Review of foam stability in porous media: The effect of coarsening

Foam stability in porous media is crucial to industrial applications such as enhanced oil recovery (EOR) and CO2 geological sequestration. Though many studies of foam stability in porous media have been carried out, the results from these studies often interpreted the combined effects of drainage, coalescence, and coarsening. This convoluted interpretation does not add to the understanding and contribution of the isolated foam instability mechanisms. Coarsening or Ostwald ripening dominates the evolution of foam structure in porous media due to the scale effect. To bridge the knowledge gap, a comprehensive review of the mass transfer fundamentals and the experimental methods for observing the foam coarsening process in porous media was conducted. Unlike an open system where bubbles can grow freely, the restriction from the geometric confinement in the rock (or rock-like) matrix decreases the foam coarsening rate. The confinement can even reverse the mass transport direction to go from larger bubbles to smaller bubbles (anti-coarsening). Various foam coarsening models based on Fick's diffusion law from the bubble-scale to the bulk-scale are reviewed to better understand the foam coarsening mechanisms. From the reviewed literature, foam coarsening dynamics are dominated by the average bubble size, the liquid film thickness between bubbles, and the gas-liquid interfacial properties. The proper selection of the foaming agents (surfactants and nanoparticles) and the gas phase (N2 and CO2) is critical to control the foam coarsening kinetics. The review of the experimental methods shows that both optical and Atomic Force Microscopy (AFM) are commonly used in studying the foam coarsening process at the bubble scale. X-ray microtomography (mu CT) provides a robust tool to investigate the temporal evolution of trapped gases at the pore scale in real rocks. Microfluidics allow seeing the foam kinetics in 2D and 2.5D feature representations of the porous media. The fabrication of 3D micromodels with controlled wettability remains a challenge. Additionally, the challenge of capturing the dynamics of the foam coarsening process with moving bubbles remains understudied and ill-resolved.