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Results from the first extensive field test of a man-made hot dry rock (HDR) geothermal reservoir in low permeability crystalline rock are presented. A reservoir with a small heat transfer area was utilized to study the characteristics of a prototype HDR system over a shortened lifetime. The resulting accelerated thermal drawdown was modeled to yield an effective area of 8000 m/sup 2/. In addition to the thermal effects, this test provided an opportunity to examine equipment operation, water permeation into the formation, geochemical interaction between the circulating fluid and the rock and flow characteristics including impedance and residence time distributions. Continuous monitoring for induced seismic effects showed that no activity to a Richter threshold of -1.0 was detected during the 75-day experiment.

A series of seismic studies was performed at the Department of Energy Multi-Well Experiment site to delineate the character of the beds within the Mesaverde group in western Colorado. The experiment consisted of a three-dimensional seismic survey of the area, a vertical seismic profile of the two wells and a cross borehole survey covering zones of special interest. The combined analysis of these data and well log and core information was used to relate the seismic records to the geology of the area.

Results of the preliminary assessment of the second hot dry rock reservoir at the Fenton Hill field site are presented. This second reservoir was created by fracturing a deeper interval of granite rock located at a depth of 2.93 km (9620 ft) in the same wellbore pair used in the creation of the first reservoir; no additional redrilling was required. The new fracture system has a vertical extent of at least 320 m (1050 ft), suggesting that the combined heat-transfer area of the old and new fracture systems is 11 times that of the old system. The virgin rock temperature at the bottom of the deeper interval was 197/sup 0/C (386/sup 0/F). Water at a flow rate of 6 l/s (100 gpm) was circulated through the reservoir for a period of 23 days. Downhole measurements of the water temperature at the reservoir outlet, as well as temperatures inferred from geothermometry, showed that the thermal drawdown of the reservoir was negligible and preliminary estimates indicate that the minimum effective heat-transfer area of the new reservoir is 45,000 m/sup 2/ (480,000 ft/sup 2/), which is six times larger than the first reservoir. The following are presented: operational plan, reservoir geometry and flowmore » paths, flow impedance, geochemistry, heat extraction, dye tracer flow distribution studies, and seismicity. (MHR)« less

Measurements of the travel time of acoustic signals transmitted between wells at the Department of Energy Multi-Well Experiment site (MWX) near Rifle, Colorado, are processed and analyzed. Interpretations relevant to sand geometry and continuity have proved possible through inspection of the signal travel time plotted against the coordinates of transmitter and receiver wellbore positions, or against the depth of receiver and ray path inclination. The continuity of several sands between wells is corroborated. A major lenticular sand terminating between wells could be inferred. To explore the possible distortions in tomographic images derived from crosswell data, synthetic tomographs are constructed from computed travel times of signals transmitted through idealized models from stratigraphy thought to be present at the MWX site. The synthetic tomographs, although preserving the general character of the model stratigraphy, are distorted enough that detailed interpretations are not possible. Horizontal distortions predominate in reconstructions of flat-lying stratigraphy. 7 refs., 9 figs.