Methane Gas Hydrate Research Articles

Kinetic and rheological study of methane hydrate formation in water-in-oil emulsion: Effects of emulsion composition and silica sands

Gas hydrate blockage in the emulsifying crude oil poses security risks to the subsea multiphase flow transportation. The kinetic and rheological properties of methane gas hydrate in the water-in-oil emulsion were in situ characterized by experiments in a high-pressure rheometer with particular focus on the effects of emulsion composition and silica sands. Results demonstrated that the methane hydrate growth rate and yield stress increased with the surfactant concentration and water fraction, while the hydrate formation in the emulsion of pure light alkanes was faster than that in the mineral oil emulsion. Moreover, the presence of silica sands shortened the hydrate nucleation time, eliminated the stochasticity of nucleation, and induced the occurrence of hysteresis by affecting the transient and steady state viscosity of hydrate-in-oil slurry. Overall, this study was of added knowledge to the hydrate formation mechanisms in the water-in-oil emulsion, which highlighted the fact that the high emulsification degree, high water fraction, and the high concentration and large particle size of silica sands in the multiphase flow would increase the risks of hydrate blockage in the oil-gas transportation pipelines.

Prediction of Methane and Ethane Gas Hydrate Formation and Their Mixture in a Porous Medium

The purpose of this paper is to study the thermodynamic modeling of the conditions for methane and ethane gas hydrate formation and their mixtures in a porous and non-porous environment. In this paper, the Van der Waals- Platteeuw thermodynamic model was used for prediction of gas hydrate formation conditions. Also, the SRK and PTV equations of state were used for calculations of driving force. In this research, the results of thermodynamic modeling in a porous were compared with the non-porous environment and laboratory data in the literature. Studies have shown that the results of the modeling are in good agreement with the laboratory data and the percentage of errors is low. The results also showed that with increasing pressure of porous and non-porous media, the equilibrium temperature increases. In addition, the effect of the pore diameter of porous media on the results of modeling was investigated for methane, ethane and their mixtures during gas hydrates formation. The results showed that by increasing the pressure for any size of the pore diameter of the porous medium, hydrate formation temperature increases. In addition, by increasing the pore diameter of the porous medium, hydrate formation temperature methane, ethane and their mixture increase at a constant pressure. The results also showed that the equilibrium temperature of the non-porous medium is higher than the equilibrium temperature of the non-porous medium. This shows that the hydrate formation in the porous medium has a deterrent effect and leads to lower temperatures and higher temperatures conditions for gas hydrate formation. The results showed that by increasing the percentage of methane in a porous or non-porous medium, the temperature of hydrate formation of the binary gas mixture of methane and ethane decreases.

Numerical simulation of the short- and long-term production behavior of the first offshore gas hydrate production test in the South China Sea

Gas hydrate-bearing sediments at the first offshore gas hydrate production test site in the Shenhu area, South China Sea, have the characteristics of high irreducible water saturation and low permeability. The stratum structure of this production test site is complex, its gas hydrate-bearing layer (GHBL), three-phase layer (TPL) and free gas layer (FGL) all contain methane gas or gas hydrates, that make the production more complicated. According to the available geological data at the first offshore gas hydrate production test site in the Shenhu area, a 2D numerical simulation model is built to study the short- and the long-term production behavior of the gas hydrates in the Shenhu area. In the short-term, the total gas production from hydrates, when the TPL gas saturation is 0.078, is determined to be 2.75 × 105 m3, which is close to the actual total gas production of 3.09 × 105 m3. When the irreducible water saturation varies from 0.40 to 0.63, the total gas production changes from 1.41 × 105 to 2.75 × 105 m3, and the total water production decreases from 1.04 × 104 to 8.7 × 103 m3, which indicates that irreducible water is a favorable factor for gas production from hydrates in clayey silt sediments. In the long-term gas production from hydrates, the gas production rate first decreases, then increases, and finally decreases again. Gas hydrate reformation occurs at the interface between the TPL and the FGL, which will adversely affect the gas production from hydrates.

