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The hypothesis pursued in this work deals with the influence that the local pressure of the reservoir could exert on the CO2/CH4 replacement and on the gas storage processes. To experimentally validate such hypothesis, the relation between initial pressure of the system and induction, formation, and dissociation time was found and described. Sixteen methane hydrates formation and dissociation tests were carried out in a lab-scale reactor. The tests were made consecutively and with the same gas-water mixture to avoid differences between the tests related to the memory effect. The main difference between experiments consisted in the initial pressure of the system, which ranged from 62.41 bar to 46.03 bar. The effect of varying the initial pressure was evaluated by considering its influence on the induction period and the time required for methane hydrates growth and dissociation.

CH4–CO2 swapping is a promising and safe technique to produce CH4 gas and store CO2 without destabilizing hydrate-bearing sediments. However, inefficient CH4 recovery due to low CO2 diffusion prevents this technique from large-scale application. Multistep depressurization (MD) is therefore performed on CH4/CO2 hydrates after hydrate swapping to improve CO2 diffusion. This work presents pore-scale visualization of CH4/CO2 hydrates by non-destructive X-ray computed tomography (CT) and combined it with mixed hydrate kinetics for hydrate morphology evolution and fluid migration during MD. Influences of particle sizes (2–5 mm) and L-methionine (3000 ppm) on dissociation parameters were examined at reducing depletion pressures (8.5–59.4 bar) and constant temperatures (0.5–1.3 °C). The results of CT images showed enlargements of hydrate or gas phases with depletion pressures below CH4 hydrate stability pressures, complying with the pressure variations during MD. These variations of hydrate and fluid phases as well as pressure responses were the most obvious for particle size of 3 mm, resulted from good pore connectivity and relatively efficient mass transfer. L-methionine had no direct effect on CH4 production but enhanced CO2 storage, with the highest both CH4 gaseous mole fraction (86.1 mol%) and CO2 storage ratio (88.8%) at depletion pressure of 22.6 bar. This work clarified five-type pressure variations coupled with phase changes, providing firm evidence for hydrate dissociation/reformation and fluid migration that greatly affected hydrate exploitation. These understandings of varying morphologies and compositions of CH4/CO2 hydrates during MD would be applicable to help hydrate energy harvest and carbon emission mitigation.

Natural gas hydrates have been an unconventional source of energy since the beginning of this century. Gas-hydrate-filled reservoirs show higher resistivity values compared with water-filled sediments. Their presence can be detected using marine controlled-source electromagnetic methods. We classify acquisition configurations into stationary and moving receiver configurations, which are described in terms of the design group, the operational details, and where they have been used successfully in the field for natural gas hydrate exploration. All configurations showed good numerical results for the detection of a 700 m long gas hydrate reservoir buried 200 m below the seafloor, but only the stationary configurations provided data that can be used to estimate the horizontal boundaries of the resistive part of the reservoir when the burial depth is known from seismic data. We discuss the operational steps of the configurations and provide the steps on how to choose a suitable configuration. Different CSEM configurations were used together with seismic data to estimate the edge of the gas hydrate reservoir and the total volume of the gas hydrates, to optimize the drilling location, to increase production safety, and to improve geological interpretations. It seems that CSEM has become a reliable method to aid in the decision-making process for gas hydrate reservoir appraisal and development.

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

Gas hydrates are nonstoichiometric solid crystalline compound, whose formation is function of several parameters, such as pressure, temperature, fluid phase composition, reservoir saturation degree and others. One of the most critical aspects related to the research on this manner stays in differences existing between experimental results reached by using different experimental apparatuses. Moreover, laboratory scale reactors often have very contained dimensions with a consequent increasing influence of the boundary conditions. In the present paper, a brief overview of reactors used worldwide for experimental research on gas hydrates formation, is provided. In particular, the surface/volume ratio was calculated for each different typology of reactor and then associated with the ratio between moles of guest compound entrapped into water cages and moles injected. Even if such ratio does not represent the process efficiency, it is proportional to it. Consequently, that comparison was useful to well define the supporting effect of a greater S/V ratio on the hydrate formation process efficiency.

Natural gas hydrates (NGH) is the largest energy reservoir in the globe which consists of natural gas and water in icy form at artic permafrost regions and deep ocean floors but is still undeveloped due to several technical and economic issues. Finding an economic thermal energy resource to make the hydrate dissociated for harvesting is one of the major difficulties for making this technology viable. In light of this issue, a new hybrid process is proposed and thermodynamically analyzed for its viability. In the proposed method, fuel cell system and gas processing system are combined with hydrate harvesting system. Dissociation heat for harvesting natural gas from NGH is supplied using the waste heat from fuel processing and fuel cell systems. In the article, the proposed method is simulated with Aspen Plus® simulator incorporating mass balance and energy balance equations at every step conforming the validity of the results as well as the system viability under steady state operating conditions. Moreover, the effect of reforming reaction yield is studied and culminated that 85 % yield gives a better system efficiency than the equilibrium conditions to reach the maximum hydrogen production however at a toll of utilizable thermal energy for heating the hydrate sediments.

