Hydrates in some crude oils have a smaller tendency to form plugs than in others, and lately this is becoming a focus of research. To study this and the action of hydrate antiagglomerants in general, hydrate surface properties must be known. To help in characterizing the surface properties by simulation, the capillary waves of clathrate hydrate surfaces in vacuum are examined in all unique crystal faces by Molecular Dynamics, and an attempt is made to estimate the surface energies in the respective crystal faces from the wave fluctuations [1]. We also attempt to estimate solid/liquid surface energies of hydrate/oil and hydrate/water for a specific face, for comparison. The forcefield OPLS_AA is used for the organic compounds, while TIP4P/ice is used for the water framework. The anisotropy of the surface energy is then estimated and the result compared to the initial growth rate of different crystal faces as found in experiment [2].

Gas hydrates in near-mudline subsea sediments present significant challenges in the production of underlying hydrocarbons, impacting wellbore integrity and placement of subsea equipment. As the fluids of an underlying reservoir flow to the mudline, heat carried by the fluids warms nearwell sediments and dissociates hydrates, which releases gas that can displace and fracture near well soil. This gas release may be calculated with numerical simulations that model heat and mass transfer in hydrate-bearing sediments. The nature and distribution of hydrates within the sediments, the melting behavior of the hydrates, the thermal and mechanical properties of these shallow sediments, and the amount of hydrates contained in the sediments are required for the model simulations. Such information can be costly to acquire and characterize with certainty for an offshore development. In this information environment, it is critical to understand what information, processes, and calculations are required in order to ensure safe, robust systems, that are not overly conservative, to produce the hydrocarbon reservoirs far below the hydrates.

The purpose of the work is to show how to employ the experimental data from geocryology and soil physics for thermodynamic calculations of gas hydrate phase equilibria by taking into account pore water behavior in sediments. In fact, thermodynamic calculation is used here to determine the amount of non-clathrated pore water content in sediments in equilibrium with gas and hydrate phases. A thermodynamic model for pore water behavior in sediments is developed. Taking into account the experimental water potential data, the model calculations show good agreement with the experimentally measured unfrozen water content for different pressure and temperature conditions. The proposed thermodynamic model is applied for calculations of three-phase equilibria: multicomponent gas phase (methane, natural gas, etc.) – pore water in clay, sand, loamy sand, etc. – bulk (or pore) hydrate. As a result, correlations have been established between unfrozen and non-clathrated water content in natural sediments.

A thermodynamic inhibitor - methanol is used for hydrates control both at gas-gathering pipelines and gas conditioning / treatment field plants of Novatek JSC. Due to severe climate conditions and absence of serious infrastructure high operation costs for hydrate control take place. For reducing inhibitor losses some new technological solutions were proposed including recycling and regeneration of saturated methanol. A small module for producing methanol at field conditions was designed. Technological schemes for methanol injection and recirculation are discussed. These technologies reduce methanol losses. Small methanol-producing plant at Yurkharovskoe gas-condensate field (12.5 million ton methanol per year) integrated with field gas treatment plant is presented. The technology includes producing converted gas (syngas) from natural gas, catalytic process for raw methanol synthesis and rectification of raw methanol at final stage. Some particularities of the integrated technology are as follows. Not needs for preliminary purification of required raw materials (natural gas and water). Dried natural gas after conditioning (without any traces of sulfuric compounds) and pure water from simplified water treatment block are used. Rectification of raw methanol is combined with rectification of saturated methanol from gas treatment plant. Economic estimations show that the integrated methanol-producing technology and optimization of methanol circulation in technological processes essentially reduce capital and operational costs for hydrate control at northern gas and gas-condensate fields.

Active marine electromagnetic methods have proven to be a powerful tool to detect resistivity anomalies associated with gas hydrate. However, because the propagation of electromagnetic fields for these methods works in the diffusive regime the spatial resolution of the resistivity structure is limited. So far only bulk electrical properties have been estimated from measured data, although hydrate bearing layers are found to be highly heterogeneous. We computed response curves for synthetic one- and two-dimensional models to investigate the resolution capabilities for various measurement geometries with respect to resistive features. Electric dipole transmitters (TXs) are used as sources. In the marine case, the in-line electric dipole-dipole configuration has proven its capabilities to detect the shallow resistive gas-hydrate. Our model study demonstrates that both the depth to a resistive feature can be resolved nicely using data for multiple TX-RX offsets. However, resolving smaller features of the resistive zone, for example if the zone is split in separate resistive layers, is extremely difficult. The resolution of the target can be improved using electrical downhole transmitters. So far there have been no reports of the detection of permafrost gas hydrate deposits with surface electromagnetic methods. Our calculations show that a similar setup to that used in the marine case is capable of detecting gas hydrate on land. The resolution, however, is lower than for the marine case, because of the significantly greater depths to the target.

