Percolation characteristics of pore fluid during hydrate depressurization dissociation from multi-phase multi-field coupling analysis

The recovery of gas from hydrate-bearing reservoirs is essentially controlled by pore fluid flow. Four numerical models with different multi-phase multi-field were established in this work based on the thermo-hydro-chemical model, considering ice phase, fluidized sand phase, depositing sand phase, and ice, fluidized sand and depositing sand phases, respectively. These above models were used to simulate the dissociation of hydrate-bearing cores by depressurization and to investigate the effects of freezing and melting behavior, sand production behavior, and their combined behavior on pore fluid migration. By comparing the simulations with and without the ice phase, it can be found that the permeability decreases sharply due to ice generation resulting in a decrease of 89.16% in pore-water velocity and of 99.95% in pore-gas velocity. This indicates an inhibition of pore fluid migration by freezing behavior. Meanwhile, it can be found that the ice melting behavior can temporarily promote pore-water migration but has no obvious promotion for pore-gas migration. It is difficult for the pore fluid to percolate in the regions prone to ice generation during hydrate dissociation. In addition, the permeability at 300 min with sand detachment is 16% higher than that without sand detachment. But at the same time, the pore water velocity is 28.44% smaller since sand detachment behavior does not directly power pore fluid migration. By comparing the simulations with and without the combined behavior, it can be concluded that the freezing behavior and the sand production behavior can jointly weaken the pore fluid mobility. Significantly, the former has a stronger negative effect on the pore fluid mobility than the latter during the depressurization dissociation of the hydrate-bearing sediment.