ИНЖЕКЦИЯ ЖИДКОГО ДИОКСИДА УГЛЕРОДА В ПЛАСТ, НАСЫЩЕННЫЙ МЕТАНОМ И ЕГО ГАЗОГИДРАТОМ

Актуальность исследования связана с разработкой теоретических основ технологий добычи газа из газогидратных месторождений методом замещения. Рассматривается метод инжекции диоксида углерода, позволяющий значительно снизить энергетические затраты на разработку природных газогидратных залежей. Целью исследования является выяснение особенностей протекания процесса замещения метана диоксидом углерода в газогидрате при инжекции жидкого диоксида углерода в газогидратный пласт. Объект: пористый пласт конечной протяженности, насыщенный метаном и его газогидратом, исходные давление и температура которого соответствуют условиям стабильного существования газогидрата метана. Методы. На основе уравнений механики сплошной среды построена математическая модель тепломассопереноса в природном пласте, сопровождающаяся замещением метана на диоксид углерода в газогидрате. Принято, что в рассматриваемом случае в пласте возникают две характерные зоны, разделенные подвижной границей фазовых переходов. В первой (ближней) зоне поры насыщены жидкой двуокисью углерода и ее газогидратом, а во второй (дальней) зоне содержатся метан и его газогидрат. Результаты. Получены численные решения для полей давления и температуры при инжекции жидкого диоксида углерода в газогидратный пласт конечной протяженности. Построены зависимости температуры на границе замещения от давления инжекции и проницаемости пласта. Установлено, что при достаточно низких значениях давления инжекции и проницаемости величина температуры на границе замещения может подниматься выше равновесного значения температуры диссоциации газогидрата метана на газ и воду. Это соответствует возникновению второй подвижной межфазной границы, на которой происходит разложение газогидрата метана. Найдена зависимость предельной температуры закачиваемой жидкой двуокиси углерода, выше которой необходимо учитывать образование смеси метана и воды, от значений давления на правой и левой границах пласта и его проницаемости. Установлено, что режим с разложением газогидрата реализуется при высоких значениях давления на правой границе пласта и низких значениях проницаемости и давления, под которым закачивается двуокись углерода. Получены зависимости скорости границы замещения, а также времени полной замены газогидрата метана на газогидрат диоксида углерода во всем пласте от давления на правой и левой границах пласта, а также от его проницаемости.

Specific Character of Natural Hazards on High-latitude Sea Transport Passages Europe-Asia-Pacific Region: Challenges for Technosphere Safety

The especial feature of high-latitude marine routes is unusual geodynamic regime of the sea bed and hydrodynamic of waters, caused by a wide spread of a submarine permafrost, enriched with methane gas-hydrates. They had been accumulated during the Ice Age, and after deglaciation the both permafrost and gas-hydrates are subject to destruction, accompanied by a runoff of methane into sea water and atmosphere. There are two mechanisms of degassing: diffusion (DDG) and flare-bubble (FDG). DDG acts permanently resulting in appearance of areas with an abnormal concentration of methane dissolved in water, decreasing its density, and so affecting a floatability of vessels. FDG appears locally and impulsively, but this type of degassing presents an essential risk for a safety of high-latitude transport communications, as well as for underwater technical infrastructure in the exploited oil and gas fields. Fast-growing gas-hydrate pingoes can change the bottom relief and generate newborn islands or shallow banks with hummock. Consequences of the blowing up of subaqueous pingoes are very hazardous, and include a formation of giant pockmarks and craters at the sea bed, an emergence of large methane bubbles to the sea surface and emission in air of the methane tails up to a thousand kilometers in length. The entry of ships into the FDG zone is fraught with flooding; the engineering facilities in these zones will be subjected to mechanical damage and fires. Due to provide both industrial and ecological safety the special preventive measures are needed.

