Hydrate Decomposition Process Research Articles

Thermal-hydro coupling model of methane hydrate reformation in porous media

Methane hydrates are crystalline compounds found in marine sediments and permafrost regions. Methane hydrates remain stable under both low-temperature and high-pressure conditions. When a methane hydrate reservoir is heated or depressurized, methane hydrates become unstable and decompose into water and methane gases. The heat absorption process during the decomposition of methane hydrates influences the temperature field. Methane hydrate reformation occurs during the extraction process, significantly reducing the hydraulic conductivity of the reservoir and hindering the long-term stable extraction of methane hydrates. In this paper, a numerical model is established by coupling seepage and heat processes. The temperature variation owing to the heat absorption process of methane hydrate decomposition is quantified based on the proposed model. The effect of spatial lithology on methane hydrate conversion is also analyzed, and the results for hydrate, water, gas, and permeability in the model are summarized. The numerical model also reflects the heterogeneity of marine sediments. Finally, the sensitivities of different physical parameters (permeability and porosity) and pressure gradients to the reformation rate of methane hydrate reforming are discussed. The results of this study provide scientific data supporting the influence of secondary hydrate formation on long-term extraction efficiency in the actual engineering hydrate extraction process.

Molecular dynamics simulations to evaluate the decomposition properties of methane hydrate under different thermodynamic conditions

Understanding the thermodynamic effect on methane hydrate decomposition is beneficial for designing controllable methane recovery processes under conditions of continental margins and oceanic sediments. In this work, we systematically examined the effects of varying temperatures and pressures on the natural gas hydrate decomposition in the presence of 20 mol% methanol through the molecular dynamics approach. Taking advantages of key parameters of angular order parameter (AOP), radial distribution function (RDF), mean square displacement (MSD), and potential energy, we mainly aimed to explore the impact of temperature and pressure on the decomposition of methane hydrate. It can be founded that high temperature is a positive factor for the methane hydrate decomposition, and as the temperature increases from 272.15 K to 277.15 K, the promotion effect becomes more obvious, resulting in a reduction of decomposition time by 3.93 ns. On the contrary, pressure has a negative effect on the methane hydrate decomposition, and as the pressure increases from 1 bar to 200 bar, the inhibitory effect becomes smaller. Additionally, the potential energy as a function of time can be used to evaluate the decomposition rates under different thermodynamic conditions. The obtained results underscore the significant dependence of methane hydrate decomposition rates on temperature and pressure, providing essential guidelines for optimizing gas hydrate decomposition processes.

Experimental study on the evolution process of hydrate deposition, blockage and decomposition in reducing pipeline

BackgroundIn the process of deep-water oil and gas extraction, due to the special structure of the reducing pipeline, the flow field of the pipeline changes here, making it easier for hydrates to form and accumulate at reduced-diameter zone and increasing the risk of pipeline blockage. MethodsAiming at the problem of hydrate deposition and blockage in reducing pipeline, an experimental study was carried out through the self-designed and reduced-diameter transparent flow loop to investigate the hydrate deposition and blockage and decomposition evolution of hydrate at reduced-diameter zone under different conditions of flow velocities and initial cyclopentane volume contents. Significant FindingsThe study found that the flow field formed at reduced-diameter zone was the main reason affecting the distribution of hydrate particles, and the flow velocity and the initial hydrate volume fraction were the main controlling factors affecting the blockage in reducing pipeline. The flow velocity of 1.6 m/s was determined as the safe boundary for cyclopentane hydrate in reducing pipeline. Based on the experimental results, the mechanism of cyclopentane hydrate deposition and blockage in reducing pipeline was revealed, and targeted prevention and control methods for hydrate blockage at reduced-diameter zone were proposed. It is of great significance to ensure the safe flow of hydrate in reducing pipeline.

