Polymer solutions, in contrast to water, exhibit non-Newtonian rheological behavior, such as in-situ shear-thinning and shear-thickening effects, leading to a varying viscosity distribution in the reservoir. Consequently, a different pressure distribution and a rather slower pressure decline rate are exhibited during Pressure Fall-Off (PFO) test (compared to a waterflooding case). Therefore, applying a different interpretation method, compared to conventional approaches for Newtonian fluids is required.
In this paper we provide a new, simple and practical method to infer the in-situ polymer rheology and the induced fracture dimensions from PFO tests performed during polymer injection. This is based on a combination of numerical flow simulations and analytical pressure transient calculations, resulting in generic type curves that are used to compute consistency index, flow behavior index, fracture dimensions, and reservoir parameters (kh, faulting, etc.) from the measured pressure derivative curves.
The novelty of this study are the analysis of interference caused by fracture sizes on the radial stabilization of polymer, as well as the use of realistic polymer rheology, combined with an analysis method that derives polymer rheology parameters based on the pressure derivative curve.
This method can be used for interpretation of PFO tests on existing EOR polymer flooding projects, where monitoring of injection performance and of in-situ effective polymer rheology are key in the success of a project.