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

This work uses X‐ray computed micro‐tomography (μCT) to monitor xenon hydrate growth in a sandpack under the excess gas condition. The μCT images give pore‐scale hydrate distribution and pore habit in space and time. We use the lattice Boltzmann method to calculate gas relative permeability (krg) as a function of hydrate saturation (Shyd) in the pore structure of the experimental hydrate‐bearing sand retrieved from μCT data. The results suggest the krg ‐ Shyd data fit well a new model krg = (1‐Shyd)·exp(–4.95·Shyd) rather than the simple Corey model. In addition, we calculate krg‐Shyd curves using digital models of hydrate‐bearing sand based on idealized grain‐attaching, coarse pore‐filling, and dispersed pore‐filling hydrate habits. Our pore‐scale measurements and modeling show that the krg‐Shyd 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 krg‐Shyd data with known porosity and average grain diameter, suggesting it is a useful model for hydrate‐bearing sand. Key Points A new method to determine krg ‐ Shyd relation in hydrate‐bearing sand at pore‐scale and a corrected Corey model for the krg ‐ Shyd data krg ‐ Shyd relation depends on whether hydrates are patchy or dispersed, rather than whether hydrates are grain‐attaching or pore‐filling Carman‐Kozeny equation is a useful model for krg ‐ Shyd relation in hydrate‐bearing sand