Hydrate Decomposition Research Articles

The effect of decomposition of natural gas hydrate on deep-water drilling

One of the significant challenges in deep-water drilling is calculating BHP during kickoff caused by natural gas hydrate decomposition. In this paper, the heat transport theory, seepage mechanics, and hydrate decomposition kinetics serve as the foundation for the hydrate decomposition and bottom hole pressure model. Research shows 42.8 m3 of gas have entered Well X cumulatively, resulting in a 1.66 MPa pressure decrease and a 6.9-hour-recovery period. The monitoring of high saturation and porosity formations needs to be strengthened, and drilling parameters need to be modified to protect the stability of the wellbore, according to the analysis of formation parameters and drilling parameters.

Reservoir stimulation of marine natural gas hydrate-a review

Low production rate, limited recovery range, and short period of stabilized production of natural gas hydrate (NGH) field trials indicate a large gap to commercial production. To improve the efficiency of gas recovery from NGH reservoir, a large number of studies on reservoir stimulation have been reported. In this review, various aspects of reservoir fracability, including brittleness, fracture toughness, and fracability index model are comprehensively analyzed to provide an overall view on the generation of fractures in marine NGH reservoir. The main reservoir stimulation methods are summarized into four aspects: hydraulic fracturing, jet breaking, overlying layer modification and split grouting, of which the working principle, advantages, and limitations are highlighted. It indicates split grouting is the most promising method for achieving two goals of both reservoir skeleton reinforcement and permeability enhancement. Besides, the production enhancement by reservoir stimulation of non-diagenetic hydrate reservoir should be further evaluated considering the evolution of artificial seepage channels during hydrate decomposition. Finally, further developments are suggested to obtain reliable results of the fracability of NGH reservoir, bridge the gap of reservoir stimulation between concept design and practical application, and highlight the combination between reservoir stimulation and NGH reservoir stability for safe and efficient gas production.

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.

Effect of pore water on the depressurization of gas hydrate in clayey silt sediments

Marine gas hydrate always occurs in clayey silt sediments partially or fully saturated with water. Pore water is involved in gas hydrate formation and decomposition, which plays an important role in gas recovery from hydrate during depressurization. In this work, the depressurization of methane hydrate in the clayey silt sediments was experimentally investigated in a one-dimensional reactor. The variation of gas production and pressure gradient under the gas-rich and water-rich condition were studied. It was found that the gas production in water-rich environment was much lower than that in gas-rich environment as the movement of hydrate dissociating front is slower in water-rich environment. The permeability of the clayey silt sediments was measured before hydrate formation and after hydrate decomposition, which indicated that there was a large depression in the permeability of the clayey silt sediments after hydrate decomposition, which could be attributed to the hydration swelling of clay minerals with the expansion of diffuse double layer caused by the release of water with lower salinity from gas hydrate dissociation. And the permeability depression of the clayey silt sediments caused by hydrate decomposition was more obvious in water-rich environment, reaching up to ∼50%, and increased with the content of clay minerals. In addition, inorganic salt-based clay stabilizer solution was pre-injected into the clayey silt gas hydrate-bearing sediments before depressurization to mitigate the formation damage during gas hydrate production. The results showed the permeability of the sediments pre-saturated with NH4Cl was maintained at ∼75% of the initial values after gas hydrate decomposition as the exchange of Na+ with NH4+ in sediments was proved to decrease the bound water in montmorillonite interlayers by NMR analysis and inhibit the hydration swelling of clay particles.

Simulation research on solid fluidization exploitation of deepwater superficial layer natural gas hydrate reservoirs based on double-layer continuous pipe

In this paper, a new method for exploiting shallow marine natural hydrate reservoirs is proposed using a double-layer continuous pipe. In order to research the multiphase flow behavior and hydrate decomposition characteristics during the hydrate mining process, a non-isothermal transient gas-liquid-solid multiphase flow model is established considering the coupling effect of multiphase flow, heat transfer and hydrate phase transition. The model's accuracy is verified by comparisons with laboratory and field data. Numerical simulation results show that the hydrate decomposition is slow in the first 2 h of mining, thus the volume fraction of each phase in the pipe changes little; with the increase of time, the volume fraction of each phase in the pipe changes significantly until it reaches a stable state after about 5 h of mining. When the mining rate exceeds 15 kg/s, the hydrate particles will not be fully and effectively transported, and a short time aggregation phenomenon will occur. Increasing the temperature of the injected seawater is beneficial to promote the hydrate decomposition, which in turn increases gas production. While, increasing the wellhead backpressure and seawater displacement facilitates in reducing gas production. The research findings will be of reference value for gaining insight into studying complex multiphase flow behaviors involving hydrate phase transition and provide a new approach to exploiting subsea shallow natural gas hydrate reservoirs.

