Injection-induced fractures and polymer injectivity in unconsolidated formations : fundamentals, experiments and modeling




Li, Zihao, Ph. D.

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Polymers are often used for enhanced oil recovery (EOR) and in-situ site remediation. Polymer injectivity is an essential indicator in the project economics. Reduced injectivity due to the high polymer viscosity can be one of the most important challenges in a field polymer flood project, more so than the cost of chemicals. Although injection of viscous polymer is expected to significantly reduce the injectivity, observations from field data of polymer floods often show higher than expected injectivity for viscoelastic HPAM polymers. One explanation for the unexpected injectivity is that the injection pressure may increase above the formation parting pressure (FPP) at which fractures are created from the wellbore. The effects of non-Newtonian rheology on fracture opening and fluid leakoff morphology is largely unexplored compared to Newtonian fluids. Comprehensive studies that explain the polymer-driven fracturing mechanisms are lacking, even in unconsolidated sands. In this study, we study polymer-driven fracture initiation through experiments and numerical simulations. First, we conduct experiments injecting Newtonian, shear-thinning and viscoelastic fluids into a sand-filled Hele-Shaw cell to study fluid invasion and fracturing patterns of sand and injected fluids. The experimental results demonstrate complex fracture patterns depending on injection velocity, fluid viscosity, boundary conditions and fluid rheology. We use digital image correlation to demonstrate that the fracturing mechanism in sand is associated with shear dilation at the tip of fracture and fracture offshoots. Compared to Newtonian fluids, shear-thinning fluids cause thinner fracture openings and more leakoff. Injecting viscoelastic fluid into sand creates a more irregular leakoff pattern, with fluid sometimes leaking mostly at the fracture tip for high injection rates. Second, we investigate the mechanisms behind polymer-driven fractures in cohesionless granular media by coupling the discrete element method (DEM) with computational fluid dynamics (CFD) at the pore/grain scale. Fluid injection can initiate a fracture orthogonal to the minimum principal stress, as expected, through viscous drag forces. However, the displacement field around the fracture is much more complex than what is expected for a linear elastic solid, showing that (1) fracture propagation requires small leak-off (a few times the fracture width) and (2) localized shear deformation ahead of the fracture tip affects fracture morphology. Finally, we simulate the injection of the polymer into the unconsolidated sand. Polymer injection can create fractures in the granular media along the direction perpendicular to the minimum principal stress, thereby reducing wellbore pressure buildup at a constant polymer injection rate. Polymer rheology, water quality, and undissolved polymer also affect the polymer injectivity. This work shows the initiation of polymer-driven fractures in a granular model and demonstrate its implications on polymer injectivity.


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