A survey of natural fractures at the Hi Vista site, Mojave Desert, California

Abstract

An array of five wells was drilled in Mesozoic granitic rock for hydrological and geophisical investigations at Hi Vista, California. A 592 m-deep well is located in the center of the array and four 183 m-deep wells were drilled 14 m from it in each of the cardinal directions. Two hundred seventy-seven fractures were seen in televiewer logs of the deep well and three significant fracture sets were identified; a north-striking high-angle set, a northeast-striking high-angle set, and a low-angle set that dips to the west-southwest. Each of the shallow wells contained some fractures within these sets; however the distributions of fracture orientations varied from well to well. These different fracture orientations may help to explain why water levels between wells vary by as much as 50 m.When these fracture distributions are compared with those from three other deep holes in crystalline rock at Black Butte, Crystallaire, and Cajon Pass, a regional trend can be identified. Each of the wells has significant clustering associated with it. Two of the clusters identified at Hi Vista are well defined in more than one deep hole. Fracture frequency at Hi Vista increases with depth; that at Cajon Pass decreases with depth; and the fracture frequencies at Black Butte and Crystallaire show no strong systematic trend with depth.

Continuation of a deep borehole stress measurement profile near the San Andreas Fault: 1. Hydraulic fracturing stress measurements at Hi Vista, Mojave Desert, California

“…As natural fractures have been shown to increase the compliance of granitic rocks [Pratt et al, 1977], the low stresses observed between 220 and 280 m and below 530 m might, for example, correspond to zones of anomalously high natural fracture density or low P wave velocity. This is especially likely if the majority of the natural fractures dip steeply (> 50ø), as is the case in the Hi Vista well [Springer and Ader, 1987], or if these fractures were sites for enhanced geochemical alteration or microcrack production [see Moos and Zoback, 1983]. A comparison between the in situ stresses, the natural fracture density, and the sonic P wave velocity ( Figure 4) Figure 7).…”