Impact of surfactant depletion on foam in porous media

Foam can be applied as an Enhanced Oil Recovery (EOR) process. Foam stability in porous mediadecreases with decreasing surfactant concentration. It is also known that foam collapses belowa “limiting water saturation” in porous media. However, there isn’t a complete theory for the relationship between surfactant concentration and the water saturation below which foam coarsens ina specific porous medium. The aim of this study was to find a relationship between the surfactantconcentration, the foam bubble radius, and the limiting water saturation. This research gathersand analyses experimental data from the literature on foam properties in porous media. The foamswere stabilized with the same anionic surfactant (AOS), at different surfactant concentrations andporous media.The experimental data shows that for a specific porous medium, the limiting water saturation exponentially decreases with increasing surfactant concentration. These results can be explainedby the surfactant depletion from the solution to the gas-water interface. This work shows that thelimiting water saturation approximates the water saturation for which the gas-water interfacial-areais equal to the surface-area that could be covered by the surfactant molecules in the surfactant solution in the given porous medium. A fundamental assumption in this calculation is that gas bubblesin the porous medium correspond to pore size, as is thought to apply to foams at water saturationsabove the limiting water saturation.The general implication of these results is that from two known parameters, the third parameter canbe calculated (the three parameters are surfactant concentration, the average foam bubble sizeand limiting water saturation). A possible implication on the modelling of the foam flood in porousmedia was investigated in this research. The observed relationship was applied to a simulation ofsurfactant-alternating-gas injection in a homogenous reservoir with a uniform residual oil saturation. In this simulation the limiting water saturation was a function of the foam bubble size and thesurfactant concentration