Fluid viscosity reduction is commonly used to gauge polymer degradation. Although viscosity reduction indicates polymer degradation, it is misleading to conclude that this reduced viscosity equates to improved fracture conductivity. Polymer fragments which are desolubilized from the gelled fluid no longer contribute to fluid viscosity but do, unfortunately, contribute significantly to proppant pack damage.
A recent study disclosed laboratory procedures to characterize the efficiency of gel breakers based upon the size distribution of the generated polymeric fragments. The study focused on 8-week evaluations of the effects of various breakers on linear, guar-based fracturing fluids from room temperature to 210 F. The studies indicated that enzyme breakers continued to degrade the polymeric molecular weight for at least eight weeks. The molecular weight reduction attributed to the enzyme breakers outperformed oxidative breakers at all conditions evaluated.
This study discloses the results of similar efforts conducted to characterize the efficiency of breakers applied in crosslinked fracturing fluids. The data yield a quantitative profile of the polymeric fragments as well as a measure of the relative degrading efficiencies of the various oxidative and enzymatic breakers. The studies were conducted with borate-crosslinked guar, zirconium-crosslinked guar, and zirconate-crosslinked CMHPG. Detailed analyses of the data are provided.
The post-treatment degradation of polymers used in fracturing fluids is commonly gauged by the observance of the viscosity of "broken gel" recovered at the surface upon return flow. The definition of a "broken gel" in terms of viscosity has been a moving target for several years. A commonly used "rule of thumb" suggests that if the recovered fluid exhibits a viscosity reduced to a value of 10 cps or less the gelled fluid has been degraded adequately to minimize fracture conductivity damage by polymeric residue. However, 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. Conversely, cleavage to reduce the polymeric molecular weight results in an exponential reduction in the solution viscosity. Thus, small reductions of the molecular weight of the polymer are reflected with large reductions in the observed solution viscosity.
Several laboratory studies have been conducted to evaluate breaker performance and the associated molecular weight reduction of fracturing fluid polymers. Almond observed that broken fracturing gels can cause significant flow reduction in porous media. Gall et. al. observed that degradation of the fluid viscosity did not ensure fluid return because the broken fluids contained sufficient amounts of partially degraded residues to damage and restrict the fracture permeability. They also derived a correlation between the log of the polymer average molecular weight and the inverse of the solution viscosity.
A recent effort disclosed the results of a study conducted to characterize the efficiency of gel breakers based upon the size distribution of the generated polymeric fragments. The study focused on 8-week evaluations of the effects of various breakers on linear, guar-based fracturing fluids from room temperature to 210 F. The study confirmed that reduced viscosity is not fully indicative of molecular weight reduction as exhibited by fluids with "adequately reduced viscosity" containing gross concentrations of fragments with average molecular weights exceeding 300,000. Linear gel degradation by oxidative breakers was observed to terminate rapidly as evidenced by minimal reduction of average molecular weight observed after 24 hours.
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