Polymer degradation is commonly gauged by fluid viscosity reduction, and viscosity reduction does indicate that polymer degradation has occurred. The conclusion that fracture conductivity and formation permeability are not damaged if the viscosity of the return fluid has achieved a given reduced level, however, may be misleading. Polymer fragments which result from the normal breaking of gelled, cross-linked fracturing fluids no longer contribute significantly to fluid viscosity but do contribute to proppant pack and/or formation permeability damage. Fluids with viscosities of less than 5 cp were found, in this study, to damage rock permeability.
Laboratory evaluations and procedures to characterize the efficiency of gel breakers, based upon the size distribution of the generated polymeric fragments, have been presented in previous studies. Results of core flow evaluations are presented in this study, and demonstrate the relationship of typical molecular weight distributions produced by degraded typical cross-linked fracturing fluids to permeability and production reduction within the rock matrix. Several ranges of core permeability were evaluated.
Data yield a quantitative profile of the extent of formation permeability damage that can be expected based upon polymer fragment distributions and the original rock permeability. In general, rocks with permeabilities in excess of 100 md are more severely damaged than are tight (permeabilities <1 md) rocks. The linkage specific enzyme breaker tested is generally more effective than are the tested nonspecific enzyme breaker and the persulfate breaker.
The inadequate degradation of polymers used in hydraulic fracturing fluids may result in damage to formation and proppant pack permeability and, hence, result in a decrease in well productivity. The degradation of the polymeric filtercake is assisted by the addition of gel breakers to fracturing fluids. Breakers degrade polymers by cleaving the polymeric macromolecule into small fragments which can be produced from the fracture during load recovery. Polymer degradation is commonly gauged by an observed reduction in the viscosity of the gelled fluid, with the assumption that the observance of a "broken gel" viscosity at the surface upon return flow is proof positive optimum gel degradation. The definition of a "broken gel" in terms of viscosity has been revised over the past several years. A broken gel has, in the past, been considered as a gel having a viscosity of less than 15 cps as measured at 511 sec on a Fann 35 viscometer. The broken gel criterion was reduced to less than 10 cps, subsequent to reports of a high degree of fracture conductivity damage caused by fracturing fluid residues by several researchers. Some completion/stimulation engineers are specifying that returned gel be broken to less than 5 cps.
Viscosity reduction does indicate polymer degradation; however, it is misleading to conclude that this reduced viscosity will equate to fracture cleanup and improved conductivity. Solution viscosity is a function of both the polymer concentration in solution and the molecular weight of the polymer. At a given constant concentration, solution viscosity exhibits an exponential relationship with the molecular weight of the polymer used to viscosify the fluid. Cleavage to reduce the polymeric molecular weight results in a exponential reduction in the solution viscosity. Guar solutions broken at 160 F with an oxidative breaker to a viscosity of 3 cps have been reported to have average polymer molecular weights in the range of 250,000 to 500,000, with about 20% of the polymer remaining essentially unbroken at a molecular weight of greater than 2 million. Viscosity reduction may also occur due to the creation of insoluble polymeric fragments by undesirable reactions.