Phase transformation studies of methane-propane hydrate using neutron scattering

Gas hydrates form readily under the pressures and temperatures experienced in deep-sea oil and gas pipelines. They form large solid plugs that can block and damage pipelines and disrupt production. Consequently the Oil and Gas industry uses chemical inhibitors in deep sea fields as a preventative for gas hydrates. As a production field matures the fraction of water in the gas flow increases. The quantity of chemical inhibitors must therefore increase resulting in additional production costs. The ability to understand the processes associated with hydrate formation would allow design engineers and operators to work to tighter margins at considerable cost savings. In-situ neutron diffraction experiments of hydrate reaction processes, using the powder diffractometer (Wombat), provide insights into the kinetics of crystallisation of gas hydrates from solid and liquid water phases. We present neutron scattering studies of the phase transformations and the phase occurrence over a range of pressure/temperature (3.0 – 4.8 MPa, 263 – 288 K) of methane-propane hydrates as a simple analogue of natural gas hydrates. Our results are mainly in accord with the thermodynamic stability of the hydrate phases, where the methane-propane mixture can produce either pure sII hydrate or a combination of sI and sII hydrates. Surprisingly of two measurements at 4.8 MPa the sI phase did not form at a temperature of 263 K but did form at a higher temperature. This result is inconsistent with the thermodynamic phase boundary of methane sI hydrate. We also demonstrated the effectiveness of two common inhibitors, monoethylene glycol and polyvinylcaprolactam. We observe that decomposition of gas hydrates just above the ice-point results in formation of ice, which subsequently decomposes slower than the hydrate. Furthermore these experiments elucidate the importance of the heat flow of the reaction processes and the role of a free liquid-gas interface. In-situ SANS experiments can probe the hydrate nucleation process. Our first SANS experiments, on Quokka, reveal rapid uptake of methane-propane gas on ice crystallites well below the accepted phase boundary, offering the prospect of valuable insights in planned future experiments using a gas-liquid flow loop, which is under development at ANSTO.