Rate Of Hydrate Decomposition Research Articles

Molecular Dynamics Simulation of the Effects of Different Initial Water Content on Methane Hydrate Decomposition

Gas hydrate is mainly distributed in deep sea-floor sediments and permafrost regions. The water content of these sediments varies with the type of reservoir and affects the rate of hydrate decomposition. In this work, the decomposition process of methane hydrate under four different initial water contents was investigated by molecular dynamics simulation. The results were analyzed by the system conformation, radial distribution function (RDF), and mean square displacement (MSD), which revealed the microscopic mechanism of the effect of the initial water content on the decomposition rate of hydrate. The results demonstrate that the hydrate decomposition starts from the boundary to the middle, and the cage structure is destroyed layer by layer. Methane molecules continue to escape from the hydrate cages as the hydrate decomposes, and subsequent decomposition of the hydrate is inhibited when its solubility in water reaches saturation. The higher the initial water content is, the faster the decomposition rate of hydrate is. The movement distance of methane gas is affected by the initial water content. The higher the initial water content, the smaller the MSD of methane molecules.

Study on the Evolution Law of Wellbore Stability Interface during Drilling of Offshore Gas Hydrate Reservoirs

The study of wellbore stability in offshore gas hydrate reservoirs is an important basis for the large-scale exploitation of natural gas hydrate resources. The wellbore stability analysis model in this study considers the evolution of the reservoir mechanical strength, wellbore temperature, and pressure parameters along the depth and uses plastic strain as a new criterion for wellbore instability. The wellbore stability model couples the hydrate phase transition near the wellbore area under the effect of the wellbore temperature and pressure field and the ‘heat–fluid–solid’ multifield evolution characteristics, and then simulates the stability evolution law of the wellbore area during the drilling process in the shallow seabed. The research results show that, owing to the low temperature of the seawater section and shallow formation, the temperature of the drilling fluid in the shallow layer of the wellbore can be maintained below the formation temperature, which effectively inhibits the decomposition of hydrates in the wellbore area. When the wellbore temperature increases or pressure decreases, the hydrate decomposition rate near the wellbore accelerates, and the unstable area of the wellbore will further expand. The research results can provide a reference for the design of drilling parameters for hydrate reservoirs.

Experimental study on the intrinsic dissociation rate of methane hydrate

This work developed a new method that can efficiently separate gas-liquid-hydrate mixture and obtain solid gas hydrate, making it possible to measure the intrinsic dissociation rate of gas hydrate directly. The results demonstrate that the classical Kim’s model can reflect the relationship between driving force and hydrate dissociation rate when at same temperature, but can’t accurately reveal the effect of temperature on hydrate decomposition rate when under the same driving force. In Kim’s model, the driving force of hydrate dissociation was expressed as the difference between equilibrium fugacity (feq) and the fugacity of guest molecules in gas phase (fg). A new hydrate dissociation model was established in this work, and the driving force of hydrate dissociation was modified into (feq-fg)/feq, which has a higher universality than feq-fg. The intrinsic rate constant and activation energy of methane hydrate decomposition were determined to be 18905.35 mol/(m2‧s) and 22.149 kJ/mol, respectively.

Effect of main functional groups of cement slurry additives on the stability of methane hydrate: Experiment and molecular dynamics simulation

