Pressure spikes and early, rapidly developing screenouts are often observed while hydraulically fracturing naturally fissured reservoirs. These occurrences are commonly attributed to fracture width restrictions or slurry dehydration. This paper describes the results of a series of laboratory experiments designed to study proppant transport and distribution in a hydraulic fracture with leakoff through discrete fissures. Mechanisms are described which explain the observed phenomena of proppant induced pressure rises and rapid screenouts. Methods of treatment design to combat these effects are also outlined. Observations made during the experiments provide insight into proppant crushing during fracture closure, poor polymer gel recovery, and final fracture conductivity.
The experiments show that lateral flow velocity into natural fissures causes proppant to migrate to the fracture wall and build up a dense proppant pack at the leakoff site. With sufficient leakoff the proppant holdup is severe enough to completely fill the main fracture channel, leading to screenout, even at low injected proppant concentrations. During buildup of the proppant bank the injected fluid is forced to flow in channels of diminishing height at very high shear rate. This unstable channelized flow leads to significantly different proppant and fluid distribution in the fracture than that predicted by uniform flow and transport models.