James N. Albright and Christopher F. Pearson
Los Alamos Scientific Laboratory
Geothermal Technology Group
Los Alamos, New Mexico
Michael C. Fehler
Oregon State University
Corvallis, Oregon
ABSTRACT
Acoustic signals transmitted between wells have been used to study the structure of the hot dry rock geothermal reservoir at Fenton Hill, New Mexico. The signals produced, using commercially available logging tools, traveled paths as long as 45.7 m (150 ft) in fractured granitic basement rock. Both P-and S-waves were transmitted, the amplitude of the latter varying with the inclination between logging positions in each well.
On pressurization of the reservoir, the signals showed changes in attenuation and waveform. The change in attenuation varied from a value of -2 dB above the fractured portion of the reservoir to -30 dB near the fracture well-bore intersections. The signals, having amplitude content in the frequency range from 6 to 16 kilohertz, were severely attenuated in the higher frequencies. Signal waveforms were limited to one of three distinctive types--the first having a pronounced S-wave arrival, the other two having either no S-wave arrival or apparent distributed P- and S-wave arrivals. The general character of each waveform seems to depend on the absence or presence of fractures along the signal travel paths, on the multiplicity of fractures, and on their pressure states.
The propagation velocity of the signals did not change perceptibly on reservoir pressurization. However, the velocity did change as much as 6% locally in the reservoir following a prolonged period of heat extraction and pressure cycling, resulting in extensive microfracturing of the bulk rock.