There are a lot of hydrates in deep-water and shallow layer, which accumulate at relatively shallow depths (<100–120 m). During cementation in the deep-water shallow formation, the free inorganic ions and organic molecules in the cement slurry filtrate enter the formation via percolation, which may affect the stability of the hydrate. The free organic matter in cement slurry acts mainly as retarder, fluid loss additive and dispersant. First, the representative fluid loss additive BXF-200 L, retarder PC-H21L, and dispersant PC-F41L were selected for infrared spectroscopy. Then, the influence of BXF-200 L, which contains all the previously mentioned functional groups, was studied for its impact on hydrate stability using an experimental method. Second, a molecular dynamics simulation method was used to study the adsorption properties of the functional groups on the surface of sI methane hydrate under canonical (NVT) ensemble, which it is the most widely distributed hydrate type in nature. What is more, in order to characterize the influence of functional groups on the decomposition of methane hydrate, the molecular conformation, mean square displacement (MSD), diffusion coefficient (D) and adsorption energy (Eadsorption) were calculated using the dynamics simulation trajectory of sI methane hydrate.The experimental results showed that the main functional groups in cement slurry filtrate additives were carboxyl, sulfonate, and acylamino groups. Further, BXF-200 L can promote hydrate decomposition. Subsequently, the simulation results showed that under the same conditions, the adsorption energy of sulfonate group on the surface of methane hydrate was the largest and the adsorption capacity was the greatest. When the concentration was ≤ 1 mol%, among the three functional groups, acylamino had the most obvious promoting effect on the decomposition of sI methane hydrate, and the hydrate decomposition rate increased by 16.31%. When the concentration was more than 1 mol%, the decomposition rate of hydrate was increased under the interaction of carboxyl, acylamino, and sulfonate groups, by 12.49%, 19.46% and 27.74%, respectively, compared with that under pure water-hydrate system.

A new process to develop marine natural gas hydrate with thermal stimulation and high-efficiency sand control

The development of marine gas hydrate from low-permeability offshore reservoirs with serious sand incursion issues is a challenging task. Currently, there are no mature technologies in the world. Therefore, we proposed a comprehensive development process, including the following measures: (1) Vertical injection-production well pattern enhanced by multi-layer fracturing and hot water injection by seawater source heat pump. (2) An advanced multi-layer hydraulic fracturing technique incorporating cold fracturing fluid, low-temperature consolidation proppant, multi-stage sliding sleeve packer, and a chemical diverting agent. (3) A series of sand control measures including hydraulic fracturing, consolidation proppant, and stand-alone screen. (4) Several rounds of hot water injection-soaking-production (huff-n-puff) on injection wells, and finally continuous injection-production, with depressurization. Then, we built a gas hydrate decomposition rate model to simulate this process. The results show that hot water injection combined with multi-layer hydraulic fracturing can significantly improve the gas hydrate dissociation rate compared with the schemes without hot water injection or fracturing. Combining hot water injection by seawater source heat pump, multi-layer horizontal fractures by advanced hydraulic fracturing technique, and a set of sand control techniques, can efficiently control sand while producing gas hydrate.

Methane Hydrate Dissociation in Quartz Sand by Depressurization Combined with Microwave Stimulation

Depressurization combined with thermal stimulation is a promising method for gas hydrate production. Owing to its advantages of rapid uniform heating, microwave radiation is an efficient source of heat. However, the mechanisms of hydrate decomposition under depressurization combined microwave stimulation are currently unclear. In this study, methane hydrate was synthesized under 6 MPa and 2 °C, with hydrate saturation of approximately 42% in natural quartz sand. We then compared hydrate decomposition via rapid depressurization and piecewise depressurization, with or without microwave stimulation at 2.45 GHz and 400 W. When using the depressurization method alone, the hydrate decomposition rate increased with the depressurization amplitude. However, in the last stage of hydrate decomposition, the external flow of gas was hindered by the Jamin effect, especially under larger depressurization amplitudes; therefore, extremely low production pressure is not justified. When combined with microwave stimulation, both depressurization methods resulted in increased reservoir temperature within a few seconds, and microwave heating provided an extra driving force for hydrate decomposition. Furthermore, microwave heating was more effective when larger amounts of undecomposed hydrate remained after depressurization. When depressurization was combined with microwave stimulation, the average gas production rate at 100% gas production was 0.269–0.601 L/min, which was significantly higher than that for depressurization alone. However, the energy efficiency ratio was approximately 1, which has no practical value. Conversely, the average gas production rate at 90% gas production was 0.452–2.945 L/min and the energy efficiency ratio was 2.9–17.6. Under the combined method, gas hydrate decomposition at a production pressure of 1.9 MPa achieved the subtle balance between the average gas generation rate and energy efficiency. Thus, optimizing the gas production pressure and microwave stimulation time can improve the average gas production rate and energy efficiency ratio according to the reservoir hydrate conditions.