FEATURES OF DECOMPOSITION OF GAS HYDRATE IN NONEQUILIBRIUM MODE IN A POROUS MEDIUM

The paper presents a mathematical model of the nonequilibrium decomposition of a gas hydrate contained in a porous reservoir of finite extent. The system of basic equations, which includes the law of conservation of mass, the heat inflow equation and Darcy’s law, also contains an equation describing the kinetics of nonequilibrium hydrate decomposition, which is determined by pressure, temperature and surface area of particles. On the basis of the numerical solution of the problem, dependences are obtained that allow us to identify the regularities of the process of nonequilibrium decomposition of gas hydrate in a natural reservoir when warm gas is injected into a porous reservoir. It is shown that at the initial moments of time after the start of injection, the extent of the region saturated with gas, hydrate and water is very small compared to the extent of the formation, which corresponds to the mode of hydrate decomposition on the frontal surface. At long injection times, the nonequilibrium hydrate decomposition process is characterized by the formation of three characteristic zones: the near one, where the pores are saturated with gas and water, the far one, saturated with methane and its gas hydrate, and the intermediate one, where gas, hydrate and water are located. It is established that the pressure and temperature in the extended region are connected by the condition of phase equilibrium. The influence of the main parameters of the system on the distributions of pressure, temperature, water saturation and hydrate saturation, as well as on the dynamics of their changes over time, is analyzed. It is shown that injection pressure, as well as reservoir permeability and porosity have a significant effect on the decomposition rate of gas hydrate. An increase in the values of these parameters contributes to an increase in the rate of nonequilibrium hydrate decomposition. An increase in the injection temperature leads only to an increase in the extent of the area containing methane and water.

Thermo-hydro-chemical modeling and analysis of methane extraction from fractured gas hydrate-bearing sediments

Hydraulic fracturing is widely used in enhancing hydrocarbon recovery efficiency of unconventional reservoirs including gas hydrate-bearing sediment (GHBS). However, the underlying mechanisms governing the influences of fractures on the intricate multi-field coupled processes during methane extraction remain poorly understood. This study develops a fully coupled thermo-hydro-chemical (THC) model for methane extraction from fractured GHBS under combined heat injection and depressurization operations. Novel dimensionless numbers are proposed to characterize the underlying heat transfer, fluid flow, and hydrate decomposition processes. The results reveal that depressurization induced fluid flow dominates methane extraction in the early stages, while heat injection induced thermal processes become the primary controlling factor in the intermediate and long-term stages. The flow field primarily governs the global-scale hydrate decomposition process, whereas the thermal field dictates the local-scale evolution of the pre-decomposition front. We find that increasing injection temperature and decreasing production pressure could improve gas recovery efficiency, although economic consideration may constrain these production strategies. Particularly, increasing injection temperature expands local hydrate decomposition zone and boosts gas production, while depressurization at the production well reduces the overall gas hydrate saturation and leads to extra gas production. Variation in well spacing show little effect on the gas recovery efficiency, but the total gas production changes for the change of reservoir scale. This study provides insights into the mechanisms of coupled thermo-hydro-chemical behaviors during methane extraction from fractured GHBS, and offers a fundamental model for the optimization of both extraction efficiency and economic viability of gas hydrate recovery.

Sensitivity Analysis of Multi‐phase Seepage Parameters Affecting the Clayey Silt Hydrate Reservoir Productivity in the Shenhu Area, South China Sea

AbstractNatural gas hydrate (NGH) is an important future resource for the 21st century and a strategic resource with potential for commercial development in the third energy transition. It is of great significance to accurately predict the productivity of hydrate‐bearing sediments (HBS). The multi‐phase seepage parameters of HBS include permeability, porosity, which is closely related to permeability, and hydrate saturation, which has a direct impact on hydrate content. Existing research has shown that these multi‐phase seepage parameters have a great impact on HBS productivity. Permeability directly affects the transmission of pressure‐drop and discharge of methane gas, porosity and initial hydrate saturation affect the amount of hydrate decomposition and transmission process of pressure‐drop, and also indirectly affect temperature variation of the reservoir. Considering the spatial heterogeneity of multi‐phase seepage parameters, a depressurization production model with layered heterogeneity is established based on the clayey silt hydrate reservoir at W11 station in the Shenhu Sea area of the South China Sea. Tough + Hydrate software was used to calculate the production model; the process of gas production and seepage parameter evolution under different multi‐phase seepage conditions were obtained. A sensitivity analysis of the parameters affecting the reservoir productivity was conducted so that: (a) a HBS model with layered heterogeneity can better describe the transmission process of pressure and thermal compensation mechanism of hydrate reservoir; (b) considering the multi‐phase seepage parameter heterogeneity, the influence degrees of the parameters on HBS productivity were permeability, porosity and initial hydrate saturation, in order from large to small, and the influence of permeability was significantly greater than that of other parameters; (c) the production potential of the clayey silt reservoir should not only be determined by hydrate content or seepage capacity, but also by the comprehensive effect of the two; and (d) time scales need to be considered when studying the effects of changes in multi‐phase seepage parameters on HBS productivity.

