Hydrate Decomposition Research Articles

Evaluation of the sealing capacity of overlying fine-grained sediments in the process of hydrate exploitation by thermal stimulation with warm water

Although natural gas hydrate is taken a potential energy resource, public concerns over environmental risks associated with hydrate exploitation are also growing, especially possible methane leakage to seafloor. However, whether hydrate exploitation can trigger disastrous methane blowout remains not well addressed. Overlying fine-grained sediments with poor seepage capacity are typically assumed to be good seals for methane. Yet, this view may be challenged when considering the geomechanical response of marine sediments, since sediment failure may provide new high-permeability pathways for methane flow. In this research, a new fracture module is supplemented into the Tough + Hydrate simulator to consider hydraulic fracturing of marine sediments. The modified simulator is employed to investigate methane behavior in the exploitation of hydrate reservoirs overlain by fine-grained sediments via the method of depressurization accompanied by warm water injection. Results suggest that overlying fine-grained sediments cannot necessarily seal methane during hydrate exploitation. The direct cause is that water injection elevates pore pressure above the critical stress of marine sediments and generates fractures. The fractured sediments provide pathways for the flow of mobile methane released from hydrate decomposition. Compared with control groups of simulations without the module executed, the results indicate that ignoring the fracturing process may cause significant underestimation of methane leakage when overlying fine-grained sediments exist. In addition, the results of simulations for the Shenhu area suggest that 0.3546–1.5432 Tg/yr may be leaked to the ocean with large-scale commercial hydrate exploitation in this area, contributing little to the global carbon budget.

Production Behavior of Hydrate-Bearing Sediments with Mixed Fracture- and Pore-Filling Hydrates

Most hydrate-bearing sediments worldwide exhibit mixed pore- and fracture-filling hydrates. Due to the high exploitation value, pore-filling hydrate production is the focus of current hydrate production research, and there is a lack of systematic research on the decomposition of fracture-filling hydrates and their effects on the evolution of temperature and pressure in hydrate-bearing sediments. If only the decomposition characteristics of pore-filling hydrates are studied while the fracture-filling hydrates decomposition and its effects on the hydrate-bearing sediments production process are ignored, the obtained research results would be inconsistent with the actual situation. Therefore, in this study, the effects of fracture-filling hydrates with different dipping angles on the hydrate production process were studied, and the necessity of considering the phenomenon of mixed pore- and fracture-filling hydrates in hydrate-bearing sediments was illustrated. On this basis, the simulation of a typical site (GMGS2-16) with mixed pore- and fracture-filling hydrates was constructed, and the production process was researched and optimized. The results indicated that: (a) fracture-filling hydrates formed in shallow fine-grained sediments and gradually approached the area of pore-filling hydrates, before a stable mixed zone was formed; (b) the occurrence of fracture-filling hydrates was conducive to the hydrate-bearing sediment depressurization production, and the promoting effect of the fracture-filling hydrate with smaller dipping angles was stronger; and (c) depressurization combined with heat injection could effectively compensate for the local low temperature and secondary hydrate caused by the mass decomposition of fracture-filled hydrates.

Experimental Investigation on Deformation and Permeability of Clayey–Silty Sediment during Hydrate Dissociation by Depressurization

The sediments in the South China sea are mainly composed of clayey silt, characterized by weak cementation, low strength, and poor permeability. These characteristics lead to slow gas and water transport and low gas production efficiency in the production process, which is not conducive to reservoir stability. Therefore, this paper studied the influence of different factors on the displacement and permeability of hydrate-bearing sediments by using remolded cores from the South China Sea. It was found that, when the depressurization method was used for hydrate decomposition, the displacement change in sediments could be divided into three stages: depressurization stage, decomposition stage, and creep stage. During the decompression stage, sediment deformation was rapid and displacement was small. During the decomposition process of hydrates, sediment deformation was slow and displacement was maximum. The creep stage had the slowest deformation and the smallest displacement. The displacement increased with the increase in initial porosity, hydrate saturation, effective pressure, and depressurization amplitude. The permeability of the sediments was lower than that of the original sediments after hydrate decomposition. This permeability damage increased with the increase in the sediment porosity, hydrate saturation, depressurization range and effective pressure. Furthermore, the displacement of sediments was positively correlated with the permeability damage.