A new class of promising biodegradable kinetic/anti-agglomerant methane hydrate inhibitors based on castor oil

This study is devoted to the evaluation of vegetable oils (in this case castor oil) as an inexpensive, sustainable, environment-friendly, natural and promising resource to design/synthesis efficient kinetic/anti-agglomerant methane hydrate inhibitors. The castor-based waterborne polyurea/urethanes (CWPUUs) were synthesized on the basis of the waterborne technique. The high-pressure autoclave cell and high-pressure micro-differential scanning calorimeter (HP-μDSC) using methane gas were applied to evaluate the inhibition performance of CWPUUs as an inhibitor for methane gas hydrate formation. The results of gas uptake tests confirm that the CWPUUs show high efficiency as kinetic hydrate inhibitors (KHIs). The CWPUUs with ∼3.2 and 6.8 kD molecular weight delayed the onset time of methane hydrate formation by a factor of 26.8 and 13 times and reduced the methane hydrate growth rate by a factor of 3.8 and 2.5 times in comparison with pure water. The results were further confirmed through DSC measurements, in which the usage of CWPUUs with ∼3.2 and 6.8 kD molecular weight decreased the mean onset temperature of methane hydrate formation on 6.1 and 4.7 °C compared to pure water, respectively. Additives of CWPUUs (1 wt % solution) increased the mean onset time of methane hydrate formation from 2.3 h for pure water to 8.7 and 6.4 h. CWPUUs also demonstrated the thermodynamic inhibition by decreasing the melting temperature of methane hydrates comparing to the pure water system. On the other hand, the torque measurements were used to assess anti-agglomerant performance of CWPUUs. During the methane hydrate formation in the presence of CWPUUs, the torque changes remained constant that suggests the hydrate particles stayed separate without aggregation. Moreover, synthesized CWPUUs can be related to biodegradable substances because their BOD5/CODcr value measured in this work is equal to 0.389. Hence, the application of CWPPUs can be a prospective option for delaying the onset time/temperature of hydrate formation, reducing hydrate growth rate and preventing agglomeration of hydrate particles, and this strategy refers to the terms of green chemistry.

Structural Transitions Range of Methane + Ethane Gas Hydrates during the Decomposition Process below the Ice Point

The structural transitions in the forming process of methane and ethane gas hydrate were reported previously, this phenomenon has also been observed during the dissociation process of methane and ethane gas hydrate. In our earlier work, the self-preservation and structural transition of 68 mol.% CH4+32 mol.% C2H6 hydrates during dissociation below the ice point have been studied. In this work, the dissociation process of a series of methane + ethane hydrate samples at atmospheric pressure and temperatures below the ice point (272.15 K-269.15 K) was analyzed using in suit Raman spectroscopic, suggesting the hydrate structures transfer from structure I to structure II over a methane vapor composition (yCH4) range of 50 mol.%-68 mol.%. Further investigation shows that the time of structure transition is reduced with the increasing yCH4 at the same decomposition temperature. Meanwhile, hydrate dissociation is retarded with decreasing temperature in this temperature region (272.15 K-269.15 K). The result of this work can be applied to hydrate exploitation in permafrost zone and seabed.

Synergy Effect Study of Poly(N-isoacrylamide) with Tetra Butyl Phosphonium Bromide on Methane Hydrate Formation

Natural gas hydrate plugging is one of the costly and challenging problems for the oil and gas industry, especially for subsea fields. One way to prevent gas hydrate formation at low cost is the use of low-dosage hydrate inhibitors (LDHI). Poly(N-isopropylmethacrylamide) (PNIPAM) with amide group have previously been shown to be an outstanding low-dosage hydrate inhibitor (LDHI). PNIPAMs are usually polymerized via radical polymerization, which can allow control over the molecular weight. We have synthesized PNIPAMs using control radical polymerization giving a fairly high degree of polymerization control. Additionally various Kinetic Hydrate Inhibitors (KHIs) were tested as CH4 hydrate inhibitors with polymeric hydrate inhibitor, PNIPAM. Furthermore, to check the synergetic effects of tetra butyl phosphonium bromide (TBPB), PNIPAM and their mixture TBPB-PNIPAM were tested as KHIs for methane gas hydrate formation. In this paper we present results on the performance of various polymeric inhibitors in KHI tests on methane gas in stirred autoclaves and on structure I hydrate formation. When PNIPAM alone was used as a single KHI, PNIPAM exhibited worst performance among the KHIs tested in this study. However the mixing of TBPB with PNIPAM further extended the induction time and reduced the CH4 hydrate growth rate. TBPB was an excellent synergist in blends with PNIPAM for kinetic hydrate inhibition of structure I forming CH4 hydrate. In summary, the mixture TBPB-PNIPAM was proven that great synergist for prevent the CH4 hydrate and good potential in achieving the industrial application of Oil & Gas production technology and therefore was a significantly meaningful discovery in the field of energy production.

