Nucleation and Dissociation Kinetics:
Flow-through sediment column experiments
A 60 cm glass column (diameter 4.8 cm) was filled with sediment and glass beads as spacers and suspended within the SPS. Methane-saturated water flow was controlled by an external HPLC pump; methane gas was injected into the sediment in some experiments (vessel measurements in inches). Results from these experiments suggest that free methane gas within the sediment, either as bubbles or at the water-gas interface, greatly increases the likelihood that hydrate will nucleate, therefore decreasing the observed induction time. Boroscope images of hydrate formation within the sediment column. Hydrate first observed as a film around bubbles in void spaces, and then later grows into sediment pore spaces.
Effects of Mineral Surfaces
The goal of our research is to investigate the effects of host sediment mineralogy on methane hydrate stability.
Hydrates are formed in the SPS by bubbling methane gas through concentrated suspensions of colloidal particles at low temperature and high pressure. When the vessel is gradually warmed, the hydrate zone temperature exhibits a plateau attributed to hydrate dissociation and absorption of latent heat. Methane hydrate dissociation conditions for water measured in the SPS compare well with model (Sloan, 1998) predictions. The presence of silica or bentonite at 34 mg/L in water appears to have little effect on hydrate dissociation conditions.
Research is being conducted to measure the effects of sediment surfaces on the dissociation of natural gas (methane + ~10% higher MW hydrocarbons) hydrates.
Previous experimental studies of methane hydrate (Vysniauskas and Bishnoi, 1983; Parent and Bishnoi, 1996) and carbon dioxide hydrate (Takeya et al., 2000; Ohmura et al., 2003) showed that formation of hydrate from liquid water depended on whether the water was previously cooled, frozen or recently used for hydrate formation and dissociation. The studies agreed that the period of time from the solution becoming metastable to when hydrate was first distinguished, called the induction time, was affected by the thermal history of water.
Recent experimental studies were conducted at ORNL, with a goal of understanding the influence of fractional addition of thawed water on nucleation of carbon dioxide hydrate in water. Water samples with different histories were examined. We considered cooled water, fractional addition of thawed water, and water from previous hydrate formation and dissociation. The study began with experiments that employed the small pressure vessel with a volume of 450 mL, where hydrate nucleation was initiated by stirring water. The entire content of the water under study was either thawed or thermally untreated. Results from those experiments were not conclusive enough to continue in studying the effects of the fractional addition of thawed water. The SPS, with injection of carbon dioxide bubbles through the bottom, was used in a subsequent set of experiments. These experiments revealed that as little as a 5% volumetric fraction of recently thawed water could considerably reduce driving force for hydrate nucleation. Visual observations of hydrate formation in a big vessel revealed changes in morphology of hydrate with the driving force. Our findings may provide additional insights into kinetics of hydrate formation as well as into production of hydrate at lower pressures with shorter induction times.
4. Probability of CO2 hydrate nucleation measured in induction
lag time for two treatments (thawed H2O and untreated H2O)
in the 450 mL Parr vessel. Induction
lag time is the amount of time taken after the equilibrium point is reached to
induce hydrate formation.
This research was supported by a grant from the Laboratory Director’s Research and Development Fund of Oak Ridge National Laboratory, and by the Gas Hydrates Program through the Office of Fossil Energy, U.S. Dept. of Energy.
Ongoing Methane Hydrate Research
Natural gas hydrates provide an abundant and virtually untapped source of energy. A majority of hydrates are found along continental margins and in permafrost regions. Destabilization of the hydrate by removing the pressure, increasing the temperature, or adding a dissociation inducing chemical leads to the release of trapped natural gas. This gas can then be produced and used to meet energy demands. However, production of natural gas from hydrates requires the development of new instruments and production strategies. The testing of novel instruments and techniques requires the use of a pressure cell that has an internal volume that approaches field scale and has the ability to maintain conditions observed in the natural environment.
Methane Production Experiments
The geometry of the SPS provides a unique opportunity to closely simulate methane production in a natural environment. The internal volume of the vessel (72 L) allows for the use of the geologic media found in hydrate bearing units. A know amount of hydrate can be formed in this media and production strategies can be tested in the SPS. Different strategies can be compared by observing the amount of the total gas produced. This approach allows for the most efficient strategy to be implemented in pilot scale hydrate production tests. The accuracy of novel instruments used for hydrate detection in the field can be validated by experiments in the SPS using known quantities of hydrate and natural gas.
Upcoming experiments include forming known quantities of hydrate in technical grade sediments (Ottawa sand). In order to calculate the quantity of hydrate that is formed in the SPS it will first be necessary to geologically characterize the hydrate bearing sediment (i.e. pore space, permeability, sediment size, mineralogy), which can be readily obtained from literature for technical grade materials. Secondly, the continuous measurement of gas flow, pressure, and temperature conditions before, during, and after hydrate formation will be obtained. This information will allow us to calculate the mass of hydrate that has formed in the sediment. An array of many hundreds of temperature sensors will allow us to spatially characterize the expanse of the hydrate in the sediment. A production strategy can then be implemented and the volume of gas produced and the change in spatial distribution of the hydrate can be measured. This experimental approach can be applied to a variety of sediments and production strategies.
Whole Vessel Hydrate-Sediment Characterization
Large volume hydrate-sediment characterization experiments will be conducted to assess the effects of sediment heterogeneity and methane flux pathways on hydrate accumulation processes. Large void spaces, sand lenses, and fine grain material will be assembled within the SPS to create model sediment columns. The rate and distribution of hydrate accumulation will be monitored using a new fiber optic distributed sensing system to make time-resolved 3-D temperature and strain measurements on the cm scale within large sediment volumes in the SPS. These experiments will also allow for cm-scale monitoring of dissociation kinetics, sediment movement, and flow paths, as well as assessment of possible ice formation as a result of production. These experiments will help us better understand the distribution of hydrate within heterogeneous sediment systems and contribute to the development of efficient production practices.
Riestenberg, D. E.; West, O. R.; Lee, S. -Y.; McCallum, S. D.; Phelps, T. J. Sediment Surface Effects on Methane
Hydrate Formation and Dissociation. Marine Geology 2003, 198, 181-190.
Zatsepina, O. Y.; Riestenberg, D. E.; McCallum, S. P.; Gborigi, M.; Brandt, C.; Buffett, B. A.; Phelps, T. J. Influence of
Water Thermal History and Overpressure on CO2-Hydrate Nucleation and Morphology. American Mineralogist 2004, 89,