Insights into climate-driven evolution of gas hydrate-bearing permafrost sediments
Vasheghani Farahani, Mehrdad
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Massive reserves of natural gas hydrates exist in permafrost and marine sediments. Dissociation of natural gas hydrates could result in enhanced emissions of methane to the atmosphere, aggravating global warming. It may also become a serious geohazard to the geomechanical stability of gas hydrate deposits. The environmental and infrastructural impacts due to climate-driven and human-induced dissociation of natural gas hydrates cannot be well predicted unless we have an accurate estimation of hydrates deposited in both marine and permafrost sediments. However, conventional seismic techniques cannot give reliable estimations of gas hydrates in permafrost sediments as they are unable to distinguish ice from hydrates due to their almost identical acoustic properties. The main objective of this thesis is to address the above challenge via shedding light on the influence of the hydrates presence on the evolution of gas hydrate-bearing permafrost sediments, and developing a coupled geophysical-geothermal scheme, for the first time, for estimation of hydrates saturation in these sediments. To achieve this, magnetic resonance imaging (MRI) was employed to investigate the kinetics of formation and spatial characteristics of methane hydrate in synthetic and natural sediment samples. The analysis of the images acquired during three consecutive thermally-induced hydrate formation/dissociation cycles indicated that in addition to the kinetics of formation, pore-scale distribution of hydrates is affected by the thermal history of the system and the host sediment type, characteristics, and particle size distribution. The results also showed that different hydrate pore-scale habits may co-exist in the system, which is essential to be considered in the models developed for the estimation of the physical properties of gas hydrate-bearing sediments. Having the above fundamental insights, the geophysical and geothermal responses of hydrate-free and hydrate-bearing sediments were characterised by measuring their elastic wave velocities and effective thermal conductivity (ETC) at different hydrate saturations (up to 55%) and effective overburden pressures (up to 6.89 MPa) at both unfrozen and frozen conditions. The results confirmed that the evolution of the elastic wave velocities as well as ETC depends on the saturation and pore-scale habit of hydrates; and ETC could interestingly assist with distinguishing ice from hydrates. The ETC measurements also revealed that the presence of hydrates in porous media is associated with three key pore-scale phenomena contributing to the efficiency of the heat transfer: the saturation and pore-scale habit of hydrates, hydrate/ice-forced heave, and unfrozen water saturation at frozen conditions. Moreover, a new physical model was developed for the prediction of ETC of hydrate-free sediments using the Free-energy Lattice Boltzmann Model (LBM) and a space renormalisation technique, and modified according to the insights from the ETC measurements to account for the effect of the above-mentioned key pore-scale phenomena. Ultimately, the coupled geophysical-geothermal scheme was developed by using the modified ETC model as the geothermal part and Ecker’s rock-physics models as the geophysical part, and its performance was validated using the measured geophysical and geothermal properties. It was demonstrated that the proposed coupled scheme is able to quantify the saturation of the co-existing phases in a wide range of hydrate saturations and at different effective overburden pressures, particularly at frozen conditions where the co-existence of hydrates, ice, and unfrozen water is essential to be captured. This scheme makes it possible to distinguish ice from gas hydrates in frozen sediments hence it could be employed for not only quantification of gas hydrates in permafrost but also further development of large-scale permafrost monitoring systems for monitoring the dynamic response of gas hydrate-bearing permafrost sediments to climate warming in cold regions.