Hydrate Production Behavior in China’s Second Marine Trial Production Site under High Irreducible Water Saturation and a Strong Sealing Effect

In the second depressurization test production of marine natural gas hydrates (NGHs) in China, gas was successfully produced from low-permeability clayey silt NGHs through a horizontal depressurization well, setting a new record for marine NGHs production. Although significant success has been achieved at the field scale, there is still limited research on its production behavior. In this paper, a layered heterogeneous horizontal well production numerical model was established based on logging data from this field production site. First, the cumulative gas production volume for 30 and 42 days was successfully fitted through numerical simulation, and the short-term production behavior under different irreducible water saturation (Sirw) was studied in detail. An increase in Sirw during short- and long-term production can increase the flow of free gas toward the production well, thereby increasing gas production. During the long-term production process, a barrier area with high hydrate saturation in the bottom of the hydrate-bearing layer (HBL) hinders the further upward expansion of the decomposition front. Meanwhile, due to the relatively large amount of movable water, the barrier area formed at low Sirw is relatively small. Through this study, it is helpful to reveal the hydrate production behavior of the horizontal well under high Sirw and a strong sealing effect.