Experimental and numerical modeling of fluid injection into unconsolidated formations

Abstract

Both the fracturing and acid cleaning processes in unconsolidated reservoirs are examined in this work. Specifically, we first focus on characterizing how the fracture morphology and fluid leakoff are affected by the injection rate, fluid rheology and formation permeability. A series of injection experiments is performed with mixtures of sand and silica flour as analog materials for unconsolidated formations. We show that as the weight percentage of the silica flour increases, the matrix permeability decreases significantly and the capillary effect becomes non-negligible. Compared with the injection experiments with pure sand, an additional dimensionless number incorporating surface tension is introduced in order to characterize the fluid-grain displacement process. Effect of the non-Newtonian fluid rheology is subsequently analyzed using the discrete element method (DEM) coupled with a pore-network model. The quasi-steady-state fluid flow algorithm is improved to enhance numerical stability in modeling fluid injection in a wellbore. It is shown that the high shear rate rheology is critical to the near-wellbore failure and fluid flow. Furthermore, a hybrid phase field method is constructed to model the fracturing process. The benefit of the phase field method is that creation of a fracture could be modeled through explicit consideration of phase change. Finally, the acid cleaning process is simulated using a hydro-chemically coupled scheme implemented in an equation based solver. Effects of the injection rate, the acid reaction rate and the fracture conductivity are examined. Outcomes from this research could serve as guidelines to optimize the field practice.