Hydrodynamic Models of the Dynamics of Methane Release by Natural Hydrates

We present results of a broad study of the dynamics of natural deposits of methane hydrates in response to global warming. Particular attention is given to the hydrodynamic aspects of the process associated with instability and deformation of boundaries and filtration of released methane to the surface. The study is computational and is based on a novel physical model that fully takes into account the nonlinear three-dimensional dynamics of the processes of heat and mass transfer that control the dissociation of natural hydrates. This particular presentation is focused on two phenomena. One is the possibility of development and growth of chimney-like vertical hydrate-free channels within the hydrate stability zone (HSZ) that provide paths of relatively unimpeded escape of methane to the surface. According to our analysis, such channels can develop and grow under geologically plausible conditions, the main of which is relatively high (above approximately 10^-12 m²)permeability of deposits. The key mechanism that determines the growth is the convection heat transfer by upward filtration of methane gas from the underlying gas-saturated layer. Simulations of the system consisting of several channels located close to each other demonstrate faster growth and merging. We also consider the dynamics of a compact hydrate-free inclusion (a bubble) within the HSZ. The goal of this investigation is to evaluate the ability of such bubbles to migrate to the surface. As a model, an isolated bubble in a boundless hydrate zone is considered. Considering the evolution of a purely spherical bubble, we find that its lifetime can be estimated as t ∼ R^{2δ -σ} , where R is the bubble radius, δ is a parameter evaluating the closeness of surrounding hydrates to dissociation, and 0<σ <1 is function of R. The more advanced model that takes into account the flow of methane gas and buoyancy force shows significant deformation and even disintegration of the bubble at high permeability of deposits and/or large size of the bubble. Work is supported by the US Civilian Research and Development Foundation through Grant RUP1-2945-PE-09 and by the Russian Fund for Fundamental Research through Grant 09-01-92505.