Abstract
Polyacrylamide-based friction reducers (FR's) - including viscosifying polyacrylamides, which are designed to decrease proppant settling by increasing molecular weight and/or active material in the FR - are used extensively in high-rate fracture stimulations. However, because polyacrylamides are difficult to break, there have been concerns about how these materials impact fracture conductivity and formation permeability. This study presents the effect of conventional and novel oxidative breakers over the viscosity and colloidal size distribution of the broken polymers.
Breakers tested include conventional persulfates, perborates and patent pending peroxides, all of which generate free radicals to degrade partially hydrolyzed polyacrylamides (PHPAs). Breakers were tested at bottomhole temperatures encountered in the Permian, Bakken, Haynesville and Eagle Ford. Changes to PHPA viscosity were determined using vibrational viscometers. Size distributions and percentage of the broken colloidal PHPA were determined by dynamic light scattering. This method can measure sizes down to 0.6 nanometers, which is within the range of even the smallest pore-throat sizes in shales.
Light scattering revealed surprising anomalies in breaker performance. When aged at temperatures typical of the Permian, each of the tested breakers at each of the varied concentrations caused similar levels of viscosity reduction but different size distributions. Some breakers had the unwanted effect of narrowing the colloidal size fractions to the lower end of the spectrum. At these small sizes, colloids are more likely to overlap with segments of the pore throat distribution in some shales, which could inhibit production. In addition, when the FR was aged at the higher temperatures encountered in the Bakken, Eagle Ford and Haynesville, some breakers were not able to uniformly break the PHPA. In these cases, FR's without breakers delivered superior performance. The results clearly demonstrate that breakers may not always have the desired effect of increasing the formation's permeability. In fact, depending on the type of breaker and the concentration, they can often have detrimental effects that ultimately hinder production.