Influence of cold-seep environments on the kinetics of methane hydrate formation

Methane hydrate (MH) is widely distributed in active deep-sea cold-seep areas. The complex environment of cold seeps influences MH formation. Investigating the effects of cold-seep environments on MH formation can provide an in-depth understanding of the formation mechanism of MH and kinetic process of cold-seep development and evolution. In this study, multiple MH formation experiments were conducted in the laboratory to investigate the effects of a cold-seep environment on the kinetics of hydrate formation. Considering the cold-seep fluid and bottom seawater as the samples, MH was formed in the capillary under the same pressure-temperature conditions as those in cold-seep environments. The entire process of MH formation was recorded by time-series Raman spectra and video, and the results demonstrated the promotion of MH formation by solid particles, especially authigenic carbonates. Compared to 20 min in seawater, the induction time of 2 min in cold-seep fluids indicated promotion of hydrate formation by tiny particles and by its lower salinity. Notably, the reduction in salinity in the cold-seep fluid may be attributed to the dilution effect of pure water produced by hydrate decomposition, indicating the presence of a hydrate memory effect. Overall, our results are in agreement with those of the in situ hydrate formation experiment in a cold-seep environment. A model of hydrate formation in a cold-seep environment has been proposed, which can aid in understanding the kinetics of hydrate formation.

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.

Dual-gas co-production behavior for hydrate-bearing coarse sediment with underlying gas via depressurization under constrained conditions

Hydrate reservoirs with underlying gas show significant potential for commercial exploitation. An experimental apparatus capable of applying confining pressure and axial pressure was built to simulate the accumulation and dissociation of naturally occurring marine hydrate and a novel method for synthesizing hydrate reservoirs with underlying gas was proposed. With the formed methane hydrate-bearing coarse sediment (HBCS) with underlying gas, hydrate dissociation experiments via depressurization under constrained conditions were carried out at different HBCS proportions. The characteristics of heat transfer and dual-gas co-production during the exploitation of hydrate reservoir at various HBCS proportions were presented. Due to the influence of hydrate decomposition, the diverging temperature variations of the underlying layer and upper HBCS layer were observed at HBCS proportions of 0.21, 0.48, and 0.77. The negative effect of the outside pressure on gas production was not observed in our experimental conditions owing to the adopted coarse quartz sand deposits. The final produced gas volume increases with the HBCS proportion increasing. The produced gas percentage is negative or positive related to the HBCS proportions respectively with the extended production time. In addition, the different production behavior of the underlying gas and hydrate decomposed gas may induce the unexpected exploitation accidents such as blowouts resulting from unstable gas production rate. These findings guide the design of exploitation scheme for reservoirs with different HBCS distribution. And an exploitation method by setting the production well in the underlying gas layer/underlying sediment layer with free gas is then proposed, which can inhibit hydrate reformation and ice generation and improve the production well stability during the hydrate exploitation process.

Spatial-temporal evolution of reservoir effective stress during marine hydrate depressurization production

In tests of marine hydrate depressurization production in many countries, accidents such as collapse and failure of boreholes have occurred, which caused by the evolution of the effective stress field have motivated further attention. However, in existing effective stress models, the pressure of the fluid in the wellbore is taken as the boundary condition, which did not account for the effects of the wellbore on the effective stress distribution. This paper is aimed at proposing a methodology to research the spatio-temporal evolution of the effective stress during the hydrate depressurization production, in order to provide a theoretical benchmark against which to analyze the borehole stability. We derived a mathematical model of the effective stress by taking into account the interaction between the wellbore and the reservoir on the basis of the theory of elasticity, and coupled with deposited infiltration using fluid-structure interaction theory, a coupled multiple field model considering the kinetic effects of hydrate decomposition was then established. And a solver was developed independently. A case study shows that the wellbore has significant effect on the effective stress within the radius of 0.5 m of the reservoir. At the borehole wall, the radial effective stress is 23.58 MPa, whereas the circumferential effective stress is 7.31 MPa, such nonuniformity of the effective stress lead to the risk of borehole failure. As the production progresses, the effective stress increases rapidly at first and then slowly. The effective stress is slightly affected by the size of the wellbore, it is found that when the outside diameter of the wellbore is increased from 100 mm to 500 mm, the change is less than 3.04%.

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.