The phase fraction measurement of gas-water-sand fluid in downhole is an important premise for safe and stable exploitation of natural gas hydrates, but the existing phase fraction measurement device for oil and natural gas exploitation can’t be directly applied to hydrate exploitation. In this work, the electrical resistivity properties of different gas-water-sand fluid were experimentally investigated using the multiphase flow loop and static solution experiments. The effect of gas phase fraction and gas bubbles distribution, sand fraction and sand particle size on the relative resistivity of the multiphase fluid were systematically studied. The measurement devices and operating parameters were also optimized. A novel combined resistivity method was developed, which demonstrated a good effect for the measurement of phase fractions of gas-water-sand fluid, and will have a good application potential in marine natural gas hydrates exploitation.

Natural gas hydrates (NGH) are potential energy resource and distributed in unconsolidated marine sediments. CH4–CO2 swapping combined with depressurization is a promising method for simultaneously enhancing CH4 recovery and CO2 storage in NGH. This combined method was previously studied on CH4/CO2 mixed hydrates in consolidated sediments but no such studies are available for unconsolidated sediments. In this study, we investigated dissociation characteristics of CH4/CO2 hydrates in unconsolidated sediments using slow multistep depressurization. CH4/CO2 hydrates were formed in unconsolidated sandpack. The effects of CH4/CO2 molar ratio (CH4-rich and CO2-rich) in hydrate, residual water saturation (9.2% to 31.7%) and reservoir temperature (−3.3 °C to 2.2 °C) on production and storage efficiencies were mainly investigated, and the results were compared to those of bulk water and consolidated sediments in literatures. The results showed that CO2 was preferably encaged in hydrate than CH4 and this phenomenon was most apparent in unconsolidated sediments. CH4-rich hydrates in unconsolidated sediments were recommended to be exploited by multistep depressurization. Higher residual water saturation was unbeneficial to CH4 gas recovery, while a lower value was not conducive to CO2 storage in hydrate. Temperature below or above freezing point mainly decided ice formation and affected CH4 gas yield and CO2 retention by mass transfer. CH4 recovery was primarily dependent on degree of depressurization with higher CH4 recovery percent at lower depletion pressure. The maximum increment of CH4 concertation (6.5 mol%) and the highest CO2 storage efficiency (88.2%) were achieved in CH4-rich hydrates at optimum residual water saturation of 17.7% and temperature above 0 °C. In addition, sediments were the key factor controlling production and storage performances, and unconsolidated sediments provided a larger contact area among CO2 gas, CH4 hydrate and mobile water than consolidated sediments and bulk water. The results in this work could be useful for future NGH exploitation after CO2 injection into CH4 hydrates in unconsolidated sediments.

This study evaluates the spatial and temporal variability of seepage detected in the main fjords of western Spitsbergen, Svalbard archipelago, as indirect hydrate indicators (geophysical attributes to the presence of gas or seepage) of the natural gas hydrate (NGH) distribution. While methane seepage and NGH distribution in the offshore provinces of Vestnesa Ridge on the continental slope west of Svalbard and Prins Karls Forland are extensively studied, their potential distribution in the Isfjorden and Van Mijenfjorden fjords is poorly constrained. 2D seismic interpretation enabled mapping the major stratigraphic units and structural elements of the fjords and further identifying the distribution of potential source rocks. We also recognized different migration pathways such as faults and igneous intrusions, presumably transporting the gas to the seabed. In addition, analysis of hydrographic datasets acquired in August 2015 and June 2021 allowed the quantification and description of gas flares in the fjords and the comparison of the gas system characteristics over the late spring and early autumn seasons. Overall, 796 flares (668 in Isfjorden and 115 in Van Mijenfjorden) have been identified in the echograms acquired in 2015, and 152 flares have been identified in Isfjorden in 2021. The observations revealed an active fluid flow system in the fjords with an evident spatio-temporal variability of the seepage. Furthermore, different morphologic expressions at the seafloor, such as pockmarks, have been spatially correlated with the flares, unveiling no direct association with the present-day seepage. Despite the achievements of this work, further efforts are needed to finally prove the presence of NGH in the fjords of the Svalbard archipelago. Besides, more work is needed to understand the relation between high flare activity areas and the atmospheric methane concentration anomalies to assess potential methane efflux (diffusion between the surface waters and the atmosphere), contributing to the atmospheric carbon pool., Master thesis 2021 part of the Master Program for Polar and Marine Science (POMOR), by the Saint Petersburg State University and Hamburg University, in collaboration with Bremen University and The University Centre in Svalbard.

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.

This paper demonstrates the applicability of machine learning algorithms in sand production problems with natural gas hydrate (NGH)-bearing sands, which have been regarded as a grave concern for commercialization. The sanding problem hinders the commercial exploration of NGH reservoirs. The common sand production prediction methods need assumptions for complicated mathematical derivations. The main contribution of this paper was to introduce machine learning into the prediction sand production by using data from laboratory experiments. Four main machine learning algorithms were selected, namely, K-Nearest Neighbor, Support Vector Regression, Boosting Tree, and Multi-Layer Perceptron. Training datasets for machine learning were collected from a sand production experiment. The experiment considered both the geological parameters and the sand control effect. The machine learning algorithms were mainly evaluated according to their mean absolute error and coefficient of determination. The evaluation results showed that the most accurate results under the given conditions were from the Boosting Tree algorithm, while the K-Nearest Neighbor had the worst prediction performance. Considering an ensemble prediction model, the Support Vector Regression and Multi-Layer Perceptron could also be applied for the prediction of sand production. The tuning process revealed that the Gaussian kernel was the proper kernel function for improving the prediction performance of SVR. In addition, the best parameters for both the Boosting Tree and Multi-Layer Perceptron were recommended for the accurate prediction of sand production. This paper also involved one case study to compare the prediction results of the machine learning models and classic numerical simulation, which showed the capability of machine learning of accurately predicting sand production, especially under stable pressure conditions.

