Natural gas hydrates are ice-like structures in which gas molecules are trapped inside water molecules that form under conditions of high pressure and low temperature. These abundant compounds exceed our resource of conventional gas reserves by at least an order of magnitude and naturally occur in the ocean bed and permafrost regions. Whilst there is interest in natural gas hydrates as a potential untapped energy resource they currently cause considerable problems to both the energy sector and the environment.
Firstly, natural gas hydrates that escape to the surface are powerful greenhouse gases. Methane, commonly present in natural gas hydrates, is 21 times more powerful than carbon dioxide as a greenhouse gas. In addition, natural gas hydrates pose a severe problem in oil and gas pipelines. If water and natural gas cool in the pipes, hydrates may form which can block the line. Deep oil and gas extraction can also disturb sites containing volatile natural gas hydrates, posing a danger to those nearby.
Given these damaging consequences there is much interest in understanding the mechanisms by which gas hydrates form to help with the design of future inhibitor technologies. Researchers from UCL, ISIS Neutron and Muon Source and BP Exploration Operating Co. Ltd have utilised neutron scattering techniques at ISIS Neutron and Muon Source and molecular dynamic simulations to study a natural gas hydrate - methane hydrate.
Using neutrons to study natural gas hydrates
The effect of dissolved solid impurity particles on the formation of methane hydrate was investigated using a variety of clay and silica nanoparticles. The nanoparticles used are such that you might typically expect to find under natural and industrial conditions.
Neutron scattering was used in conjunction with hydrogen-deuterium isotopic labelling using NIMROD and SANDALS time-of flight neutron diffractometers to study the formation process of methane hydrate. The instruments, based at ISIS Neutron and Muon Source, are optimised for studies of liquids and amorphous materials containing a high proportion of hydrogen.
Impact of impurities on methane hydrate formation
The surprising results from the experiments found that the formation of methane hydrate was insensitive to the addition of the nanoparticle impurities. This is a somewhat unexpected finding, as the presence of particles generally enhances pure ice formation by orders of magnitude.
The explanation behind this result centres on the different chemical natures of methane and water. Whilst water molecules are polar and form relatively strong hydrogen bonds, methane molecules are non-polar and interact through much weaker interactions. The different chemical natures of methane and water mean it is unlikely that the surface of a dissolved particle will display a strong affinity for both water and methane, and it therefore won't promote mixing and hydrate formation.
Future inhibitor technologies
Molecular dynamic simulations were also undertaken in this research. The results found that methane hydrate forms away from the solid impurity/liquid interface - a useful result for the design of inhibitor technologies. As hydrate formation occurs away from mineral surfaces it was suggested that future studies could focus on weakening the affinity of inhibitors to mineral surfaces to improve their effectiveness.
A snapshot from a molecular dynamics simulation of clathrate formation. Credit: Steve Cox.
In summary, these results have not only found that nanoparticle impurities have little impact on the formation of methane hydrate, but the results from molecular dynamic simulations have shed light on where hydrate formation takes place. This research will help in the design of future inhibitor technologies which could stop the damaging environmental and industrial consequences of these abundant natural compounds.
Read the full research article: Stephen J. Cox, et al. “Formation of methane hydrate in the presence of natural and synthetic nanoparticles" Journal of the American Chemical Society Just Accepted Manuscript DOI: 10.1021/jacs.7b12050
Learn more on ISIS Neutron and Muon Source instruments NIMROD and SANDALS.
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