Abstract

Damage to proppant packs is known to negatively impact well productivity. The damage is often the result of inadequate degradation of the polymers used to viscosify fracturing fluids. Gel breakers are used to degrade the polymer by cleaving the macro-molecule into smaller fragments, ideally to a size which can easily be produced during load recovery. Fluid viscosity reduction is commonly used to gauge polymer degradation. However, although viscosity reduction indicates polymer degradation, it is misleading to conclude that this reduced viscosity equates to improved fracture conductivity. Polymer fragments which are desolublized from the gelled fluid no longer contribute to fluid viscosity but do, unfortunately contribute significantly to proppant pack damage.

Several new breaker technologies have been introduced in efforts to improve polymer degradation, and hence, improve fracture conductivity and ultimately well productivity. Many production case histories have been offered as evidence of the utility of the new technologies to improve well productivity. However, the facility to quantitatively determine the polymer degrading efficiency of the breakers has heretofore been lacking. Laboratory procedures, both wet chemical and instrumental, have recently been developed to address characterization of the relative degrading efficiency of the various breakers and breaking mechanisms. The analysis of the combined data provide both a qualitative size distribution, as well as a quantitative profile of the polymer fragments. Extensive studies were conducted employing the new procedures to compare the degrading efficiency of various oxidative and enzymatic breakers, Detailed analysis of the results are provided.

Introduction

Guar-based polymers are used extensively in hydraulic fracturing applications to provide the necessary transport properties to deploy the proppant into the fracture. The polymers are too large to penetrate the typical formation matrix and are consequently concentrated within the fracture due to dynamic fluid loss during the pumping treatment and fracture volume reduction upon closure. Several studies have shown that the polymer can be concentrated from 5 to upwards of 20-fold the surface gelling agent concentration.

The inadequate degradation of the polymers used in hydraulic fracturing fluids is known to negatively impact well productivity. The addition of gel breakers to fracturing fluids is recommended to promote degradation of the polymeric filter cake. The breakers ideally degrade the polymer by cleaving the polymeric macromolecule into small fragments which can be produced from the fracture during load recovery. Viscosity reduction of the gelled fluid is commonly used to gauge the polymer degradation, with the assumption that the observance of a "broken gel" viscosity at the surface upon return flow is proof positive for optimum gel degradation. The definition of a "brokengel" in terms of viscosity has been a moving target for several years. As recently as 15 years ago, a broken gel was defined as one having the viscosity reduced to a value of less than 15 cps as measured at 511 sec-1 on a Fann 35viscometer. Subsequent reports on the high degree of fracture conductivity damage caused by fracturing fluid residues led to the broken gel criteria being reduced to less than 10 cps. Some stimulation engineers are now specifying that returned gels be broken to less than 5 cps.

Although viscosity reduction indicates polymer degradation, 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 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 an exponential reduction in the solution viscosity. Guar solutions broken to a viscosity 3 cps at 160 F with an oxidative breaker have been reported to have average polymer molecular weights in the range of 250,000 to 500,000, with as much as 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. The polymer fragments which are desolublized from the fluid no longer contribute to fluid viscosity but do, unfortunately contribute significantly to proppant pack damage.

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