Spatial-temporal variations of the gas hydrate stability zone and hydrate accumulation models in the Dongsha region, China

It is of great significance to study the spatial-temporal variations of the thickness of the gas hydrate stability zone (GHSZ) to understand the decomposition, migration, accumulation and dissipation of gas hydrate, the corresponding relationship between bottom-simulating reflectors (BSRs) and gas hydrate, and the distribution of heterogeneous gas hydrate. We selected the Dongsha region in the South China Sea (SCS) as the research object to calculate the spatial-temporal variation of the GHSZ since 10 Ma, analyzed the main factors affecting the thickness of the GHSZ, discussed the dynamic accumulation processes of gas hydrate, and proposed an accumulation model of gas hydrate in the Dongsha region. The results show that the thicknesses of the GHSZ in the study area were between 0 m and 100 m from 10 to 5.11 Ma, and the relatively higher bottom water temperature (BWT) was the key factor leading to the thinner thickness of the GHSZ during this period. From 5.11-0 Ma, the thickness of the GHSZ gradually increased but showed several fluctuations in thickness due to changes in the geothermal gradient, seawater depth, BWT, and other factors. The decrease in the BWT was the main factor leading to GHSZ thickening from 5.11 to 0 Ma. The thicknesses of the GHSZ are between 110 m and 415 m at present. The present spatial distribution features show the following characteristics. The GHSZ in the deep canyon area is relatively thick, with thicknesses generally between 225 m and 415 m, while the GHSZ in other areas is relatively thin, with thicknesses between 110 m and 225 m. Based on the characteristics of the GHSZ, two hydrate accumulation models are proposed: a double-BSRs model due to thinning of the GHSZ and a multilayer hydrate model due to thickness changes of the GHSZ, with single or multiple BSRs.

The Morphological Characteristics of Authigenic Pyrite Formed in Marine Sediments

Pyrites are widely distributed in marine sediments, the morphology of which is applied as a proxy to infer the redox conditions of bottom water, and identify diagenetic stages and hydrocarbon leakage activities. In this review, the methods used for the morphological study of pyrite are summarized. The textural and size characteristics of euhedral pyrite and pyrite aggregates, as the formation and evolution mechanism of pyrite are discussed for their significance in reconstructing the geochemical environment. The morphological study of pyrite includes shape observation, size estimation, and surface feature analysis. Scanning electron microscope and optical microscope are the main methods for morphological observation; transmission electron microscope and scanning tunneling microscope are applicable to observe nanoscale morphological structures and crystal growth on the crystal surface, and X-ray computed tomography is capable of measuring pyrite size distribution at the scale of a micrometer. Under the marine sedimentary condition, the single crystal of pyrite appears in cube, octahedron, dodecahedron, and their intermediates, the size of which ranges from several nanometers to more than 100 µm. The morphology of euhedral pyrite is controlled by temperature, pH, the chemical composition of interstitial water, etc., and might have been experienced in later reformation processes. The pyrite aggregates occur as framboid, rod-like, fossil-infilling, etc., characterized by the comparatively large size of several microns to several millimeters. It is found that certain textures correspond with different formation mechanisms and geochemical environments. Particularly, under special geological conditions, for instance, the methane leakage and/or decomposition of gas hydrate, pyrite is anomaly enriched with morphological textures of massive framboid cluster, rod-like aggregates, etc., and framboid is found with a large mean diameter (>20 µm) and standard deviation (>10 µm). These typical features can be employed to ascertain the position of the paleo sulfate methane transition zone (SMTZ).

