The formation of channels in propped fractures, in the presence of fibers, has been demonstrated experimentally under a wide variety of conditions. The presence of channels can have a significant impact on the effective fracture conductivity and consequently the productivity of a given well. While single channels are observed in experiments, a question is how many channels may be present in real proppant packs and how long do they grow? The length is especially important when proppant flow back control additives are used as "tail-ins" with the last part of the proppant pack. To complement the experimental study, the understanding of channel propagation and the interaction between multiple growing channels was modeled using finite difference codes.
Modeling was performed using a 2-dimensional explicit finite difference program. The formation of channels was predicted by using two parameters—a critical flow rate and a curvature limit. Results demonstrated that the curvature limit is a critical parameter in determining the stability of channels. Calibration of the model with laboratory experiments with proppant and fibers provided clear explanation as to why channel formation provides proppant pack stability at high flow rates.
For a given flow rate, channels grew until a stable length was attained; increasing the flow rate would cause the channel to grow further until a subsequent stable length was attained. Multiple channels were generated but a dominant channel grew once the length of the channel was greater than the spacing between channels. The presence of one, or several widely spaced, long channels will have a much larger impact on productivity than many short channels. Bilinear flow conditions resulted in channel growth to a stable equilibrium length. This is expected to occur in real fractures. A consequence of this behaviour is that fractures should be flowed back at high rates to grow channels that will then be stable at subsequent production rates.
Fibers have been used successfully for several years to control proppant flow back following hydraulic fracturing treatments. Field experience has shown that fibers allow immediate rapid flow back of the fracture resulting in lower flow back costs, earlier gas to sales and better productivity.1–3 The improved productivity has been attributed to higher polymer recovery and retention of near wellbore conductivity.
However, in laboratory proppant flow back tests,4–5 proppant packs containing fibers routinely form high permeability channels, once a critical flow rate is reached. In many field cases channels do not form, as the critical flowrate cannot be reached. The channels are stable to very high flow rates and to closure stress cycling. The formation of channels has two major benefits. Firstly, the presence of open channels in a proppant pack will significantly increase the fracture conductivity in that area. In addition the channels will allow polymer to be removed from further along the fracture length leading to an increased effective fracture length. Secondly, the formation of self-stabilising channels is a mechanism by which fibers can control proppant flow back in very high rate wells. In field appliCat ions fibers have controlled proppant flow back in wells with multiphase flow at rates of 60mmcfd with 4000bbl condensate per day from a 60 ft perforated interval. At equivalent rates in laboratory tests channels would have formed.