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.

Highlights • Code comparisons build confidence in simulators to model interdependent processes. • International hydrate reservoir simulators are compared over five complex problems. • Geomechanical processes significantly impact response of gas hydrate reservoirs. • Simulators yielded comparable results, however many differences are noted. • Equivalent constitutive models are required to achieve agreement across simulators. Geologic reservoirs containing gas hydrate occur beneath permafrost environments and within marine continental slope sediments, representing a potentially vast natural gas source. Numerical simulators provide scientists and engineers with tools for understanding how production efficiency depends on the numerous, interdependent (coupled) processes associated with potential production strategies for these gas hydrate reservoirs. Confidence in the modeling and forecasting abilities of these gas hydrate reservoir simulators (GHRSs) grows with successful comparisons against laboratory and field test results, but such results are rare, particularly in natural settings. The hydrate community recognized another approach to building confidence in the GHRS: comparing simulation results between independently developed and executed computer codes on structured problems specifically tailored to the interdependent processes relevant for gas hydrate-bearing systems. The United States Department of Energy, National Energy Technology Laboratory, (DOE/NETL), sponsored the first international gas hydrate code comparison study, IGHCCS1, in the early 2000s. IGHCCS1 focused on coupled thermal and hydrologic processes associated with producing gas hydrates from geologic reservoirs via depressurization and thermal stimulation. Subsequently, GHRSs have advanced to model more complex production technologies and incorporate geomechanical processes into the existing framework of coupled thermal and hydrologic modeling. This paper contributes to the validation of these recent GHRS developments by providing results from a second GHRS code comparison study, IGHCCS2, also sponsored by DOE/NETL. IGHCCS2 includes participants from an international collection of universities, research institutes, industry, national laboratories, and national geologic surveys. Study participants developed a series of five benchmark problems principally involving gas hydrate processes with geomechanical components. The five problems range from simple geometries with analytical solutions to a representation of the world's first offshore production test of methane hydrates, which was conducted with the depressurization method off the coast of Japan. To identify strengths and limitations in the various GHRSs, study participants submitted solutions for the benchmark problems and discussed differing results via teleconferences. The GHRSs evolved over the course of IGHCCS2 as researchers modified their simulators to reflect new insights, lessons learned, and suggested performance enhancements. The five benchmark problems, final sample solutions, and lessons learned that are presented here document the study outcomes and serve as a reference guide for developing and testing gas hydrate reservoir simulators.

Geologic reservoirs containing gas hydrate occur beneath permafrost environments and within marine continental slope sediments, representing a potentially vast natural gas source. Numerical simulators provide scientists and engineers with tools for understanding how production efficiency depends on the numerous, interdependent (coupled) processes associated with potential production strategies for these gas hydrate reservoirs. Confidence in the modeling and forecasting abilities of these gas hydrate reservoir simulators (GHRSs) grows with successful comparisons against laboratory and field test results, but such results are rare, particularly in natural settings. The hydrate community recognized another approach to building confidence in the GHRS: comparing simulation results between independently developed and executed computer codes on structured problems specifically tailored to the interdependent processes relevant for gas hydrate-bearing systems. The United States Department of Energy, National Energy Technology Laboratory, (DOE/NETL), sponsored the first international gas hydrate code comparison study, IGHCCS1, in the early 2000s. IGHCCS1 focused on coupled thermal and hydrologic processes associated with producing gas hydrates from geologic reservoirs via depressurization and thermal stimulation. Subsequently, GHRSs have advanced to model more complex production technologies and incorporate geomechanical processes into the existing framework of coupled thermal and hydrologic modeling. This paper contributes to the validation of these recent GHRS developments by providing results from a second GHRS code comparison study, IGHCCS2, also sponsored by DOE/NETL. IGHCCS2 includes participants from an international collection of universities, research institutes, industry, national laboratories, and national geologic surveys. Study participants developed a series of five benchmark problems principally involving gas hydrate processes with geomechanical components. The five problems range from simple geometries with analytical solutions to a representation of the world's first offshore production test of methane hydrates, which was conducted with the depressurization method off the coast of Japan. To identify strengths and limitations in the various GHRSs, study participants submitted solutions for the benchmark problems and discussed differing results via teleconferences. The GHRSs evolved over the course of IGHCCS2 as researchers modified their simulators to reflect new insights, lessons learned, and suggested performance enhancements. The five benchmark problems, final sample solutions, and lessons learned that are presented here document the study outcomes and serve as a reference guide for developing and testing gas hydrate reservoir simulators.

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.