Optimization of the depressurization rate and stepwise strategy for hydrate exploitation using a genetic algorithm-based depressurization method

Depressurization is one of the most effective methods for hydrate exploitation. Optimization of the pressure-drop strategy for improving the exploitation efficiency requires immediate attention. In this study, a hydrate decomposition model was for the first time combined with a genetic algorithm. The pressure-drop rate and strategy were optimized based on the exploitation efficiency. The depressurization rate of the continuous slow depressurization method was optimized. Subsequently, the best pressure-drop rate for stepwise depressurization was hypothesized and validated. In addition, the stepwise strategy was optimized. For the two depressurization methods, increasing the pressure-drop rate increased the exploitation efficiency, while increasing the number of pressure-drop stages in the stepwise depressurization method adversely impacted the exploitation efficiency. Prolonged constant-pressure intervals reduced the efficiency when there were few pressure-drop stages; however, the opposite trend emerged for a high number of depressurization steps. This study provides guidance for optimizing the production efficiency of hydrate exploitation via depressurization.

Gas permeability variation during methane hydrate dissociation by depressurization in marine sediments

Accurate acquisition of permeability change in hydrate-bearing sediments (HBSs) is the key to predicting reservoirs' multiphase fluids distribution and production capacity. Hydrate saturation and effective stress present significant effects on reservoir permeability. Thus, the influence of hydrate saturation and effective stress on gas permeability change of remolded core during decompression was studied. The results showed that effective stress compression and hydrate decomposition determine the flow channel structure within hydrate sediments. Under unconsolidated and low effective stress(σ' = 1.2 MPa) conditions, the gas permeability increased gradually as the decomposition of hydrate, and the magnitude of permeability variation before and after decomposition increased with increasing hydrate saturation, indicating that the effect of hydrate saturation on permeability dynamic change dominated at low effective stresses. When σ′ was increased to 3.2 MPa and 5.2 MPa, the results were contrary, indicating that the effective stress variation on gas permeability dynamic change dominated at higher effective stresses. In addition, it was found that compression led to higher heat transfer efficiency in high-saturation samples. When hydrate saturation was increased from 23.37% to 48.7%, the effect of increasing effective stress on gas production changed from inhibition to facilitation.

Numerical simulation on the evolution of physical and mechanical characteristics of natural gas hydrate reservoir during depressurization production

Changes in the physical and mechanical characteristics of the natural gas hydrate reservoir during depressurization production can affect safe and efficient production. In order to reveal the evolution law of reservoir physical and mechanical characteristics, based on the establishment of thermo-hydro-mechanical-chemical (THMC) multi-field coupling theoretical model, taking SH2 drilling platform in Shenhu sea area of the South China Sea as an example, COMSOL multiphysics is used to simulate the processes of depressurization production with a single horizontal well. The results show that, after the bottom hole pressure began to decrease, the gas and water production rates immediately increased from zero to their respective peaks, and then decreased rapidly. The decomposition of hydrate is an endothermic process. The changes of temperature and pressure conditions have a significant impact on the decomposition of hydrate. Effective stress and Mises stress appear to be concentrated in the area of complete hydrate decomposition. Mises stress rises sharply at the location of the leading edge of decomposition, which needs to be alert to the risk of landslide. In the process of depressurization production, the top of the reservoir gradually appears settlement behavior. The upper area of the horizontal well has a large amount of subsidence, and reservoir modification can be implemented during production to improve the mechanical stability of the reservoir. The results are an important guide to achieve stable and continuous gas production.

Phase change material microcapsules for smart temperature regulation of drilling fluids for gas hydrate reservoirs

In natural gas hydrate drilling operations, hydrate decomposition can lead to serious wellbore instability. In this study, the use of encapsulated phase change materials (PCMs) is proposed to smartly regulate the temperature of drilling fluids to inhibit hydrate decomposition. PCM microcapsules containing tetradecane in the core and a melamine-urea-formaldehyde resin as the shell were prepared by in-situ polymerization. The structure and properties of the PCM microcapsules were characterized, the temperature-regulating performance of the microcapsule and its effect on borehole stability were analyzed, and the compatibility was evaluated. The results showed that the melting point and latent heat of the PCM microcapsules were 6.7 °C and 116 J/g, respectively. The surface of microcapsules was hydrophilic and negatively charged, and the shell had good shear strength and sealability. PCM microcapsules effectively reduced the heating/cooling rate and the highest temperature of aqueous suspensions, thus decreasing the borehole enlargement rate. The temperature-regulating performance was unchanged after 15 heating/cooling cycles. After adding 5–10 wt% microcapsules, the rheological and filtration properties of the water-based drilling fluid remained in a reasonable range, indicating that the PCM microcapsules were compatible with drilling fluids. This study provides a new method for inhibiting hydrate decomposition in gas hydrate drilling.