Numerical study of the gas production process from a gas hydrate deposit in the presence of thermal and depression effects

Today the issue of gas production technology from existing gas hydrate deposits discovered on the shelf of the World Ocean and in permafrost areas is still very significant since the methane reserves in the free state are significantly inferior to its reserves in the form of its gas hydrates. One of the tasks for possible gas production from a hydrate-containing porous medium is to study the process of gas hydrate decomposition under thermal and depression effects since they are most commonly used ones. It is necessary to conduct a theoretical study including the development of a mathematical mode and its algorithmization, the creation of a computational program and the conduct of numerical experiments. The paper presents one-dimensional axisymmetric problem of heating and/or pressure reduction at the bottom of a well passing through the entire thickness of a porous formation when its pores are initially filled with methane and its hydrate. The utilized mathematical model includes the continuity equations for methane, its hydrate and water; the equation of the gas phase motion in a porous medium as the Darcy filtration law; the state equation of methane and water, the energy conservation equation considering the Joule–Thomson effects and adiabatic cooling for gas, the latent heat of the “gas hydrate  methane + water” phase transition. A numerical implementation of the proposed mathematical model and a numerical study of the thermal and/or depression impact on the studied hydrate-bearing deposit are carried out. The results of calculations show that the size of a zone containing only the gas hydrate decomposition products (gas and water) slightly increases with a smaller length of a porous layer. They also show that the thermal effect (increasing the temperature at the bottomhole of production well) on the hydrate-saturated reservoir simultaneously with the depression effect is not efficient enough due to the intensive flow of cold gas (with a temperature equal to the initial temperature of the reservoir) from the hydrate-containing deposit to the well.

Characterization of the quasi-liquid layer on gas hydrates with molecular dynamics simulations

Understanding the properties of the surficial quasi-liquid layer (QLL) is a prerequisite in proper handling of gas hydrates, a potential alternative future energy resource. Despite of its critical importance, the characterization of QLL has long been known to be highly challenging. In this work, the evolution of QLL during hydrate decomposition was systematically investigated by Molecular Dynamics simulations. Using the instant displacement of water molecules as a measure of the fluidity of the molecular layer adjacent to hydrate surfaces, QLL thickness was accurately measured using the dynamic properties of water molecules for the first time. The QLL thickness obtained by this new molecular dynamics-based approach takes amorphous water molecules close to hydrate surfaces into account, which was often ignored in the previous results based on the static structural order parameters. The variation of QLL thickness throughout the hydrate decomposition process at different temperatures and pressures was then evaluated. The thickness of QLL was found to increase with elevated temperature and pressure, owing to the varied changes of the amorphous and hydrate-like molecular content in this important layer. The results regarding the dynamics of the QLL with molecular resolution improved the current understanding on the stability of gas hydrate and shed new light on the physical fundamentals relevant to future hydrate exploration as well as anti-hydrate materials design.

THMC Fully Coupled Model of Natural Gas Hydrate under Damage Effect and Parameter Sensitivity Analysis

In order to study the influence of damage on the gas production of natural gas hydrate, a multi-physical field theoretical model considering damage effect and coupling thermal-hydraulic-mechanical-chemical (THMC) was established by theoretical analysis and numerical simulation. The THMC model establishes the relationship between the elastic modulus of hydrate sediment and hydrate saturation during the whole process of hydrate decomposition. The THC (thermal-hydraulic-chemical) and THMC fully coupled models not considering or considering the damage effect were compared and analyzed, and the reliability of the THMC fully coupled model was verified. On this basis, the deformation, permeability and damage of hydrate sediments under different initial hydrate saturations and different depressurization amplitudes, as well as the hydrate gas production rate and cumulative gas production, are analyzed. The results showed that higher initial hydrate saturation inhibited the development of damage, maintained stable gas production and increased cumulative gas production. Larger depressurization promoted damage and increased cumulative gas production, but it was easy to cause stability problems.

