Formation, phase equilibrium and gas exchange of methane hydrates
Creators: name: Bian, Hao
Publisher: name: Imperial College London
Year: 2017

description: The phase behaviour and the formation/gas exchange kinetics of methane hydrates have been studied at elevated pressures and low temperatures, conditions under which naturally occurring hydrates exist or synthetic gas hydrates are formed during exploration operations and transportations. To this end, experimental measurements of hydrate phase equilibria for the binary methane and water mixture were made in mesoporous and macroporous silica, a stirred slurry reactor was used to study hydrate formation kinetics under various driving forces and water-based media, and a new Raman imaging technique was devised to study the exchange kinetics of methane hydrates with carbon dioxide under reservoir conditions. The formation kinetics of methane hydrates were measured using a high-pressure low-temperature autoclave system with a special focus on (a) the physical state of the water phase from which hydrate is formed and (b) the effect of available gas-water interfacial area for mass transfer and reaction during gas hydrate formation. The initial formation rate of methane hydrates in an agitated binary methane and water system was shown to have a universal exponential dependence on the driving chemical potential difference at temperatures between 276.5 K and 283.5 K and pressures between 5 MPa and 10.5 MPa. The hydrate formation rate was also studied for a methane-micronised ice particle system and a methane-‘Dry Water’ system consisting of a nanoparticle-stabilised high internal volume ‘dry’ water emulsion. The results indicated that the large differences in reactive surface area and gas diffusion rates for these three different pre-cursor systems resulted in pronounced effects on the initial formation rate of methane hydrates. Physical mechanisms were proposed to rationalize the observed behavior; it is anticipated that the results could be used to predict the hydrate formation rate for different combinations of water precursor state, reaction geometry and agitation conditions. As a model for natural gas hydrates formed in porous mineral sediments, the phase equilibrium behaviour of synthetic methane hydrates were measured, using high-pressure differential scanning micro-calorimetry, in mesoporous and macroporous silica that had controlled pore sizes ranging from 8.5 nm to 195.7 nm. The effect of pore surface chemistry was also qualified by studying the phase equilibrium in surface functionalised porous media. An oscillating dynamic method was used to essentially fully convert the ice/water pre-cursor into hydrates followed by slow heating in the micro-DSC to determine the decomposition conditions of methane hydrates in the porous silica over a wider range of pressures and pore sizes than previous studies. Significant shifts in dissociation temperatures with pore size were observed and rationalised using a modified Gibbs-Thomson equation. The experimental data up to 50 MPa indicated that the confinement effect in porous media showed a significant pressure dependency, most likely due to the interfacial energy. The effects of interfacial energy on the phase equilibria were investigated quantitatively by grafting different chemical groups onto the silica surface. The dissociation temperatures of methane hydrates in hydrophilic and hydrophobically-modified silica pores were significantly different; hydrates in strongly hydrophobic porous silica tended to behave like bulk methane hydrates regarding the melting temperature, with no significant effect of confinement. The measured phase equilibria, interfacial energy and enthalpy data can be used as parameters in gas hydrate simulation models. For the Raman studies, a high-pressure low-temperature optical reactor was designed, commissioned and applied. The cell accommodated samples to be analyzed over a temperature range of 288 K to 353 K and at pressures up to 50 MPa. Fast temperature response (up to 5 K/s), and stable temperature control (± 20 mK) were facilitated by using four Peltier elements. A large optical path allowed the acquisition of in-situ 3D Raman images to reveal the molecular structure and (CH4-CO2) exchange kinetics of gas hydrates in both single crystal and polycrystalline methane hydrates. An oscillating dynamic method was used to form polycrystalline hydrates at pressures up to 15 MPa and by contrast single crystal hydrates were formed under a small thermal driving force (around 3 K) at pressures up to 14 MPa. The polycrystalline hydrates exhibited a much faster CH4-CO2 exchange rate compared to the single crystal hydrates; defects and crystal imperfections also had a significant effect on this rate. The morphology of methane hydrates hence plays a critical role in the exchange process, which helps to explain the discrepancy of exchange rates for similar conditions reported in the literature. These time-resolved Raman results indicated a diffusion-controlled guest molecule substitution process. The Raman spectra were also used to quantify the exchange kinetics and the cage occupancy of the methane hydrates. This work has provided new insight and data for the rate of methane hydrate formation, their phase behavior under confinement and rates of gas exchange as a function of hydrate physical state under pressure and temperature conditions relevant to natural gas hydrate systems and their potential exploitation. The research represents a significant step forward in our fundamental understanding of the way gas hydrates are formed and behave. The collected data and derived parameters offer valuable inputs to process simulators concerning field exploration/production and potential uses in gas transportation and separation. Areas in which the research could be usefully extended have been identified.