Enzyme type gel breakers have been widely used in completion and stimulation work to reduce the viscosity of various polysaccharide polymers in a controlled manner. Major applications are in gravel packing, fracturing and well control. Mixtures of enzymes are utilized to reduce the polymer strands to low molecular weight fragments for flow back from the formation and wellbore. Recent laboratory studies have found that under certain conditions, enzymes are removed from the polymer solutions by adsorption when injected into sandstone or coal matrices. The implications of this original finding were that gelled fluid lost to these type reservoirs would not break completely and could cause permeability impairment. Continued research found that the phenomenon appeared restricted to hydroxyethylcellulose (HEC) gelling agents and did not occur to a great extent with guar based gels. Fortunately, the presence of an initial oil saturation interferes with the adsorption process except in the cases of low paraffin condensates. Conclusions suggest that care should be taken in application of HEC gels with enzyme breakers in "dry" gas reservoirs and coal seams. Misleading results in laboratory experiments can also result when retained gel samples are used to predict viscosity decline of gel within a core matrix. This is especially true for the practice of using "cleaned" core samples where solvents are used to remove any oil saturation prior to use.

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