Imaging of compositional gradients during in situ emulsification using X-ray micro-tomography

The rate of emulsification in surfactant/oil/water systems is influenced by transport of chemicals and mixing of the fluid phases. In porous media applications, complex flow regimes are generated due to three-dimensional connectivity and irregular cross-sections of the pores facilitating the mixing for emulsification. The properties of the resulting emulsified phase depend on the interplay of flow, mixing and emulsification kinetics of the surfactant/oil/water system. Emulsification can be relatively quick. Direct visualization of the process and compositional gradients in three-dimensional pore space during flow requires imaging at few seconds time intervals. In this study, a flow unit was integrated in a synchrotron beamline-based fast X-ray computed micro-tomography set-up. Non-destructive three-dimensional visualization of multi phase flow inside a porous rock at flow conditions became viable. An oil saturated rock sample was first flooded with water, followed by surfactant solution to mobilize the remaining oil by miscible displacement. The sample was continuously imaged during injection; the scans were made at time intervals of 7-60 s. The presence of an emulsified phase in addition to the oil and the aqueous phases required a more advanced image processing workflow compared to the workflows used for the immiscible fluid systems. A newly developed image processing technique was adopted; the grey levels in the images were correlated with the local oil content in the emulsified fluid regions. The visual extractions of the pore space showed that the emulsification occurred within seconds. Compositional gradients were observed in the emulsified phase as the injected surfactant solution reached the remote locations in the pore space. While a significant fraction of the oil was displaced within few seconds, the compositional gradients persisted over several millimeter length for several minutes, illustrating a sequence of mobilization and solubilization of the oil phase. The ability to interpret such compositional gradients in real time in porous space brings capability to study interfacial phenomena in applications where in situ emulsification occurs under flow. (C) 2019 Elsevier Inc. All rights reserved.