Criteria for proppant placement and fracture conductivity

The design of a hydraulic fracturing treatment must include the recipe of the treatment and an estimation of the expected productivity improvement. For such a frac-job design, one must decide which criteria should be applied for proppant admittance and placement. From the resulting propping-agent concentration in the fracture, a conductivity value must be derived for predicting the productivity improvement. For low-viscosity fracturing fluids and proppant concentrations up to 5 lb/gal, this study recommends a value of 2.0 for the admittance criterion, which is defined as the ratio between fracture width and maximum proppant diameter. High-viscosity fracturing fluids require a ratio of 2.6 for proppant concentrations up to 8 lb/gal. Fluids carrying this concentration of proppant can be pumped without triggering internal bridging. The latter value is recommended as a maximum placement criterion. Empirical relations have been developed which allow a sufficiently accurate estimate of the conductivity for fractures propped with sand, glass beads, and walnut hulls. (11 refs.)