Scaling of Mechanical Degradation of EOR Polymers: From Field-Scale Chokes to Capillary Tubes

Abstract

Polymer flooding is a well-known method for enhanced oil recovery (EOR). Synthetic EOR polymers are susceptible to mechanical degradation. Understanding and mitigating mechanical degradation is a key issue for successful polymer flooding. The main concern of this work is mechanical degradation during choking. Offshore, it is necessary to have control over the injection pressure to each well. During traditional waterflooding, this is achieved by choking the fluid stream by choke valves. In choke valves, there will be sudden change in the flow field, strongly indicating that choke valves will cause mechanical degradation of polymers and thereby reduce the EOR potential of the polymer flood. In this study we investigate mechanical degradation of conventional synthetic EOR polymers. The experiments were performed in commercial chokes and pipes with internal diameters (IDs) from 0.127 to 35 mm, lengths from 13.5 mm to 400 m, and flow rates from 0.3×10−3 to 600 dm3/min, covering several magnitudes of Reynolds number, linear velocities, shear rates, and pressure drop. Using friction factors, f, and Reynolds number, Re, we derived a simple and practical expression for a scaling parameter, τw/η, for degradation of shear thinning polymers in circular tubes at turbulent and laminar flow, where τw=f8ρ⟨v⟩2. At laminar flow, the friction factor is ∝1Re, resulting in a scaling parameter proportional to the velocity-to-radius ratio, ∝⟨v⟩R, equal to the shear rate at the wall and practically independent of viscosity. At turbulent flow, the friction factor is ∝1Reβ, resulting in a scaling parameter at turbulent flow, which is a function of density and viscosity and valid only for high shear rates where the viscosity of shear thinning polymers approaches a fixed value. During the large-scale test, several methods for mitigating or decreasing degradation as a function of pressure drop were identified: decreasing the pressure with several chokes in series, each below a critical pressure drop; decreasing the pressure over a long distance in a linear pressure reducer (LPR); or by choking concentrated polymer solution.