The role of residual oil in the mechanistic simulation of foam flow in porous media: Experiment and simulation with the population-balance method

Since the late 1950's, coinjection of dilute aqueous surfactant solutions into oil reservoirs with drive gases has been recognized as a method of decreasing the gas flow mobility and increasing the oil recovery from the reservoir. This is because a very low mobility foam is generated in situ. However, actual use of foam in enhanced oil recovery at the field scale has been limited due to the lack of understanding of the complex flow properties of foam preventing thea priori design and optimization of proposed foam processes. Recently, the population-balance method, which tracks the pore-level structure of foam and relates that structure to the continuum gas-phase flow resistance, has successfully modeled a variety of foam flow experiments in the absence of oil. Since in most cases oil destabilizes foam, the effect of oil on foam flow is an important, but previously missing, element of the population-balance model.; The goal of this dissertation is to extend the current population-balance model to include mechanistically the effects of residual oil on foam flow in porous media. The primary effect of residual oil is the destabilization of foam lamellae as they move across oil globules trapped within the porous medium. Because a universally accepted mechanism of foam coalescence by oil is not currently available, we use a physical model to study experimentally the movement of foam lamellae across nonwetting surfaces that emulate oil droplets. We find two rupture mechanisms. Above a critical velocity, foam ruptures by a Plateau-border-depletion mechanism, whereas at lower velocities, typical of enhanced oil recovery, rupture is by a Plateau-border pinch-off mechanism. Based on the observed pinch-off mechanism, we develop a mechanistic rate of foam coalescence due to oil for use in the population-balance model. The rate expression is based on gas velocity, oil saturation, and the interfacial properties of the gas, surfactant solution, and oil.; We conduct foam-flow experiments at velocities less than 1 m/day in Berea sandstone with permeabilities ranging from 0.28 to 10 μm2, both in the absence and presence of residual oil. Transient saturation and pressure profiles are measured with a scanning microwave attenuation apparatus and pressure transducers placed along the core length. Good agreement is seen between experiment and the extended population-balance model. Both experiment and simulations demonstrate that foam is more stable and has a lower relative mobility in high-permeability media than in low-permeability media. This is because capillary pressure is higher in low-permeability media, and foam films and pseudoemulsion films separating gas bubbles from oil droplets are less stable. This behavior indicates that foams can be designed to divert flow from previously swept high-permeability regions of an oil reservoir and direct flow to oil-containing low-permeability regions. The extended population-balance model provides a new tool for designing foam displacement processes in oil reservoirs.