We investigate the mode 1 fracture toughness and its anisotropy of Poorman Schist rocks recovered from the Enhanced Geothermal Systems Collaboration (EGS Collab) Experiment 1 site. The EGS Collab team is conducting a series of intermediate (10–20 m) scale stimulation and inter-well flow tests with comprehensive instrumentation and characterization at the Sanford Underground Research Facility to validate existing theories and description of hydraulic fractures propagation and associated fluid flow. An important parameter to constrain is how the fracture toughness varies depending on the orientation of the fracture and the direction of fracture propagation, which may have controls on hydraulic fracture propagation. Fracture toughness relative to foliation orientation was determined through the utilization of Cracked Chevron Notched Brazilian Disk (CCNBD) samples in three different orientations (Divider, Arrester, and Foliation Splitting/Short Transverse). Each sample group contains at least three 25.4 mm diameter and 12.7 mm thick CCNBD samples, one of each sample type. Arrester and Foliation Splitting samples were obtained from the same sub-core while Divider samples were obtained from a separate sub-core obtained in close proximity. We found fracture toughness to be weakest in the Foliation Splitting orientation and strongest in the Divider orientation, similar to findings from anisotropic fracture toughness measured in shale rocks. Our findings on the influence of foliation orientation on fracture toughness are presented here.
The Enhanced Geothermal Systems (EGS) Collaboration project's goal is to improve the feasibility of large-scale EGS resources by better understanding the relation between permeability enhancement and fractures in crystalline rock. To better understand fracture stimulation methods, fracture geometries, and processes controlling heat transfer between crystalline rock and stimulated fractures, a group of test beds (10–20 m) were established for stimulation and testing (Kneafsey et al., 2018).
Experiment 1 of the EGS Collaboration project takes place in the Poorman schist at the Sanford Underground Research Facility (SURF) in Lead, South Dakota. The Poorman schist is a foliated unit consisting of calcite, muscovite, dolomite, biotite, quartz, and chlorite. Foliations can be observed in planar and tightly folded orientations. These folds vary in size from centimeters to meters.