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A new borehole method has been recently suggested for estimating stresses remotely at long distances from the operator and under any stress configuration. This method, called “hydraulic fracturing,” is based on a technique used by the petroleum industry to stimulate oil wells. The basic required equipment is available commercially through renting or purchasing, and the experience of oil field service companies is accessible to ensure correct functioning. The method itself consists of pressurizing a sealed-off interval of the tested borehole until fracture occurs. Additional pumping is then undertaken to open and extend the fracture. After the pressure is released, an “impression packer” is used to obtain an oriented imprint of the hole showing the inclination and azimuth of the hydraulic fracture. Using the recorded fracturing pressures and the fracture impression, the principal stresses and their directions are calculated. Laboratory experimental work has confirmed most of the theoretical assumptions and results. Stresses calculated from field hydraulic fracturing tests have demonstrated the great potential of the method. In particular, a hydraulic fracturing test was conducted at 6280 ft (1915 m) below the surface at Rangely, Colo. The in situ stresses as determined by the method were in accord with the expected condition for the type and the slip direction of an existing fault, and were used to predict the critical pore pressure necessary to trigger local earthquakes. Following the successful experiment, more field tests have been planned, and a number of research groups have purchased the necessary equipment, planning to use hydrofracturing on a routine basis in the future.
rocks, rock mechanics, determination of stress, ground stress, hydraulic fracturing, field tests, instruments
Associate professor of rock mechanics, University of Wisconsin, Madison, Wis.