Large-scale experimental study of heterogeneity in different types of hydrate reservoirs by horizontal well depressurization method

The heterogeneity of hydrate distribution and decomposition in sandy reservoirs is an important issue. Because large hydrate simulators are scarce and visualization difficult, heterogeneity in large-scale hydrate reservoirs has been consistently underestimated. In this study, a pilot-scale hydrate simulator with an effective volume of 117.8 L was used to simulate the horizontal well depressurization decomposition of two different types of hydrate reservoirs: gas-saturated and water-saturated. The production performance and heterogeneity of these two hydrate reservoirs were analyzed during the hydrate decomposition process. For the first time, hydrate decomposition heterogeneity was classified into planar heterogeneity and interlayer heterogeneity based on the heterogeneous characteristics in the horizontal and vertical directions. Based on the pressure difference and duration, a dimensionless factor was proposed to quantify the intensity of the heterogeneity at a specific location. Furthermore, this study related the hydrate decomposition heterogeneity intensity variation process to the variations in the two-dimensional geometric model for hydrate distribution in the pore space. The experimental results showed that horizontal well extraction using the depressurization method was highly efficient. During the hydrate decomposition process, planar heterogeneity was stronger in the water-saturated hydrate reservoir than in the gas-saturated hydrate reservoir. In gas-saturated hydrate reservoir, hydrate saturation and heterogeneity increased from top to bottom. In water-saturated hydrate reservoir, hydrate saturation and interlayer heterogeneity increased first and then decreased from top to bottom. The study of heterogeneity was critical for optimizing the hydrate extraction scheme, and emphasis should be placed on the placement of production enhancement facilities in regions with significant heterogeneity.

Heat utilization efficiency analysis of gas production from hydrate reservoir by depressurization in conjunction with heat stimulation

The heat utilization efficiency of heat injection is a key factor for the available heat stimulation method in natural gas hydrate, and the heat transfer characteristics during natural gas production from hydrate reservoir require further analysis. In this work, based on the laboratory tests by the Pilot Hydrate Stimulator (PHS), the heat transfer process of hydrate decomposition in the PHS was investigated, and the real-time decomposition rate and the energy utilization efficiency of the hydrate decomposition were quantitatively studied. The experimental results showed that the energy consumption of hydrate decomposition was mainly obtained from the sensible heat hydrate bearing sediments and the heat transfer from the environment in the depressurization process. In the initial stage of heat injection, the energy consumption of hydrate decomposition mainly came from the heat contained in the injected hot water and the heat transfer through the boundaries. In the later stage of heat injection, all of the energy consumption for hydrate decomposition was obtained from heat injection. In this work, the heat transfer coefficient between the reservoir and the environment was first proposed to quantify the heat transfer through the boundary. The results showed that the heat transfer coefficient presented large fluctuations during the initial stage of the depressurization and the period when the reservoir temperature reached the environment temperature in the heat injection process. Meanwhile, the coefficient remained stable at the later stage of heat injection. These results indicated that the change in gas-water hydrate inside the reservoir and the change in reservoir temperature affected the heat transfer coefficient between the reservoir and the environment. Furthermore, the energy utilization efficiency and gas production rate were optimized for hydrate production when the reservoir temperature was controlled within the range close to the environmental temperature. These results may be of great significance for heat injection control during field production from hydrate reservoirs.

Mathematical Model of the Decomposition of Unstable Gas Hydrate Accumulations in the Cryolithozone

We present a generalization of the mathematical model of gas discharge from frozen rocks containing gas-saturated ice and gas hydrates in a metastable state (due to the self-preservation effect) caused by the drop in external stress associated with various geodynamic factors. These factors can be attributed, for example, to a decrease in hydrostatic pressure on a gas-bearing formation due to glacier melting, causing an isostatic rise, or to the formation of linear depressions in the bottom topography on the shelf due to iceberg ploughing. A change in external pressure can also be associated with seismic and tectonic deformation waves propagating in the lithosphere as a result of ongoing strong earthquakes. Starting from the existing hydrate destruction model, operating at the scale of individual granules, we consider a low-permeable hydrate and ice-saturated horizontal reservoir. Generalization is associated with the introduction of a finite threshold for the external pressure drop, which causes the destruction of the gas hydrate and gas-saturated microcavities of supramolecular size. This makes it possible to take into account the effect of anomalously high pressures occurring in the released gas as a result of partial hydrate dissociation. Numerical and approximate analytical solutions to the problem were found in the self-similar formulation. A parametric study of the solution was carried out, and regularities of the hydrate decomposition process were revealed.

