Numerical analysis of depressurization production of natural gas hydrate from different lithology oceanic reservoirs with isotropic and anisotropic permeability

Natural gas hydrate (NGH) is a promising alternative energy and mainly distributes in deep oceanic sediments. The intrinsic permeability of hydrate-bearing layers can potentially influence heat and pressure transfer during depressurization-induced NGH dissociation. By taking siltstone, sand and clay reservoirs in Shenhu area of South China Sea as examples, this study numerically investigates the effects of the magnitude and anisotropy of reservoir permeability on NGH production process and main physical field evolution. Results reveal that permeability anisotropy could impede advective interaction of fluids in vertical direction, significantly changing temperature and pressure evolution during NGH dissociation. Consequently, NGH dissociation slows down, delaying the coming of peak gas production rate. Some degrees of permeability anisotropy even lead to much earlier termination of NGH dissociation. In this case, the more permeable sand reservoir could possibly have a lower gas production potential than the less permeable siltstone and clay reservoirs. The permeable overburden limits the fluid pressure reduction and weakens the efforts of depressurization production. The re-production by further lowering well pressure only lasts for a short time. Comparatively, the depressurization of constant production mass rate is more effective but only provides a small contribution for the total gas yield.