Abstract

Fracturing treatments in tight-gas sands often suffer disappointing production results. Conductivity damage caused by immobile gel filtercakes is often cited as a probable damage mechanism. Breaker requirements for reducing the viscosity of filtercakes remain a challenge, particularly at suspected filtercake concentrations of greater than 600 lbm/1,000 gal. Previous studies measured the rheology of non-crosslinked hydroxypropyl guar (HPG) across broad ranges of molecular weight (MW), temperature, concentration, and shear rate. The reported rheology model provided a means to experimentally evaluate breaker requirements to achieve a target uncrosslinked filtercake viscosity. Experiments were conducted to determine kinetics of polymer degradation by simple hydrolysis at temperature and degradation by sodium chlorite and sodium persulfate breakers. Temperatures ranged from 85 to 280°F, while breaker concentrations ranged from 0 to 80 lbm/1,000 gal. HPG concentration was held at 1 lbm/gal of 260 kg/mole HPG. The viscosity profiles during the experiments were matched with the rheology model to infer MW decline profiles and thereby determine the rate laws for reaction with the polymer.

It was found that efficiencies of breakers on breaking backbone linkages of HPG were only 2 to 5%, with the remaining breaker consumption apparently not reducing the molecular weight of the polymer. The rate laws for the breakers were found to be first order in breaker and first order in linkage concentration. The rate laws were used to make projections on the amount of breaker required to reduce concentrated gels to an arbitrary target viscosity.

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