Pore-Scale Mechanistic Study of the Preferential Mode of Hydrate Formation in Sediments: Fluid Flow Aspects

Authors: Javad Behseresht, Masa Prodanovic, and Steven Bryant, University of Texas at Austin.

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

Abstract: A spectrum of behavior is encountered in ocean sediments bearing methane hydrates, ranging from essentially static accumulations where hydrate and brine co-exist, 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 (Jain and Juanes), the researchers describe methods 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. The researchers will describe a novel implementation of the level set method to determine the capillarity-controlled displacement of brine by gas from sediment and from fractures within the sediment. Predictions of fluid configurations in infinite-acting-model sediments indicate that the brine in drained sediment (after invasion by methane gas) is better connected than previously believed. This increases the availability of water and the rate of counter-diffusion of salinity ions, thus relaxing the limit on hydrate build-up within the gas-invaded grain matrix. Simulated drainage of a fracture in sediment shows that points of contact between fracture faces are crucial. They allow residual water saturation to remain within an otherwise gas-filled fracture. Simulations of imbibition—which can occur, for example, after drainage into surrounding sediment reduces gas phase pressure in the fracture—indicate that the gas/water interfaces at contact points significantly shift the threshold pressures for withdrawal of gas. During both drainage and imbibition, the contact points greatly increase water availability for hydrate formation within the fracture. The researchers will discuss coupling this capillarity-controlled displacement model with a discrete element model for grain-scale mechanics. The coupled model provides a basis for evaluating the macroscopic conditions (thickness of gas accumulation below the hydrate stability zone, average sediment grain size, principal earth stresses) favoring co-existence of methane gas and hydrate in the HSZ. Explaining the range of behavior is useful in assessing resource volumes and evaluating pore-to-core scale flow paths in production strategies.

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)