SUBMARINE SLOPE STABILITY DURING DEPRESSURIZATION AND THERMAL STIMULATION HYDRATE PRODUCTION WITH HORIZONTAL WELLS

Gas hydrate is extensively found in the voids of sediments under deep sea. It is considered one of the promising sources of future clean energy, and has attracted global attentions of research and development. Submarine slope instability could be triggered during gas production from oceanic hydrate reservoirs due to reduction in soil strength and build-up of pore pressure. This paper investigates the impact of hydrate extraction on submarine slope stability within the framework of the limit equilibrium slope stability analysis considering the thermo-hydro-chemical (THC) coupled processes of hydrate dissociation. A THC coupled simulator TOUGH + HYDRATE was employed to simulate the process of hydrate production under depressurization and thermal stimulation with horizontal wells. The simulation captured the propagation of hydrate dissociation front and the evolution of pore pressure during hydrate production. The safety factor of the slope stability during and after production was then acquired from the limit equilibrium slope analysis by means of SLOPE/W. A parametric study was conducted to investigate the effect of well locations and production methods on the slope stability. The results indicate that, for a steep slope with a sandy reservoir covered by a tight overburden layer, the slope is stabilized during single-well depressurization production due to increase in the effective stress caused by depressurization. Compared to wells installed elsewhere, a depressurization well installed at the mid-height of the slope provides the highest safety factor during production. As the pore pressure recovers back to hydrostatic conditions and the cohesion of the host soils decreases, the slope becomes less stable after production. For scenarios using double-well thermal stimulation production, the critical slip surface passes through dissociated zones, and the safety factor of the slope significantly drops due to high pressure build-up developed around the injection well. Submarine landslides could be triggered by hydrate production with thermal stimulation.