Stability of hydrate slope in vertical multi-well dynamic mining

Natural gas hydrate is a clean energy occurring in the pores of seabed sediments. It has a large content and does not cause great harm to the environment, thus it has a good prospect and research value. Vertical wells as a main way of gas hydrates mining, the exploitation activities may alter the environmental conditions of the natural gas hydrate sediments, under which a large amount of gas and water would be released and result in overpressure in the strata, and the excessive pore pressure could reduce sediment consolidation strength, destroy the stability of the sediments, and induce submarine landslides. We established a high-precision underwater slope model with the aid of Flac3D software, and numerically simulated the slope stability in the process of vertical multi-well dynamic hydrate mining in finite difference method. The strata mechanical response and displacement changes caused by different influencing factors such as hydrate decomposition and well pressure change under different exploitation schemes were simulated. Using the safety factor method, the slope stability caused by hydrate mining was preliminarily analyzed. The results show that the stability of submarine slope decreases gradually with the increasing degree of hydrate decomposition under multi-well mining conditions. When the degree of hydrate decomposition reaches 80%, the safety factor will be lower than 1.0, and the slope will become unstable. With the decrease in well pressure, the stability of submarine slope also decreases gradually. When the well pressure drops below 4 Mpa, the safety factor will drop below 1.05, and the slope becomes less stable with a risk of submarine landslide.