Hydrate Formation and Dissociation in Simulated and Field Samples
Last Reviewed 5/15/2012
The goal of this research is to characterize natural and simulated sediment samples, and to use these sediments as hosts to form methane hydrate and to investigate the kinetics of hydrate formation and dissociation
Oak Ridge National Laboratory – Oak Ridge, TN 37831
In nature, gas hydrates are observed either as disseminated small particles of hydrate within sediments (often fine grained clays) or as massive nodules or vein-like sheets within fractures or faults. From an economic viewpoint, locating and utilizing massive hydrate deposits is the key to producing methane hydrates as an energy resource. However, the bulk of gas hydrate within sea-floor sediments is likely to be found as disseminated particles that would be difficult to utilize economically. Therefore, if sea-floor gas hydrates are to be used as a hydrocarbon resource, it is necessary to understand how massive hydrate deposits form in the sea-floor and the geologic controls governing their distribution.
In this study, methane hydrate accumulation processes and controls will be examined in the laboratory through hydrate accumulation experiments using free methane gas bubbles percolated through simulated and natural sediment systems. These experiments aim to simulate hydrate accumulation processes that may be occurring within sea-floor sediments, allowing observation of hydrate accumulation and growth in the laboratory.
Potential Impact of this research
This research project, which utilizes man-made and natural core samples to produce methane hydrates, will provide fundamental insights into where to explore for potentially viable commercial methane hydrate accumulations, assess potential fields for production scenarios, and assess stability and safety issues.
The Sea-floor Process Simulator (SPS) at Oak Ridge National Laboratory is a unique experimental facility, ideally suited for the determination of kinetic, thermophysical, and mechanical properties of methane hydrates that are relevant to understanding their environmental significance and effects on the mechanical stability of the sea-floor. Not only can the physical conditions be controlled, but the size and design of the vessel permit direct observation of the hydrate formation processes, and investigation of how these processes are influenced by the heterogeneities that can be expected in nature.
(A) 72-liter Seafloor Process Simulator (SPS) pressure vessel used in the experiments. The SPS has >30 access ports and windows for instrumentation and observation of experiments. The sediment column (B) was suspended within the vessel and submerged in distilled water throughout the experiments. Methane gas was introduced into the column either through the bottom endcap or a capillary placed within the sediment. In some experiments methane saturated water was also circulated through the column using an external HPLC pump and collected in a secondary reservoir within the vessel (C).
The fiber optics-based Luna® Distributed Sensing System (DSS) has been incorporated into the SPS and records temperature/strain values at 1 cm intervals along the fibers (~ 2 m long) at specified time intervals (e.g., every 60 seconds) allowing for time-resolved 3-D monitoring of hydrate formation and dissociation processes within large volume sediment samples. Several homogeneous and heterogeneous sediment experiments have been conducted with the integrated SPS/DSS system and the results have been recorded in progress reports (e.g., Heterogeneous Sediment Experiments Interim Report 2008), meeting proceedings (e.g., International Conference on Gas Hydrates (ICGH) 2008), presentations (e.g., ICGH 2008, AGU 2009, Atlanta Hydrates Meeting 2010, Goldschmidt 2010), and a peer reviewed journal publication (Rev. of Scientific Instruments (accepted for publication Oct. 2010)). These experiments generate extremely large data sets. (For example, an experiment lasting four days with data collected every 60 seconds would generate 345,600 data points. Since each fiber has roughly 200 sensors and 4 (or more) fibers are generally used for each experiment, a total of 276,480,000 data points would be generated per experiment.) In order to analyze the large data sets, time-lapse movies are generated that permit a several day experiment to be viewed in less than 30 minutes. In addition to plotting the data by sensor number in a linear manner, new analysis tools also allow the data to be viewed using polar coordinates reflecting the spiral configuration in which optical fibers are physically positioned within the cylindrical SPS. A paper describing the data analysis has been accepted by the peer reviewed journal Computers & Geosciences. In FY10 an experiment in the SPS evaluated the energy input required to induce methane release from methane hydrates (presented at the AGU 2010 Annual Meeting).
Experiments have been conducted within a partially transparent cylinder (60 cm length, 4.8 cm diameter) using commercially available Ottawa sand, black aquarium sand (<500 micron grain size), and silt, as well as natural sediments collected by ODP Leg 204 at Hydrate Ridge and the Hot Ice I drilling project in Alaska’s North Slope permafrost gas zone. Pressure and temperature conditions were maintained for at least 12 hours prior to experiments to allow the water of the SPS to become saturated with methane. Results from the earlier experiments [see Topical Report, June 2007] demonstrate that in systems containing free methane gas, hydrate is likely to nucleate on the surface of methane gas bubbles, forming a film of methane hydrate. This is likely due to the super-saturation of methane at the bubble/water interface as a result of sluggish methane diffusion into surrounding water. If bubbles accumulate in the sediments within void spaces or at interfaces between sediment types, massive hydrate growth is likely to occur. Therefore, bubble accumulation points are likely to control where massive hydrate nodules and deposits will form in systems with a free gas phase.
