The Impact of Contractional Flow on the Extensional Viscosity of Polyacrylamide Polymers in Fracturing Applications

Abstract

Abstract High molecular weight polyacrylamide polymers have been used for many years in water floods for enhanced oil recovery (EOR) and as friction reducers in hydraulic fracturing. More recently, so called "high viscosity" versions of these polymers are being used in fracturing operations for friction reduction and also for fracture creation and proppant transport (Hu, 2018). These polymers have more complex viscous properties than materials such as guar gum or cellulose. In addition to the typical shear thinning behavior (shear viscosity), they also may have extensional viscosity thickening in convergent geometries. Separating and quantifying these properties for polyacrylamide polymers is difficult but is needed to predict their behavior not only for flow in porous media but for slot flow in hydraulic fracturing applications. Extensional viscosity in polyacrylamide polymers may be generated in a flow field that experiences a sudden change in geometry such as a contraction. The elongational flow triggers a transition in the conformation of the molecule from a coiled to a stretched state. This conformation change can result in a sudden change in resistance to flow of several magnitudes. Screen-factor measurement used in determining the resistance factor for polymers in EOR applications have been shown to have components of both shear thinning viscosity and extensional viscosity (Lim, 1986). A technique has been developed using contraction flow which can separate shear viscosity from apparent extensional viscosity. The contraction flow technique is useful in determining the effect on the extensional viscosity as a result of changes in molecular weight, copolymer composition, salt effects and shear degradation of polyacrylamide polymers. Examples are given which illustrate how the results are quite useful in the selection of appropriate polymers for fracturing or water flood operations. While the extensional properties are known and the effects of the change in extensional viscosity have been better defined for flow in porous media, the definition and change in these properties have not previously been well defined for fracturing applications. Particularly, the duration of the extensional viscosity change and the effect of extensional viscosity on proppant transport and fracture geometry have not been determined.