부산대학교 도서관

In support of the U.S. DOE SubTER Crosscut initiative, we established a field test facility in a deep mine and designed and carried outin situ hydraulic fracturing experiments relevant to enhanced geothermal systems (EGS) in crystalline rock to characterize the stressfield, understand the effects of rock fabric on fracturing, and gain experience in monitoring using geophysical methods. The project alsoincluded pre- and post-fracturing simulation and analysis, and laboratory measurements and experiments. The kISMET (permeability(k) and Induced Seismicity Management for Energy Technologies) site was established in the West Access Drift of the SanfordUnderground Research Facility (SURF) 4757 ft (1450 m) below ground (on the 4850 ft level (4850L)) in phyllite of the PrecambrianPoorman Formation. We drilled and continuously cored five near-vertical boreholes in a line on 3 m (10 ft) spacing, deviating the twooutermost boreholes slightly to create a five-spot pattern around the test borehole centered in the test volume 40 m below the drift invert(floor) at a total depth of ~1490 m (4890 ft). Laboratory measurements of core from the center test borehole showed P-wave velocityheterogeneity along each core indicating strong, fine-scale (~1 cm or smaller) changes in the mechanical properties of the rock. Fieldmeasurements of the stress field by hydraulic fracturing showed that the minimum horizontal stress at the kISMET site averages 21.7MPa (3146 psi) trending approximately N-S (356 degrees azimuth) and plunging slightly NNW at 12°. The vertical and horizontalmaximum stresses are similar in magnitude at 42-44 MPa (6090-6380 psi) for the depths of testing, which averaged approximately 1530m (5030 ft). Hydraulic fractures were remarkably uniform suggesting core-scale and larger rock fabric did not play a role in controllingfracture orientation. Analytical solutions suggest that the fracture radius of the large fracture (stimulation test) was more than 6 m (20ft), depending on the unknown amount of leak-off.