Temperature response mechanism of methane hydrate decomposition coupled with icing and melting under variational thermodynamic conditions

High-efficient decomposition is the precondition of commercial production of methane hydrates. Under variational thermodynamic conditions, icing and melting may occur during hydrate decomposition, inducing complex heat and mass transfer. In this study, hydrates formed under temperature gradient of 0.012, 0.020 and 0.026 °C/mm in a 2 L reactor, decomposed by long depressurization to 1.0 MPa at constant exhaust rate from 0.11 to 1.07 ln/min. Temperature of hydrate-bearing area was in a specific relationship called thermodynamic non-equilibrium effect with pressure, controlled by hydrate decomposition. When pressure reached around 2.1–2.3 MPa, instant icing caused rapid temperature rise, and accelerated hydrate decomposition generating pressure rise of 2.36 MPa at most coordinated by limited exhaust rate. Isothermy stage occurred in double phase transition system with hydrate decomposition and ice melting, temperature slightly raised after most hydrates had decomposed. Temperature gradient only worked on non-hydrate/ice area because the temperature response of hydrate decomposition couple with icing and melting is first dominated by thermodynamics and then affected by heat and mass transfer. The control effect of ice melting in thermodynamic is stronger than hydrate decomposition. These results are significant for systematic analysis among gas production, hydrate decomposition, temperature gradient and ice behavior of actual hydrate exploitation.

Numerical Simulating the Influences of Hydrate Decomposition on Wellhead Stability

Natural gas hydrate reservoir has been identified as a new alternative energy resource which has characteristics of weak cementation, low reservoir strength and shallow overburden depth. Thus, the stability of subsea equipment and formation can be affected during the drilling process. To quantitatively assess the vertical displacement of the formation induced by hydrate decomposition and clearly identify the influence laws of various factors on wellhead stability, this study established a fully coupled thermo-hydro-mechanical-chemical (THMC) model by using ABAQUS software. The important factor that affects the wellhead stability is the decomposition range of hydrates. Based on this, the orthogonal experimental design method was utilized to analyze the influence laws of some factors on wellhead stability, including the thickness of hydrate formation, initial hydrate saturation, overburden depth of hydrate sediment, and mudline temperature. The results revealed that the decomposition of hydrate weakens the mechanical properties of the hydrate formation, thus leading to the compression of the hydrate formation, further causing the wellhead subsidence. When the duration of drilling operations was 24 h and no decomposition of natural gas hydrate occurs, the wellhead subsidence is recorded at 0.053 m, this value increases with an increase in drilling fluid temperature. The factors were listed in descending order as following, according to their significance of influences on wellhead stability: the thickness of hydrate formation, initial hydrate saturation, overburden depth of hydrate sediment, and mudline temperature. Among the above factors, statistical significance of the mudline temperature was less than 15% confidence level, suggesting that the effect of mudline temperature on wellhead stability is negligible. These findings not only confirm the influence of hydrate decomposition on wellhead stability, but also suggest important implications for the drilling of hydrate-bearing formation.

Experimental and model study on the weakening of gas hydrate-bearing sand considering dissociation

Hydrate decomposition changes the mechanical behavior of reservoirs, which threatens the safety of mining wells. Therefore, this study determines the weakening behavior of gas hydrate-bearing sands by performing triaxial shear tests considering decomposition based on drilling data from the Shenhu Sea. The results indicate that gas hydrate-bearing sands, both before and after hydrate decomposition, show a strain hardening trend, and the strength and cohesion increase with hydrate saturation. The dilatancy of the nondissociated specimen increases with hydrate saturation, while that of the dissociated specimen decreases. Moreover, hydrate decomposition causes the strength, cohesion and dilatancy of the specimen to decrease, exhibiting significant weakening behavior. Furthermore, a statistical damage constitutive model is proposed by considering that the microunit strength obeys the Drucker‒Prager criterion and Weibull distribution, and the model well describes the deformation and damage of gas hydrate-bearing sands due to decomposition.

