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.
A practical understanding of factors which determine hydraulic fracture behavior is critical for the design of fracture treatments. The process of hydrofracture initiation and propagation, and the ultimate fracture geometry, are determined by a complex combination of material properties, fracture fluid behavior, and the in-situ stress environment. Many of these properties are poorly defined in potential stimulation sites, and most are beyond the control of the reservoir engineer. Nevertheless, a working knowledge of the impacts of different factors on hydrofracture behavior can guide treatment design, and provide a basis for understanding deviations from anticipated performance.
More important, however, parameters such as fracture fluid rheology and injection rate can be modified in the field, and it is possible that hydrofracture treatments can be enhanced by judicious choices. For example, recent experimental studies reveal the existence of a zone behind the hydrofracture tip, into which the fracture fluid does not penetrate. Morita et al. attribute this behavior to the buildup of solids from dehydration of the fracture fluid, for which significant increases in fracture propagation pressures have been calculated. The variation in requisite hydrofracture pressure were related to variations in an effective hydrofracture toughness parameter.
The amount of solids buildup in a hydrofracture is a function of fluid leakoff rate, which can be altered by fracture fluid gel chemistry and permeability-reducing solids content, and a quantitative description of this process could lead to guidelines for customized fracture treatments for better control of fracture geometries. This process also provides a basis for understanding large discrepancies between reported field fracture fluid pressure and values computed on the basis of available numerical models.
Unfortunately, variations in observed hydrofracture behavior are usually a consequence of several coupled processes.