Measurements of capillary pressure and relative permeability curves for foam transport in porous media-A capillary bundle approach

Abstract

Foam mobility in porous media is governed by bubble texture, foam trapping, bubble generation, and bubble coalescence. Published reports showed that foam mobility is dominated by foam trapping in the transient flow regime or low capillary pressures (Pc), while bubble coalescence dominates foam flow in the steady state regime or higher Pc. An efficient foam model is expected to capture all these pore scale phenomena. However, most of the available models do not adequately account for foam trapping behavior because their model parameters are usually determined from coreflood experiments at steady state flow regimes. Hence, they are unsuitable for simulating transient foam flow behavior. Some mechanistic population balance models incorporate the effect of foam trapping on gas mobility through the relative permeability (Kr) function such that the Kr of the flowing foam is a function of only the flowing foam saturation. However, a constitutive equation is required to predict the flowing foam fraction. Some of the variables in this equation are also obtained from experimental data at steady state, while there is no known experimental method to measure other variables. In this paper, a novel coreflood method is presented that allows flowing foam fraction to be measured directly at different Pc and water saturations during foam flow. A consolidated porous medium was treated like a bundle of capillaries of various sizes, by modifying the surfactant alternating gas (SAG) method of foam injection. An electrical resistivity tool monitored in-situ saturation changes while a high resolution pressure transducer measured the pressure drop across rock samples. Multiple SAG cycles generated a data array of trapped and flowing foam as a function of water saturation and capillary pressure, which was then used to generate Pc and Kr curves. The Pc and Kr curves exhibit behavior that are consistent with established physics of foam flow in porous media. The measured Pc curves also compared well with porous plate Pc measurements, which thus validates the accuracy of the methodology in treating a consolidated porous medium as a bundle of capillaries. The measured foam limiting capillary pressure values also showed a good trend with those reported in the literature. The critical water saturation for foam was much higher than the connate water saturation measured from the porous plate method. Finally, it was shown that this method has a strong potential to be implemented at a field scale.