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Natural gas hydrates are enormous energy resources occurring in the permafrost and under deep ocean sediments. However, the commercial or sustained production of this resource with currently available technology remains a technical, environmental, and economic challenge, albeit a few production tests have been conducted to date. One of the major challenges has been sand production due to the unconsolidated nature of hydrate bearing formations. This review presents progress in methane gas production from natural gas hydrate deposits, specifically addressing the technology, field production and simulation tests, challenges, and the market outlook. Amongst the production techniques, the depressurization method of dissociating natural gas hydrates is widely accepted as the most feasible option and it has been used the most in field test trials and simulation studies. The market for natural gas hydrates looks promising considering the increasing demand for energy globally, limited availability of conventional fossil fuels, and the low carbon footprint when using natural gas compared to liquid and solid fossil fuels. The major market setback currently is cheap gas from shale and conventional oil and gas reservoirs. Cited as: Shaibu, R., Sambo, C., Guo, B., Dudun, A. An assessment of methane gas production from natural gas hydrates: Challenges, technology and market outlook. Advances in Geo-Energy Research, 2021, 5(3): 318-332, doi: 10.46690/ager.2021.03.07

Natural gas hydrates represent a valid opportunity for the permanent storage of CO2. In this work thermal stimulation technique has been deepened for replacing CH4 with CO2. Using a lab-scale reactor, eight experimental tests were carried out, characterized by two phases. The first one consists in the methane hydrates formation and is the same for all experiments. The second one consists in the replacement process and has been conducted with two different methods. In the first four tests CO2 was first introduced to reach a solid phase of methane hydrates and a gaseous phase composed of both CH4 and CO2; subsequently the temperature was increased. In the other four tests, the first step consisted in increasing temperature and then introducing CO2. In this case, the temperature required to move the thermodynamic conditions from a region suitable for the formation of both CH4 and CO2 hydrate to a region where the formation process can only involve CO2, was achieved without additional thermal energy; in fact, when the CO2 hydrates began to form, an immediate and massive release of heat took place. The results show that the methane yield and the amount of CO2 that can be stored do not differ between the two types of tests, but the thermal energy required to complete the process is much lower in the tests conducted with the second method.

International audience; Natural Gas Hydrates (NGH) collected during the Ghass cruise 2015 in the Western Black Sea onboard the R/V Pourquoi pas? are characterized by a suite of techniques. Gas Chromatography and Raman spectroscopy are used for the identification of the nature of the gas source, the hydrate structure and spatial variability of cage occupancies. The nature and source of hydrate forming gases primarily reveal a high methane content (99.6 mol%) and small amount of nitrogen (>0.29 mol%) and CO 2 (0.056 mol%). Isotopic analyses from the hydrate-bound methane and recently published results from Pape et al. (2020) clearly indicate a microbial source of gas supplying the hydrate deposit generated by the reduction of carbon. For the first time, Raman imaging spectroscopy was applied on NGH recovered in the Western Black Sea. The results show a heterogeneous distribution of the encapsulated guest molecules (CH 4 , N 2 and H 2 S), which is associated with a spatial variability of the guest-gas composition at the micron-scale. Some portions of the 2D-Raman images clearly exhibit a relative N 2-enrichment (with a concentration exceeding 6 mol% N 2 at some positions), while H 2 S shows a rather minor contribution on all the spectral maps investigated. A correlation is then established between the composition of the gas in the NGH and its impact on the CH 4 cage occupancy, with a ratio of θ LC /θ SC (large cage/small cage) between ~ 0.5 and 1.26 depending on the positions analyzed. The departure from the expected ratio in pure methane hydrate is attributed to the preferential encasement of N 2 in the large cage of the NGH structure. In addition, the occurrence of carotenoids identified in sediment-rich zones show a minor impact on the CH 4 cage occupancies. The results are discussed within the context of natural gas resource estimates in NGH to emphasize how the measured cage occupancies may impact the volumetric conversion factor commonly used with other geologic parameters to determine the resource endowment and global volume of methane. The small-scale heterogeneities revealed by the 2D-Raman images point out the importance to better understand stages of hydrates formation in methanerich seafloor environment.

