Effects of Horizontal Well Placement on Gas Production and Methane Leakage during Depressurization in Unsealed Hydrate Reservoirs

Unlike conventional oil and gas reservoirs, hydrate-bearing reservoirs are generally unsealed, making methane leakage control critical for environmental safety. Previous studies have mainly examined hydrate dissociation and production enhancement in sealed systems, whereas leakage behavior in unsealed reservoirs remains poorly understood. Here, a self-developed experimental simulator was used to construct a submarine-stratified environment and assess how different placements of multilateral horizontal wells influence hydrate dissociation and methane leakage. Results showed that varying the placement of a single horizontal well did not alter final gas and water production, but it modified the flow space of low-temperature overlying water, thereby affecting hydrate dissociation efficiency and methane leakage. The upper-well configuration reduced ineffective sensible heat loss and shortened reservoir temperature recovery, but also produced the highest methane leakage ratio of 13.55%. Under the dual horizontal well configuration, randomly occurring delayed failure of the overlying sediment layer reduced secondary hydrate formation caused by seawater intrusion, improved production continuity and final gas yield, and lowered methane leakage to 1.21%. These findings provide a theoretical basis for the safe and efficient development of marine natural gas hydrates.