Mechanical degradation of polyacrylamide at ultra high deformation rates during hydraulic fracturing

Abstract

Degradation of drag reducer polyacrylamide under high volume hydraulic fracturing (HVHF) conditions alters its polymer size, distribution and chemical composition, potentially affecting the toxicity and treatability of the resulting wastewater. This study focused on a non-chemical pathway-mechanical degradation of polyacrylamide under ultra-high fluid strain conditions (∼10[superscript 7] s[superscript −1]) that uniquely exist during HVHF but has not yet been explored experimentally. PAM solutions were subjected to an abrupt contraction into a narrow capillary driven by a high-pressure precision pump (∼10 000 psi). The change in polyacrylamide size distribution was evaluated by size exclusion chromatography. The peak polymer molecular weight (MW) after a single-pass through the capillary decreased from 10[superscript 7] to 7 × 10[superscript 5] Da at deformation rate V/R = 4 × 10[superscript 6] s[superscript −1]. The extent of degradation increased with V/R, approximately following an empirical scaling relationship of MW ∝ V[superscript −0:69]/R for the polyacrylamide with an initial MW ≈ 10[superscript 7] Da. Degraded PAM with lower MW (<10[superscript 6] Da) showed minimal degradation during multiple flow passes even at high deformation rates, suggesting that most mechanical degradation occurs at the first entrance into the fracture. Relative to chemical degradation, mechanical degradation caused a narrowing of the MW distribution due to greater degradation of the larger MW polymers and preferential mid-chain polymer scission. In addition, we saw no detectable change in chemical composition during mechanical scission, in contrast to the generation of carbonyl groups during oxygenic radical induced chemical degradation. Combining both chemical and mechanical mechanisms during HVHF operation, we propose an initial mechanical breakage of polymer chain by fluid strain, followed by chemical degradation under the high temperature and appropriate mineralogical conditions. These findings provide critical information for understanding the nature of degradation byproducts from polyacrylamide, and the treatability of polyacrylamide fragment-containing wastewaters.