An Experimental and Numerical Investigation of Crossflow Effects in Two-Phase Displacements

Summary In this paper, we present flow visualization experiments and numerical simulations that demonstrate the combined effects of viscous and capillary forces and gravity segregation on crossflow that occurs in two-phase displacements in layered porous media. We report results of a series of immiscible flooding experiments in 2D, two-layered glass bead models. Favorable mobility-ratio imbibition and unfavorable mobility-ratio drainage experiments were performed. We used pre-equilibrated immiscible phases from a ternary isooctane/isopropanol/water system, which allowed control of the interfacial tension (IFT) by varying the isopropanol concentration. Experiments were performed for a wide range of capillary and gravity numbers. The experimental results illustrate the transitions from flow dominated by capillary pressure at high IFT to flow dominated by gravity and viscous forces at low IFT. The experiments also illustrate the complex interplay of capillary, gravity, and viscous forces that controls crossflow. The experimental results confirm that the transition ranges of scaling groups suggested by Zhou et al. (1994) are appropriate/valid. We report also results of simulations of the displacement experiments by two different numerical techniques: finite-difference and streamline methods. The numerical simulation results agree well with experimental observations when gravity and viscous forces were most important. For capillary-dominated flows, the simulation results are in reasonable agreement with experimental observations. Introduction Streamline methods are very efficient numerical techniques for field-scale reservoir simulation, but they are not without limitations. They treat flow along each streamline as independent of adjacent streamlines and therefore do not typically represent crossflow in the simulations. If users of streamline methods are to interpret simulation results reliably, they will need to assess whether any of the mechanisms not modeled in the simulations ar. important enough to limit the accuracy of the simulations appreciably. Transfer of fluid in the direction transverse to streamlines can result from diffusion and dispersion, crossflow caused by viscous and capillary forces, and gravity segregation. The scaling of diffusion and dispersion has been investigated in a number of previous studies. If the injected gas is miscible or partially miscible with the oil, diffusion and dispersion mechanisms may play a significant role in the displacement (Mohanty and Johnson 1993; Fayers and Lee 1994; Tchelepi 1994; Jiang and Butler 1994; Burger and Mohanty 1997). In particular, Burger and Mohanty (1997) showed that diffusion through the oil phase can limit mass transfer from oil residing in low-permeability regions. Similar arguments can also apply to other mechanisms of crossflow: viscous and capillary crossflow as well as gravity segregation (Fayers and Lee 1994.Burger and Mohanty 1997; Zapata and Lake 1981; Zhou et al. 1994).