Pore scale investigation of decomposition methods dependent fluid flow properties in hydrate-bearing sediments

Depressurization (PD) and thermal stimulation (TS) are the primary methods for producing gas from natural gas hydrate sediments. Fluid flow properties, such as permeability, of the hydrate sediment are fundamental parameters for simulating both processes. Most of the existing formulated permeability models are based on numerical or experimental investigation of hydrate morphology evolution without considering the decomposition methods. In this study, we investigate the hydrate decomposition methods (PD and TS processes) dependent fluid flow properties of hydrate sediments, which is achieved by micro-computed tomography (micro-CT) scanning of hydrate morphology evolution during PD and TS induced decomposition, as well as pore scale modelling of fluid flow in the extracted 3D fluid-rock-hydrate images.
We find that the decomposition behaviour during TS is much more complicated than that during PD. The retardation zone in the PD sample increases the heterogeneity of the pore structure, while the secondary hydrates generated
during TS cause even more heterogeneity in the pore space. The better facilitation of the TS method on hydrate split is favourable for the continuity of gas phase. The pore scale fluid flow simulation shows that the modified K-C
model is the best to describe the evolution of the normalized permeability with hydrate saturation during PD. However, a single model is not sufficient to describe the normalized permeability during TS decomposition due to the stronger heterogeneous porous structure reformed by local accumulation of secondary hydrates. The two-phase flow capability is best at the initial stage of PD decomposition. While the two-phase flow region becomes wider as TS decomposition progresses. These results provide significant references for the simulation of the natural hydrate extraction process using different decomposition methods.