Permeability analysis of hydrate-bearing sediments considering the effect of phase transition during the hydrate dissociation process

Abstract

Permeability is the most important factor affecting water and gas production in hydrate-bearing sediments (HBS). In this study, we combined in-situ microfocus X-ray computed tomography (CT) and pore network model simulation to obtain insights into the dynamic permeability of the hydrate dissociation process. Microfocus X-ray CT is used to visualize the three-dimensional pore structures of the specimen. Then an equivalent pore network can be extracted, which is used as input to pore network flow simulator to simulate the water and gas flow process. Results show that secondary hydrate formation causes effective permeability keff decreases and effective connate water saturation increases as hydrate saturation Sh increases from 0.40 to 0.48. The water relative permeability krw decreases and gas relative permeability krg increases due to larger patchy hydrates (Sh = 0.33). The krw and krg are largely affected by the pore and throat sizes and pore space connectivity. Hydrate dissociation in HBS also influences its permeability anisotropy. Secondary hydrate formation causes keff decreases in the x, y, and z directions when Sh increase from 0.40 to 0.48. Large patchy hydrates influence preferential flow direction for water flow. Predicting the dynamic permeability of the hydrate dissociation process would improve pore-scale water and gas flow analyses in HBS.