SYNOPSISUltracentrifugal techniques have been employed to investigate the degradation of hydroxypropyl guar (HPG) solutions, and of gels formed with a delayed zirconium cross-linker, by ammonium persulphate at 90°C. Molecular weights, M , for various samples of degraded polymer were determined by sedimentation/ diffusion, and an empirical relationship was established between M and the sedimentation coefficient at a single, low concentration. This enabled the straightforward determination of M as functions of persulphate concentration, cross-linker concentration, and time at temperature. It was shown that large decreases in M occur within the first hour at temperature, but that complete degradation is not achieved. It was further shown that, when a delayed zirconium cross-linker is used, degradation may be less effective when gels are formed and degraded at high shear rates.
I NTRO DUCT10 NHydraulic fracturing is employed to increase the flow of crude oil or gas from a reservoir. A fluid containing "proppants," typically sand or ceramic beads, is pumped into the reservoir under high pressure to fracture the rock. The proppants prevent the newly created channels from subsequently closing up. A water-soluble polymer is included in the fluid and cross-linked in situ to form a gel. This ensures effective proppant placement and helps control fluid loss. After fracturing, a "breaker" brings about degradation of the polymer to give a low viscosity fluid that can be withdrawn leaving behind the proppants. Both the timing and the efficiency of degradation are crucial; if breakdown of the gel occurs too fast, the proppant may drop out of suspension prematurely, whereas if breakdown is incomplete, residues may remain in the fracture that impede the flow of oil or gas.Gall and Raible 'v2 have previously used size-exclusion chromatography (SEC) to study the degradation behavior of fracturing fluid polymers. In the present work, ultracentrifugal techniques3 are employed for the first time in a n investigation of hydroxypropyl guar ( H P G ) as a fracturing fluid polymer. Advantages of ultracentrifugal methods over SEC are that ( i ) no filtration of solutions is required prior to analysis and (ii) absolute rather than relative molecular weights may be obtained. HPG is a derivative of guar gum, a polysaccharide obtained from the seed of the guar plant, Cyamopsis tetragonolobus. Chemically, guar gum consists of a backbone of D-mannose units joined by /3-1,4 linkages, with D-galactose units attached a t intervals by a-1,6 linkages ( Fig. 1). In HPG, 40% or less of the sugar hydroxyls are substituted with hydroxypropyl group^.^ HPG forms a viscous solution a t low concentrations in water, which may be converted to a gel by the addition of cross-linking agents such as polyvalent cations. Degradation of the polymer in reservoirs a t temperatures of 60-115°C may be brought about with oxidant breakers such as ammonium persulphate.