Simulation study on multiphase flow considered seawater salinity in wellbore for deep-water bearing hydrate formation drilling

The gas-liquid-solid multiphase flow caused by decomposition of hydrate cuttings is a new problem facing the drilling of marine hydrate formations. In this paper, a gas-liquid-solid non-isothermal transient flow model is proposed considering the coupling relationships between hydrate decomposition, and heat and mass transfer in multiphase flow. The accuracy of the model in predicting gas void fraction, friction pressure and wellbore temperature is verified by using indoor experimental data and field measured data. Using this model, the influence of seawater salinity on the gas-liquid-solid flow behaviors is investigated. Numerical simulation results show that the decomposition area and decomposition rate of hydrate in the wellbore gradually increase with the increase of seawater salinity. Moreover, the higher the salinity of seawater, the greater the reduction in bottomhole pressure and the higher the decomposed gas void fraction reaching the wellhead. Increasing wellhead backpressure and lowering inlet temperature of seawater can effectively control the amount of hydrate decomposed in the wellbore. To ensure wellhead safety and reduce the rate of hydrate decomposition, the wellhead backpressure should be greater than 3.5 MPa. Compared with the wellhead backpressure of 2 MPa, the gas void fraction at the wellhead decreases 1.75 times when the wellhead backpressure is 5 MPa. When the seawater inlet temperature is in the range of 15 °C–30 °C, the seawater salinity control in the range of 8‰∼19‰ can manage the maximum gas phase volume fraction at the wellhead within 10%. Compared with the inlet temperature of 15 °C, the hydrate decomposition rate at the wellhead is 1.6 times higher when the inlet temperature is 30 °C.

Numerical Modeling on Dissociation and Transportation of Natural Gas Hydrate Considering the Effects of the Geo-Stress

A deep understanding of the dissociation and transportation mechanism of natural gas hydrate (NGH), taking into account the effects of geo-stress, contributes to optimizing the development strategy and increases the exploitation efficiency of NGH. In this paper, the mathematical model, coupled with fluid heat and mass transfer, multiphase flow mechanics, and reaction kinetics with phase change in the process of hydrate decomposition was established. An axisymmetric two-dimensional model was developed to simulate the depressurization decomposition process of natural gas hydrate in the Berea sandstones. FLUENT software was used to solve the fundamental governing equations of the multi-phase flow, and UDF programming was employed to program the hydrate decomposition model and the modified permeability model in the dissociation and transportation of NGH. The simulation results were then validated by Masuda’s experimental data. The effects of gas saturation, outlet pressure, temperature, absolute permeability and geo-stress on the decomposition of natural gas hydrate were studied. The results indicated that a higher absolute permeability, higher initial gas saturation, lower outlet pressure, and higher initial temperature advance the decomposition rate of hydrate. Thus, an optimized production plan is essential to promote the extraction efficiency of the NGH. The geo-stress causes a decrease in the porosity and permeability of the porous rock, which restricts the efficiency of the heat and mass transfer by the fluid flow, leading to a slower dissociation and transportation rate of the NGH. Thus, it is important to take geo-stress into consideration and balance the extracting efficiency and the well pressure, especially when the NGH is developed by depressurization.

A developed transient gas–liquid–solid flow model with hydrate phase transition for solid fluidization exploitation of marine natural gas hydrate reservoirs

The multiphase flow characteristic is one of the most concerning problems during solid fluidization exploitation of marine natural gas hydrate reservoirs. In this research, a new transient gas–liquid–solid multiphase flow model with hydrate phase transition was developed. Meanwhile, this model considered the coupling relationship among convective heat transfer, hydrate dynamic decomposition, and multiphase flow. The model can simulate the change of flow pattern from solid−liquid to gas–liquid–solid flow, and describe the distribution character of volume fraction of phase, wellbore temperature and pressure, and hydrate decomposition rate during transportation. The simulation results indicate that the hydrate decomposition region in the wellbore gradually expands, but the hydrate decomposition rate gradually decreases during the solid fluidization exploitation of hydrate. When mining time lasts for 4 h, and the bottom hole pressure decreases by about 0.4 MPa. Increasing NaCl concentration in seawater helps expand hydrate decomposition regions and improves the wellbore hydrate decomposition rate. When the NaCl mass fraction in seawater reaches 15%, it will raise the hydrate decomposition regions to the whole wellbore. In addition, the higher the wellhead backpressure, the lower the decomposition area and decomposition rate of hydrate in the wellbore. When wellhead backpressure reaches 2 MPa, the volume fraction of gas near the wellhead will reduce to about 12%. This work is expected to provide a theoretical basis for the development of marine hydrate reservoirs.

