Influences of Dead‐End Pores in Porous Media on Viscous Fingering Instabilities and Cleanup of NAPLs in Miscible Displacements

Abstract

AbstractImproving the understanding of mechanisms involved in a low miscible displacement efficiency is significant for a wide spectrum of applications in subsurface from environment such as groundwater remediation and sequestration to energy extraction such as enhanced oil recovery and geothermal recovery. Two key limiting factors to the efficiency are viscous fingering (VF) instability and dead‐end pores in porous media. Previous research on VF simply assumes all pores are well connected and fluids can be mobilized by convection. However, fluids trapped in dead‐end pores, such as nonaqueous phase liquids (NAPLs) in groundwater remediation, are inaccessible to convection, resulting in even less efficient displacements. Instead of the classic convection‐diffusion/dispersion equation, in this work, we use a fundamentally different capacitance model to incorporate the mass transfer between two pore types in miscible displacements. The hybrid pseudospectral and high‐order finite difference methods are employed to solve the governing equations in a fixed reference frame for simulating the flow dynamics. We found that the viscous fingering instabilities in well‐connected pore network led to an unstable, nonuniform distribution of trapped NAPLs in dead‐end pores network. Different with a traditional view that NAPLs are easily cleaned up, our study indicates the existence of dead‐end pores results in an inefficient cleanup of NAPLs in swept area because of the slow mass transfer of trapped NAPLs between two pore types. A simple model is developed to accurately predict the NAPL concentration behind finger trailing front, which has not been previously examined. Six flow regimes, four of which are new, are then identified.