Characterizing Rock Fractures and Physical Properties for Experiment 2 of the EGS Collab Project, Sanford Underground Research Facility

Abstract

ABSTRACT: The EGS Collab project is focused on understanding and predicting permeability enhancement and evolution in crystalline rocks to support geothermal energy production. To support this effort, the project is creating a suite of intermediate-scale test beds coupled with stimulation and interwell flow tests that will provide a basis to better understand the fracture geometries and processes that control heat transfer between rock and stimulated fractures. Therefore, high level site characterization is paramount for building models to estimate potential interwell flow rates and heat exchanges. For the second experimental testbed, the EGS Collab team visually mapped the distribution, orientation, and nature of open and healed fractures exposed along the drift wall in the Sanford Underground Research Facility 4100 feet below ground surface, and within the eleven boreholes drilled for this test bed. Continuous core logging of each borehole and a suite of geophysical wireline logs was collected to characterize the spatial variability of rock properties and fracture orientations. All of the data were then compiled into multiple 3D visualization software packages for interpretation and further analyzed for slip and dilation tendencies that will be incorporated into coupled-process geomechanical flow and transport models to better constrain the planned flow and tracer tests. 1. INTRODUCTION Following the completion of Experiment 1 at 4850 feet below ground surface (the 4850 level), the EGS Collab team initiated the development of a second testbed for Experiment 2 (E2) in the Sanford Underground Research Facility (SURF and former Homestake Gold Mine) in Lead, South Dakota, USA. The overarching goal of the EGS Collab project is to create field test beds to host stimulation and interwell flow tests that will provide improved understanding of fracture stimulation methods, resulting fracture geometries, and processes that control heat transfer between rock and stimulated fractures for Enhanced Geothermal Systems (EGS). This experiment will contrast from Experiment 1 in that its goal is to create a series of hydro shear fractures for interwell flow tests in support of the EGS Collab project (Kneafsey et al., 2018) at the intermediate-scale (~10-20 m). In the winter-spring of 2021, the EGS Collab team started to develop the E2 testbed on the 4100 level (1.25 km below the ground surface).