Deep in-Situ Stress Measurements by Hydrofracturing

Abstract Hydrofracturing is a recently developed in-situ stress-measuring method. It consists of hydraulically pressurizing a sealed-off interval inside of a borehole until fracture initiates. The fracture is then extended by additional pumping of fluids from the surface. The pressures recorded during the test can be directly related to the magnitudes of the in-situ principal stresses. The orientation of the fracture in the borehole yields the directions of the principal stresses. The method overcomes some limitations of the more conventional strain-relief techniques: stresses can be determined at any distance from the access point (limited only by hole length), stresses are measured directly, no elastic parameters are required in determining the minimum principal stress, no delicate instruments are used in the borehole, no overcoring is necessary. The hydrofracturing method was theoretically developed for both high- and low-permeability rocks, and was extensively tested in the laboratory under simulated in-situ conditions. These tests confirmed the theoretical assumptions and results. Field measurements were conducted in some ten states within the continental U.S., at depths varying from 100 m (Nevada Test Site) to 1900 m (Rangely, Colorado). The hydrofracturing results were very consistent within each site, and the calculated principal stresses and their directions could, in most cases, be correlated to tectonic phenomena or other measurements and observations. In particular, the stresses determined at Rangely were in accord with the expected condition for the right lateral slip of the existing strike-slip fault, and were used to correctly predict the critical pore pressure necessary to trigger local earthquakes. At the Nevada Test Site, the results of twelve hydrofracturing measurements conducted both from the surface and from underground tunnels indicated a linear increase with depth in all the principal stresses. Hydrofracturing is a simple method of stress determination. It utilizes commercially available packers, pressure lines and pumps, which can be either rented or purchased. It is now being tested in fault zones, in anomalously hot formations, in extremely anisotropic rocks. It is being considered as a major tool in plate-tectonics studies, earthquake control, geothermal-energy extraction from hot dry rock, and new methods of mining.