Enormous amount of latent heat generates/absorbs at the formation/dissociation process of gas hydrates and controlls their thermal condition themselves. In this paper we investigated the effect of ethane concentration on dissociation heat of mixed-gas (methane and ethane) hydrate. It has been reported by researchers that a structure II gas hydrate appears in appropriate gas composition of methane and ethane. We confirmed by using Raman spectroscopy that our samples had the following three patterns: structure I only, structure II only and mixture of structures I and II. Dissociation heats of the mixed-gas hydrates were within the range between those of pure methane and ethane hydrates and increased with ethane concentration. In most cases two peaks of heat flow appeared and the dissociation process was divided into two parts. This can be understood in the following explanation that (1) the sample contained both crystal structures, and/or (2) ethane-rich gas hydrate formed simultaneously from dissociated gas and showed the second peak of heat flow.

Dissociation and specific heats of synthetic methane and ethane hydrates were measured under high-pressure condition by using a heat-flow type calorimeter to understand thermodynamic properties of gas hydrates under submarine/sublacustrine environments. Ice powder was put into the sample cell and pressurized by methane and ethane up to 5MPa and 2MPa, respectively. After the completion of gas hydrate formation, samples were heated from 263K to 288K at the rate of 0.01 K min-1. Large negative peaks of heat flow corresponded to the dissociation of gas hydrates were detected in a temperature range 279-282K at a pressure of 5MPa for methane hydrate and 283-286K at 2MPa for ethane hydrate, respectively. We also obtained the specific heats of gas hydrates in the range 264-276K for methane and 264-282K for ethane under pressure.

Natural gas hydrates occur on most continental margins in organic-rich sediments at water depths >450 m (in polar regions >150 m). Gas hydrate distribution and abundance, however, varies significantly from margin to margin and with tectonic environment. The National Gas Hydrate Program (NGHP) Expedition 01 cored 10 sites in the Krishna-Godawari (K-G) basin, located on the southeastern passive margin of India. The drilling at the K-G basin was comprehensive, providing an ideal location to address questions regarding processes that lead to variations in gas hydrate concentration and distribution in marine sediments. Pore fluids recovered from both pressurized and non-pressurized cores were analyzed for salinity, Cl-, SO4 2-, alkalinity, Ca2+, Mg2+, Sr2+, Ba2+, Na+, and Li+ concentrations, as well as 􀀂13C-DIC, 􀀂18O, and 87/86Sr isotope ratios. This comprehensive suite of pore fluid concentration and isotopic profiles places important constraints on the fluid/gas sources, transport pathways, and CH4 fluxes, and their impact on gas hydrate concentration and distribution. Based on the Cl- and 􀀂18􀀁 depth profiles, catwalk infrared images, pressure core CH4 concentrations, and direct gas hydrate sampling, we show that the occurrence and concentration of gas hydrate varies considerably between sites. Gas hydrate was detected at all 10 sites, and occurs between 50 mbsf and the base of the gas hydrate stability zone (BGHSZ). In all but three sites cored, gas hydrate is mainly disseminated within the pore space with typical pore space occupancies being 􀀁2%. Massive occurrences of gas hydrate are controlled by high-angle fractures in clay/silt sediments at three sites, and locally by lithology (sand/silt) at the more “diffuse” sites with a maximum pore space occupancy of ~67%. Though a majority of the sites cored contained sand/silt horizons, little gas hydrate was observed in most of these intervals. At two sites in the K-G basin, we observe higher than seawater Cl- concentrations between the sulfate-methane transition (SMT) and ~80 mbsf, suggesting active gas hydrate formation at rates faster than Cl- diffusion and pore fluid advection. The fluids sampled within this depth range are chemically distinct from the fluids sampled below, and likely have been advected from a different source depth. These geochemical results provide the framework for a regional gas hydrate reservoir model that links the geology, geochemistry, and subsurface hydrology of the basin, with implications for the lateral heterogeneity of gas hydrate occurrence in continental margins.

Natural gas hydrates store a substantial portion of the Earth's organic carbon, although their occurrence is restricted by thermobaric boundaries and the availability of methane-rich fluids. The complexity of geological systems and the multiphase flow processes promoting hydrate formation can result in a mismatch between the predicted and the observed hydrate distribution. The purpose of this research is to achieve a better comprehension of the factors that influence the distribution of gas hydrates and the mechanism of fluid movements beneath and across the gas hydrate stability zone (GHSZ). Therefore, this study integrates seismic, petrophysical and geochemical data from different gas hydrate provinces. This work provides evidence that hydrates can occur below bottom-simulating reflectors, in the presence of sourcing thermogenic hydrocarbons. The relationship between fluid-escape pipes and gas hydrates is further explored, and pipe-like features are suggested to host a significant volume of hydrates. The host lithology also represents a critical factor influencing hydrate and free gas distribution and, in evaluating a natural gas hydrate system, needs to be considered in conjunction with the spatial variability in the methane supply. The three-dimensional distribution of gas hydrate deposits in coarse-grained sediments, representing the current target for hydrate exploration, is shown to be correlated with that of the underlying free gas zone, reflecting sourcing mechanisms dominated by a long-range advection. In such systems, the free gas invasion into the GHSZ appears controlled by the competition between overpressure and sealing capacity of the gas hydrate-bearing sediments. Globally, the thickness of the free gas zones is regulated by the methane supply and by different multi-phase flow processes, including fracturing, capillary invasion and possibly diffusion. In conclusion, this research indicates that geological, fluid flow and stability factors interweave at multiple scales in natural gas hydrate systems.