В настоящее время разрабатываемые месторождения природного газа характеризуются значительной истощенностью. Одним из возможных промышленных объектов, способных кратно увеличить ресурсную базу и максимально раскрыть потенциал действующих месторождений, могут быть газогидраты, особенно учитывая то, что это самый большой неосвоенный газовый ресурс из всех нетрадиционных. На сегодняшний день отечественная наука сильно отстает от мировых тенденций в области изучения природных газогидратных отложений, а тем временем ведущие страны в ближайшие годы планируют выйти на стадию опытнопромышленной разработки. По этой причине данная тема представляет особый интерес. Объектом исследования являются газогидратные скопления Западной Сибири, с которыми связаны основные перспективы. Цель проекта заключается в обосновании способа выделения и оценке перспективных газогидратных залежей, опирающихся на детальное изучение геологических условий залегания, геологопромыслового анализа и свойств гидратонасыщенных пород, который может быть тиражирован, а также в оценке ресурсов гидратного газа, потенциала его извлечения и оценке экономической эффективности на примере месторождения ПАО НК Роснефть . В результате проделанной работы были получены следующие результаты: произведен анализ и систематизация российского и зарубежного опыта по выявлению, изучению и освоению газогидратных скоплений выделены основные критерии наличия газогидратных скоплений выделена зона образования газогидратных скоплений Западной Сибири, перспективная для поиска УВ предложены и обоснованы способы выделения газогидратных залежей, а также оценки коэффициентов пористости и насыщенности на рассматриваемом месторождении выделена газогидратная залежь, оценены возможные ресурсы гидратного газа и перспективы их освоения, а также оценена экономическая эффективность. Текущий статус реализации проекта дает представление о геологогеофизических особенностях газогидратов, принципах их поиска, выделения и оценки перспектив ресурсов гидратного газа и перспектив его освоения на конкретном примере месторождения ПАО НК Роснефть ., To date, producing natural gas fields are substantially depleted. Gas hydrate fields are promising production targets, which can enable manifold increase of the resource base and unlock the potential of existing fields under development, particularly given that this is the largest undeveloped unconventional gas source. Russia is currently lagging too far behind global trends in natural gas hydrate research, while some countries are planning to proceed to the pilot development stage in the nearest future. That is why this issue is of particular interest. This study is targeted at gas hydrate accumulations of West Siberia associated with the highest production potential. The purpose of this project is to validate a replicable method for identification and evaluation of promising gas hydrate deposits based on detailed study of mode of occurrence, geological and field analysis and properties of gas hydratebearing rocks. Other objectives include assessment of gashydrate resources, production potential and costeffectiveness analysis through the example of NK Rosneft field. The study involved analyses and systematization of Russian and international experience in identification, characterization and development of gas hydrate accumulations definition of the main criteria of the presence of gas hydrate accumulations, identification of potentially productive gas hydrate accumulations in West Siberia. Methods for identification of gas hydrate accumulations and estimation of porosity and saturation were proposed and validated. A gas hydrate accumulation was identified within the field under study a simplified reservoir simulation model was used to assess the resource potential of gas hydrate inplace and development opportunities. The project is economically viable. Results obtained to date provide significant insight into geological and geophysical aspects of gas hydrates, principles of prospecting, identification and assessment of resource potential of gas hydrate accumulations as demonstrated by the example of NK Rosnefts field., №3(19) (2019)

The paper considers the research results of natural gas (NG) hydrate formation process in systems consisting of paraffin oil and distilled water at various ratios. Thermobaric conditions of hydrates phase transitions in water-oil emulsions were determined by the method of high-pressure differential scanning calorimetry (DSC). It was ascertained that the equilibrium curve in systems under study, compared to the process of hydrate formation of this gas in distilled water, is shifted in the area of low pressures and high temperatures. According to DSC analysis results and volume analysis, it was determined that with increase of the water content in emulsions by more than 40 % of mass, the degree of water conversion into hydrate in emulsions decreases. It was concluded that the degree of water conversion into hydrate in emulsions can be used as an index of its stability.

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.

This study investigates the geomechanical, hydraulic and thermal characteristics of natural sandy sediments collected during the Ulleung Basin gas hydrate expedition 2, East Sea, offshore Korea. The studied sediment formation is considered as a potential target reservoir for natural gas production. The sediments contained silt, clay and sand fractions of 21%, 1.3% and 77.7%, respectively, as well as diatomaceous minerals with internal pores. The peak friction angle and critical state (or residual state) friction angle under drained conditions were ~26° and ~22°, respectively. There was minimal or no apparent cohesion intercept. Stress- and strain-dependent elastic moduli, such as tangential modulus and secant modulus, were identified. The sediment stiffness increased with increasing confining stress, but degraded with increasing strain regime. Variations in water permeability with water saturation were obtained by fitting experimental matric suction-water saturation data to the Maulem-van Genuchen model. A significant reduction in thermal conductivity (from ~1.4–1.6 to ~0.5–0.7 W·m−1·K−1) was observed when water saturation decreased from 100% to ~10%–20%. In addition, the electrical resistance increased quasi-linearly with decreasing water saturation. The geomechanical, hydraulic and thermal properties of the hydrate-free sediments reported herein can be used as the baseline when predicting properties and behavior of the sediments containing hydrates, and when the hydrates dissociate during gas production. The variations in thermal and hydraulic properties with changing water and gas saturation can be used to assess gas production rates from hydrate-bearing deposits. In addition, while depressurization of hydrate-bearing sediments inevitably causes deformation of sediments under drained conditions, the obtained strength and stiffness properties and stress-strain responses of the sedimentary formation under drained loading conditions can be effectively used to assess sediment responses to depressurization to ensure safe gas production operations in this potential target reservoir.