Natural gas hydrates represent a valid opportunity in terms of energy supplying, carbon dioxide permanent storage and climate change contrast. Research is more and more involved in performing CO2 replacement competitive strategies. In this context, the inhibitor effect of sodium chloride on hydrate formation and stability needs to be investigated in depth. The present work analyses how NaCl intervenes on CO2 hydrate formation, comparing results with the same typology of tests carried out with methane, in order to highlight the influence that salt produced on hydrate equilibrium conditions and possibilities which arise from here for improving the replacement process efficiency. Sodium chloride influence was then tested on five CO2/CH4 replacement tests, carried out via depressurization. In relation with the same typology of tests, realised in pure demineralised water and available elsewhere in literature, three main differences were found. Before the replacement phase, CH4 hydrate formation was particularly contained; moles of methane involved were in the range 0.059–0.103 mol. On the contrary, carbon dioxide moles entrapped into water cages were 0.085–0.206 mol or a significantly higher quantity. That may be justified by the greater presence of space and free water due to the lower CH4 hydrate formation, which led to a more massive new hydrate structure formation. Moreover, only a small part of methane moles remained entrapped into hydrates after the replacement phase (in the range of 0.023–0.042 mol), proving that, in presence of sodium chloride, CO2/CH4 exchange interested the greater part of hydrates. Thus, the possibility to conclude that sodium chloride presence during the CO2 replacement process provided positive and encouraging results in terms of methane recovery, carbon dioxide permanent storage and, consequently, replacement process efficiency.

Carbon dioxide injection into natural gas hydrate reservoirs represents a promising opportunity to predispose a theoretically carbon neutral energy source. This technique allows to replace methane molecules with an equal number of carbon dioxide molecules and, consequently, to balance in advance emissions associated to methane utilization. While the direct CH4/CO2 replacement has been widely investigated, more data and scientific evidences are required to well define the feasibility of recovering methane by replacing it with CO2-based gaseous mixtures. In this sense, the most promising opportunity consists in flue-gas mixtures. In some cases, the presence of nitrogen was found capable to improve the overall efficiency, due to the direct competition between CH4 and N2 molecules to fill small cages characterizing hydrate structures. Moreover, these mixtures are extremely less-expensive than pure carbon dioxide. In this work, a binary CO2/N2 (50/50 vol%) gaseous mixture was used to recover methane contained into hydrate structures. Experiments were carried out in a small-scale experimental apparatus, designed to simulate a natural gas hydrate reservoir and to intervene on it with replacement techniques. Composition of gaseous mixtures present into hydrates and in the gaseous phase present immediately above, where defined via gas-chromatographic analyses. Finally, results were compared with data currently present in literature, in order to validate their consistency.

In the present study, the effect of different kinds of impellers with different baffles or no baffle was investigated. Up-pumping pitched blade turbine (PBTU) and Rushton turbine (RT) were the two types of impellers tested. The reactor was equipped with different designs of baffles: full, half and surface baffles or no baffles. Single (PBTU or RT) and dual (PBTU/PBTU or RT/RT) use of impellers with full (FB), half (HB), surface (SB) and no baffle (NB) combinations formed two sets of 16 experiments. There was estimation of rate of hydrate formation, induction time, hydrate productivity, overall power consumption, split fraction and separation factor. In both single and dual impellers, the results showed that RT experiments are better compared to PBTU in rate of hydrate formation. The induction time is almost the same since we are deep in the equilibrium line while hydrate productivity values are higher in PBTU compared to RT experiments. As general view RT experiments consume more energy compared to PBTU experiments.