Phase equilibrium for clathrate hydrates formed in the (methane, carbon dioxide or ethane) + water + ammonium chloride system

Methane hydrates are one of unconventional energy resources. There are vast depositions of methane hydrates on the sea floor and under permafrost. The methane hydrates are stable under high pressure and low temperature conditions. Depressurization method is being developed for methane gas production from deep sea methane hydrate layers. In this method, the methane hydrates are depressurized by a pump, and dissociated to be methane gas and water. There is a risk of plugging in the production well by reformed methane hydrates such as at stagnation points and during an operation stop. To manage the plugging risk, inhibitors for methane hydrate formation are necessary. Among several chemical inhibitors, thermodynamic hydrate inhibitors (THIs) are a unique technique that can dissociate methane hydrates. Ammonium salts are one of THIs which naturally occur in oceans and vastly available in the deep sea area. In this study, inhibition effects of ammonium chloride on gas hydrates of methane, carbon dioxide and ethane are investigated. Three phase (gas–hydrate–aqueous) equilibrium conditions are measured by a temperature search method. The equilibrium data show that ammonium chloride suppresses equilibrium temperatures of these gas hydrates until it reaches solubility limit in water. The comparison with molecular inhibitors showed that ammonium chloride inhibits methane hydrate formation more than urea, but less than methanol. The present data clarified the inhibition effect of ammonium chloride on subsea natural gas hydrates and CO2 injection and sequestration techniques.

Insights into induction time and agglomeration of methane hydrate formation in diesel oil dominated dispersed systems

Induction time and agglomeration of methane hydrate formation in dispersed systems play an important role in exploitation of natural gas hydrate, prevention of gas hydrate plug, and application of hydrate-based technologies. In this work, an autoclave with particle video microscope (PVM) probe was used to detect induction time of methane hydrate formation as a function of the water cut, dosage of sorbitan monolaurate (Span 20), and subcooling. Forty-one experiments and thirty-six experiments of induction time have been conducted for methane hydrate formation at constant pressure and at nonconstant pressure, respectively. The results showed subcooling was the major factor that affects induction time during methane hydrate formation process. Subcooling of 4 K can be seen as an inflection point because the average methane hydrate formation time was less than 200 min when the subcooling was greater than 4 K, while methane hydrate formation time exhibited more stochastic when the subcooling was less than 4 K. The results also suggested that there exists a transformation range of subcooling (TRS) during methane hydrate formation process. The agglomerated mechanism of methane gas hydrate may be changed when the subcooling is greater than TRS, and subcooling of 4 K is included in the TRS.

Evaluation of Tetraethylammonium Chloride on Methane Gas Hydrate Phase Conditions

In this work, hydrate liquid vapor equilibrium conditions for methane (CH4) gas is measured in the absence and presence of different aqueous TEACl concentrations. The temperature cycle (T-cycle) method has been used for the hydrate equilibrium temperature of CH4 + H2O + TEACl system measurement within the pressure ranges of 3.90 – 8.30 MPa. Results reveal that, for 10 wt% aqueous TEACl average of suppression temperature (ΔT) (inhibition effect) appeared of 0.9 K for CH4 hydrate. Additionally, the enthalpy of dissociation for methane gas hydrate with and without the present of inhibitor is calculated using Clausius-Clapeyron equation.