Over the last decade global natural gas consumption has steadily increased since many industrialized countries are substituting natural gas for coal to generate electricity. There is also significant industrialization and economic growth of the heavily populated Asian countries of India and China. The general consensus is that there are vast quantities of natural gas trapped in hydrate deposits in geological systems, and this has resulted in the emerging importance of hydrates as a potential energy resource and an accompanying proliferation of recent studies on the technical and economic feasibility of gas production from hydrates. There are then the associated environmental concerns. This study reviews the state of knowledge with respect to natural gas hydrates and outlines remaining challenges and knowledge gaps.

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.

Desde hace cerca de dos siglos, los hidratos de gas han ganado un rol importante en la ingeniería de procesos, debido a su impacto económico y ambiental en la industria -- Cada día, más compañías e ingenieros ganan interés en este tema, a medida que nuevos desafíos muestran a los hidratos de gas como un factor crucial, haciendo su estudio una solución para un futuro próximo -- Los gases de hidrato son estructuras similares al hielo, compuestos de moléculas huéspedes de agua conteniendo compuestos gaseosos -- Existen naturalmente en condiciones de presiones altas y bajas temperaturas, condiciones típicas de algunos procesos químicos y petroquímicos [1] -- Basado en el trabajo doctoral de Windmeier [2] y el trabajo doctoral the Rock [3], la descripción termodinámica de las fases de los hidratos de gas es implementada siguiendo el estado del arte de la ciencia y la tecnología -- Con ayuda del Dortmund Data Bank (DDB) y el paquete de software correspondiente (DDBSP) [26], el desempeño del método fue mejorado y comparado con una gran cantidad de datos publicados alrededor del mundo -- También, la aplicabilidad de la predicción de los hidratos de gas fue estudiada enfocada en la ingeniería de procesos, con un caso de estudio relacionado con la extracción, producción y transporte del gas natural -- Fue determinado que la predicción de los hidratos de gas es crucial en el diseño del proceso del gas natural -- Donde, en las etapas de tratamiento del gas y procesamiento de líquido no se presenta ninguna formación, en la etapa de deshidratación una temperatura mínima de 290.15 K es crítica y para la extracción y transporte el uso de inhibidores es esencial -- Una composición másica de 40% de etilenglicol fue encontrada apropiada para prevenir la formación de hidrato de gas en la extracción y una composición másica de 20% de metanol en el transporte

Dissociation by pressure depletion, thermal stimulation and/or injection of inhibitors are three known methods for gas production from natural gas hydrates. Challenges concerning these production methods are water and sand production as well as a potential loss of geo-mechanical stability, which may cause subsidence and landslides. A fourth production scheme, studied in this thesis, is based on the exchange process taking place when methane hydrate is introduced to the more thermodynamically stable guest molecule carbon dioxide. Sequestration of CO2 in hydrate follows as an added benefit to released methane gas. This thesis investigates experimentally the impact of temperature on CH4-CO2 exchange within hydrate-bearing sandstone. All experiments have been conducted at the Department of Physics and Technology, University of Bergen. The experimental work performed covered three main procedures; 1) methane hydrate formation in consolidated sandstone cores, 2) exchange between methane stored in hydrate and injected liquid carbon dioxide, and 3) hydrate dissociation either by depressurization or thermal stimulation. Time and effort was used on improving the hydrate laboratory with regards to safety, more accurate measurements, and ease of use. To achieve this goal, new instruments and solutions were implemented, service on old equipment was carried out, and new tubing systems were implemented where needed. Methane hydrate was successfully formed in eleven Bentheim sandstone cores with similar initial water saturation (Swi=0.4) and brine salinity (0.1 wt%), providing equal initial conditions for the CH4-CO2 exchange process. Repetitiveness in post hydrate results were observed at equal initial properties. A consistent trend was also observed where increased initial water saturations (Swi>0.6) and increased salinity (3.5 wt%) resulted in a decrease in hydrate saturation when compared with earlier experimental results. Following the completed hydrate formation, six CH4-CO2 exchange experiments were conducted at two different temperatures (4° C and 9.6° C). Liquid carbon dioxide was injected into the core at a slow rate (1.2ml/h) and effluent, composition and mass were measured. A spontaneous exchange process was proven in several experiments, consequently producing methane from the solid hydrate and sequestrating carbon dioxide. Based on the experimental results there is a strong indication towards an increase in both exchange rate as well as total methane recovery with an increase in temperature, especially when close to the equilibrium line where the hydrate is less stable. A new approach of adding heat to the system was tested by co-injecting carbon dioxide with a potential catalyst for CH4-CO2 exchange; methyldiethanolamine (MDEA) or monoethanolamine (MEA). Injection of these liquids post pure CO2 injection and CH4-CO2 exchange showed no enhanced recovery of methane from hydrate. A modified setup was designed where MEA could be injected by a separate pump and therefore be introduced ahead of the carbon dioxide. A high methane recovery was observed at a high recovery rate. This was most likely due to melting of the hydrate which can be verified by observation of water in the production line. Dissociation of both pure and mixed hydrate (CH4 and CO2) by thermal stimulation with a stepwise increase in temperature showed different equilibrium temperatures. The mixed hydrate dissociated at a range of higher temperatures, compared to pure methane hydrate. This verifies presence of carbon dioxide and the increase in stability for a mixed hydrate where carbon dioxide is present. MAMN-PETR PTEK399

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.