Mathematical Model Research on the Influence of Fluid Parameter Characteristics on the Solid Fluidization Exploration Process of Hydrate Reservoir

Objective: In order to further study the characteristics of flow parameters in the wellbore during solid fluidization drilling, based on the physical model of solid fluidization exploration of hydrate reservoir, a gas-liquid-solid three-phase flow model is established during the drilling process. Methods: The parameter changes of wellbore pressure, section gas holdup, liquid volume fraction, temperature and wellbore hydrate decomposition rate under the influence of different wellhead choke pressure, displacement and drilling fluid density during solid fluidization are analyzed. Results: The analysis results of the case will show that under different wellhead choke pressures, hydrate begins to decompose at 400m to 500m, and with the increase in wellhead choke pressure, the wellbore pressure and the volume fraction of liquid and solid phase at the same position increase significantly, and the gas production rate and the gas volume fraction decrease. The results also show that under different drilling fluid rates, hydrate begins to decompose at 500m to 550m, and with the increase in drilling fluid rate, bottomhole pressure and wellbore temperature increase, the situation of hydrate decomposition becomes better, and gas volume fraction decreases. Conclusion: With the increase of drilling fluid density, hydrate begins to decompose at 500m to 600m, the wellbore temperature increases, and the hydrate decomposition is delayed. Meanwhile, the difference in the location of hydrate decomposition becomes smaller.

Heat and mass transfer at interfaces in decomposition of methane hydrate under combustion

The thermodynamic and kinetic mechanism of decomposition of gas hydrate with burning is still in its infancy. The heat and mass transfer are analyzed based on accurate positioning of interfaces in the decomposition of methane hydrate under combustion by ReaxFF molecular dynamics simulation. There is a transition region between the solid–liquid interface of decomposition and the burning boundary. Results show that the water layer in transition region largely hinders the diffusion of methane produced by decomposed hydrate so that hydrate cannot burn steadily. Heat flux at burning boundary is larger than at solid–liquid interface in the early stage of decomposition, so thermal transport in burning boundary drives decomposition of hydrate. The heat flux at solid–liquid interface is larger than at burning boundary in the later stage, and the diffusion and extrusion of liquid water with heat transport through the transition region become the new driving force. The interfacial thermal resistance of solid–liquid interface decrease rapidly to the order of 10−10 m2·K/W at different temperatures. 23.9 % of the heat from burning process at 1600 K can maintain entire decomposition of hydrate within 3000 fs. Although the formation of transition region doesn’t significantly reduce the decomposition rate of hydrate, it is unfavorable for burning. The discharge of water layer in transition region should be focused on in the future.

Analysis of factors affecting microwave heating of natural gas hydrate combined with numerical simulation method

As a clean and abundant unconventional natural gas resource, natural gas hydrate (NGH) holds the characteristics of safety, high efficiency and sustainable exploitation, which helps to alleviate the energy shortage of China, reduce the foreign-trade dependence of oil and gas, and ensure the national energy security. Microwave heating is a significant method that has been used in natural gas hydrate exploration. By using the microwave heating, the NGH in the reservoir formations would be heated, decomposed and stimulated thanks to taking advantage of microwave heating's unique characters: efficiency, high speed, clean and pollution-free. This paper established the temperature under microwave heating gas hydrate distribution theory model, and by using the finite element method for simulating temperature field of microwave heating gas hydrate, this paper analyzed the natural gas hydrate in the microwave field temperature distribution in the influencing factors. Microwave has a significant heating effect on the hydrate reservoir in the immediate vicinity of wellbore, and it is not affected by the initial conditions of reservoir. The temperature can rise to above 50°C within 1 h which is higher than the phase equilibrium temperature at the time of hydrate decomposition and is helpful to improve the decomposition rate of hydrate. The frequency is set at 915 MHz, and the feed port has a spiral arrangement with a length of 10 mm, which greatly expands the microwave heating range.