Formation of a Mixed Region of Carbon Dioxide and Methane Hydrates during the Injection of Liquid Carbon Dioxide to a Reservoir Saturated with Methane and Water

Based on the equations of continuous medium mechanics, a mathematical model is constructed for the injection of liquid carbon dioxide into the porous medium saturated with methane and water for the case when the injection is accompanied by the formation of gas hydrates of carbon dioxide and methane. Self-similar solutions to the one-dimensional problem describing the evolution of hydrodynamic and temperature fields are constructed. The dynamics of movement of frontal boundaries of phase transitions with respect to injection pressure and temperature, as well as with respect to permeability and initial pressure of the porous medium is investigated. It is shown that intensity of the formation of methane and carbon dioxide gas hydrates increases with an increase in injection pressure and reservoir permeability, and at rather high values of permeability, pressure drop in the reservoir and injection temperature, the frontal formation boundaries of methane and carbon dioxide gas hydrates may merge. The dependence of the limit values of injection pressure and temperature corresponding to the merger of the phase transition boundaries on the permeability and initial pressure is studied in this work.

China's conventional and unconventional natural gas resources: Potential and exploration targets

A series of significant breakthroughs in exploring conventional and unconventional natural gas resources have been achieved in new strata and new domains. However, the quality of the increased reserves has declined with the entry of unconventional resources into the reserves sequence. Therefore, the potential of conventional and unconventional natural gas resources as well as important exploration regions of the remaining natural gas resources need to be clarified, and favorable directions need to be established. On the basis of oil and gas exploration results, discoveries in petroleum geology, and exploration data of the past 10 years acquired by PetroChina, an entire system of resource assessment for conventional and unconventional oil and gas resources has been established. The fourth round of resource assessment has been systematically executed. The results indicate that the total conventional natural gas resources of China are 78 × 1012 m3, which shows an increase of 8 × 1012 m3 over the results of the third round of resource assessment. Unconventional resources include four types of resources: tight gas, shale gas, coalbed methane, and natural gas hydrates. Among them, tight gas reservoirs make up 21.86 × 1012 m3 (Jiyang, Dongpu, Nanxiang, Subei, and others were excluded from the assessed regions). In addition, shale gas, coalbed methane, and natural gas hydrate reservoirs make up 80.21 × 1012 m3, 29.82 × 1012 m3, and 153.06 × 1012 m3, respectively. The remaining resources for onshore conventional natural gas are distributed mainly in four significant domains, which are lithology–strata (clastic rock), marine carbonatites, foreland thrust belts, and complex tectonic belts. Of these, marine carbonatites and foreland thrust belts are the dominant domains. The resources for offshore natural gas are concentrated mainly in three domains, which are offshore structures, organic reefs, and lithology in deep water. The total remaining natural gas resources in marine carbonatites, foreland, lithology–strata, complex tectonic belts, and the South China Sea amount to 59.83 × 1012 m3, which is 94% of the remaining natural gas resources in China. Based on this assessment, favorable targets are selected including 10 targets for conventional resources, 4 for tight gas, 4 for coalbed methane, and 6 for shale gas.

Experimental investigation of the behavior of methane gas hydrates during depressurization-assisted CO2 replacement

CO2 replacement with depressurization assistance is a crossover concept that has been proposed in recent years for natural gas hydrate development. It combines the merits of two traditional methods to improve production and reduce risks. To verify the feasibility of this new method and evaluate the corresponding efficiency, experimental apparatus has been designed to simulate the horizontal interface between a CO2 injection well and a CH4 production well based on the independent extraction mechanism and well structure design. To prepare natural gas hydrate-bearing samples under conditions that conform to actual field conditions, silica sand is packed into a core-holding tube with methane gas and distilled water at appropriate pressures and temperatures. Then a horizontal extraction process with an injection well and a production well are simulated, and the parameters influencing the pressure system, such as inlet pressure (pressure of injection well), outlet pressure (pressure of production well), and confining pressure (pressure of the overburden layer), are controlled as variables and analyzed. The ratio of generated methane and injected CO2 is used to evaluate the corresponding utilization efficiency (UE) in this work. Results indicate that all these pressure parameters have certain effects on CO2 replacement behavior. Generally, the fluid flow driven by pressure differences, and the special phase equilibria properties are the essence of their influence. The feasibility of enhancing CO2 utilization efficiency with depressurization assistance is verified based on experimental data. In consideration of time and cost, the relative pressure parameters should be optimized comprehensively to maximize commercial efficiency before field application.

