Kamaljit Singh, Tom Bultreys , Ali Q. Raeini, Mosayeb Shams, Martin J Blunt 

The displacement of a non-wetting fluid by a wetting fluid in porous media, called imbibition, is important in many natural and industrial processes. During imbibition, the wetting fluid invades the pore space through a series of competitions between piston-like displacement, film and corner flow, snap-off, pore bypassing and trapping. Our understanding of these fundamental pore-scale displacement processes in natural three-dimensional porous media with complex pore geometries is still incomplete. Here, we use time-resolved three-dimensional synchrotron X-ray micro-tomography to visualize and quantify these processes in a water-wet carbonate rock sample. We utilize a pore-throat network extracted from the segmented pore space and superimpose it on experimental three-dimensional time-series datasets to obtain properties of pores and throats involved in 183 displacement events. Our findings suggest that the occurrence of snap-off or $I_n$ pore-filling events strongly correlates with the aspect ratio (the ratio of pore radius to throat radius involved in the displacement event), and weakly with the throat shape factor and other pore-throat parameters. In addition, we have found a new type of snap-off event that occurs in pores rather than in throats, which is related to the pinning of fluid-fluid interfaces at rough surfaces resulting in a contact angle more than 90$^\circ$ during displacement. This type of pore snap-off is independent of the aspect ratio and leads to the formation of a non-wetting fluid ganglion occupying a fraction of a single pore body. By incorporating these new events and the correlation of displacement events with pore-throat geometry, existing models can be improved and validated.