Influence of Solid Particles on Wellbore Parameters in Solid Fluidization Production Process

Objective: To efficiently exploit the hydrate reservoir, based on the problem of a large amount of sand production in the current hydrate reservoir exploration process, combined with the solid fluidization well testing and production method. Methods: A gas-liquid-solid three-phase flow model is established and hydrate decomposition is taken into consideration. The changes in parameters of wellbore pressure, gas volume fraction, liquid volume fraction, temperature and the decomposition rates of wellbore hydrate under the conditions of different rates of sand production are analyzed. Results: The results show that with the increasing rate of sand production during the solid fluidization well testing and production, the pressure of the wellbore is lifted, gas volume fraction declines, liquid volume fraction declines, and the temperature of the wellbore does not change. Moreover, increasing the rate of sand production also leads to the delay of the position where hydrate begins to decompose. Conclusion: With the decrease in hydrate saturation, the pressure of the wellbore goes up, gas volume fraction is lifted, solid volume fraction rises, and decomposition rate of hydrate declines.

Effect of biofriendly amino acids on methane hydrate decomposition: Insights from molecular dynamics simulations

Controllable mining is a prerequisite for commercial hydrate mining and the addition of chemicals makes the mining process more efficient. However, traditional chemical additives are environmentally harmful and give rise to damage to the environment. Amino acids are green additives, and their effect on the decomposition of hydrates remains to be unequivocally understood. To reveal the influencing mechanism of amino acids on the decomposition of methane hydrates, molecular dynamics (MD) simulations were performed to systematically investigate the influences of the amino acid types, concentrations, temperature, and pressure on hydrates decomposition. The results demonstrated that glycine had shown excellent performance on accelerating the decomposition of hydrates, followed by valine, serine, alanine, and glutamic acid. 5 mol% glycine was most conducive to the decomposition of hydrates. Temperature was a favorable factor for the decomposition of methane hydrate, and the decomposition rate had a nonlinear relationship with the increase in temperature. Additionally, there were two factors affecting the decomposition of hydrates under varying pressure conditions. The first was the inhibition of pressure on the decomposition of hydrates, called inhibiting effect. The second was that pressure had an effect on the solubility of amino acids which could promote the decomposition of hydrates, called the facilitation effect. The synergistic effect of inhibiting effect and facilitation effect determined the final hydrate decomposition rate. The obtained results provided a qualitative and quantitative analysis of the decomposition process of the amino acid-hydrate system on the molecular scale, and also laid a basis for the selection and design of novel amino acid molecules promoting hydrates decomposition.

Numerical Simulation Study on the Effect of Horizontal Well Reservoir Stimulation for Gas Hydrate Production

AbstractA new gas hydrate reservoir stimulation method of in‐situ fracturing with transient heating is proposed, in line with analysis of the technological bottlenecks faced by marine gas hydrate production. This method injects the developed chemical reagents into a hydrate reservoir through hydraulic fracturing, releasing heat during the chemical reaction to increase the hydrate decomposition rate. The chemical reaction product furthermore has a honeycomb structure to support fractures and increase reservoir permeability. Based on the geological model of natural gas hydrate in the South China Sea, three development methods are simulated to evaluate hydrate production capacity, consisting of horizontal well, fractured horizontal well and in‐situ fracturing with transient heating well. Compared with the horizontal well, the simulation results show that the cumulative gas production of the fractured horizontal well in one year is 7 times that of the horizontal well, while the cumulative gas production of in‐situ fracturing with transient heating well is 12 times that of the horizontal well, which significantly improves daily efficiency and cumulative gas production. In addition, the variation patterns of hydrate saturation and temperature‐pressure fields with production time for the three exploitation plans are presented, it being found that three sensitive parameters of fracture conductivity, fracture half‐length and fracture number are positively correlated with hydrate production enhancement. Through the simulations, basic data and theoretical support for the optimization of gas hydrate reservoir stimulation scheme has been provided.

