Assessing Methane Migration Mechanisms at Walker Ridge, Gulf of Mexico, via 3D Methane Hydrate Reservoir Modeling

Abstract:

We employ a 3D methane hydrate reservoir simulator to model marine methane hydrate systems. Our simulator couples highly nonlinear heat and mass transport equations and includes heterogeneous sedimentation, in-situ organic methanogenesis, and the influences of both pore size contrast and salt exclusion from the hydrate phase on solubility gradients. Using environmental parameters of Walker Ridge, Gulf of Mexico, we first simulate hydrate formation in and around a thin, dipping, planar sand stratum surrounded by clay lithology as it is buried to 295mbsf. With sufficient methane supplied by methanogenesis in the clays, a 200x sand-clay pore size contrast allows for a strong enough concentration gradient to significantly drop the concentration of hydrate in clays immediately surrounding a thin sand, a phenomenon observed in corresponding well log data. Building upon previous work, our simulations account for a depth-wise increase in sand-clay solubility contrast from about 1.6% near the seafloor to 8.6% at depth, progressively strengthening the diffusive flux of methane with time. An exponentially decaying methanogenesis input to the clay lithology decreases the methane supplied to clays surrounding the sand layer with time, further enhancing the sand-clay hydrate saturation contrast. Significant diffusive methane transport occurs in a clay interval of about 11m above the sand and 4m below it, matching well log observations. Clay-sand pore size contrast alone is not enough to create hydrate-free zones seen in logs, because the corresponding diffusive methane flux is slower than the rate at which methanogenesis supplies methane. Therefore, it is likely that additional mechanisms are at play, notably bound water activity reduction in clays. Three-dimensionality allows for inclusion of lithologic heterogeneities, which focus flow and allow for heterogeneity in locally dominant methane migration mechanisms. Incorporating recent 3D seismic data to inform the model structure, we show that even with deep advective sourcing of methane, local hydrate accumulations can be sourced advectively or diffusively. Advectively sourced hydrates accumulate evenly in highly permeable strata, while diffusively sourced hydrates are characterized by thin strata-bound intervals of high clay-sand pore size contrasts.