An Analytical Tool to Predict Fracture Extension and Elastic Desaturation for Polymer Field Projects

Abstract

Induced fractures often play a key role in achieving acceptable injectivity during polymer floods, especially for vertical injection wells. However, fracture extension must be controlled to prevent severe channeling between the wells and compromise the flood performance. This paper presents a physics-based analytical model to predict polymer injectivity and fracture length as a function of polymer rheology, injection rate, and reservoir geomechanical properties. The analytical injectivity model is based on the unified viscoelastic model by Delshad et al. (2008). The injectivity model is coupled with 2-D fracture models: Perkins-Kern-Nordgren (PKN) and Kristianovich-Geertsma- de Klerk (KGD). In addition, the model is coupled with the elastic desaturation curve to predict additional oil recovery due to polymer viscoelasticity as a function of the leak-off rate through the fracture faces. Finally, a sensitivity study is conducted on reservoir properties and polymer rheology to understand the dominant factors that control fracture extension. The analytical model shows good agreement in injectivity and fracture length with two other fracture numerical simulation models (Gadde and Sharma 2001, Ma and McClure 2017). The degree of fracture extension is a strong function of formation permeability, with relatively short fractures predicted for the high permeability characteristics of most commercial-scale polymer floods. We also examine conditions when relatively high leak-off rates through fracture faces might allow the viscoelastic nature of HPAM solutions to displace capillary-trapped residual oil. This is the first analytical solution for coupled polymer injectivity and fracture-length based on real HPAM rheology that can be used by a simple mathematical software or Excel worksheet. The developed tool can assist field operators in reducing the uncertainty and risk in polymer injectivity and quantifying fracture extension in the reservoir.