A simple hydraulic fracturing technique is proposed for determination of the fracture toughness of rock. A preliminary set of experiments have been performed on a variety of rock types. The experimental procedure consists of repeatedly propagating a stable fracture away from a jacketed borehole in a uniaxially stressed prismatic sample. The borehole pressure at which fracture propagation occurs is determined by monitoring acoustic emission activity. This method allows fracture toughness to be determined without measurement of fracture length. Plexiglass samples, in which fracture length could be monitored, were used to confirm the analysis.
Linear elastic fracture mechanics is increasingly used as a method for analyzing fracture growth in geologic materials. This is especially true regarding growth of hydraulic fractures in the earth''s crust. Conceptually, hydraulic fractures are excellent examples of fractures in which the displacement of the fracture walls is perpendicular to the plane of the fracture (or Mode I fractures in the nomenclature of Paris and Sih, 1966). A principal advantage of using linear elastic fracture mechanics to analyze fracture growth is that in order to predict when fracture extension will occur it is not necessary to prescribe the detailed manner of energy dissipation in the region of the fracture tips. Rather, the criteria controlling growth is merely comparison of the stress intensity factor with the critical stress intensity factor, or fracture toughness of the material. Unfortunately, very few data exist on the fracture toughness of rock. Clifton et al (1976) report values for several samples of sandstone, siltstone, and shale, Schmidt (1975) reports values for a limestone, and Peng and Johnson(1 972) report the fracture toughness of Chelmsford granite. The purpose of this work is to provide fracture toughness data for several rocks and to establish the validity of a relatively simple technique for making such measurements.
