Analysis of Reservoir Stability during Natural Gas Hydrate Exploitation under Incline Seafloor

Abstract

Natural gas hydrates (NGHs) have been recognized as a potential substitute for traditional fossil fuels. Mining NGH reservoirs can decrease the strength of the reservoirs, especially while improving production, and the double-well mining of NGHs also significantly reduces the strength of reservoirs. This study develops a thermofluid-solid multifield coupling model for mining NGHs through depressurization while considering the NGH decomposition kinetics and physical properties of NGH reservoirs. The influence of the formation responses and burial conditions on the slope stability in the depressurization process of NGHs is analyzed by combining it with the finite-element strength-reduction method. Results show that the decomposition zones of NGHs are nonuniformly distributed in space and have an irregular prismatic shape. The pore pressure propagates from the wellbores to the surrounding areas, forming cylindrical high-pressure-drop zones. Plastic zones first appear in the decomposition zones of NGHs; then, they gradually spread to the slope shoulder and toe, eventually coalescing to form a plastic zone. The stability of submarine slopes declines with the increasing slope angle, reservoir thickness, and initial saturation of the reservoir, while it increases with the growing burial depth of the reservoir. The seabed settlement grows with the growing slope angle and initial saturation, and thickness of reservoirs, while it decreases with the rising burial depth of the reservoir.