FEAB105

Mesoscale Characterization of Natural and Synthetic Gas Hydrates

FEAB105

Project Goal

Develop the information needed to model various hydrate recovery schemes and assess and predict seafloor stability.

Background

Prior to this project, Oak Ridge National Laboratory (ORNL) had constructed a pressure cell to synthesize gas hydrates from water and gas and characterize the resulting atomic structures with in-situ neutron powder diffraction. As part of this earlier effort ORNL studied the structure of laboratory synthesized clathrate hydrates. However, these experiments had not provided a complete understanding of the complex multiphase assemblages found in nature where hydrates with several types of hydrocarbon molecules are found in a mixture with ice and sediments. The main objective of this project was to characterize naturally occurring samples from two Green Canyon (Gulf of Mexico) locations (designated GC232 and GC234) collected by researchers at Texas A&M University.

Performers

Oak Ridge National Laboratory – project management and research products

Location

Oak Ridge, Tennessee 37830

Potential Impact

This project will develop an experimental methodology to characterize structural and interfacial properties of natural gas hydrates and implement that methodology to characterize both natural and synthetic gas hydrates on a mesoscopic scale (dimensions intermediate between macroscopic and atomic).

In addition, it will determine thermal transport properties of hydrates necessary to model, predict, and simulate hydrate behavior under subsea and permafrost conditions. The results of these experiments will be directly used to develop better physical property measurements that will be used to develop models. Industry can utilize these models to develop the resources and/or evaluate the safety issues that deposits may have on current operations in the energy sector.

Results
Gulf of Mexico hydrate samples were characterized by in-situ x-ray powder diffraction, thermal conductivity measurements, Infrared camera imaging, inelastic neutron scattering (INS), Raman studies, and small angle neutron scattering (SANS).

Low-temperature x-ray powder diffraction was used to determine the phases present, decomposition temperatures, and instantaneous coefficient of thermal expansion. The resulting diffraction patterns showed that both samples were structure II hydrates with ice present as a secondary phase. X-ray powder diffraction data collected separately on sediments included in the samples identified quartz (SiO2), aragonite (CaCO3), and halite (NaCl) constituents. Decomposition temperature and an instantaneous coefficient of thermal expansion were also determined.

A thermal conductivity study was conducted to determine the low temperature thermal transport properties of natural clathrate hydrates. The standard Hot Disk method combined with a liquid nitrogen bath was used to measure thermal conductivities of ice, synthetic hydrate, and natural hydrate. The tests conducted on synthetic structure I hydrate showed an average value of 0.58 W/mK at a temperature of -34°C and 0.49 W/mK at liquid nitrogen temperature (-194°C). The natural hydrate samples showed an even lower average thermal conductivity at liquid nitrogen temperature; 0.20 W/mK with relatively large scatter (15-20%).

ORNL also explored the use of an infrared camera to monitor sample temperatures while in a high-pressure sapphire cell, a non-contact temperature measurement that could be used while collecting other data. Even at constant pressure, the IR camera observed melting of the sample, especially in areas in contact with the cell wall and supporting rod. The results showed this method can be used as an effective tool to monitor sample temperature changes during data collection.

Vibrational spectroscopy of hydrates can be used to link microscopic properties to macroscopic properties such as thermal conductivity. Vibrational spectroscopy done by inelastic neutron scattering (INS) generally offers poorer resolution than Fourier Transform Infrared (FTIR) or Raman spectroscopy, but has fewer restrictions on its applicability. ORNL examined the two natural hydrate samples using INS, in order to elucidate the underlying lattice structure.

Raman spectra were also obtained for both hydrate samples. Raman spectroscopy is useful in characterizing gas hydrates since it can directly measure the vibrational energies of the interstitial gas molecules non-destructively. Sample GC232 had more petroleum deposits than sample GC234, resulting in a slightly higher background for this sample, but Raman bands from both samples were essentially identical: methane and ethane along with trace amounts of isobutene and trans-butane.

Small angle neutron scattering (SANS) experiments were carried out on both natural hydrate samples. Data was also collected on sediment samples from the vicinities of both the GC232 and GC234 mounds. The sediment portion of the GC232 sample, which remained after the decomposition of the hydrates and gases, was also studied. Results confirmed that GC234 is a two-phase hydrate and ice mixture well described by the Debye-Bueche model.

Current Status and Remaining Tasks
In July 2003 additional funding of $35,000 was provided to enable the collaboration of ORNL with Colorado School of Mines researchers synthesizing gas hydrate samples under different conditions. They will be determining the hydrate number of samples using refined site occupancies from structural refinements on neutron powder diffraction data.

FY04 funding was added to continue the support of the facility and capabilities to current NETL gas hydrate research projects.

FY05 funding was added for continued support to current NETL gas hydrate research projects to analyze cores from the Gulf of Mexico.

Work on this project has been completed.

Project Start: September 1, 2001
Project End: September 30, 2005

DOE Contribution: $450,000
Performer Contribution: $0

Contact Information:
NETL – Thomas Mroz (TMROZ@netl.doe.gov or 304-285-4071)
ORNL – Claudia Rawn (rawncj@ornl.gov or 865-574-3184)

Additional Information:
Report - Mesoscale Characterization of Natural and Synthetic Gas Hydrates [PDF-836KB]

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