| Summary: | This study aimed to investigate the free gas accumulation behavior in a reservoir using a multiple-well system for methane hydrate production achieved by depressurization. Twenty-year simulations of gas production from a large-scale 3D methane hydrate reservoir model with different reservoir permeabilities were conducted, and the effects of different reservoir and operating conditions on the free gas accumulation behavior were fully examined. The simulation results indicated that the free gas accumulation behavior was affected by the reservoir permeability, and methane gas was inclined to accumulate within a certain permeability range, which was defined as the “free gas accumulation zone” for the first time. For an actual methane hydrate reservoir with a porosity of 0.31–0.51 and an initial hydrate saturation of 0.34–0.54, the free gas accumulation zone was estimated to be 37–145 mD at most. On the other hand, a low wellbore pressure could contribute to enhancing gas recovery by narrowing the free gas accumulation zone. In addition, the free gas accumulation zone was dramatically enlarged with the increase in well spacing, so a proper well spacing should be carefully designed to avoid the free gas accumulation zone. The prediction method proposed in this study could be applied to future commercial gas production from actual methane hydrate deposits achieved by depressurization using multiple-well systems. Methane hydrate; Free gas; Multiple-well system; Blind area effect; Gas accumulation zone;
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