A potentially important component of the total pressure drop from a fracture into a formation during hydraulic fracture stimulation is that across a polymer invaded zone. Following experimental work with long cores (up to 60 cm-long) and with permeabilities up to 5 md it was determined that the polymer invasion for a zirconate-crosslinked HPG-system has not particular significance and is thus neglegible compared to the very dominant filtercake. The observed early-and late-time filtration phenomena were decribed using a numerical cake layer model and verified by experimental measurements. It is shown that the early-tiine ’’hyperbola" is not a result of polymer invasion but is caused by a changing pressure differential across the filtercake at the beginning of the filtration experiment.

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