Molecular simulation study of methane hydrate decomposition in the presence of hydrophilic and hydrophobic solid surfaces

Natural gas hydrates (NGHs) exist mainly in marine muddy porous media, and it is of great significance to understand the hydrate decomposition in microporous sediments for hydrate exploration. In the paper, molecular dynamics (MD) simulations have been performed for CH4 hydrate decomposition in the slit nanopores of graphite and hydroxylated-silica surfaces. As expected, the higher the temperature, the faster the decomposition rate. Compared with the hydrate decomposition in the free system, the solid surface can enhance hydrate decomposition; the hydroxylated silica surface has better enhancement effect, and this effect is more obvious at lower temperatures. At 330 K, 340 K, 350 K and 360 K, the decomposition rates of hydrate in HS-H systems are 50.8%, 36.4%, 14.7% and 11.7%, respectively, which are higher than those in G-H systems. In the slit-nanopores, hydrate decomposition starts from the surface and then grows towards the bulk region, the hydrate near the pore surface decomposes faster. However, due to the strong ability of the hydrophilic surface to adsorb water molecules, the hydrophilic-silica pore has a more vital role in enhancing hydrate decomposition near the surface in the early stage. The higher the temperature, the stronger the diffusion ability of methane molecules and water molecules, and methane molecules diffuse more easily in pores in directions parallel to the surface than perpendicular to it. Due to the difference in wettability, the contact angles of nanobubbles on the surface are different, resulting in different forms of nanobubbles. For the G-H system, both surface cap bubbles and bulk spherical bubbles were formed at higher temperatures, while only surface cap bubbles were formed at lower temperatures. For the HS-H system, bulk spherical bubbles were formed over the temperature range studied. The formation conditions of the bulk spherical bubbles are more lenient than surface cap bubbles. The hydrates in the graphene pores proceed in an isotropic and uniform decomposition. At 330 K, the liquid methane concentration of HS-H system (0.03181–0.03397) is higher than that of G-H system (0.02452–0.02559) at t = 4–6 ns, while the decomposition rate is higher than that of G-H system, which is due to the formation of bulk spherical bubbles. Bulk spherical bubbles in the hydroxylated-silica pores leads to the uneven decomposition of the hydrates, this inhomogeneity leads to local differences in hydrate dissociation rate, and the difference in hydrate dissociation rates in different regions can be up to 4.3 times. The above findings provide the theoretical basis for the safe and efficient exploitation and transportation of hydrate in marine sediments.

Probing the Effects of Ultrasound-Generated Nanobubbles on Hydrate Nucleation: Implications for the Memory Effect

Understanding the microscopic hydrate phase transition process is of crucial importance to the application of hydrate-based techniques. This process is nevertheless significantly limited by the insufficient enrichment of hydrophobic gas molecules in the solution. Here, in this work, ultrasonic technology was applied to generate stable and large amount of nanobubbles in pure water to promote hydrate formation kinetics. The particle size distribution of the nanobubbles was following the log-normal distribution, with a concentration of ∼109 mL–1 and a diameter concentrated in 100–120 nm. The concentration of the nanobubbles increased with the ultrasonic duration and power. Notably, the concentrations of nanobubbles produced by hydrate decomposition were slightly higher with the bubble size concentrated around 200 nm. The effect of nanobubbles on the induction time of hydrate formation was further analyzed by forming from the nanobubble solution with different concentrations and particle size distributions. Hydrate formation induction time was reduced by up to 61.13%. We further analyzed the mechanism of the nanobubble dominant hydrate memory effect. The results proved ultrasound as an approach to enhance the solubility of gases in water and thus promote hydrate formation kinetics, providing evidence for the dominant role of nanobubbles in the memory effect.

Analysis of Influencing Factors and Kinetic Characteristics of Spherical Methane Hydrate Decomposition

Herein, several molecular systems are simulated by molecular dynamics to study the decomposition process and fluctuation-dissipation characteristics of spherical methane hydrates under different conditions. The spherical radius and the movement of the hydrate-liquid water interface during decomposition are measured. Different fitted formulas of the variation of methane numbers are obtained from the decomposition of spherical and bulk methane hydrates. Fluctuation-dissipation characteristics for spherical methane hydrates with different radii are analyzed, which show that increasing the scale of hydrates can increase the relaxation time and slow down the fluctuation process. The variations of the hydrogen bond and hydrogen-bond lifetime are calculated. For hydrate phase water, the peak of the hydrogen-bond lifetime lies between 8 and 10 ps. After complete decomposition, the hydrogen-bond lifetime mainly distributes in 0 and 2 ps and the peak disappears. The effects of temperature, cage occupancy, liquid phase environment, and spherical hydrate scale are explored. The decomposition activation energy for the spherical hydrate with a radius of 20 Å is calculated to be 52.23 kJ/mol. It can speed up the decomposition rate as well as the diffusion of methane and water molecules with a lower cage occupancy. For the effect of the liquid phase environment, it is found that the number of liquid water rarely affects the decomposition. However, when the Na+ and Cl- concentrations change from 0 to 10%, the decomposition time reduces from ∼511 to ∼369 ps, which indicates that there is an obviously positive impact on decomposition.

