Microfluidic visualisation and analysis of multiphase flow phenomena at the pore scale

Abstract

Micro-fluidic devices have been designed and used to investigate immiscible fluid-fluid displacement processes. Due to their optically transparent nature, such devices allow direct visualisation of pore scale events using light microscopy. In this thesis, we focus on imaging multiphase flow phenomena at the pore scale within specifically designed micro-models using optical and Raman microscopy. The flow channel designs have been selected to investigate fundamental questions relating to fluid displacement mechanisms.

The Lucas-Washburn equation predicts that the distance a meniscus travels within a smooth cylindrical capillary is proportional to the square root of time. However, it is poorly understood how surface roughness of the capillary influences the rate of imbibition. This is of great importance for flow in real porous media. Therefore, the displacement dynamics within a series of single capillaries of increasing surface roughness was investigated. It was found that as the roughness increased, the rate of penetration decreased. The rate of decrease with increasing roughness was observed to be different from recent computer simulation studies.

Using specifically designed single-junction micro-models, the role of pore geometry on fluid displacement during drainage and imbibition processes is investigated via quasi-static and spontaneous experiments under ambient conditions. The experimental results are directly compared to theory and Lattice Boltzmann Model (LBM) simulations. The experimental critical pore filling pressures observed for the quasi-static experiments agree well to those predicted by the Young-Laplace equation and follow the expected filling sequence. However, the experimental spontaneous imbibition results were found to be different from those predicted by the Young-Laplace equation: instead of entering the narrowest available downstream throat, the wetting phase enters an adjacent throat first. Thus, pore geometry plays a vital role as it becomes the main deciding factor in the displacement sequence. This observation may have serious implications for the prediction of displacement processes at the Darcy scale: current Pore Network Models (PNMs) adopt the Young-Laplace displacement sequence for spontaneous imbibition processes and may therefore need to be revised. To further consider these observations, more complex models consisting of multiple connected junctions, are being investigated.

Finally, experiments have been conducted to investigate Raman spectroscopy to understand surfactant enhanced oil recovery (EOR) processes within micro-models. This may prove particularly useful for EOR as micro-emulsion formation is difficult to discern optically. The work presented demonstrates that Raman spectroscopy is an effective technique in visualising pore scale fluid flow processes and distributions.