Experimental investigation of trapped oil clusters in a water-wet bead pack using X-ray microtomography

Uncertainties in the quantification of transport properties associated with multiphase flow in porous systems often make the prediction of fluid residence and migration a difficult task. Movement and trapping of immiscible fluids in permeable formations depends upon a complex combination of fluid properties, rock properties, fluid-solid interactions, and forcing conditions. This work consists of using X-rays and visualization techniques to map the distribution of immiscible fluids, particularly trapped oil clusters, residing in a glass bead pack subject to different flow conditions. We analyze the effect of flowing conditions on the evolution of fluid microstructures using X-ray microtomography. Spherical glass beads (0.425-0.600 mm in diameter), a water-wet porous medium, were packed inside a specially designed core holder. High-resolution imaging provides detailed mapping of pore structures resulting from bead packing, and characterization of fluid microstructures formed during sequential water and oil injections. We present spatial distribution of trapped oil clusters for the entire bead pack, as well as mechanistic explanations leading to the fluid configurations observed. We also present simple statistical analyses of blob size, shape, and surface area at the end of different fluid injection cycles. Trapped oil clusters appear in sizes that range from 5.923 × 10-5 mm3 to 3.119 × 103 mm3, where 0.01-0.50 mm3 clusters are most common. About 98% of the total trapped oil at the end of drainage and imbibition cycles corresponds to blobs that are smaller than 1 mm3. It is also shown that most blobs are larger than the mean pore size (0.03 mm3). The mean oil blob size is about 5 times larger than the average pore. A typical blob extends through various interconnected pores, exhibiting elongated of ramified shapes that include multiple voids and constrictions at the same time. The mean aspect ratio of these clusters is less than 2, and the surface area to volume ratio is constant for those larger than 0.1 mm3. Experimental methods and findings presented in this paper are expected to lead to powerful calibration mechanisms for multiphase flow models.