Recoverable gas from hydrate-bearing sediments: Pore network model simulation and macroscale analyses

The volume of hydrate expands into a significantly larger volume of water and gas upon dissociation. Gas recovery and capillary-trapped residual gas saturation are investigated by simulating hydrate dissociation within pore networks. A fluid pressure-controlled boundary condition is used to determine the amount of recovered gas as a function of volume expansion; in this form, results are applicable to gas production by either thermal stimulation or depressurization when production rates prevent secondary hydrate or ice formation. Simulation results show that gas recovery is proportional to gas expansion, initial hydrate saturation, and the sediment pore size distribution (i.e., capillary pressure). Gas recovery is not affected by pore size in coarse-grained sediments with pores larger than 1 mu m. Hydrate-bearing sediments with low hydrate saturation yield low gas recovery. Macroscale close form solutions, validated using the numerical results, provide estimates for recoverable gas as a function of the initial hydrate saturation and the fluid expansion factor.