ABSTRACT
Several geophysical logs were run in a 3-kilometer borehole, that had been drilled for the purpose of extracting hest from a hot-dry granitic formation. Rock characterization tests were performed, both in the laboratory and field, to assist in the design of the hydrofracturing experiment and in improving drilling technology. Most of the properties determined from interpretation of the in situ geophysical measurements showed reasonably good agreement with the ones obtained under similar laboratory conditions. The minor discrepancies can be explained either by the presence of small open cracks in the rock core that may have been absent or closed in the rock mass, or by differences in the moisture environment. Greater discrepancies encountered in determining the porosity of the rock formation probably were due to the inapplicability of empirical formulas derived from experience gained in testing soft porous rock masses.
INTRODUCTION
In December 1974, the Los Alamos Scientific Laboratory completed the drilling of the GT-2 deep exploratory well located on the western flank of the Valles Caldera, Jemez Mountains, New Mexico. The purpose of the well was to investigate the feasibility of the hot dry-rock concept of extracting geothermal energy (Laughlin, 1975). The final depth of this first 'energy extraction hole' was 2,929 meters and bottom hole temperatures reached 196° C. Requirements of the system were that the rock be competent and nearly impermeable uphole to prevent loss of injected fluids and that a fracture system be induced at depth to provide sufficient heat exchange surface to the injected fluid. These and drilling requirements established a need for rock mechanics data to characterize strength and deformational properties of the rock at this site. Under a cooperative exchange of information, the Los Alamos Scientific Laboratory performed the drilling of the borehole, downhole logging and in situ testing, in general, while the Federal Bureau of Mines performed laboratory investigations to determine the properties on the recovered core.
DIAMETRAL LOGGING TEST
Anisotropy of the rock core at different depths was examined by an onsite acoustic method immediately following core recovery. Measurements were conducted at 45° intervals across the diameter of the core through 180° at each interval of depth. It was hoped that this might indicate weakness planes and, consequently, preferred fracture orientation during the subsequent hydrofracturing experiments. Further correlation would then be sought by examining the directional variation of the strengths, the constitutive minerals and even the preexisting fracturing pattern. The technique and pulse velocity apparatus were field adaptations of those described elsewhere by Thill, et al, (1968). No large anisotropic character was apparent throughout the granitic formation with the exception of a region situated at a depth of 960 meters which showed an anisotropy index1 of 14 pct. The lower velocities recorded through this section and other logs suggested, however, a fractured rock structure, and this section was cemented in to prevent loss of drill fluids. Average velocity for all the cores was 5.688 km/sec with a standard deviation of 0.356.
DENSITY
Several density measurements were performed on small rock specimens and compared with the gamma-density log. The correspondence was close.
