Numerical investigation on abnormally elevated pressure in laboratory-scale porous media caused by depressurized hydrate dissociation

摘要

Hydrate dissociation induced by artificial or environmental factors in submarine sediments may lead to drilling risks, submarine landslides, and the collapse of offshore platforms-all of which are associated with suddenly increased pore pressure. This study establishes a coupled model for the two-phase flow of fluids in porous media in which the pore pressure of water and gas are used as dependent variables and heat transfer and hydrate dissociation equations are combined. The simulation results of the model are consistent with Masuda's experimental dataset. A quantifiable controlling hydrate dissociation specific reaction surface area (SRSA) model and an absolute permeability model are proposed to conduct numerical simulations of laboratory-scale hydrate cores, with the aim to investigate the mechanism of hydrate dissociation-induced geological hazard initiation and a law of their evolution in sediments. The results show that i) suddenly depressurized hydrate dissociation in porous media can be divided into three phases-pressure induction, transition, and temperature-controlled-and that the first two are much shorter than the third. Moreover, the lower initial permeability results in a shorter holding period for the first stage, and the transition phase becomes indistinguishable; conversely, the larger initial permeability makes the three stages more distinguishable; ii) hardly increasing the core permeability during the intermediate stage of hydrate dissociation makes the pore pressure rebound. However, the steep increase in SRSA does not result in a significant pore gas pressure rebound; conversely, a larger SRSA does facilitate pore pressure dissipation. This work provides theoretical and laboratory-scale model analyses for the safe development of natural gas hydrates bearing in marine sediments.