Role of Critical Gas Saturation in Methane Production from Hydrate Dissociation at the Pore-Network Scale

Abstract

Abstract Hydrates are crystalline structures that are formed under conditions of low temperatures or high pressures and consist of a lattice made up of hydrogen-bonded water molecules, containing cavities. These cavities can be stabilized by van der Waals forces if occupied by certain types of guest molecules (e.g. CH4). Hydrates are very important compounds due to their capacity to store large volumes of gases. They are studied as a possible source of energy, since an enormous reservoir of carbon is deposited in worldwide accumulations of hydrates, containing predominantly methane, both on-shore (under the permafrost), and off-shore (in marine sediments). In this study, a 2-D pore-network simulation based on concepts from Invasion Percolation in a Gradient is discussed to study the dissociation of methane hydrate in porous media. By using reported experimental values of porosity and permeability that are found in oceanic sediments we reconstruct a porous medium with similar properties by varying appropriately the size range of throat radii and the ratio pore/throat radii. The reconstructed porous medium can be either fully or partially saturated with hydrate. In this work emphasis is placed on examining the patterns formed by the release of methane gas in a partially saturated porous medium following hydrate dissociation. We investigate how the various patterns are affected by parameters such as the viscous pressure drop applied to the system, the size range of throat radii, hydrate saturation, and pore size distribution. Note that in order for the methane gas to be produced from the system, the critical gas saturation (defined as the volume fraction of the gas phase at the onset of bulk gas flow) must be reached. This is an issue of significant importance that delineates the possible range of parameters where methane production can be economically viable. For the critical gas saturation to occur the produced gas clusters must: either (i) connect to each other forming a sample-spanning gas cluster through which the gas phase can be produced, or (ii) be mobilized by the presence of viscous or buoyancy forces and thus arrive at the producing end of the network (this is a limiting case where continuum models may break down because of the formation and mobilization of gas bubbles). In order to obtain a better understanding of the production efficiency for the commercial development of hydrate deposits we address the significant question whether the resulting dissociating system (at lower hydrate saturation values) is below or above the critical gas saturation and how the aforementioned parameters affect the value of the critical gas saturation.