Gas hydrates are molecular host-guest mixtures where guest gas species are encapsulated in host water networks. They play an important role in gas storage in aqueous environments at relatively low pressures, and their stabilities are determined by weak interactions of the guest species with their respective host water frameworks. Thus, the size and the amount of the guest species vary, depending on the size of the empty space provided by the host water structures. The systems studied here are noble gas (He, Ne, Ar) and diatomic (H2) hydrates. Because of the similarity of the guests' sizes between the noble gases and the di-atomic gases, the noble gas hydrates act as simple models for the di-atomic gas hydrates. For example, He, Ne and H2 have approximately the same size. Density functional theory calculations are used to obtain the ground state formation enthalpies of each gas hydrate, as a function of host network, guest stoichiometry, and pressure. Dispersion effects are investigated by comparing various dispersion corrections in the exchange-correlation functionals (semi-local PBE, semi-empirical D2 pair correction, and non-local density functionals i.e. vdW-DF family). Results show that the predicted stability ranges of various phases agree qualitatively, although having quantitative difference, irrespective of the methods of the dispersion corrections in the exchange-correlation functionals. Additionally, it is shown in gas-water dimer interaction calculations that all DFT dispersion-corrected functionals overbind significantly than the interaction acquired by the coupled-cluster calculations, at the CCSD(T) level, which is commonly accepted to provide the most accurate estimation of the actual interaction energy. This could lead to an overestimation of the stability of the hydrate mixtures. Further study in the gas-water cluster indicates that less overbinding effect is found in the cluster than in the dimer. This implies that the overbinding energy caused by DFT might become less pronounce in the solid phase. Graph invariant topology and a program based on a graph theory are used to assign protons based on the 'ice rule' to fulfill the incomplete experimental structural data such as unknown/unclear positions of protons in the host water lattices. These methods help constructing host water networks for computational calculations. Several configurations of the host water structures are tested. Those configurations having lowest enthalpies are used as the host water networks in this research. Furthermore, the enthalpic spread between the configurations having the highest and the lowest enthalpy in the pure water ice network is very small (about 10 meV per water molecule). Nevertheless, it is still unclear to conclude that this protonic effect is also trivial in the gas-water compound. Therefore, this study also calculates the enthalpies of the gas-water mixtures having various proton configurations in the host water networks. Results indicate that very small enthalpic distributions among the proton configurations are found in the compounds as well. Furthermore, the enthalpic spread is almost constant as pressure increases. This suggests there is no pressure effect in the enthalpy gap amoung the proton distributions in both pure water ice and the gas-water compounds. Predicted stable phases for the noble gas compound systems are based on four host water networks, namely, ice Ih, II and Ic, and the novel host water network S!. The He-water system adopts ice Ih, II and Ic network upon increasing pressure. In the Ne-water system, a phase sequence of Sx/ice-Ih, II and Ic with a competitive hydrate phase in the S! host network at very low pressure is found. This is similar to the phase evolution of the H2-water system. For the Ar-water mixture, only a partially occupied hydrate in the Sx host network is found stable. This Sx phase becomes metastable if taking the traditional clathrates (sI and sII) into account. This result agrees very well with the experiment suggesting only two-third filling is found the large guest gases i.e. CO2. For the diatomic guest gas compound systems, the traditional clathrate structure (sII) that found to be existed experimentally in the H2-H2O system is also included in this study together with those four host water networks. Predicted phase stability sequence as elevated pressure is as follows: Sx, ice-Ih, II and Ic. This computationally prediction agrees very well with experiment. Results in this work suggest that the compound based on the traditional clathrate structure II (sII) host water framework is found to be metastable with respect to the decomposition constituents - in this case, they are pure water ice and the S!. The metastability of the hydrogen hydrates based on the sII structure might due to zero-point motions or other dynamic/entropic mechanisms uncovered in this research. Dynamic studies concerning the transition states of the hydrogen guest molecules in three competitive phases at very low pressure (less than 10 kbar), based on Sx, ice-Ih, and ice-II host water network, are considered. The energy barriers required by the hydrogen guest molecules in those three host frameworks are calculated by using Nudged Elastic Band (NEB) method. Results suggest that the hydrogen molecules are more mobile in the Sx than the other two host structures significantly. In the S! host water network, the energy barrier is about 25 meV/hydrogen molecule. This energy is about the room temperature suggesting that the hydrogen guest molecules are easily mobile in the Sx host water network if there is an empty site adjacent to them.