Decomposition characteristics of methane hydrate in porous media under continuous and intermittent microwave heating

Microwave stimulation is a new method for natural gas hydrate exploitation. In this study, methane hydrate was synthesized in quartz sand with a particle size of 106–150 μm, and the decomposition characteristics were investigated using continuous and intermittent microwave heating. During the initial stage of continuous microwave heating, methane hydrate decomposed immediately due to microwave stimulation. Subsequently, due to a decrease in microwave penetration depth, microwaves did not affect the areas further away from the microwave source and macroscopic heat transfer became the main heat source for hydrate decomposition. The microwave power used during the study ranged from 200 W to 500 W. The results indicated that hydrate decomposition time decreased as power increased, and the average gas production rate increased at the same time. However, the energy efficiency ratio did not vary with the microwave power. The maximum energy efficiency ratio was 1.320 at a microwave power of 400 W under continuous microwave heating. Compared with continuous microwave heating, intermittent microwave heating improved the efficiency ratio by 59.32 %, with a heating cycle involving microwave stimulation for 5 min (Microwave ON time) and no stimulation for 1 min (Microwave OFF period). As microwaves provide heat for hydrate decomposition, a high-speed gas production can be obtained. Heat reservoirs were utilized to maintain hydrate decomposition during the OFF period, which also resulted in energy saving. Although intermittent microwave heating reduced the average gas production rate slightly, it improved the energy efficiency significantly. Therefore, the number of heating cycles and microwave heating periods should be carefully considered to obtain high hydrate exploitation efficiency. Intermittent microwave heating is recommended for hydrate exploitation to improve extraction efficiency and save energy.

Trigger Mechanisms of Gas Hydrate Decomposition, Methane Emissions, and Glacier Breakups in Polar Regions as a Result of Tectonic Wave Deformation

Trigger mechanisms are proposed for gas hydrate decomposition, methane emissions, and glacier collapse in polar regions. These mechanisms are due to tectonic deformation waves in the lithosphere–asthenosphere system, caused by large earthquakes in subduction zones, located near the polar regions: the Aleutian arc, closest to the Arctic, and the Antarctica–Chilean and Tonga–Kermadec–Macquarie subduction zones. Disturbances of the lithosphere are transmitted over long distances (of the order of 2000–3000 km and more) at a speed of about 100 km/year. Additional stresses associated with them come to the Arctic and Antarctica several decades after the occurrence of seismic events. On the Arctic shelf, additional stresses destroy the microstructure of metastable gas hydrates located in frozen rocks at shallow depths, releasing the methane trapped in them and leading to filtration and emissions. In West Antarctica, these wave stresses lead to decreases in the adhesions of the covered glaciers with underlying bedrock, sharp accelerations of their sliding into the sea, and fault occurrences, reducing pressure on the underlying rocks containing gas hydrates, which leads to their decomposition and methane emissions.

Study on Gas Hydrate Reservoir Reconstruction for Enhanced Gas Production in the Shenhu Area of the South China Sea Based on a 3D Heterogeneous Geological Model

