Measurements of effective fracture toughness values for hydraulic fracture: Dependence on pressure and fluid rheology

Results from a series of laboratory hydrofracture tests, in which different fracture fluids, fracture fluid injection rates, confining pressures, and test specimen mechanical and fluid transport properties were used, are presented and discussed. Fracture fluids included various simple linear viscous (Newtonian) fluids and different combinations of three types of commercial fracture fluids: linear gels, cross-linked gels, and combinations of gels with suspended solids. Systematic variations in the behavior of fracture initiation pressures, and fracture fluid pressures during stages of stable fracture propagation, are observed to depend on fluid flow resistance, leak-off rate, and fracture fluid rheology. Post-test inspections of hydrofracture surfaces indicate the presence of a dry zone near the fracture tip. The effective hydrofracture toughness parameters determined from the tests with fracture fluid gels are substantially higher than the normal Mode-I toughness values, determined for the same material. The behavior is interpreted to be the consequence of a build-up of solids at the fracture tip, from dehydration of fracture fluids containing gels and/or solids.