Grain-Scale Study of Hydrate Formation in Sediments from Methane Gas: A Coupled Fluid-Solid Interaction Model

Authors: Jain, Antone K., and Juanes, Ruben, Massachusetts Institute of Technology

Venue: American Geophysical Union fall meeting, San Francisco, CA, December 10-14, 2007 (http://www.agu.org/meetings/fm07/ [external site]).

Abstract: Ocean sediments bearing methane hydrates exhibit a range of behavior, from cold seeps, where solid and gas phases co-exist in the hydrate stability zone (HSZ), to essentially static accumulations where solid and liquid co-exist. This paper and its companion paper (Behseresht, Prodanovic, and Bryant) describe the development and application of models for grain-scale phenomena governing in situ gas-to-hydrate conversion. The motivation is the following hypothesis: as gas phase pore pressure varies, the competition between brine displacement and sediment fracturing determines the extent of conversion of methane gas entering the HSZ to hydrate. The researchers present a discrete element method (DEM) to model the strong coupling that takes place between the pore fluids (brine and methane gas) and the mechanical behavior of the sediment. In a DEM, each element or grain is an individual entity, identified by its size, mass, and moments of inertia. Newton’s second law dictates the motion of the assembly of grains. For dry systems, the grain-scale forces are limited to interactions at grain contacts. In contrast, when one or more fluids are present, additional pore-scale forces play a significant role: a set of forces due to pore fluid pressure and another set due to surface tension between fluids. The researchers developed a self-consistent fluid-solid interaction (FSI) model at the grain scale in which these additional sets of forces are introduced rigorously. Their computational model captures the two-way coupling between multiphase fluid flow and sediment mechanics, which is validated by means of triaxial laboratory experiments. In particular, this allows them to determine the conditions under which gas invasion fractures the sediment. 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
The goal of the related NETL project DE-FC26-06NT43067, “Mechanisms Leading to Co-existence of Gas and Hydrate in Ocean Sediments,” is to quantitatively describe and understand the manner in which methane is transported within the HSZ and, consequently, the growth behavior of methane hydrates at both the grain scale and bed scale.

NETL Project Contacts
NETL - Robert Vagnetti (Robert.Vagnetti@netl.doe.gov or 304-285-1334)
UT-Austin – Steven Bryant (steven_bryant@mail.utexas.edu or 512-471-3250)