DOE/NETL METHANE HYDRATE PROJECTS
 
Mechanisms for Methane Transport and Hydrate Accumulation in Coarse-Grained Reservoirs Last Reviewed 12/24/2014

DE-FE0013919

Goal
The project goal is to evaluate whether the transport of methane, and the specific mechanism by which it is transported, are the primary controls on the development of persistent, massive hydrate accumulations in sediments below the seabed. The dissolved methane flux and time required to develop the accumulations observed at WR 313 by long-distance updip migration or by short-distance local migration will be studied and defined within the scope of this research.  Researchers will also determine whether there is enough methane in the dissolved phase in the fine-grained sediments to form the observed hydrate deposits or whether a gas phase is present and, if so, what the conditions are for three-phase equilibrium.

Performers
University of Texas at Austin, Austin, TX 78713-7726
Ohio State University, Columbus, OH 43210
Lamont-Doherty Earth Observatory (Columbia University), Palisades, NY 10964

Background
Massive hydrate deposits, defined as thick (>5 feet) accumulations of high hydrate saturation (>50 percent), have been encountered in many regions worldwide. This project will focus specifically on accumulations found at Walker Ridge Block 313 in the northern Gulf of Mexico during the Gulf of Mexico Gas Hydrate Joint Industry Project Leg 2. Hydrates may be thought of broadly within a petroleum systems framework, requiring a methane source, migration mechanisms, a reservoir, and an appropriate seal. Hydrate reservoirs and seals are defined by thermodynamics rather than buoyancy as in the case of conventional oil and gas. Hydrates form most easily within coarse-grained sediments within the methane hydrate stability zone, the depth interval in which pressure and temperature favor hydrate as the stable phase. Methane sources may include microbial activity as well as thermogenic sources. The focus will be on migration mechanisms in marine hydrate reservoirs as they represent some of the least understood processes in hydrate systems, but at the same time represent a crucial link between methane generation sites and hydrate reservoirs.

Potential Impact
Successful completion of this project will provide valuable insight into conditions necessary for the development of massive gas hydrate accumulations and the role of free gas in their persistence. This, in turn, will advance understanding of the transport and fate of methane in the subsurface; carbon cycling associated with hydrates; and role of a free gas phase in the formation and persistence of hydrate deposits.

Accomplishments
Preliminary 2-D modeling has shown that microbial methanogenesis is necessary to form hydrate in shallow sands that are far from the base of the hydrate stability zone, such those as observed at Walker Ridge Block 313. Using the constraints on rates of microbial methanogenesis, the project team determined that the hydrate saturations inferred from downhole logs could form within a few hundreds of thousands of years, which is consistent with sediment age. A supply of methane from deep sources below the hydrate stability zone makes little difference in this case. These results were presented at the American Geophysical Union Fall Meeting in December 2014.

1-D reactive transport modeling has shown that sedimentation is a key component for developing hydrate deposits by short migration, since the amount of methane dissolved in the pore fluid increases as sediment as buried, providing a greater driver of diffusive flux. These results were presented at the American Geophysical Union Fall Meeting in December 2014.

Current Status (December 2014)
The project team completed development of the reservoir simulator to include microbial methanogenesis, sedimentation, and salinity and pore size effects on hydrate stability. The simulator is currently being benchmarked against results of previous models. In addition, the log data from Walker Ridge Block 313 and other shallow wells in the northern Gulf of Mexico are being analyzed to develop algorithms for computing permeability and pore size from downhole logs. This information will be used to provide inputs to the model.

The team is currently working with a 1-D reactive transport model to analyze microbial methanogenesis and hydrate formation in a subsiding coarse-grained layer. The results will be used to constrain methanogenesis rates in the reservoir model.

Project Start: October 1, 2013
Project End: September 30, 2017

Project Cost Information:
DOE Contribution: $1,679,137
Performer Contribution: $448,001

Contact Information
NETL – John Terneus (John.Terneus@netl.doe.gov or 304-285-4254)
University of Texas at Austin – Hugh Daigle (daigle@austin.utexas.edu or 512-471-3775)

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