Spatial and temporal patterns in particle retention in vuggy porous media

Permeability interpretation of well logs in carbonates is difficult due to the vuggy porosity and its connectivity and, on top of it, potential formation damage in and around the vugs. Here we perform an experimental study to determine how single and multiple vugs alter particle retention and ultimately permeability. We previously devised a way to create repeatable vuggy media with different vug spatial distribution (Khan et al., 2019). Vugs are created by adding dissolvable salt inclusions to a 1.0 mm glass bead mixture placed in a graphite mold and sintering in a muffle furnace in the presence of air. Flooding the core after it cools dissolves the salt and creates void spaces. We then inject smaller-sized (50 micron) glass beads in the top-down direction in the porous medium, and perform time-lapsecomputed tomography (CT) scans for measuring spatial distribution of injected particles. We find that initially the injected particles deposit at the upstream side of the vug, with minimum deposition in the matrix. Deeper in the core and at later times, the injected particles start depositing as a layer on the wall of the vug, reducing its cross-sectional area. Thickness of the layer increases with time, rough undulations fill out first, and results in a smooth exterior surface. The behavior rapidly changes as we go past the center of the vug (largest vug cross-section), with little to no particle deposition observed on the walls of the vug. The maximum change in this region is observed at the bottom of the vug, where the particles are trapped as they are exiting the vug. Deposition in the matrix increases with time, with particles being deposited later in the regions furthest away from the vug. We conclude that the flow converges at the top of the vug, resulting in higher particle-particle interactions and therefore higher deposition. The permeability contrast bends the streamlines towards the top half of the vug, with particles being deposited on the walls of the vug. In the bottom half of the vug, the streamlines are exiting along the side walls of the vug. Gravity dominates particle transport and they drop to the bottom of the vug. We present a novel study that shows the development of particle retention patterns in a vug-matrix system over time and space and the influence of a vug on particle deposition in the matrix and/or the other vugs. This could potentially revolutionize permeability interpretation of well logs in vuggy carbonates.