Gas Production and Reservoir Settlement in NGH Deposits under Horizontal-Well Depressurization

Identifying geohazards such as landslides and methane leakage is crucial during gas extraction from natural gas hydrate (NGH) reservoirs, and understanding reservoir settlement behavior is central to this assessment. Horizontal wells can enlarge the pressure relief zone within the formation, improving single-well productivity, and are therefore considered a promising approach for NGH development. This study examines the settlement response of hydrate-bearing sediments during depressurization using horizontal wells. A fully coupled thermal, hydraulic, mechanical, and chemical (THMC) model with representative reservoir properties (Shenhu region in the South China Sea) is presented accordingly. The simulations show that lower production pressures, while increasing gas output, also intensify formation settlement. The maximum difference in settlement between the lowest and highest production pressures reaches 0.016 m, contributing to more pronounced differential subsidence. Optimal well placement, specifically targeting a low-saturation hydrate zone containing free gas and situated adjacent to a high-saturation hydrate layer, markedly improves both gas production rate and cumulative yield, while reducing overall settlement and limiting changes in effective stress.