We simulate gas hydrate and free gas accumulation in heterogeneous marine sediments over geologic time scales. Simulations with a vertical fracture network, which extends through the gas hydrate stability zone and has permeability 100...
moreWe simulate gas hydrate and free gas accumulation in heterogeneous marine sediments over geologic time scales. Simulations with a vertical fracture network, which extends through the gas hydrate stability zone and has permeability 100 times greater than the surrounding shale formation, show that focused fluid flow causes higher hydrate (25-55%) and free gas saturation (30-45%) within the fracture network compared to the surrounding, lower permeability shale. Systems with high permeability, dipping sand layers also show localized, elevated saturations of hydrate (60%) and free gas (40%) within the sand layers due to focused fluid flow. Permeability anisotropy, with a vertical to horizontal permeability ratio on the order of 10-2, enhances hydrate concentrations within high permeability conduits because anisotropy enhances transport of methane-charged fluid to high permeability conduits. Our two-dimensional (2-D), heterogeneous models quantify how focused fluid flow through high permeability zones affects local hydrate accumulation and saturation. We also show increased fluid flux and deep source methane input result in enhanced concentrations of hydrate and free gas, and also increase the flow focusing effects. From our 2-D results, we determine that the hydrate and free gas saturations can be characterized by the local Peclet number (localized, focused, advective flux relative to diffusion); which is consistent with Peclet number characterization in one-dimensional (1-D) systems. This
characterization suggests that even in lithologically complex systems, local hydrate and free gas saturations can be characterized by basic parameters (local flux and diffusivity).
