14 - Methane hydrates in porous layers: Gas formation and convection

Abstract

This chapter provides a review of recent analytical and numerical work on the generation and flow of methane gas through a layer of porous medium impregnated with solid clathrate hydrates. The porous layer is depressurized suddenly on its lower plane and the phase-change front advances under the influence of heat conduction and convection. The chapter describes a simplified analytical solution based on a unidirectional phase-change model in which the conduction in the gas-filled region behind the front is neglected. It continues with numerical results for the evolution of the unidirectional phase-change process. Both methods lead to the conclusion that the rate of gas flow through the depressurized (bottom) plane of the layer decreases. Further numerical modeling shows that the presence of a vertical geothermal gradient has a significant effect on the rate of gas generation. Numerical results for phase change and gas generation in porous sediment with non-uniform porosity and permeability are also reported. The aim is to draw attention to an important new area of fundamental and applied research on convection in porous media–the extraction of methane gas from clathrate hydrates. Vast deposits of methane hydrates have been found all over the globe, under the oceanic floor, and under permafrost. The basic scales of the process characterize the rate at which the dissociated lens advances into the porous medium filled with solid hydrate, and the rate at which gas is being produced and captured in an unsteady, time-dependent fashion. There are two problems–two phenomena–that join hands during this process. One problem is the dissociation and advancement of the phase-change front into the hydrate-filled medium. This problem can be studied first as a 1D, time-dependent heat, and fluid flow phenomenon. The second problem is the nearly radial gas flow through the lower porous layer and into the well. Numerical modeling shows that the presence of a vertical geothermal gradient has a significant effect on the rate of gas generation. Numerical results for phase change and gas generation in a porous sediment with non-uniform porosity and permeability are also reported.