Experimental Study on Influences of Self-Preservation Effects and Memory Effects on the Hydrate Decomposition Process

Abstract In order to explore the characteristics of the hydrate decomposition behavior at the pore scale, this study carries out a pore-scale experimental study of methane hydrate decomposition based on the high-pressure visual model under the etched glass. A mathematical model is also constructed to analyze the behavioral characteristic of the self-preservation effect and memory effect during the hydrate decomposition period. This study draws the following conclusions: (1) The self-preservation effect and memory effect exist during the methane hydrate decomposition lead by depressurization, which generally inhibits the hydrate decomposition process. (2) The hydrate self-preservation effect is a transition of the surface water film’s phase state to inhibit the methane hydrate decomposition, in which the liquid phase transforms into a metastable “quasiliquid film”. (3) The multiple syntheses induced by the hydrate memory effect are a periodic attenuation process. The times of synthesis are a critical factor affecting the hydrate gas production and decomposition rate. (4) The self-preservation and memory effects during the hydrate decomposition period are associated with each other. The two are correlated at some degree with playing a dominant role alternately at different stages. The self-preservation effect is an abnormal behavior of the hydrate, which refers to the hydrate’s transition from the solid phase to the gas-liquid mixed phase. The memory effect is another abnormal behavior, which refers to the transition from the gas-liquid mixed phase to the solid phase. The sustaining pressure drop is the key reason of the disappearance of the two effects. This research was aimed at providing a theoretical basis for the exploitation and optimization of marine natural gas hydrates.

Hydrate decomposition front within porous media under thermal stimulation and depressurization conditions: Macroscale to microscale

Natural gas hydrates are unstable and easily to be decomposed under warming or depressurizing condition. The complicated hydrate decomposition process is generally characterized by a moving decomposition front, yet the essence of decomposition front evolution is unclear and the evidence is lacking. In this study, X-ray computed tomography (X-CT) technology is used to reveal the dynamic process of macroscale and microscale evolution of hydrate decomposition front. The hydrate-bearing porous samples are synthesized in the high-pressure reactor and then decomposed by the thermal stimulation method and the depressurization method with X-CT scans in real time. Macroscale results show that isolated hydrates beyond the decomposition front remain undecomposed under both two decomposition conditions, indicating the oversimplification of the decomposition front as the boundary where gas hydrates are completely decomposed. Hence, the decomposition front is redefined as the boundary which divides the hydrate-bearing sediment into an interconnected zone and an isolated zone based on the interconnectivity of hydrate particles. Microscale hydrate decomposition behaviors show that, the heat transfer differences among pores, solid media, and gas hydrates under thermal stimulation condition result in the remnant of isolated hydrates beyond the outside-in moving decomposition front. The gas fugacity differences in gas phase, water layer, and dead-end pores under depressurization condition result in the remnant of isolated hydrates beyond the top-down moving decomposition front. More importantly, we found that the remnant grain-cementing hydrates could alter the thermal- hydro-mechanical-chemical (THMC) processes outside the decomposition front, and play important roles in formation stability and ecological environment by supporting the skeleton structure of hydrate-bearing sediments. These findings may provide implications and visual evidences for production efficiency improvement and environmental protection in future field tests of hydrate exploitation.

