Pore scale mechanistic study of the preferential mode of hydrate formation in sediments: Coupling of fluid flow and sediment mechanics

Authors: Antone K. Jain and Ruben Juanes

Venue: International Conference on Gas Hydrates, Vancouver, Canada, July 7-10, 2008. ( http://www.icgh.org [external site] )

Abstract: Methane hydrates in ocean sediments range from essentially static accumulations of hydrate and brine, to active cold seeps where hydrate and a methane gas phase co-exist in the hydrate stability zone (HSZ). In this and a companion paper (Behseresht, Prodanovic and Bryant) methods are described to test the following hypothesis: the coupling between drainage and fracturing (both induced by pore pressure) determines whether methane gas entering the HSZ is converted completely to hydrate. H A discrete element method (DEM) is presented to model the strong coupling that takes place between the pore fluids and the mechanical behavior of the sediment.

In the discrete element method each element or grain is an individual entity identified by its size, mass and moments of inertia. The movement of a grain is dictated by the net force and moment acting on it. For systems in which the pore pressure is negligible, the interactions between particles are limited to grain contacts. Here, in contrast, the method rigorously accounts for the presence of one or more fluids in the pore space by incorporating two additional sets of forces: one set due to pore fluid pressures, and another due to surface tension between fluids. The ability of DEM to reproduce core-scale behavior is demonstrated, as measured by triaxial laboratory experiments. The proposed methodology elucidates the depositional environments (grain size, depth, earth stresses) under which migration of methane gas by fracturing of the sediment is favored over capillary invasion. This determines the distribution of methane gas and hydrate which, in turn, has direct implications on the likelihood that gas and hydrate will co-exist, and on the overall size of the energy resource.

Work is under way to couple this grain mechanics model with a capillarity-controlled displacement model, described in the companion paper by Behseresht, Prodanovic and Bryant.

Related NETL Project
This presentation is related to the NETL project DE-FC26-06NT43067, “Mechanisms Leading to Co-Existence of Gas and Hydrate in Ocean Sediments.” The goal of this project is to quantitatively describe and understand the manner in which methane is transported within the Hydrate Stability Zone (HSZ).

Project Contacts
NETL – Robert Vagnetti (Robert.Vagnetti@netl.doe.gov or 304-285-1334)
University of Texas at Austin – Steven Bryant (Steven_Bryant@mail.utexas.edu or 512-471-3250)