A thermo-hydro-chemo-mechanical coupled model for natural gas hydrate-bearing sediments considering gravity effect

Natural gas hydrates have attracted many attentions recently as a promising energy, the exploitation of which will cause complicated multifield coupled behavior of hydrate-bearing sediments. As sediments usually vary from tens to hundreds of meters, the gravity effect on gas-liquid migration and soil deformation may not be completely ignored. This paper develops a new thermo-hydro-chemo-mechanical model to investigate the sediment behavior during the hydrate dissociation. The equations of gas-liquid migration are numerical solved with explicit incorporation of hydrate dissociation process. The numerical stability and efficiency have been improved by expanding the Taylor series of the source terms and making the first-order approximation. Furtherly, pre-calculation procedures have been considered to obtain the initial state of field variables. Pilot-scale model results show that the gas-liquid migration, soil deformation and NGH dissociation are accelerated when the gravity effect is present. During the exploitation, a dissociation front can be observed, and gas-liquid migration and hydrate dissociation dominate the process alternatively, leading to first decrease and subsequent increase of gas saturation and continuous rise of liquid saturation. Moreover, it is inferred that marginal enhancement of gas production can be achieved with the increase of wellbore lengths, but it should not exceed 75% of the reservoir thickness.