Report on televiewer log and stress measurements in holes USW G-3 and Ue-25p1, Yucca Mountain, Nye County, Nevada

Hydraulic fracturing stress measurements and televiewer observations were made in drill holes USW G-3 and Ue-25p1 on Yucca Mountain, Nevada, as part of a continuing investigation of the tectonic stress field in the vicinity of a proposed site for the disposal of nuclear waste. The results from USW G-3 are similar to the results reported previously from USW G-1 and USW G-2 with a low least horizontal principal stress in a direction approximately N65°W. This indicates a normal faulting stress regime consistent with the inferred regional extension in this part of the Basin and Range province. The number of reliable hydraulic fracturing tests in Ue-25p1 is not sufficient to reliably characterize the state of stress at this site, but some definite conclusions can be drawn from these data. The pressure curves observed during many of the frac tests are significantly different from the curves observed in USW G-1, USW G-2, and USW G-3. The breakdown pressures are higher, suggesting a higher least horizontal principal stress and/or a substantially higher apparent tensile strength for the rocks in this hole. The instantaneous shut-in pressure does not clearly reveal the least horizontal principal stress in most of the frac tests, but two pressure versus volume pumping tests in the Paleozoic rocks show a fracture opening pressure close to the vertical stress, suggesting that the least horizontal principal stress could possibly be as high as the vertical stress. One good hydraulic fracturing test reveals a least horizontal stress much less than the vertical stress, with a greatest horizontal principal stress close to but less than the vertical stress. No drilling-induced hydraulic fractures were observed in this hole, in contrast to the three USW G holes which had drilling-induced hydrulic fractures in the upper portions of the televiewer logs