| Title | Hydraulic Fracturing of Crystalline Basement Rock: Insights from SP Monitoring |
|---|---|
| Authors | E. Schill, N. Nakata, E. Um, M. Wilt, E. Nichols, T. Wood, A. Nayak |
| Year | 2026 |
| Conference | World Geothermal Congress |
| Keywords | |
| Abstract | Hydraulic fracturing of crystalline basement rock is key to advancing Enhanced Geothermal Systems (EGS) in the United States. This study presents findings from underground laboratory and reservoir-scale experiments, focusing on fracture development and associated self-potential (SP) signals. At the Äspö Hard Rock Laboratory, a fracture was created via a single-step injection and extended through four hydraulic re-fracturing (re-frac) stages. Sensor arrays at depths of 410 m and 280 m recorded acoustic emissions (AE), tilt, and SP data. Near-field SP began at ~0 mV, while far-field SP remained elevated due to residual effects from a prior experiment. SP signals changed notably after the main fracture formed. In the near-field, SP increased uniformly along the electrode array, with maxima aligning with the primary fracture plane. When secondary fractures developed, AE activity and seismic energy increased, and SP displayed localized minima. Far-field SP responses varied spatially: northern electrodes (SP2–4) showed increases, southern electrodes (SP7–8) showed decreases, and central electrodes remained stable. This polarity pattern reflects the geometry of the induced fracture. At the reservoir scale, SP monitoring has also shown promise. Previous studies (e.g., Darnet et al., 2004) reported SP increases during hydraulic shearing and shut-in. We now present new SP data from an opening-mode hydraulic fracturing experiment in Utah’s crystalline basement. Here, electrode arrays were deployed parallel to a horizontal well section and monitored during multiple fracturing stages, providing valuable insight into SP evolution during stimulation. |