Pore-scale modeling of flow in particle packs containing grain-coating and pore-filling hydrates: Verification of a Kozeny-Carman-based permeability reduction model

The permeability of the methane gas hydrate-bearing sand, which reflects the hydrate saturation and morphology of the sediments, significantly influences the rate of gas production. In this study, we formulated permeability reduction models for cubic packs of cylinders and spheres and random sphere packs with the grain-coating (GC) and pore-filling (PF) hydrates. Our models were based on the Kozeny Carman equation. Three assumptions were made: (1) electrical tortuosity could be used in place of hydraulic tortuosity, (2) the effect of hydrate saturation on the shape factor could be ignored, and (3) the presence of an overlapping surface area of the GC hydrate could also be ignored. To confirm the validity of these assumptions, we conducted a series of computational fluid dynamics simulations of particle packs with the GC and PF hydrates generated by the discrete element method. Assumptions (2) and (3) caused the simulated normalized permeability of the GC hydrate to diverge from the proposed models because the simulated hydraulic tortuosity and shape factor of the PF hydrate were different from their analytical counterparts. However, because these differences canceled out at low hydrate saturations, the simulated normalized permeability of the PF hydrate agreed well with the proposed model. This agreement disappeared as the hydrate saturation increased because the cancellation effect atrophied. We compared the prediction accuracy of the proposed models with that of existing models in our simulations and with published experimental results. The proposed models agreed well with the simulation results and the experimental data. We showed that the parameters of the proposed model had a physical meaning: the average size of small clusters of PF hydrates. Analyzing the values of this parameter, we found that the PF type was the dominant morphology in natural core samples used in previous experimental studies. (C) 2017 Elsevier B.V. All rights reserved.