Natural gas hydrate (NGH) has the huge exploitation potential for clean energy in the current world energy pattern. Its total organic carbon storage is about twice the sum of carbon content of oil, gas, and coal and hence attracts the attention of the world. China has carried out some gas hydrate exploitation works and delineation of three gas hydrate deposits whose reserve scale is over 100 billion cubic meters, and the NGH resource is abundant. The NGH reservoir in the South China Sea is of low porosity and permeability, is unconsolidated, has poor cementation, and is easy to fragment in hydrate production. On the one hand, the condition of low porosity and permeability around the well would severely reduce the production effect; on the other hand, the hydrate decomposition would more easily bring wellbore instability caused by stratum stress change. Therefore, to achieve safe and efficient exploitation of NGH, proper reservoir reconstruct technology needs to be chosen for enhancing the reservoir’s permeability and stability. This paper chose the Shenhu area of South China Sea as the research area, constructed the three-dimensional heterogeneous geological model of hydrate-bearing sediments which can be approximated realistically to depict, then used high-pressure jet grouting (HPJG) technology to reconstruct the hydrate reservoir, optimized the grouting hole direction, location, and spacing, and assessed the productivity increasing effect. The research results show the following: (a) Because of the higher hydrate saturation (SH) and lower permeability (k) around the well, the HPJG in the vertical direction was profitable for reservoir productivity. (b) Considering the heterogeneous SH and k in the horizontal direction, the HPJG in the front of the horizontal well was profitable for the reservoir productivity. (c) The HPJG spacing needs to be properly designed in combination with the absolute standard (VP), the relative standard (RGM), and the specific production index (J). (d) Under the conditions of this paper setting, the optimal HPJG reconstruction scheme is as follows: the HPJG in the vertical direction was arranged at the front of the horizontal well, and the spacing between the holes was 10 m. The productivity after reconstruction ranged up to 55.69% compared with before.

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.

Role of different types of water in bentonite clay on hydrate formation and decomposition

The equilibrium and kinetic of hydrate in sediments can be affected by the presence of external components like bentonite with a relatively large surface area. To investigate the hydrate formation and decomposition behaviors in bentonite clay, the experiments of methane hydrate formation and decomposition using the multi-step decomposition method in bentonite with different water contents of 20%, 40% and 60% (mass) were carried out. The contents of bound, capillary and gravity water in bentonite clay and their roles during hydrate formation and decomposition were analyzed. In bentonite with water content of 20% (mass), the hydrate formation rate keeps fast during the whole formation process, and the final gas consumption under different initial formation pressures is similar. In bentonite with the water contents of 40% and 60% (mass), the hydrate formation rate declines significantly at the later stage of the hydrate formation. The final gas consumption of bentonite with the water contents of 40% and 60% (mass) is significantly higher than that with the water content of 20% (mass). During the decomposition process, the stable pressure increases with the decrease of the water content. Hydrate mainly forms in free water in bentonite clay. In bentonite clay with the water contents of 20% and 40% (mass), the hydrate forms in capillary water. In bentonite clay with the water content of 60% (mass), the hydrate forms both in capillary water and gravity water. The bound water of dry bentonite clay is about 3.93% (mass) and the content of capillary water ranges from 42.37% to 48.21% (mass) of the dry bentonite clay.

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.

Influence of pipeline steel surface on the thermal stability of methane hydrate

The thermal stability and surface adhesion of natural gas hydrate are critical for the safety of oil and gas pipelines. The roughness and hydrophobicity of the pipe surface often vary during long-distance transportation, but it remains unclear about how these variances influence the hydrate stability. In this study, twelve molecular models of solid steel pipeline surfaces with random morphology were evaluated and molecular dynamics simulations were performed to gain insights into the kinetics of methane hydrate dissociation, the nucleation and growth of gas bubbles during hydrate decomposition, and the free energy of hydrate adhesion to the solid steel surface. Results demonstrated that the stability of methane hydrate could be decreased by up to 85% by increasing the hydrophobicity of the pipe surface by 52%. The bubble nucleation site of the gas released from hydrate decomposition shifted from bulk water to the solid surface by increasing the surface hydrophobicity (εsw: 3.73–5.74 kJ mol−1), but a highly hydrophobic surface (εsw: 2.73 kJ mol−1) made it hard to form gas bubble on either smooth or rough surface. Moreover, the free energy of hydrate adhesion also depended on the roughness and hydrophobicity of the solid surface, while the largest energy barrier for the adhesion of methane hydrate was found on the hydrophobic surface with high roughness. The findings from this study provided theoretical support for better understanding the methane hydrate evolution principles when the surface properties of the pipe wall changed from naturally occurred events (e.g., metal corrosion) or artificial treatment (e.g. chemical coating).

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.

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.