, there are logistical criteria that will also influence site selection, including: 1) site availability/access, 2) logistical support, e.g., power, internet, water, operating rail system; 3) drift size and orientation compatible with drilling and site operations; 4) appropriate ground support. There are only two deep levels at SURF that can be accessed by the Yates Shaft: the 4100 and the 4850, so we limited our search to these two levels. Currently, three candidate sites are being evaluated: the existing Experiment 1 site and its surroundings on the West Drift on the 4850 level, and two locations on the 4100 level. This paper will evaluate advantages, disadvantages, and risks associated with these three options at SURF.