New methane hydrate phase boundary data in the presence of aqueous amino acids

Currently there are few amino acids hydrate based thermodynamic studies compared to kinetic studies, thus, the thermodynamic effect of four amino acids on methane hydrate formation are presented herein. The amino acids (valine, threonine, asparagine, and phenylalanine) were tested as gas hydrate inhibitors using the isochoric pressure search method in a high pressure stirring reactor. They were tested at 1 wt% and 5 wt% in the temperature and pressure ranges of 275.71–286.10 K and 3.52–10.25 MPa, respectively. All studied amino acids thermodynamically inhibited methane gas hydrate formation, with valine exhibiting the best inhibition impact with an average depression temperature of 0.529 K at 5 wt%. Amino acids side chain properties were found to cause the variations in their inhibition impact. This study is useful to fully understand the thermodynamic inhibition effect of amino acids on gas hydrate formation. Inferring from the results in this study and literature, amino acids are thermodynamic gas hydrate inhibitors, while most of them are kinetic promoters.

Efficient parameter estimation for a methane hydrate model with active subspaces

Methane gas hydrates have increasingly become a topic of interest because of their potential as a future energy resource. There are significant economical and environmental risks associated with extraction from hydrate reservoirs, so a variety of multiphysics models have been developed to analyze prospective risks and benefits. These models generally have a large number of empirical parameters which are not known a priori. Traditional optimization-based parameter estimation frameworks may be ill-posed or computationally prohibitive. Bayesian inference methods have increasingly been found effective for estimating parameters in complex geophysical systems. These methods often are not viable in cases of computationally expensive models and high-dimensional parameter spaces. Recently, methods have been developed to effectively reduce the dimension of Bayesian inverse problems by identifying low-dimensional structures that are most informed by data. Active subspaces is one of the most generally applicable methods of performing this dimension reduction. In this paper, Bayesian inference of the parameters of a state-of-the-art mathematical model for methane hydrates based on experimental data from a triaxial compression test with gas hydrate-bearing sand is performed in an efficient way by utilizing active subspaces. Active subspaces are used to identify low-dimensional structure in the parameter space which is exploited by generating a cheap regression-based surrogate model and implementing a modified Markov chain Monte Carlo algorithm. Posterior densities having means that match the experimental data are approximated in a computationally efficient way.

Cost-effective alkylammonium formate-based protic ionic liquids for methane hydrate inhibition

In light of current concerns about environmental impact, ionic liquids have been tested in a number of industrial applications, including gas purification, lubrication, and catalysis. In this work, cost-effective alkylammonium-based protic ionic liquids (PIL) were utilized as high-dosage gas hydrate inhibitors in order to achieve an inexpensive feedstock. Three structurally different PILs, ethylammonium formate ([EA][Of]), dimethylammonium formate ([DMA][Of]), and dimethylethylammonium formate ([DMEA][Of]), were studied to determine their inhibitory effect on methane gas hydrates. All of the studied PILs were initially tested for thermodynamic gas hydrate inhibition at 5 wt% and at 10 wt% to understand the effect of concentration by increasing the dosage. These PILs were produced by adding primary, secondary, and tertiary amines to formic acid, which is mildly acidic in nature and has biological significance. The PILs showed similar thermodynamic hydrate inhibition (THI) effects at the studied concentrations. The THI trend ([EA][Of] ≥ [DMA][Of] ≥ [DMEA][Of]) reflects the small effect of the structural variation of the cation, with the common formate anion providing the dominant inhibitory action due to presence of a carboxylic group. The pressure-temperature (P-T) equilibrium curve data of the investigated PILs were compared with the existing literature data for ILs including [TBMA][Of] at 10 wt% and 5 wt%. The comparison plots reveal that the THI effects of the studied PILs are better than or comparable to the other ILs, which is the matter of discussion over cost effective operation. Additionally, experimental kinetic hydrate inhibition (KHI) data were collected for 5 wt% PIL systems using polyethylene oxide (1 wt%) as an additive to demonstrate its synergistic effect.

