Effects of Oxygen on Fracturing Fluids

Abstract

The stability of polysaccharide gels at high temperature is limited by such factors as pH, mechanical degradation, and oxidants. Oxygen is unavoidably placed in fracturing fluids through dissolution of air. To prevent premature degradation of the fracturing fluid by this oxidant, oxygen scavengers are commonly used. In this paper, the effects of oxygen and various oxygen scavengers on gel stability will be presented. Mechanical removal of oxygen resulted in surprisingly stable fracturing gels at 275 F. However, chemical removal of oxygen gave mixed results. Test data from sodium thiosulfate, sodium sulfite, and sodium erythorbate used as oxygen scavengers/gel stabilizers showed that the efficiency of oxygen removal from gels did not directly coincide with the viscosity retention of the gel, and large excesses of additives were necessary to provide optimum gel stabilization. The inability of some oxygen scavengers to stabilize the gel was the result of products created from the interaction of oxygen with the oxygen scavenger, which in turn, produced species that degraded the gel. The ideal oxygen scavenger should provide superior gel stabilization without creating detrimental side reaction products. Of the materials tested, sodium thiosulfate appeared to be the most beneficial. Introduction Polysaccharides used in the preparation of fracturing fluids degrade in solution, resulting in premature loss of fluid viscosity and reduced efficiency of the fluid to create the fracture, maintain fracture width, control leakoff, and transport proppant. The fluid viscosity loss is directly associated with the reduction of the polymer size in solution. The molecular weight of a polymer can be reduced through several methods: acid-catalyzed hydrolysis, oxidation, enzymatic action, and mechanical shear degradation. Acid-catalyzed hydrolysis of the connective bonding, a glycosidic linkage of polysaccharides, is a well-documented phenomenon that serves as a basis for the commercial processes that produce various molecular weight fractions from the polymer. The polysaccharide backbone's degree of degradation is dependent on acid concentration, duration, and temperature. This process is the mechanism by which fluids made from guar (a polysaccharide) prematurely lose viscosity in downhole fracturing operations. P. 339

Fracturing fluid cleanup by controlled release of enzymes from polyelectrolyte complex nanoparticles

“…Using 10-20 lbm/1000 gal of sodium thiosulfate is recommended as the most effective stabilizer. [33,34] Gulbis et al [1] reported that even though the mechanism for the stabilizers is not fully understood, it is believed that they prevent the degradation of gel caused by dissolved oxygen by acting as oxygen scavenger. They also stated that high pH fluids (pH=9-11) must be used if a long-term fluid stability is required since guar and its derivatives are hydrolyzed at low pH especially at temperatures higher than 200 °F.…”

A Sulfur-Free and Biodegradable Gel Stabilizer for High Temperature Fracturing Applications

“…Oxygen is unavoidably introduced in fracturing fluids through entrainment of air while pumping the fluid downhole. In these conditions, the diatomic oxygen molecule exists as a diradical, which is prone to react with the polysaccharide, and sets up a radical reaction to initiate a chain scission within the polymer that yields smaller chains at temperatures greater than 200°F (Walker et al 1995).…”