Laboratory Experiments on Hydrofracture and the Permeability of Hot Granite
It has been proposed that an artificial geothermal reservoir could be created by injecting water under high pressure through a hole drilled into a hot dry batholith. By drilling a second hole to intersect the plane created by hydraulic fracture, a fluid circulation system could be created by pumping cold water into one hole and extracting hot water or steam through the other hole. The authors have carried out a number of laboratory experiments to investigate various aspects of this project. While it is usually assumed that a single tension hydraulic fracture is formed with its plane parallel to the direction of maximum principal stress, the authors have found in laboratory experiments that, if the rock is under shear stress and the fluid is injected slowly enough, shear fractures are formed with their planes oriented about 30 degrees to the direction of maximum principal stress. The results suggest that it may be possible, in regions of high tectonic stress, to increase the fracture surface area simply by varying the fluid injection rate to create not only a tension fracture but shear fractures as well. The authors found that the spacial distribution of the fracture planes is accurately determinable from the location of the acoustic emission events that occur during fracture. The authors state that this distribution should also be calculable from the change in magnetic field at the earth's surface caused by the injection of high-magnetic-susceptibility material into the fracture. Results are described of measurements of the permeability of granite under confining pressure and differential stress at temperatures to 400ºC, along with how they relate to the way a circulating system's permeability changes with time. 4 refs.
- Research Organization:
- U.S. Geological Survey, Menlo Park, CA
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 887396
- Report Number(s):
- SGP-TR-20-28
- Country of Publication:
- United States
- Language:
- English
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