Effects of different concentrations of methanol on the decomposition of methane hydrate: insights from molecular dynamics simulations

As one of the most promising alternative energy, natural gas hydrate has aroused much attention owing to its controllable exploitation with the aid of chemical injections. Although alcohols are commonly used as promoters for hydrate decomposition, few studies have been carried out to investigate the effect of different concentrations of methanol on methane hydrate decomposition at a molecular level. For the first time, we systematically combined the parameters of angular order parameter (AOP), radial distribution function (RDF), and mean square displacement (MSD) to reveal the role of methanol in the decomposition mechanism of methane hydrates through the molecular dynamics approach. The results demonstrated that methanol concentration had an optimal value for hydrate decomposition, which was 20% methanol solution for the best efficiency for the hydrate decomposition and had been examined by analyzing the parameter of AOP. This can be attributed to the promotion effect of methanol on the deconstruction of the hydrogen bond networks of water molecules and the inhibition effect of methanol on the mass transfer effect of methane diffusion, both of which determined the decomposition of methane hydrates. These effects were further quantitatively evaluated by the structural parameters of RDF and MSD. The obtained results reveal the important influence of methanol concentrations on methane hydrates decomposition on molecular scale, and provide useful guidelines for highly efficient exploitation of natural gas hydrate reservoir.

Monitoring method of phase transition interface for natural gas hydrate based on thermoacoustic effect

In the drilling and exploitation for natural gas hydrate, the decomposition of combustible ices creates the interface between their solid state and liquid state, which would play an important role in the dynamic monitoring while exploitation. In this paper, a monitoring method based on the thermoacoustic effect is proposed to detect the position of the interface. The mechanism of the thermoacoustic coupling is introduced into the natural gas hydrate, that is, the physical model of thermoacoustic effect is established and detailed when the hydrate is excited by a strong electrical pulse. Two kinds of acoustic sources, the thermoelastic source and the phase transition acoustic source, are proposed to explain the obviously enhanced acoustic signals. By modeling and simulating, it is verified that the acoustic signals can be generated by the phase transition around the interface in hydrate. At the same time, many influencing factors on the thermoacoustic signals are researched based on simulations, such as the size of hydrate, the latent heat absorption coefficient and the amplitude of the transient electrical pulse. Finally, the experimental system both for hydrate generation and for thermoacoustic monitoring is built. We prepared carbon dioxide hydrate in a high-pressure reactor at an initial pressure of 5 MPa and temperature of −5 °C. Then take the hydrate into the environment of room temperature and standard pressure, apply the transient electrical pulse to the hydrate and use the ultrasonic probe to detect the acoustic signals. The results show that acoustic signals emitted from the position of the solid-liquid interface can be employed to monitor the hydrate decomposition experimentally and actually.

Increasing Gas Production of Continuous Freeze-Thaw Hydrate Deposits Based on Local Reservoir Reconstruction

The fracturing technology for increasing gas production based on hydrate reservoir reconstruction has been proven to have the application prospect of hydrate production. The main content of this work is to study the influence mechanism between multiple production wells after fracturing the reservoirs around horizontal wells. Simulation results indicated that the gas production rates and cumulative volume of released gas by using hydraulic fracturing were both higher than that by using the traditional depressurization method. The average gas-to-water ratio in this work has broken through the technical bottleneck of exploiting hydrate only by the traditional depressurization method. After the rock matrix around the horizontal well was fractured, the permeability of the reservoir in the fracturing area would be increased; thus, the propagation distance of pressure drop would be promoted. In addition, the penetration of the hydrate decomposition area between horizontal production wells was conducive to promoting the flow of decomposed gas and water to production wells. Besides, the hydraulic fracturing method to increase the permeability of the reservoir around the production well is very effective for the gas production of low-permeability natural hydrate deposits. The best distance between production wells was recommended to be 45~60 m, and it was recommended to provide appropriate heat in the area between the two wells to accelerate the decomposition of hydrate.