ALGORITHM FOR SOLVING THE PROBLEM OF METHANE HYDRATE DECOMPOSITION IN A CLOSED HYDRATE-CONTAINING REGION OF A POROUS MEDIUM

Russia has a law on limiting greenhouse gas emissions, signed by the President of the Russian Federation in July 2021. The list of greenhouse gasses includes methane, and its reserves in the form of gas hydrates largely prevail over the reserves of free methane. The formation of gas hydrates occurs at sufficiently low temperatures and high pressures. A significant part of the “sleeping giants”, as journalists dubbed the methane-containing deposits, are in the Arctic region, and they are very sensitive to warming. The release of this greenhouse potential would have very negative consequences for the Earth’s climate. Thus, it is necessary to study the process of gas hydrate decomposition under thermal impact on a hydrate-containing porous medium. It is worth noting the importance of theoretical research to solve this problem, which includes the construction of a mathematical model of the process under study, algorithmization, software implementation and computational experiments. The paper presents a two-dimensional approximation of the formulated problem of heating at the upper boundary of the porous medium’s closed region containing methane and its hydrate initially. A mathematical model is presented, which is based on the mass conservation equations for methane, water and gas hydrate; Darcy’s law for the motion of the gas and liquid phases; the equation of a real gas state, the energy conservation equation considering thermal conductivity, convection, adiabatic cooling, the Joule-Thomson effect and absorption of latent heat during hydrate formation. An algorithm for the numerical implementation of the mathematical model is constructed and a computer code is developed to calculate main parameters of the process in the work, the computer program is updated to calculate the main parameters of the process researched in the article.

Особенности разложения газовых гидратов в присутствии метанола при атмосферном давлении

The study of the decomposition process of gas hydrates at atmospheric pressure and temperatures below 0°C revealed that methanol could affect this process in different ways, depending on its saturation with environmental components. Indeed, dueto the absorption of methane from the hydrate by methanol, the onset of its decomposition is observed at lower temperatures.Nevertheless, decomposition proceeds more slowly than with pure methane hydrate. When the methanol surrounding the methane hydrate is saturated with other medium components, the hydrate dissociation occurs at the equilibrium temperature (when intersecting the hydrate-ice-gas curve in a system without additives) regardless of the alcohol concentration. A similar situation is observed with hydrate obtained from a methane-propane gas mixture; however, under experimental conditions, ice beginsto melt at a lower temperature compared to the dissociation point of methane-propane hydrate (in the case of methane hydrate, the situation is reversed: the hydrate is less stable). High concentrations of methanol (above 40 mass%) lead to a significant decrease in the temperature of hydrate decomposition. The data obtained show that methanol in low dosages (about 10 mass%) can be usedfor gas storage and transportation since, under certain conditions, it does not shift the equilibrium curve of hydrate formation and slows down the process of methane hydrate decomposition.

Effects of H2/N2 on CO2 hydrate film growth: Morphology and microstructure

The addition of small-molecules gases such as H2/N2 can significantly improve CO2-CH4 hydrate replacement efficiency. However, the physical/chemical mechanism behind this phenomenon is not completely clear. Since CO2-CH4 hydrate replacement is the sequential process of CH4 hydrate decomposition and CO2 hydrate reformation, the microscopes and Raman spectroscopy were combined to clarify the effects of H2/N2 gas on CO2 hydrate film growth. The experimental results showed that H2 gas significantly affected the initial morphology, thickening and compactness of CO2 hydrate films. With the increase of H2 gas concentration, the size of the initial single hydrate crystal increased, the granulation phenomenon weakened, and hydrate films became rougher after thickening. Raman experiments showed that the presence of H2 made the CO2 hydrate film looser, which was conducive to the sustainability of hydrate growth. However, their growth rates did not change monotonously with the H2 concentration. We consider that H2 played roles in reducing the local CO2 gas concentration, slowing down the CO2 supply rate to the reaction interface, and delaying the time of the gas pores being blocked by further formed CO2 hydrate. N2 gas could also play the similar roles as H2 gas, however, due to its participation in CO2/N2 mixed hydrate formation, the growth of hydrate film changed adaptively. The results obtained are not only important for understanding the mechanism of CH4 hydrate replacement by CO2 and small molecule mixed gas, but also referential for other hydrate technologies involving mixed hydrate formation.