ORNL also conducted a variety of diffraction studies (x-ray and neutron) focused on determining the effects of temperature, pressure, and time on the structural properties of relevant hydrate materials. In August 2007, Mt. Elbert core samples were received from Lawrence Berkeley National Laboratory and low temperature x-ray powder diffraction data were collected (no hydrate was present but the sediments were characterized). Time/temperature-dependent x-ray powder diffraction studies have also been performed on some of the Gulf of Mexico Green Canyon samples supplied by Texas A&M and on methane hydrate samples synthesized in-house at ORNL. The phase fractions (wt% ice vs. hydrate) and lattice parameters for both ice and hydrate have been refined from the data and plotted as a function of temperature (allowing for the thermal expansion to be calculated). Time-dependent data at specified temperatures have also been collected to determine the time it takes for a sample to decompose. The results for the sample from the Gulf of Mexico were presented in a poster and proceedings paper at the 2008 International Conference on Gas Hydrates in Vancouver, Canada.
ORNL has synthesized CO2 hydrate (starting with H2O or D2O ice) in smaller Parr vessels for characterization with x-ray and neutron diffraction. During FY09, ORNL used high pressure neutron powder diffraction studies to characterize CO2 hydrate samples that were synthesized in-house. CO2 was initially used due to safety issues with methane and to make an initial determination of what the experiments entail and how the samples must be maintained to avoid decomposition while loading them into the pressure vessels with closed-cycle He refrigeration equipment. During the first high pressure neutron powder diffraction experiment, data were collected at 225 K up to 1 kbar in a titanium-zirconium (TiZr) null scattering pressure cell; and during the second high pressure neutron powder diffraction experiment, data were collected at 153 K up to 3 kbar in an Al pressure cell. The bulk modulus (compressibility) of the CO2 hydrate was calculated using the refined lattice parameters and was determined to be similar to ice. These results were presented at the American Geophysical Union Fall meeting in December 2009.
Temperature and pressure data collected from experiment using natural sediments from Hydrate Ridge. Time zero represents the point of initial pressurization with methane gas through the sediment column. The temperature increase during pressurization is due to hydrate formation, an exothermic process. The plateau in temperature data and change in slope in the pressure data at approximately 12 hours after pressurization are due to hydrate dissociation, an endothermic reaction.
ORNL collaborated with the Georgia Institute of Technology to conduct experiments within the SPS where the SPS controls the thermodynamic conditions (i.e., the overall pressure and temperature). The large volume of the SPS and the numerous ports make it ideal for housing an instrumented internal cell (IIC) developed by the Georgia Institute of Technology collaborators. In addition to containing sediment the instrumented internal cell (IIC) houses all the instrumentation used to apply an effective stress to the sediment and instrumentation to monitor shear wave velocity, temperature, pressure, and volume change during hydrate formation and gas production. Four tests with different sediment types and procedures have been conducted and the results have been summarized in a draft publication. The Georgia Institute of Technology has pre-designed several IIC systems that will be utilized for future collaborative efforts to maximize the potential of the SPS.
Large volume hydrate-sediment characterization experiments were conducted to assess the effects of sediment heterogeneity and methane flux pathways on hydrate accumulation processes and on localized heating effects. Different sediment configurations (e.g., large void spaces, sand lenses, fine/coarse grain material in different geometries) have been, and will continue to be, assembled within the SPS to create model sediment columns. The rate and distribution of hydrate accumulation were monitored using the DSS to make time-resolved 3-D temperature and strain measurements on the cm scale within large sediment volumes in the SPS. These experiments allowed cm-scale monitoring of dissociation kinetics, sediment movement, and flow paths, as well as assessment of possible ice formation as a result of production to provide a better understanding of the distribution of hydrate within homogeneous and heterogeneous sediment systems and contribute to the development of efficient production practices.
Temperature-dependent neutron and x-ray scattering experiments were used to determine the thermal expansion and/or bulk modulus of relevant hydrate compounds. Time-resolved x-ray diffraction studies on methane hydrate were conducted and the data analyzed to quantitatively measure the phase analysis of ice vs. hydrate as well as the hydrate lattice parameters as a function of temperature. In addition to providing physical characteristics (i.e., thermal expansion and bulk modulus), the diffraction studies can be used to better understand hydrate nucleation and dissociation, to observe phase transitions, and to measure reaction kinetics.
Current Status (May 2012)
The project is complete.
Project Start: July 1, 2002
Project End: December 31, 2011
Project Cost Information:
All DOE Funding
Total Funding to Date - $1,771,000
NETL – John Terneus (John.Terneus@netl.doe.gov or 304-285-4254)
ORNL – Tommy Joe Phelps (firstname.lastname@example.org or 865-574-7290)
In addition to the information provided here, a full listing of project related publications and presentations as well as a listing of funded students can be found in the Methane Hydrate Program Bibliography [PDF].
Interim Report - Heterogeneous Sediment Experiments [PDF-307KB] - 2008
2008 Hydrate Peer Review [PDF-4.76MB]
Topical Report - Experimental Formation of Massive Hydrate Deposits From Accumulation of CH4 Gas Bubbles Within Synthetic and Natural Sediments [PDF-1.06MB] - June, 2007
Fire in the Ice article [PDF-1.01MB] "New Sensing Technology at Oak Ridge National Lab Expands Capabilities for Meso-Scale Hydrate Research" By Megan Elwood Madden, Oak Ridge National Laboratory - Winter 2007 edition, pg. 8
Interim Report - Hydrate Formation and Dissociation via Depressurization in Simulated and Field Samples [PDF-846KB] - June, 2006
Fire in the Ice article [PDF-972KB] "Oak Ridge Facilities Well Suited For Both Education and Collaborative Research" By Tommy J. Phelps and Claudia J. Rawn, Oak Ridge National Laboratory - Fall 2004 edition, pg. 4