Experimental study and recognition of natural gas hydrate chemical reagent - CO2 exploitation

Abstract In order to solve the problems of slow replacement rate and low degree of substitution in the process of CO2 displacement by natural gas hydrate, a method of chemical reagent-CO2 displacement combined mining experiment was proposed. Taking the thermodynamic methanol inhibitor as an example, three groups of experiments were carried out: critical temperature pressure condition for CH4 decomposition at different methanol concentrations, CH4 hydrate decomposition under the condition of methanol reagent at 30% concentration, and methanol reagent-CO2 combined mining. Experiments show that: (1) The concentration of methanol plays a decisive role in hydrate recovery. Methanol reagent has the property of highly reducing the formation temperature of hydrate. Methanol solution concentration is positively correlated with the critical temperature drop of hydrate decomposition and formation pressure. (2) The methanol solution decomposition of hydrate can be roughly divided into three stages: the initial stage of rapid decomposition, the competitive stage of hydrate decomposition and stable transformation, and the stable stage of decomposition. Through experimental simulation, it is concluded that formation pressure is a direct factor affecting the decomposition ability of methanol reagent to hydrate. By changing formation pressure, the rate of CH4 hydrate recovery can be improved. (3) A higher hydrate decomposition rate can be obtained based on the methanol reagent-CO2 displacement mining method. In the action of the methanol reagent, the subsequent injection of CO2 still promotes the decomposition of CH4, that is, the displacement of CO2 hydrate after the decomposition of the hydrate based on the methanol reagent is feasible. The research results are of great significance especially for large-scale mine test mining of permafrost natural gas hydrate chemical reagent method.

Characteristics of hydrate formation, decomposition, and phase equilibrium in the transition area formed by the high-pressure jet breaking and sand filling method

Marine hydrates mainly occur in clayey silt sediments, and their occurrence environment has the characteristics of a high clay content and low permeability, which seriously inhibit an increase in the gas production rate. This study presents a new method, the high-pressure jet breaking and sand filling method, to transform the natural gas hydrate (NGH) reservoir near the production well, whereby a water jet is used to form an area of sediment with large particle diameters and high permeability. Based on indoor experiments, we report the characteristics of hydrate formation, decomposition, and phase equilibrium in the transition area formed by this method under different sand particle sizes and mass fractions. The addition of sand at a high mass fraction with a large particle diameter significantly shortened the time required for rapid hydrate formation, prolonged slow hydrate formation, and reduced the increase in temperature. With an increase in the mass fraction of large-diameter sand, the free water content of pores increased, which made it easier to form hydrates with large particles, thus delaying the hydrate decomposition rate. The phase equilibrium curves of hydrates in clayey silt sediments with sand added at mass fractions of 30% and 50% were close to that of bulk hydrate in pure water at lower temperatures. As the temperature increased, the phase equilibrium curves gradually approached those of hydrates in clayey silt sediments.

Enhanced CH4 recovery from hydrate-bearing sand packs via CO2 replacement assisted thermal stimulation method