Texture, composition and properties of plugs formed by carbon dioxide hydrate and wax

Gas hydrates and wax are the major flow assurance problems for the transportation of produced hydrocarbons through pipelines. However, in most research works both these two problems are studied separately. Although simultaneous precipitation or deposition of these compounds in pipelines can lead to different mitigation/prevention strategies, the investigations in which both these problems are considered simultaneously appeared only recently. There is no information in the literature on the texture/composition and features of decomposition process of mixed wax/hydrate plugs. At the same time, this information could be useful to understand how to treat the problem of formation of these plugs. In this work, three wax/gas hydrate plugs were collected at quasi-static conditions from a water-in-oil emulsion to study their texture, composition and the features of decomposition process. Powder X-ray diffraction and IR (infrared spectroscopy) analyses showed that the plugs consisted of wax and gas hydrate. Thermovolumetric and DSC (Differential Scanning Calorimetry) experiments showed that the main part of gas hydrate in the plugs at the ambient pressure started to decompose at about 268 K. This temperature was higher than the equilibrium temperature of carbon dioxide hydrate at this pressure, indicating that the gas hydrate in the plugs could be effectively preserved at temperatures below the ice melting point (273.2 K). It was found through observation of the hydrate decomposition process in the plugs under the microscope that the gas in the samples released in small bubbles, while the hydrate particles were not visible at this magnification, indicating that the hydrate was indeed highly dispersed in the samples. A residual wax was jelly-like after decomposition of hydrate in all the cases. Rheological experiments showed that the plugs residues after decomposition of the hydrates had higher yield points and viscosities than the initial waxy crude oil originally used for the experiments.

MRI investigation of hydrate pore habits and dynamic seepage characteristics in natural gas hydrates sand matrix

A clear understanding of methane (CH4) hydrate pore habits and gas seepage behaviors in hydrate bearing sediments is critical for the exploitation of natural gas hydrate (NGH) reservoir. In this paper, we presented the experimental data of CH4 hydrate formation and decomposition in sand packs with the aid of magnetic resonance imaging (MRI) technique. The results showed that in homogeneous sand packs CH4 hydrates preferentially formed in the places where the water saturation was lower than the others, whereas in heterogeneous sand packs, the small grain size parts were the preferred locations for hydrate nucleation and the final hydrate saturation was much higher than the other parts. In this study, the CH4 hydrate decomposition process was found to consist of three stages, the initiation stage, the rapid decomposition stage and the post-decomposition stage. The relationship between gas permeability and hydrate saturation was obtained by analyzing the MRI scanned images during percolation experiments. The dynamic gas permeability indicated that the hydrate occurrence mode shifted from grain-coating and pore-filling co-existing to grain-coating predominant with the decrease of hydrate saturation. Moreover, the flow obstruction caused by the presence of hydrate barrier and sand migration was much severer in NGH reservoir.

Effects of temperature holding on methane hydrate decomposition process by thermal stimulation

Methane hydrate is an important clean energy source. In this study, the decomposition curve of methane hydrate in the thermal stimulation process with the heating rate of (0.9, 1.8, and 2.7) K·min−1 is found and summarized. When the temperature holding process occurs during the thermal stimulation process, the hydrate decomposition cannot be stopped immediately. The pressure of the reactor deviates from the thermal decomposition curve of methane hydrate without temperature holding process due to the impact of temperature fluctuation. It was found that when the holding temperature increased from 277.56 K to 281.15 K at the heating rate of 1.8 K·min−1, the maximum temperature fluctuation increased from 0.46 K to 1.25 K. Thus, the decomposition pressure of methane hydrate with a temperature holding process is higher than that of methane hydrate without a temperature holding process. After the temperature fluctuation process, the methane hydrate equilibrium curve under the equilibrium state reached by the hydrate at the holding temperature is summarized. When the temperature holding process is over, the temperature of the reactor continues to rise, leading to the decomposition of methane hydrate. The heat in the reactor is slowly transferred again. Then, the methane hydrate decomposition curve with and without temperature holding process coincide. It was found that when the holding temperature increases from 277.56 K to 281.15 K at the heating rate of 1.8 K·min−1, the percentage of pressure difference between the equilibrium curve and the decomposition curve of methane hydrate increases from 4.34% to 8.81%. The initial heating temperature and heating step have no effect on the decomposition characteristics of methane hydrate with a temperature holding process. The heating rates can affect the slope of the hydrate decomposition curve. This study is of great significance for the safe application of methane hydrate thermal stimulation process.