The effect of rock permeability and pore structure on foam in carbonate rocks

Abstract

Foam is considered a successful method for providing conformance control during enhanced oil recovery. It helps reduce gravity segregation, viscous fingering, and minimizes injected fluid loss into high permeability zones. Foam rheology has been studied and well understood in sandstones but its behavior in carbonate rocks is not well defined. This study aims to quantify the impact of pore properties represented by permeability, pore throat size (dT), pore body size (dB) and aspect ratio (R) on foam rheology in carbonate rocks. The results are contrasted with foam rheology in natural sandstone rocks. The studied carbonates possess complex pore structures, with bimodal distributions of pore throat and pore body sizes. Pore properties were shown to affect the magnitude of foam viscosity (μa)-foam quality relationship, but the observed trend is unchanged in different rocks. μa was found to linearly correlate with (dT2) and permeability. It was shown to increase exponentially with dB with a marked increase at dB > 30 μm. All the carbonates had aspect ratios R ≫ 2.5 indicating plentiful foam germination sites. As a result, the aspect ratio does not correlate strongly with μa. Pore structure properties were shown to have a strong impact on the μa-shear rate (γ̇a) relationship, by directly impacting the magnitude of both μa and γ̇a, and also by influencing the foam texture. Finally, pore properties were shown to impact factors governing foam transient behavior. Rocks with smaller (dT) are shown to have weaker foam generation and propagation, as well as a larger specific surface area leading to larger dynamic adsorption. Overall, the results of this work indicate that the successful implementation of foam for mobility control in carbonate reservoirs will depend on a good understanding of the rock pore structure.