An Experimental Investigation of Polymer Mechanical Degradation at the Centimeter and Meter Scale

Abstract

Summary In this work, we examine the common understanding that mechanical degradation of polymers takes place at the rock surface or within the first few millimeters of the rock. The effect of core length on mechanical degradation of synthetic enhanced-oil-recovery (EOR) polymers was investigated. We constructed a novel experimental setup for studying mechanical degradation at different flow velocities as a function of distances traveled. The setup enabled us to evaluate degradation in serial mounted core segments of 3, 5, 8, and 13 cm individually or combined. By recycling, we could also evaluate degradation at effective distances up to 20 m. Using low-velocity reinjection of a polymer solution previously degraded at a higher rate, we simulated the effect of radial flow on degradation. Experiments were performed with two different polymers [high-molecular-weight (MW) hydrolyzed polyacrylamide (HPAM) and low-MW acrylamide tertiary butyl sulfonic acid (ATBS)] in two different brines [0.5% NaCl and synthetic seawater (SSW)]. In the linear flow at high shear rates, we observed a decline in degradation rate with distance traveled. Even after 20 m, some degradation occurred. However, the observed degradation was associated with high pressure gradients of 100 bar/m, which at field scale is not realistic. It is possible that oxidative degradation played a significant role during our experiments, where the polymer was cycled many times through a core. This occurrence could significantly affect our suggestion that mechanical degradation still occurs after 20 m or more of flow through a porous medium. The MW of the degraded polymer could be matched with a power-law dependency, MWD ≈ L–x, where x for the HPAM was 0.07 and x for the ATBS was 0.03. In the radial flow, where the velocity decreases by length, the mechanical degradation occurs close to the sandface with only minor degradation deeper in the formation. The length at which degradation reaches a stable condition is not determined. We confirmed previous findings that degradation depends on salinity (Maerker 1975) and MW (Stavland et al. 2010), and results show that in all experiments with significant degradation, most of the degradation takes place in the first core segment. Moreover, the higher the shear rate and degradation, the higher the fraction of degradation that occurs in the first core segment.