Pore-Scale Determination of Gas Relative Permeability in Hydrate-Bearing Sediments Using X-Ray Computed Micro-Tomography and Lattice Boltzmann Method

摘要

This work uses X-ray computed micro-tomography (mu CT) to monitor xenon hydrate growth in a sandpack under the excess gas condition. The mu CT images give pore-scale hydrate distribution and pore habit in space and time. We use the lattice Boltzmann method to calculate gas relative permeability (k(rg)) as a function of hydrate saturation (S-hyd) in the pore structure of the experimental hydrate-bearing sand retrieved from mu CT data. The results suggest the k(rg)-S-hyd data fit well a new model k(rg)=(1-S-hyd).exp(-4.95.S-hyd) rather than the simple Corey model. In addition, we calculate k(rg)-S(hyd)z curves using digital models of hydrate-bearing sand based on idealized grain-attaching, coarse pore-filling, and dispersed porefilling hydrate habits. Our pore-scale measurements and modeling show that the k(rg)-S-hyd curves are similar regardless of whether hydrate crystals develop grain-attaching or coarse pore-filling habits. The dispersed pore filling habit exhibits much lower gas relative permeability than the other two, but it is not observed in the experiment and not compatible with Ostwald ripening mechanisms. We find that a single grain-shape factor can be used in the Carman-Kozeny equation to calculate k(rg)-S-hyd data with known porosity and average grain diameter, suggesting it is a useful model for hydrate-bearing sand.