En los últimos años se ha incrementado el interés por el estudio de los volcanes de fango ya que se ha demostrado su frecuente relación con hidrocarburos situados en profundidad. El Golfo de Cádiz, en él que se han descubierto más de 30 volcanes de fango, es una de las zonas más importantes para el estudio de los volcanes de fango y de los procesos sedimentarios, biológicos y bioquímicos asociados a estas estructuras. Relacionados con la actividad de estos volcanes se ha observado la formación de carbonatos autigénicos asociados a la presencia de metano en el sedimento. En esta Tesis Doctoral se presentan los resultados obtenidos en el estudio de carbonatos (costras, brechas y concreciones) pertenecientes a tres zonas del Golfo de Cádiz: la zona de Diasom, dónde se estudian concreciones de los montículos de fango Ibérico, Cornide y Arcos, la zona del Complejo de Volcanes de fango Hespérides (denominada en la presente tesis como Bosta), y el Escarpe Pen Duick, situado en el Complejo de Volcanes El Arraiche. El estudio de estos carbonatos nos ha proporcionado información sobre el origen del metano, ya sea termogénico o biogénico del Golfo de Cádiz, y los procesos bioquímicos implicados en la formación de estos carbonatos.Los objetivos generales definidos para esta tesis son: a) La caracterización de los procesos bioquímicos implicados en la génesis de los carbonatos; b) La identificación de los fluidos parentales de las costras, brechas y concreciones carbonatadas del Golfo de Cádiz, y c) La determinación del origen del metano asociado a la formación de estos carbonatos. Para poder alcanzar estos objetivos se definieron otros más concretos: a) El análisis de la morfología externa y estructuras internas de los carbonatos; b) La caracterización petrográfica, mineralógica y geoquímica de las muestras, c) La identificación de todos los productos diagenéticos (generaciones de cementos, porosidad), d) La datación de los carbonatos y de las generaciones de cementos diagenéticos con el objeto de establecer la secuencia paragenética, y e) La definición y caracterización de los estadios de precipitación observados en cada uno de los tipos de carbonato.Los principales minerales observados en los carbonatos del Golfo de Cádiz son calcita con bajo contenido en magnesio, calcita con alto contenido en magnesio, dolomita, dolomita ferruginosa, ankerita y como minerales accesorios óxidos de hierro (secundarios de pirita preexistente), barita y como componentes terrígenos cuarzo y feldespatos. La actividad biológica de organismos bioturbadores y de organismos bioerosionadores es el principal factor de control tanto para la morfología externa (concreciones de Diasom) como para la morfología o estructura interna (costras de Bosta y costras de Pen Duick) de estos carbonatos. El origen del metano implicado en la mayoría de los carbonatos estudiados del Golfo de Cádiz tiene un origen termogénico y solamente en el caso de las costras de Pen Duick y en escasas concreciones del montículo de fango Ibérico y Cornide (zona Diasom) se ha apreciado la influencia de metano biogénico. Los principales fluidos implicados son agua marina, agua marina modificada por la interacción de roca:agua y fluidos enriquecidos en 18O asociados a la desestabilización de hidratos de gas. La formación de los carbonatos estudiados se produjo a profundidades varias. La mayoría de ellos se formaron cerca de la interfase agua-sedimento, excepto en el caso las costras carbonáticas de Pen Duick, que se formaron en la zona de metanogénesis, lejos del sulfato marino y bajo la influencia de fluidos asociados a la desestabilización de hidratos de gas.PALABRAS CLAVE: Carbonatos antigénicos, volcanes de fango, golfo de Cádiz, Metano, Hidratos de gas, Desestabilización - hidratos de gas, Metanogénesis, PH.D. THESIS ABSTRACT "Origin of authigenic carbonates associated to mud volcanoes from the Gulf of Cadiz (SW Spain): influence of biochemical processes and destabilization of gas hydrates ".TEXT:During the last years, it has grown a progressive interest in the study of mud volcanoes, since it has been highlighted the recurrent relationship between these mud volcanoes and the presence of deep hydrocarbons. The Gulf of Cadiz, where more than 30 mud volcanoes have been discovered, is one of the most important zones to study the sedimentological, biological and biochemical processes related to these mud volcanoes. Authigenic carbonate formation has been related to the activity of the mud volcanoes, owing to the presence of methane.The present Ph.D. Thesis presents the results derived from the carbonate characterization (crusts, breccias and concretions) from three areas of the Gulf of Cadiz: Diasom zone (mud mounds concretions from Ibérico, Cornide and Arcos), Hespérides mud volcanoes complex zone (identified as Bosta in the present Ph.D. Thesis), and Pen Duick escarpment zone, located in El Arraiche mud volcanoes complex area. The characterization of the carbonate samples from these three zones has provided insights about the methane origin (thermogenic or biogenic) and the biochemical processes involved in their formation.The main goals of this Ph.D. Thesis are: a) characterization of the biochemical processes responsible of the carbonate formation, b) Identification of the parental fluids of the carbonate crusts, breccias and concretions present in the Gulf of Cadiz, and c) Determination of the methane origin responsible of the carbonate formation. In order to fully achieve these main goals, a number of more specific objectives have been established: a) External and internal morphological descriptions of the carbonate samples, b) Petrographical, mineralogical and geochemical characterization of the carbonate samples, c) Identification of all diagenetic products (cement generations, porosity), d) Radiometric dating of the carbonate samples and of the diagenetic cement generations in order to establish the paragenetic sequence, and e) Definition and characterization of the precipitation stages for the different carbonate types.

