Natural fractures and their relationship to the EGS Collab Project in the underground of the Sanford Underground Research Facility (SURF)

The EGS Collab Project (Enhanced Geothermal Systems) is conducting experiments on creating a 10-m scale reservoir and performing non-isothermal fluid circulation tests to assess heat transfer from the rock. The work is being performed in the Poorman Fm., a phyllite exposed on the 4850 Level (1.5 km below the surface) of the Sanford Underground Research Facility (SURF) in northwestern South Dakota. The current phase of the project involves drilling eight closely spaced 60 m long, horizontal holes that are drilled over a range of 69 m along a portion of a drift, a horizontal passageway, on that level. The injection and production holes are drilled 10 meters apart in the direction of the minimum principal stress. Pre-existing natural fractures encountered during drilling appear to be sparse and highly discontinuous and may serve to arrest hydraulic fracture growth, or shear if orientated in critical orientations to the in situ stresses. The general trend of the fracture systems is toward the northwest and is roughly parallel to nearby rhyolite dikes to the northeast. Widely spaced seepage zones intersect the drift near the experiment and in core it is expressed as a 3 m wide zone of partially healed breccia with both open fractures containing calcite crystals and vuggy porosity. Hydro-structural modeling using Golder Associates’ FracMan code shows that the seepage zone largely disappears about 50 m from the drift wall. Pressure build-up tests in nearby boreholes show that the systems appear to have their greatest permeability vertically and often do not connect laterally.