|Mechanisms for Methane Transport and Hydrate Accumulation in Coarse-Grained Reservoirs
||Last Reviewed 6/1/2015
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.
University of Texas at Austin, Austin, TX 78713-7726
Ohio State University, Columbus, OH 43210
Lamont-Doherty Earth Observatory (Columbia University), Palisades, NY 10964
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.
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.
An existing reservoir simulator has been altered to include methanogenesis, sedimentation, salinity, and pore size effects on hydrate stability. A method was developed to calculate permeability and pore size based on downhole log data (gamma ray and porosity) from wells in the northern Gulf of Mexico (Walker Ridge Block 313). Calculated sediment properties were applied as input parameters of the model and simple simulations have been completed using sediment properties similar to those encountered at Walker Ridge Block 313. The modified simulator has been successfully benchmarked against published 1-D and 2-D simulations and found to be consistent within 1% in time and hydrate saturation given the same input parameters. 2-D model results indicate that microbial methanogenesis is necessary to form hydrate in shallow sands that are far from the base of the hydrate stability zone, such as those 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.
A 1-D reactive transport model was created to couple sedimentation and microbial methanogenesis in order to assess the time constraints on the accumulation of hydrate in a subsiding sand layer. 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. The reactive transport model has been benchmarked against existing data under steady-state conditions. Results indicate that observed hydrate accumulations at Walker Ridge Block 313 would require ~1.3 million years to form with short, diffusive migration as the only source of methane assuming seafloor organic concentrations and methanogenesis rates are equal to expected values for the central Gulf of Mexico.
Current Status (June 2015)
The reactive transport model is currently being altered to include porosity as a function of depth in order to account for the effect of compaction on the burial velocities of fluid and solid components. Incorporation of burial effects on porosity will allow reactive transport results to be compared with 1-D reservoir simulations, allowing results to be computed rapidly for use in sensitively analyses. The basin-scale reservoir simulator is being modified for long distance, updip migration of dissolved methane and diffusive flux in sediments.
Project Start: October 1, 2013
Project End: September 30, 2017
Project Cost Information:
DOE Contribution: $1,679,137
Performer Contribution: $448,001
NETL – Stephen Henry (email@example.com or 304-285-2083)
University of Texas at Austin – Hugh Daigle (firstname.lastname@example.org or 512-471-3775)
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