[eng] Gas hydrates have been recently recognized as a key factor affecting a number of global processes such as the climatic change, sea floor stability, etc. In this thesis we present the multidisciplinary study of gas hydrate rich sediments recovered during ODP Leg 204. The main objective of this thesis is to study how the textural characteristics of marine sediments can affect the main pathways and intensity of fluid flow and how fluid flow determines the distribution of gas hydrates in the continental margins, as well as the main geochemical processes that occur during early diagenesis.To reach these objectives, a complete sedimentary and geochemical study of 581 sediment samples from southern Hydrate Ridge was carried out. The methods and techniques that were applied include: complete textural analyses, mineralogy, physical properties and geochemistry.The southern Hydrate Ridge sediments are mainly made up of four lithofacies defined as: hemipelagites, turbidites, ash layers and debrites. Mass‐transport deposits such as turbidites and debrites are more abundant in Lithostratigraphic Unit III and II, as well as in Lithostratigraphic Unit IA in the slope basin of southern Hydrate Ridge. Some increasing trends with depth can be observed in the smectite content in the clay mineral assemblages. These features suggest that the transport in suspension of fine sediments through the California Current was more effective during the Pliocene and early Pleistocene period. Bedload transport of coarse material from local and distal areas was more effective during the middle Pleistocene and Holocene due to the tectonic reactivation of the southern Hydrate Ridge uplift. During the Pleistocene and owing to the pervasive fluctuation of sealevel, gas hydrate dissociation together with the seismic movements in the Oregon margin seems a plausible triggering mechanism for mass‐movements.The results presented here confirm that the sedimentation patterns in the Hydrate Ridge region are controlled by climate and tectonic parameters such as the regional intensity of the California Current or the local tectonic movements that lead to the uplift of the Ridge. These parameters mainly control the clay mineral distribution as well as the sedimentary facies that were produced.The sedimentary fabric of gas hydrate‐rich intervals is disturbed during core recovery due to gas hydrate dissociation. The two main disturbance fabrics generated through this process are mousselike and soupy. The gas hydrate‐rich sediments analyzed for this thesis are coarser grained in respect to the hemipelagite sediments. The coarse‐grained layers such as turbidites and ash layers could act as conduits for fluids in the southern Hydrate Ridge region because of their higher porosity and permeability. In this context, methane‐rich fluids migrate through these layers from deep in the sedimentary sequence and into the gas hydrate stability zone. A number of barium fronts have been identified in southern Hydrate Ridge sediments and interstitial waters. Barite fronts were formed as a result of the barite recycling process during early diagenesis, which is controlled by the availability of methane‐rich fluids, in situ decomposition of organic matter and the sulphate gradient. Modelling of these data shows that these processes were active at southern Hydrate Ridge for a period of up to one thousand years.A number of geochemical and sedimentological processes are proposed in this thesis as plausible mechanisms to allow the survival of the barite fronts during diagenesis. The sedimentary texture plays an important role in controlling the major fluid flow pathways in the continental margins. The temporal evolution of the fluid flow can be studied in a given area through the distribution of the mineral phases that form during early diagenesis, as well as the interstitial water composition., [cat] Aquesta tesi integra els resultats de l'anàlisi sedimentológica i geoquímica de sediment marins rics en hidrats de gas, recuperats durant la campanya "Ocean Drilling Program" Leg 204 en el marge d'Oregon (USA). L'objectiu principal d'aquest estudi és conèixer les característiques sedimentàries que afecten el fluxe de fluids i gasos a través del sediment i com els fluids afecten la distribució d'hidrats de gas en aquesta àrea, així com alguns processos geoquímics que operen durant la diagènesi inicial. Els mètodes i tècniques aplicats inclouen l'anàlisi de sedimentològia, mineralogia, susceptibilitat magnètica i geoquímica.Els sediments de southern Hydrate Ridge estan formats per 4 litofàcies: hemipelagita, turbidita, cendra volcànica i debrita. La sedimentació està controlada per factors climàtics i tectònics com ara la intensitat del corrent oceànic Californià o moviments tectònics locals. Aquests paràmetres exerceixen un control fonamental en la distribució dels minerals d'argila i de les fàcies sedimentàries en el marge continental. La seva evolució des del Pliocè és discutida en aquesta tesi.Els sediments analitzats rics en hidrats de gas són més grollers que els sediments hemipelàgics. Els sediments més grollers actuen com a conductes preferents per a la circulació de fluids degut a la seva porositat i permeabilitat. En aquest context, fluids rics en metà migren des dels sediments profunds cap a la zona d'estabilitat dels hidrats de gas, on possibiliten la seva formació.Diversos fronts de barita han estat identificats en els sediments de southern Hydrate Ridge. Es formen com a resultat del reciclatge de barita durant la diagènesi inicial, controlada per la presència de fluids rics en metà, la degradació de matèria orgànica i la presència de sulfat. La modelització de les dades obtinguda mostra que aquest procés va ser actiu durant un període de >1000 anys. En aquesta tesi, es discuteixen els possibles processos geoquímics i sedimentaris que permetrien la supervivència de la barita durant la diagènesis.La textura sedimentària juga un paper molt important en el flux de fluids als marges continentals. La seva evolució temporal en una àrea determinada pot ser deduïda estudiant les fases minerals que es formen durant la diagènesi.

