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Experimental and theoretical study of the formation and decomposition conditions for multicomponent natural gas hydrates hosted in subsea sediments of the bathypelagic zone: the case of the subsea mountain Anaximander

Marinakis Dimitrios

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Summary

In the 21st century, the world will have to take crucial decisions with respect to the use of its energy resources. The increase of world’s population, which has recently reached the 7 billion mark, combined with the economic growth of countries heavily populated such as China and India, escalate the demand for energy, most of which is still based on fossil fuels. With growing social reservations over the use of nuclear power, particularly after Fukushima'snuclear plant disaster, and the arguments about the role of the fossil fuels in the greenhouse effect being as strong as ever, the energy question becomes very acute. Since almost all the giant oil and natural gas reservoirs have passed their peak production using conventional techniques, the oil extraction industry desperately needs to focus on the exploitation of new and unconventional resources. In this context, the natural gas hydrates deposits are expected to playa key role as a promising energy resource in the years to come, while at the same time they can also be considered as potential environmental hazard.Natural gas hydrate deposits, consisting mostly of methane gas, are estimated to contain a total energy several times greater than the oil and natural gas conventional reservoirs.Consequently, even if only a small percentage of the global hydrate resources ultimately proves to be economically viable for exploitation, its contribution to the worldʼs energy reserves will be substantial. On the other hand, potential dissociation of gas hydrates, due to global warming, can bear significant impact on the climate. An uncontrolled decomposition of the hydrate deposits will result in releasing massive amounts of methane gas to the atmosphere, whichcontributes more than 20 times than carbon dioxide to the greenhouse effect. Moreover, dissociation of gas hydrates in marine bearing sediments can lead to subsea landslides and mechanical failures of the overburden subsea formations, due to the cementing role of hydrates for the unconsolidated sediment grains.Favorable conditions for gas hydrate formation can be found in permafrost regions and mainly in marine environments. Over 95% of the gas hydrates estimated globally is considered to be found below the seabed. The subsea hydrate deposits, which are sensitive to temperature fluctuations, are expected to be found in the bathypelagic zone and fairly close to the seafloor.The bathypelagic zone extents from 1000 down to 4000 meters below the sea level. The almost anoxic conditions of the aforementioned zone help to preserve any occurring hydrocarbon gas products having originated from thermogenic and/or biogenic reactions in the subsea geoenvironment. Due to the high pressure conditions that prevail in the deep sea environment, gas hydrates formed from natural gas molecules are thermodynamically stable at fairly higher temperatures than the hydrates found at much shallower water depths. As a consequence, athigher temperatures, the hydrate stability zone, i.e the subsea geological formation where gas hydrates can form, begins at greater formation depths and extends up to the seabed. A significant part of this zone lies at conditions well inside the thermodynamic stability boundary of gas hydrates, where no free vapour phase could exist. While both methane and multi component natural gas can be enclathrated in the hydrate phase, research up to now focusesprimarily on single gas component hydrates. The scope of this study was to perform static experiments and numerical simulation of multi-component gas hydrate mixtures with compositions similar to the ones of gases recovered from bathypelagic zone sediments. Due to the wide range of possible conditions and gas mixture compositions, the study focused on the representative case of the Amsterdam mud volcano on the Anaximander sea-mountains of the East Mediterranean sea, where gas hydrates were found from exploration cruises at an average depth of 2000 m below sea level and atwater temperatures of 285-287K. A synthetic ternary gas (C1 to C3 ), resembling to the one of the studied area, was used in order to simulate the behavior of gas hydrates at conditions well inside the hydrate formation envelope.A series of experiments were conducted to form hydrates in an autoclave reactor with stirring, from the aforementioned synthetic gas and excess water. Preliminary tests were necessary to optimize the in-vitro procedure of formation in order to produce homogeneous hydrate crystals. The concentration of the dissolved gas species in the water was measured when the latter was brought at equlibria with hydrates at pressures ranging from 8 to 20 MPa, temperatures from 278 to 298 K and salinity concentrations from 0 to 4% w.t. in NaCl. The results revealed that gas hydrate formation is possible in nature, even in the absence of a vaporphase. The composition of the hydrates depends primarily on the variation of solubilities of the gas species in water with respect to temperature.The experimental study was further extended to study the behavior of gas hydrates formed inside geological formations. In-situ recovered marine sediment, as well as artificial porous media of berea sandstone and glass beads were used as a host formation, in order to simulate the effect that the geoenvironment bears on the hydrates formation. The in-situ recovered marine sediment was a clayish sample which has been retrieved from Kula and Amsterdam mud volcanoes' seabed. The host formation was partly saturated with hydrates at a pressure of 20 MPa by using the synthetic gas mixture with excess amount of water and wassubsequently subjected to a gradual dissociation of its hydrates, either by stepwise isothermal depressurization, or by slow isobaric heating. Hydrate phase boundary and gas composition, together with pore pressure data were collected during this test for all types of sentiments used.The investigation of the experimental data revealed differences in the thermodynamic behavior of the hydrate according to the nature of the hosting porous media. Significant differences were also observed with respect to the pore pressure build-up inside the host formation as a result of the degree of hydrate dissociation within the formation.Key properties of the sediment, such as permeability and compressive strength, which are affected by the presence of hydrates, were also experimentally studied. By measuring permeability values of the order of μDarcy (10-18 m2) in the host formation, it was found that hydrate dissociation bears a moderate effect on the permeability of the clayish sediment. On the contrary, hydrate dissociation triggers a more profound effect on the compressive strength of the sediment. The results indicate that gradual dissociation of the hydrates could have asignificant impact on the mechanical stability of the deep-sea sediments, confirming thus the role of hydrate dissociation as a possible cause to subsea landslides.The second major objective of this dissertation was to develop a mathematical model to simulate the equilibria of hydrates with fluid and solid phases. The model is partly based on Ballard's (2002) multiphase model, which is implemented in the CSMGem established program for multiphase hydrate equilibria, for the thermodynamic description of the individual phases.The models for the individual phases are combined in a novel double stage simulation algorithm in order to enhance the robustness of the equlibria predictions. A new formulation for the stability criterion of the Helmholtz energy was derived and used in order to be able to detect all the possible phases of the equlibria that are thermodynamically defined by the same cubic equation of state. The simulation procedure applies a minimization routine on a novel objectivefunction, which is derived from the stability criterion of the systems’ Gibbs energy for solving the phase split problem in multiphase systems without the requirement of any reference phase.By reformulating the phase split problem and by detecting all the possible equilibrium phases using stability criteria, the model does not require the stability coefficients

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