Investigation of methane gas bubble dynamics and hydrate film growth during hydrate formation using 4-D time-lapse synchrotron X-ray computed tomography

We present a time-lapse 4-D high-resolution synchrotron imaging study of the morphological evolution of methane gas bubbles and hydrate film growth on these bubbles. Methane gas and partially water-saturated sand were used to form hydrate with a maximum hydrate saturation of 60%. We investigated the transient evolution of gas bubble size distribution during hydrate formation and observed three distinct stages: a) nucleation and hydrate film formation, b) rapid bubble break-up, c) gas bubble coalescence and hydrate framework formation. Our results show that the average gas bubble size distribution decreases from 34.17 mu m (during hydrate nucleation) to 8.87 mu m (during secondary bubble formation). The small-size methane bubble population (mean diameter below 10 mu m) initially increases at the expense of the larger methane bubble population (mean diameter above 50 mu m) due to breakage of the larger bubbles and coalescence of the smaller bubbles. We quantified that the average hydrate film thickness increases from 3.51 to 14.7 mu m by tracking the evolution of a particular gas bubble. This thickness increase agrees with an analytical model with an average deviation error of 3.3%. This study provides insights into gas bubble distribution and hydrate film growth during hydrate formation, both of which impact the geophysical and mechanical properties of hydrate-bearing sediments.