Effect of clay on methane hydrate formation and dissociation in sediment: Implications for energy recovery from clayey-sandy hydrate reservoirs

Natural gas hydrate (NGH) is an unconventional energy source with high energy density, huge reserves and worldwide distribution. Sand-dominated hydrate-bearing sediments (HBS) are believed to be the most economically and technically feasible category of HBS for exploitation. Clay is one major mineral component of the natural HBS apart from sands. Due to the inhibition effect of clay on thermodynamics and kinetics of hydrate formation, it is challenging to synthesize representative clayey-sandy HBS samples in laboratory, and thus the corresponding fluid production behavior remains poorly understood. In this study, a sample synthesis method dedicated to clayey-sandy HBS (20 wt% clay) was developed. Two synergistic strategies including pre-wetting and multiple-site water injection were employed to enhance hydrate formation kinetics, increasing the final hydrate saturation from 0.285 to 0.473. Water-rich clayey-sandy HBS samples were formed with no dry sediment zones and improved hydrate distribution homogeneity. This optimized HBS synthesis method fills the gap between the conventional HBS synthesis methods and the need to form more representative clayey-sandy HBS samples. Energy recovery and fluid production from sandy and clayey-sandy HBS samples were investigated by depressurizing to 3.0 MPa at 6.0 °C. Unique features were observed for hydrate dissociation in clayey-sandy HBS: immediate hydrate decomposition upon depressurization and a great pressure difference (∼1.0 MPa) between sediment and wellbore during the production period. This pressure gradient, likely caused by the aggregation and clogging of clays near the wellbore, slowed hydrate decomposition compared to sandy HBS. Additionally, the gas recovery increased from 81.7% to 88.5%, whereas the water recovery substantially decreased from 25.3% to 8.1%. The significant difference in fluid production behavior between clayey-sandy HBS and sandy HBS calls for more thorough investigations into clay-containing HBS to develop specific production strategies for energy recovery from this type of HBS.

Influences of pore fluid on gas production from hydrate-bearing reservoir by depressurization

In addition to the temperature and pressure conditions, the pore fluid composition and migration behavior are also crucial to control hydrate decomposition in the exploitation process. In this work, to investigate the effects of these factors, a series of depressurization experiments were carried out in a visible one-dimensional reactor, using hydrate reservoir samples with water saturations ranging from 20% to 65%. The results showed a linear relationship between gas production rates and gas saturations of the reservoir, suggesting that a larger gas-phase space was conducive to hydrate decomposition and gas outflow. Therefore, the rapid water production in the early stage of hydrate exploitation could release more gas-phase space in the water-rich reservoir, which in turn improved the gas production efficiency. Meanwhile, the spatiotemporal evolution of pore fluids could lead to partial accelerated decomposition or secondary formation of hydrates. In the unsealed reservoir, the peripheral water infiltration kept reservoir at a high water saturation, which hindered the overall production process and caused higher water production. Importantly, depressurization assisted with the N2 sweep could displace the pore water rapidly. According to the results, it is recommended that using the short-term N2 sweep as an auxiliary means in the early stage of depressurization to expand the gas-phase space in order to achieve the highest production efficiency.

Stability and structure of multiply occupied sII CO2 clathrate hydrates: A possibility for carbon capturing

The hydrate method is an important technology that has been widely studied for carbon dioxide (CO2) capture, utilization and storage (CCUS). Improving the gas reserves per unit volume is the key goal for this technology. Therefore, the occurrence behaviors of large CO2 hydrate cages, the structure II, occupied with 1–5 gas molecules were investigated via molecular dynamics (MD) in this study. The occupancy rate, stability and decomposition laws of CO2 hydrate were systematically analyzed. The results indicate that the large hydrate cages occupied with 4–5 molecules were directly broken due to excessively large intermolecular repulsion. In contrast, the large hydrate cages occupied with 3 molecules showed an obvious four-rupture-stage: (1) a small part of cages rupturing; (2) metastable cages formation along with the escape of CO2 molecules, and the cages do not continue to break until all the CO2 molecules escape; (3) pure residual cages rupturing, with no CO2 gas existing in cages in this stage; and (4) complete decomposition of the CO2 hydrate. It is interesting that the large hydrate cages occupied with 2 gas molecules are stable, and 2 is the maximum occupancy number. The pressure is more important for improving the stability of the double-occupancy hydrate, and a higher pressure (20 MPa) strengthens the stability of the CO2 hydrate structure. The findings in this study indicate that the sII CO2 hydrate under sufficiently high pressure can maximally increase the storage efficiency by 36.5 wt%.