Authors: Javad Behseresht, Yao Peng, Maša Prodanovic, Steven L. Bryant

Venue: 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008. ( http://www.icgh.org [external site] )

Abstract: Ocean sediments bearing methane hydrates exhibit a range of behavior, from cold seeps where solid and gas phases coexist in the hydrate stability zone (HSZ), to essentially static accumulations where solid and liquid co-exist. This and the companion paper by Jain and Juanes [1] 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. Here the level set method is implemented to determine the capillarity-controlled displacement of brine by gas from sediment and from fractures within the sediment. Reduction of gas phase pressure, for example due to disconnection from the source gas accumulation, allows imbibition to occur. Drainage into infinite-acting model sediments indicate that the brine in drained sediment (after invasion by methane gas) is better connected than previously believed, thus facilitating hydrate formation within sediment. Nevertheless drainage to the endpoint condition of irreducible brine saturation is unlikely to account for co-existence of free gas and hydrate because it implies a large free gas saturation, which is not observed. Nor does the large gas saturation at the drainage endpoint lead to large hydrate saturations such as those reported for the Mallik well, because insufficient water is present and the requisite water can only enter the sediment by imbibition. Several drainage/imbibition cycles would be needed instead.

Work is underway to couple this capillarity-controlled displacement model with a discrete element model for grain-scale mechanics. A simple kinematic version of this coupling is presented here. The qualitative effect is to lower the percolation threshold and to increase irreducible water saturation. This would diffuse the propagation of a fracture into the surrounding sediment and reduce the free gas saturation preceding hydrate formation, but the conditions leading to gas phase and hydrate coexistence cannot be readily ascertained.

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)