Natural gas exploitation from natural gas hydrate (NGH) reservoir by thermal stimulation combined with CO2 replacement is a promising method to enhance the natural gas recovery as well as sequestrate CO2 permanently. However, the operation scheme of thermal stimulation is crucial for the efficiency of both gas recovery and CO2 sequestration. In this study, a novel CO2–CH4 replacement assisted thermal stimulation method was proposed to enhance both CH4 recovery and CO2 sequestration simultaneously. Experiments were conducted in the CH4 hydrate unstable zone with pressure and temperature range from 1.97 to 3.22 MPa and 273.63–276.17 K, respectively. One step, continuous and stepwise thermal stimulation schemes with and without CO2 replacement were applied to evaluate the effect of different operation schemes on gas recovery and CO2 sequestration. The results indicated that the CO2–CH4 replacement process significantly increased the CH4 recovery percentage by 8.21%–34.79%. Overall, higher pressure and temperature were able to provide higher CO2–CH4 exchange driving force, which leaded to higher CH4 recovery ratio and CO2 sequestration efficiency. Among three thermal stimulation schemes, the stepwise thermal stimulation showed the best CH4 recovery of 62.60% and CO2 sequestration efficiency of 71.23% on average. The maximum CH4 recovery percentage of 69.51% and CO2 sequestration efficiency of 88.29% were obtained at 3.21 MPa and 276.13 K with stepwise thermal stimulation. The mechanism of this recovery enhancement can be concluded to that CH4 hydrate decomposition rate was enhanced by repeatedly imposed heat fluctuation in stepwise thermal stimulation and eventually the CH4–CO2 replacement process in hydrate was promoted. By comparison with published data, the CH4 recovery efficiency in this work increased by 19.2% on average. We expected that the proposed combination method could be applied in the late stage of NGH reservoir exploitation to obtain an enhanced CH4 gas production yield with sequestration of CO2 simultaneously.

Simulation of upward gas—hydrate slurry multiphase flow in a vertical concentric annulus for natural gas hydrate solid fluidization exploitation

The accurate simulation of upward multiphase flow of hydrate slurry in the annulus is one of the key scientific unsolved issues in natural gas hydrate solid fluidization exploitation. In this work, the upward multiphase flow of hydrate slurry in a vertical concentric annulus is simulated. The hydrate slurry hydrodynamic models suitable for pseudo-single-phase flow, bubbly flow, slug flow, and annular flow are proposed, respectively. Finally, the hydrate decomposition kinetic model is combined with the established annulus hydrate slurry multiphase flow model to simulate the multiphase flow of hydrate slurry in the annulus. The factors affecting flow behaviors are analyzed. During the upward flow in the annulus, the hydrate slurry temperature first decreases and then increases. As the inlet temperature increases, the fluid temperature, hydrate decomposition rate, and gas superficial velocity increase. During the upward flow in the annulus, hydrate may be formed again, which indicates that the error may be magnified due to ignoring hydrate formation. The larger the flow rate, the smaller the length of the slug flow. The larger the hydrate volume fraction, the higher the starting point of hydrate decomposition. These findings are of practical value to give a further understanding of hydrate slurry multiphase flow, which can promote further engineering application of natural gas hydrate solid fluidization exploitation.

Experimental study of methane hydrate formation and decomposition in the porous medium with different thermal conductivities and grain sizes

Gas hydrate found in nature is mainly existed in deposit, such as marine deposit and permafrost. The thermophysical properties of the deposit are largely influenced by the formation features of gas hydrate, thereby affecting the hydrate production. In this manuscript, the experiments of formation and decomposition behavior of gas hydrate in the deposit with different grain sizes (40–60 mesh, 80–120 mesh, 325–400 mesh) and different thermal conductivities (0.926 W/m.K, 28.8 W/m.K, 41.9 W/m.K) were conducted in the small cubic hydrate simulator, and the coupling effect of the heat and mass transport on the hydrate formation and dissociation were researched. It was concluded that the mass transport rate in the deposit dominated the hydrate formation. The formation of gas hydrate was initial at the contact surfacing of gas–water and grew gradually in the gas-rich region, and the hydrate formation amount in the water-rich region was little. In the deposit with the grain size of 80–120 mesh and 325–400 mesh, there was no obvious induction time for the hydrate formation that occurred during gas injection. Be different from the hydrate formation, the heat transfer rate of the deposit restricted chiefly methane hydrate dissociation. With the raise of grain size and thermal conductivity of deposit, methane hydrate decomposition rate enhanced. It’s also found that the formed hydrate acted as cementation in porous medium with the grain size of 80–120 mesh and 325–400 mesh. The heat transfer rate of the deposit would significantly decrease when most of the cementation between the porous medium was disappeared as a result of hydrate dissociation, and the hydrate saturation in sediments at this time was defined as the critical hydrate saturation (50–80% of the initial hydrate saturation). The critical hydrate saturation is meaningful for gas hydrate resource prospecting and the risk assessment of gas hydrate production in actual fields.