Submitted by Boris Flegr (boris@uerj.br) on 2021-01-07T14:41:06Z No. of bitstreams: 1 DissertacaoFernandaBarbosaPovoleriBDTD.pdf: 1069836 bytes, checksum: e13b94ee43a688b7278a620967346cb7 (MD5) Made available in DSpace on 2021-01-07T14:41:06Z (GMT). No. of bitstreams: 1 DissertacaoFernandaBarbosaPovoleriBDTD.pdf: 1069836 bytes, checksum: e13b94ee43a688b7278a620967346cb7 (MD5) Previous issue date: 2007-08-31 Coordenação de Aperfeiçoamento de Pessoal de Nível Superior The objective of the present work is to present a study of solid-vapor-liquid three-phase equilibrium for methane hydrates. The analysis of three-phase equilibrium has several applications for water-hydrocarbon systems, since it permits, for example, determination of stability region for methane hydrates and natural gas hydrates. We have started seeking in literature about the state-of-art for thermodynamic behaviour and phase equilibrium for hydrates. And then the models proposed by Ballard (2002) and Zhang et al. (2005) were implemented. Zhang et al. (2005) have proposed a phase equilibrium for single-guest gas hydrates at temperatures below 300 K. Their approach has combined the van der Waals Platteeuw theory for the hydrate phase and the Peng Robinson equation of state for both fluid phases (vapor and aqueous phase) (1976) modified by Stryjek and Vera (1986). Ballard s (2000) approach has allowed the hydrate distortion from its standard state and has gave a more accurate composition of the hydrate and has improved hydrate formation predictions at high pressures. As a direct result of accounting for a changing hydrate volume, the cage radii were functions of the hydrate volume. Thus, Ballard have proposed the hydrate phase equilibrium by Gibbs energy minimization in a multi-phase flash routine. Thus, this work presents the Zhang et al. (2005) and Ballard s (2002) approaches for hydrate thermodynamic behavior and makes an analysis and comparison of them. To compute the flash problem, we use the tool lsqnonlin (built-in of MATLAB® software). The algorithm lsqnonlin is based on the Levenberg-Marquadt method. O objetivo do presente trabalho é apresentar um estudo sobre o equilíbrio de fases sólido-líquido-vapor para hidratos de metano. A análise do equilíbrio trifásico sólido-líquido-vapor tem encontrado diversas aplicações para sistemas hidrocarboneto-água, uma vez que permite, por exemplo, a determinação da região de estabilidade de hidratos de metano e hidratos de gás natural. Inicialmente foi feita uma pesquisa sobre o estado da arte no que diz respeito ao comportamento termodinâmico e equilíbrio de fases de hidratos. Foram implementados os modelos apresentados por Ballard (2002) e Zhang et al. (2005). A proposta de Zhang et al. (2005) é aplicável para equilíbrios de fases a temperaturas abaixo de 300 K. Sua abordagem combinou a teoria de van der Waals e Platteeuw para a fase hidrato com a equação do estado de Peng-Robinson (1976) modificada por Stryjek e Vera (1986) para ambas as fases fluidas (fase vapor e fase aquosa). A abordagem de Ballard (2000) considerou a distorção do hidrato do seu estado padrão, o que fornece uma exata composição do hidrato e melhora a previsão da formação dos hidratos a altas pressões. Ao esclarecer a mudança de volume no hidrato, o raio da gaiola do hidrato é uma função do seu volume. Com isso, Ballard propôs uma nova abordagem considerando tal variação de volume e gerou um equilíbrio de fases em uma rotina de flash multifásico através da minimização da energia livre de Gibbs. Assim, o presente trabalho apresenta as abordagens de Zhang et al. (2005) e Ballard (2002) para o comportamento termodinâmico de hidratos e faz uma análise e comparação entre eles. Para resolver o problema do flash computacionalmente, foi utilizada a ferramenta lsqnonlin (built-in do software MATLAB®). O lsqnonlin é um algoritmo baseado no método de Levenberg-Marquadt.

Despite the fact that relatively little is known about the ultimate resource potential of natural gas hydrates, it is certain that gas hydrates are a vast storehouse of natural gas and significant technical challenges need to be met before this enormous resource can be considered an economically producible reserve. In this theoretical study, a simulation scheme was suggested to produce NGH in an industrial scale using pure water as a carrier and seawater as a cooling source. Parametric study was carried out and rigorous design calculations for different operating parameters were investigated. Further more and economical evaluation was done taken data of locally produced LNG as a comparison. Production rates, storage and transportation from production region to consumer's ends were investigated. Results obtained suggested that NGH with little consideration can be a good alternative for fuel gas carrier.