HR: 11:05h
AN: OS22A-04    [Abstracts]
TI: Pore Scale Mechanistic Study of the Preferential Mode of Hydrate Formation in Sediments: Fluid Flow Aspects
AU: * Behseresht, J
EM: jbehseresht@mail.utexas.edu
AF: University of Texas at Austin, Department of Petroleum and Geosystems Engineering, 1 University Station, C0300, Austin, TX 78712, United States
AU: Prodanović, M
EM: masha@ices.utexas.edu
AF: University of Texas at Austin, Center for Petroleum and Geosystems Engineering, 1 University Station, C0304, Austin, TX 78712, United States
AU: Bryant, S L
EM: steven_bryant@mail.utexas.edu
AF: University of Texas at Austin, Department of Petroleum and Geosystems Engineering, 1 University Station, C0300, Austin, TX 78712, United States
AB: 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) we 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. Here we describe a novel implementation of the level set method (LSM) 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 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 shifts 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. We 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.
DE: 0702 Permafrost (0475)
DE: 0704 Seasonally frozen ground
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 3004 Gas and hydrate systems
DE: 3036 Ocean drilling
SC: Ocean Sciences [OS]
MN: 2007 Fall Meeting