The issue of hydrate formation during gas processing is a challenging problem for the oil and gas industries. Billions of dollars spent annually to get rid of the blockage of pipelines due to hydrate clogging by applying various methodologies [1]. Among them injecting chemical inhibitors is the most widely accepted solution; however, choosing a chemical is a difficult task which depends a lot on the gravity of the situation. Generally methanol, glycols and salts also know as thermodynamic inhibitors (THIs) are used which needs to be added in large quantities (50 wt% or above) making them expensive and non-environmental friendly. Also, because methanol is flammable there is always risk and cost associated to its storage. Currently, a new class of inhibitors (generally polymers and surfactants) known as low dosage hydrate inhibitors (LDHIs) are getting much attention which can be used in much less amount (1–3 wt% or less) making them economically and environmentally feasible [2]. Industries are always looking for chemicals which are economic, environmental friendly and have peculiar set of properties. Currently, ionic liquids (ILs) have attracted the attention due to their potential for fulfilling the industrial demands [3]. In this work, we have shown that how a slight variation in the structure of ammonium ILs result in peculiar behavior towards methane gas hydrates.Five ionic liquids (ILs) belonging to the same ammonium family but structurally and functionally different were tested for their hydrate inhibition ability using a meso-scale rocking-rig apparatus. The first IL studied was tetra-methylammonium acetate (TMAA) which is very similar to a tetra-alkylammonium salt; well known hydrate inhibitors/promoters. The other four ILs belong to choline, also known as, substituted alkyl-ammonium family where one of the alkyl substitutions of TMA is replaced by hydroxy-ethyl functionality. The four choline ILs were attached to anions which are different in nature viz., butyrate and iso-butyrate are isomeric counterparts; whereas hexanoate and octanoate has a difference in the chain length. It has been shown in this work that any slight structural variation in the compound used for hydrate inhibition resulted in a unique behavior. The working concentration and pressure range are also some important factors. It has been shown that at 1 wt% and at higher pressures the Ch-Oct, Ch-But and TMAA act as hydrate promoters. Whereas, at 5 wt% in the whole experimental pressure range (40–120 bar) all the studied ILs act similarly as hydrate inhibitors with TMAA showing the best performance almost comparable to methanol at high pressure. Hydrate suppression temperatures were determined and the performance of individual IL has been discussed quantitatively. Molar hydrate dissociation enthalpies were calculated and their values were interpreted in the light of the thermodynamic inhibition results which indicate that the ILs do not participate in the hydrate cages. Induction time analysis shows that Ch-Oct due to its micelles forming ability acts as a strong kinetic inhibitor which delays the hydrate formation time by more than an hour. Thus, it must be emphasized that a slight structural variations in the structure of ILs reveals their doubly dual nature for methane hydrates system viz., thermodynamic inhibition, hydrate promoter, kinetic inhibition and surfactant character. Since, the used ILs are also biocompatible, non-toxic and biodegradable it make them an excellent alternative class of inhibitors compared to their conventional counterparts.

The dissertation addresses mineralogical characteristics of natural gas hydrates from cold seeps in the Gulf of Mexico and the eastern Black Sea. The investigated properties are the crystal structure, the crystallite sizes and size distributions, the compositions of the hydrate-forming gases, the hydrate porosity as well as the grain boundary networks. That was accomplished using X-ray diffraction, gas chromatography, Raman-spectroscopy, and scanning electron microscopy. "Bragg tomography" was applied for crystallite size measurements of hydrates.Computer tomography using Synchrotron radiation was used for the first time for gas hydrate inspections on a micro-scale.The crystallite sizes of natural hydrates are governed by the velocity of crystal growth, therefore time is an important factor. One important result of this thesis is that the gas hydrate crystallite size evolution is also affected by more factors, which are impurities in the grain boundary network such as oil and salts. Gas hydrates from the northern Gulf of Mexico crystallize to structure type II hydrate (sII) from methane and a fraction of up to 30 mol% of C2 through C5 hydrocarbons. One important result of this thesis is that the lower molecular mobility of C2-C5 hydrocarbons compared to methane incorporated in hydrates is displayed phenomenologically by dense, non-porous surfaces of the gas hydrates on a micrometer scale. Instead, hydrates which were formed from volatiles like methane, often crystallizing to sI, are characterized by a micro-porosity. A further objective of this thesis was to investigate the coexistence of sI and sII at the Chapopote asphalt volcano in the southern Gulf of Mexico. sI and sII hydrates trapped below viscous asphalt are intimately associated on short distances within individual crystallite-agglomerates. Micro-computer tomograms of the Chapopote hydrates showed small bubbles as well as genetic changes of the hydrate outside hydrate stability. The presence of bubbles indicates that free gas co-occurs together with the gas hydrate. An important outcome is that also well-preserved hydrates can be accompanied by an ice fraction not deriving from hydrate dissociation; accordingly, ice is not an exclusive indicator for hydrate decomposition. The potential effect hydrates may exert on climate was investigated at the Batumi gas seep, eastern Black Sea. In the study area (0.5 km2) the hydrate-bound carbon within the upper 2.65 m below seafloor was estimated on 10.7 kt. The rough seafloor topography suggests episodic detachment of gas hydrate chunks from the seafloor, which float upwards due to the positive buoyancy.