Understanding the Mechanism of Breaking Polyacrylamide Friction Reducers

Abstract

Partially hydrolyzed polyacrylamide (PHPAM)-based friction reducers (FRs) are widely used in stimulation treatments because of their favorable economics and operational simplicity. The use of these materials, particularly at higher concentrations, has increased substantially in the past two years. FR materials have extremely high molecular weight; consequently, breakers are recommended to reduce polymer size and to mitigate potential damage to fracture conductivity, microfractures or secondary fractures, and the formation. A better understanding of the breaking mechanism of FRs and the performance of various types of common oilfield breakers will help to improve job designs and to maximize the benefit of slickwater fracturing. Radical-generating breakers (RGBs), persulfate- and non-sulfate-containing peroxygen breakers, and non-radical-generating breaker (NRGB), bromate breaker, were studied in the breaking of a PHPAM-based FR. Static break tests were performed at 150, 200, and 290°F with a setpoint of 8 gal/Mgal FR using two RGBs, two NRGBs, and their corresponding encapsulated versions. The efficiency of the breakers, at various concentrations and temperatures, was evaluated by measuring broken fluid viscosities, determining molecular weight, and performing regained permeability testing. The kinetics of the breaking reaction was determined by studying the molecular weight profile by gel permeation chromatography-multiangle laser light scattering (GPC-MALLS) of a fluid over time. This study reveals that RGBs break PHPAM rapidly and more effectively than NRGBs. At 290°F, it is shown that NRGBs must be used at eight times the concentration of RGBs to achieve the same break quality and time. Encapsulated RGBs perform more slowly than non-encapsulated RGBs and may provide an effective method of retaining viscosity below 200°F, which may aid in proppant transport and placement. Encapsulated NRGBs are extremely sluggish in reducing fluid viscosity and require very high loadings to achieve results comparable to RGBs. Fluid formulations designed with the correct RGBs, at an optimal concentration, render a steady viscosity decrease and a sufficiently low final viscosity, near that of water, to aid in cleanup and flowback. FR broken with RGBs offer high regained permeability, indicating effective and complete breaking of polymer chains. These results are significant for well productivity and confirm that the degradation of FRs proceeds in a radical pathway, rather than in a pure oxidation manner. Strong oxidants, if incapable of generating radicals, are not good candidate breakers for FRs, regardless of their oxidative potentials. Random chain-scission seems to be the most efficient means of breaking PHPAM FRs. This study clearly demonstrates that RGBs degrade polyacrylamide FRs effectively, whereas NRGBs are not recommended under the same treatment conditions, even at significantly higher concentrations. An Ubbelohde capillary viscometer has proven to be useful in distinguishing minimal viscosity differences between low viscosity fluids. A comparison of regained permeability results for a control and broken fluid confirm the need for an effective breaker for FR. The GPC-MALLS method enables degraded FR fragments to be studied at a molecular level to provide insight into further improvements of the slickwater fracturing fluid design.