Mathematical model for carbon dioxide injection into porous medium saturated with methane and gas hydrate

A mathematical model of the injection of warm carbon dioxide into a layer of finite length the pores of which are saturated in the initial state with methane and its gas hydrate is constructed. A solution of the problem is presented for the case of the formation of two moving interphase boundaries. In this case, at two different frontal boundaries there occurs the decomposition of the CH4 hydrate into methane, water and the formation of CO2 hydrate from carbon dioxide and liberated water. The dependence of the rate of decomposition of methane gas hydrate on the temperature of the injected carbon dioxide and the initial temperature of the formation is numerically studied, as well as on the pressure values on the right and left boundaries of the formation. It is shown that forming two different boundaries of phase transitions is realized at high temperatures of the formation and injected carbon dioxide. A critical diagram is constructed that determines the “injection temperature-initial reservoir temperature” and “injection pressure-initial pressure of the formation” on the parameter planes in the region of existence of two different process flow regimes. The dependence of the coordinates of both boundaries of phase transitions on time is investigated. It has been established that over time the formation boundaries of CO2 hydrate and the decomposition boundaries of the CH4 hydrate can be merged. The influence of heat released during the formation of the CO2 hydrate on the acceleration of the decomposition of the CH4 hydrate was studied. It was found that the latent heat of formation of the CO2 hydrate markedly increases the rate of decomposition of CH4 hydrate, and the contribution of this heat to the rate of decomposition of the hydrate increases with decreasing temperature of the injected gas.

Thermodynamic modeling of methane hydrate formation in the presence of imidazolium-based ionic liquids using two-step hydrate formation theory and CPA EoS

In this study, thermodynamic modeling is performed to predict the P-T equilibrium conditions of methane gas hydrate formation in presence of ionic liquids. The imidazolium-based ionic liquids including [EMIM][HSO4], [EMIM][EtSO4], [BMIM][BF4], [OH-EMIM][BF4], [BMIM][Cl] and [BMIM][Br] are modeled through two-step hydrate formation theory of Chen and Guo coupled with CPA EoS for calculating the fugacities of species in liquid and gas phase. The CPA pure component parameters for ionic liquids and also the majority of binary interaction parameters are adjusted using the liquid density and binary VLE data, respectively. The thermodynamic modeling is carried out in the presence of 0.1 weight fractions of all the studied imidazolium based ionic liquids. Moreover, the effect of three various concentrations of [BMIM][BF4] on the P-T equilibrium diagram of methane hydrate are predicted at 0.1, 0.15 and 0.20 weight fractions of this ionic liquid. The results indicate that the CPA EoS can satisfactorily model the phase behavior of aqueous solutions of imidazolium based ionic liquids considering 2B scheme for the association term. In addition, the results indicate good predictions of the proposed model for methane hydrate equilibrium conditions in the presence of imidazolium-based studied ionic liquids including concentration changes such that the absolute average deviations of methane hydrate formation pressures is obtained below 4.2%.

Salinity Effects on Pore‐Scale Methane Gas Hydrate Dissociation

AbstractMethane gas hydrate may become a significant source of methane gas in the global energy mix for the next decades. The widespread distribution of methane gas hydrate, primarily in subsea sediments on continental margins, makes the crystalline compound attractive for countries with shorelines that seek self‐sustainable energy. Fundamental understanding of pore‐level methane gas hydrate distribution and dissociation pattern in reservoirs is important to anticipate the methane production rate and overall efficiency. Specifically, the local salinity gradients occurring during methane gas hydrate dissociation, and its impact on local dissociation characteristics, must be understood as the aqueous phase in most reservoirs is saline. We experimentally evaluate the salinity effect on methane gas hydrate dissociation using high‐pressure silicon‐wafer micromodels with realistic sandstone grain characteristics. Methane gas hydrate was formed for a range of brine salinities (0–5 wt% NaCl), and we report variations in dissociation patterns during depressurization and thermal stimulation as a function of brine salinity. A strong correlation between initial methane gas hydrate distribution and dissociation characteristic, and subsequent release and mobilization of methane gas, was observed. Local water salinities affected the methane gas hydrate structure leading to distinct dissociation patterns of self‐preservation due to water freshening.