Projection–Subtraction X-ray Imaging Scheme for Studying Fast Fluid-Dynamics Processes in Porous Media

Imaging of fluid flow at the pore scale in permeable media requires high spatial resolution to observe the topology of fluid in the pore system, along with high temporal resolution to study dynamic processes. The two most popular imaging techniques in modern experiments are microfluidic device imaging and X-ray micro-computed tomography, both having significant limitations as applied to the micro-level. In particular, microfluidic experiments examine flow in quasi-2D system of pores instead of natural 3D geometry of permeable media, whereas X-ray computed tomography (reconstruction of a 3D object representation from a set of 2D projections collected at different rotation angles) is considerably slow when studying fast pore-scale events. In this work, we present a novel approach to examination of local fluid dynamics by combining traditional fast X-ray microtomography and radiographic analysis of successive projections. After initial tomographic imaging of the 3D pore structure, we perform projection-wise analysis comparing differences between two successive projections. As a result, we obtain flow visualization with time resolution determined by the projection time, which is orders of magnitude faster than standard microtomographic scan time. To confirm the effectiveness of this approach, we investigate the pore-scale mechanisms of unstable water migration that occurs during gas-hydrate formation in coal media. We first show that the displacement of brine by methane gas due to cryogenic suction can lead to multiple snap-off events of brine flow in pores. Second, we study a fast local drainage process accompanied by the formation of the gradually swelling gas bubble in the center of the pore. The measured maximum interfacial velocity in our experiments varies from 1.3 to 5.2 mm/s. We also simulate this outflow process accompanied by the bubble expansion and estimate the average brine flow rate during brine-methane displacement.