The Role of Critical Nonwetting Fluid Saturation in Darcy-Based Models of Two-Phase Primary Drainage

Abstract:

Primary drainage is the displacement of a wetting phase by a nonwetting phase where the initial condition is fully saturated with wetting phase. The typical approach to simulating this process involves the solution of coupled mass conservation equations with Darcy-based flux terms, but this method ignores the complex pore-scale processes that influence the propagation rate and shape of drainage fronts. Therefore, we explore weaknesses in the practical application of the continuum-scale Darcy approach for modeling primary drainage by comparing 1D numerical simulations to laboratory core-scale observations. The multiphase properties of cylindrical bead packs are characterized by stepped outflow experiments and standard hydrological models are used to describe water retention and relative permeability, such as van Genuchten-Mualem. Subsequently, we generate predictions of drainage front breakthrough time with Tough2 and compare them to observations of gravitationally stabilized primary drainage at low capillary number. We find that Corey’s critical nonwetting saturation parameter must be used in relative permeability curves in order to match observations, and that the best-fit value is flow-rate dependent and grid-size independent. We also argue that the value of this parameter is not constrained by current experimental methods or physical arguments, and that it is an important but irreducible source of uncertainty in the standard approach to multiphase flow. This highlights the need for a scalable model that incorporates dynamic percolation thresholds with dependency on pore-scale processes.