Shear Degradation of Partially Hydrolyzed Polyacrylamide Solutions

Partially hydrolyzed polyacrylamide solutions are highly shear degradable and may lose much of their effectiveness in reducing water mobility when sheared by flow through porous rock in the vicinity of an injection well. Degradation is investigated by forcing polymer solutions, prepared in brines of various salinities, through consolidated sandstone plugs differing in length and permeability, over a plugs differing in length and permeability, over a wide range of flow rates. A correlation for degradation based on a theoretical viscoelastic fluid model is developed that extends predictive capability to situations not easily reproduced in the laboratory. Mobility-reduction losses in field cores at reservoir flow rates are measured following degradation and are found to depend strongly on formation permeability. Consideration of field applications shows that injection into typical wellbore geometries can lead to more than an 80-percent loss of the mobility reduction provided by undegraded solutions. Also discussed are consequences for incremental oil recovery and the possibility of injecting through propped fractures. possibility of injecting through propped fractures Introduction Susceptibility of commercially available, partially hydrolyzed polyacrylamides to mechanical, or shear, degradation represents a serious problem regarding their applicability as mobility-control fluids for secondary and tertiary oil recovery applications. The approach taken in this work assumes that surface handling equipment in the field (pumps, flow controllers, etc.) have been adequately designed to minimize effects of shear degradation in all operations preceding actual delivery of the polymer solution to the sand face. The remaining problem is to assess the mechanical degradation a polymer solution experiences when it enters the porous matrix at the high fluxes prevailing around injection wells. Ability to predict the degree of mobility-control loss based on a laboratory investigation of the relevant parameters is desirable. White et al. were the first to attempt prediction of matrix-induced degradation, but the result was only a recommended injection-rate limit for minimizing polymer degradation for two specific wellbore completions. More recent papers offer limited data supporting the contention that matrix-induced degradation of polyacrylamide solutions results in significant loss polyacrylamide solutions results in significant loss of mobility control . This paper investigates the cause of mechanical degradation in dilute polymer solutions and presents experimental data on the effects of polymer concentration, water salinity, permeability, flow rate, and flow distance. permeability, flow rate, and flow distance. Several interesting and unexpected conclusions are drawn from the results. BACKGROUND - THEORETICAL CONCEPT The mechanical degradation of polymer solutions occurs when fluid stresses developed during deformation, or flow, become large enough to break the polymer molecular chains. Historically, the feeling has been that shearing stresses in laminar shear flow or turbulent pipe flow were responsible for chain scission. However, recent data reported by Culter et al. suggest that degradation of viscoelastic polymer solutions in capillary tubes may be dominated by large elongational or normal caresses occurring at the entrance to the squared-off capillaries. Such stresses result from Lagrangian unsteady flow, or elongational deformation, at the tube entrance. Flow through porous media also generates velocity fields that are sufficiently unsteady, in the Lagrangian sense, to lead one to anticipate large viscoelastic normal stresses. Viscoelastic fluids are materials that behave like viscous liquids at low rates of deformation and partially like elastic solids at high rates of partially like elastic solids at high rates of deformation. Several constitutive models are available for describing the stress-strain behavior of such fluids. SPEJ P. 311