This paper summarizes part of the results of an investigation of fracture clean-up mechanisms undertaken under a Joint Industry Project active since the year 2002. It is well documented in the literature that hydraulic fractures, although successful, often underperform: Frac and Pack completions exhibit positive skin values, and traditional hydraulic fracture completions show discrepancies between the placed propped length and the effective production fracture length. Ineffective fracture clean-up is often cited as a likely culprit.

The main results presented in this paper were obtained using a modified conductivity cell to allow polymer concentration via leakoff, and measurements of flow initiation gradients. The paper will discuss the experimental set-up and some of the artifacts that had to be removed prior to ensuring more reliable data. The results highlight the crucial role played by the filter cake and present new data that would significantly change the common industry practice of relying simply on an average polymer concentration factor.1-3 

It is shown that contrary to the current method that calculates an average polymer concentration,the polymer,in practice, concentrates only in the filter cake. It is also shown that the filter cake thickness compared to the fracture thickness plays a critical role in creating significant yield stress effects,which could be either avoided through adequate design or used to estimate the resulting productivity loss.

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