| Title | Impacts of Basement Shear Zones on Enhanced Geothermal Systems (EGS): Best Practices for Avoiding Induced Seismicity, Premature Thermal Breakthrough, and Excessive Leakoff |
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| Authors | Greg LEVEILLE and Mark ZOBACK |
| Year | 2025 |
| Conference | Stanford Geothermal Workshop |
| Keywords | EGS, shear zones, induced seismicity, thermal longevity, fluid leakoff, economic outcomes, horizontal wells, hydraulic fracturing |
| Abstract | With hydraulically fractured horizontal wells having proven to be a highly effective method for creating an Enhanced Geothermal System (EGS), the descrption of what defines an ideal EGS prospect needs to be updated. Previously, when hydraulically fractured vertical wells were the tool of choice, an ideal prospect tended to be defined as one with abundant natural fractures that could be either propped open or induced to shear during stimulation to achieve sufficient fracture conductivity to deliver commercial flow rates and thermal longevity. However, when using horizontal wells, experience from unconventional oil & gas reservoirs indicate pre-existing shear zones often diminish productivity by hijacking the bulk of the pumped fluid and proppant, leaving much of the reservoir under-stimulated. Additionally, for EGS developments, if shear zones intersect both injection and production wells, the numerous secondary fractures in the damage zones could act as fluid flow superhighways, causing premature thermal-breakthrough and unacceptably high losses of injected fluid. Fluid-injection-related induced seismicity could also pose a significant risk when pre-existing shear zones are present. For these reasons, when siting hydraulically fractured horizontal wells, it is best to avoid extensively faulted and fractured areas. Doing so will result in reduced risk of induced seismicity, greater thermal longevity, and less fluid leakoff. We of course recognize that identifying shear zones prior to drilling can be challenging given the limited ability of current geophysical methods to detect steeply-dipping shear zones in the crystalline basement rocks commonly targeted in EGS developments. The high cost of drilling and multi-stage hydraulic fracturing requires that this challenge be overcome if risks are to be reduced and economic outcomes optimized. |