Particle straining in vuggy porous media

Khan, Hasan Javed
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Formation damage is the loss of inherent permeability of porous media and is of vital importance in petroleum engineering. One important mechanism of formation damage is particulate straining, where suspended solid particles jam the flow pathways and reduce its flow capability. Most studies of particulate transport have been performed in porous media of uniform pore size, such as sandstones and sandpacks. Carbonates often contain large openings, called vugs, which can affect the flow and transport properties (particle straining) of the porous medium. In this study, particle straining experiments are performed on multiple configurations of synthetic vuggy media. A novel method is introduced to generate synthetic vuggy glass bead cores: glass beads, with dissolvable inclusions, are sintered in the presence of air in a muffle furnace and later dissolved with a core flood. Smaller-sized glass beads are injected at multiple flow rate and injection concentrations and changes in porosity, permeability, vug size, and particle effluent volume are monitored using continuous pressure measurements and computed tomography scanning. The results are combined with quasi-2D streamline simulations to understand the particle deposition patterns in these vuggy media. Furthermore, spontaneous imbibition experiments are conducted on multiple configurations of these synthetic vuggy media and capillary rise is measured. Rudimentary vug-pore flow models are generated, and solved for spontaneous imbibition using a computational fluid dynamics solver, to better explain the behavior observed in the experiments. The results can be summarized as follows: (i) particles penetrate and deposit at a deeper depth when a vug is present; (ii) the particles deposit on the vug-matrix boundary and result in a smaller and smoother vug; (iii) the maximum change in the vug is observed at the bottom of the vug; and (iv) more particle deposition occurs in the matrix around the vug. The high permeability vugs cause flow convergence, which increases the particle-particle and particle-matrix interaction, and results in an increased number of particles dropping out of the flow stream. Each vug has a sphere of influence within which it will affect the particle flow pathway; if the sphere of influence of two vugs overlap, the particle will be affected by both the vugs, with the dominance depending on the separation between the particle and the vug. The results suggest that for the vug conditions studied, the vugs in series focus the flow and increase the depth of particle deposition and the total volume of particles deposited in the core. The vugs in parallel acts independently as separate vugs, with their separation greater than their individual sphere of influence, and do not dictate the deposition of injected particles. In future, this study can be carried forward by conducting experiments inside a CT machine, to enable time-lapse particle deposition maps, on proxy vuggy media or real rocks. Coupled with real-time porosity, permeability, and possibly resistivity measurements, new near-wellbore interpretative models can be envisioned for improved formation evaluation of vuggy carbonates