Investigation of Miocene Methane Hydrate Generation Potential in the Transylvanian Basin, Romania

In geology we often revise theoretical models;upon finding new evidence,such as the discovery of methane hydrates,the initial model will be challenged immediately.Hereby the authors put forward two postulates:1)There is a third,previously unexplored source of methane in the Transylvanian Basin,based on a new theoretical approach on methane hydrate formation;2)The dissociation of methane hydrates creates a strong chlorinity anomaly.Based on a recent analogy with the Black Sea basin model,we apply our statements to the Transylvanian Basin.Using direct and indirect indicators and the published system tract analysis,we claim that there are substantial grounds to believe that this model of methane hydrate formation applies to the Miocene Transylvanian Basin.Due to the increase of the geothermal gradient as a result of the volcanic activity from the Eastern Carpathians,the clathrates dissociated into methane and freshwater.This process of dilution resulted in a chlorinity anomaly that can be spotted in the formation waters of several gas fields from the Transylvanian Basin.

Experimental study on characteristics of pore water conversion during methane hydrates formation in unsaturated sand

Understanding the pore water conversion characteristics during hydrate formation in porous media is important to study the accumulation mechanism of marine gas hydrate.In this study,low-field NMR was used to study the pore water conversion characteristics during methane hydrate formation in unsaturated sand samples.Results show that the signal intensity of T_(2) distribution isn’t affected by sediment type and pore pressure,but is affected by temperature.The increase in the pressure of hydrogen-containing gas can cause the increase in the signal intensity of T_(2) distribution.The heterogeneity of pore structure is aggravated due to the hydrate formation in porous media.The water conversion rate fluctuates during the hydrate formation.The sand size affects the water conversion ratio and rate by affecting the specific surface of sand in unsaturated porous media.For the fine sand sample,the large specific surface causes a large gas-water contact area resulting in a higher water conversion rate,but causes a large water-sand contact area resulting in a low water conversion ratio(C_(w)=96.2%).The clay can reduce the water conversion rate and ratio,especially montmorillonite(C_(w)=95.8%).The crystal layer of montmorillonite affects the pore water conversion characteristics by hindering the conversion of interlayer water.

Experimental and modeling study of the kinetics of methane hydrate formation and dissociation

In this work,several experiments were conducted at isobaric and isothermal condition in a CSTR reactor to study the kinetics of methane hydrate formation and dissociation.Experiments were performed at five temperatures and three pressure levels(corresponding to equilibrium pressure).Methane hydrate formation and dissociation rates were modeled using mass transfer limited kinetic models and mass transfer coefficients for both formation and dissociation were calculated.Comparison of results,shows that mass transfer coefficients for methane hydrate dissociation are one order greater than formation conditions.Mass transfer coefficients were correlated by polynomials as relations of pressure and temperature.The results and the method can be applied for prediction of methane production from naturally occurring methane hydrate deposits.

Formation mechanism,experimental method,and property characterization of graindisplacing methane hydrates in marine sediment:A review

Grain-displacing hydrate deposits exist at many marine sites,which constitute an important part of methane hydrate resources worldwide.Attributed to the difficulties in acquiring field data and synthesizing experimental samples,the formation and property characterization of grain-displacing hydrate remains less understood and characterized than the pore-filling hydrate in current literature.This study reviews the formation mechanisms of grain-displacing hydrate from the perspective of geological accumulation and microscale sedimentary property.The experimental methods of synthesizing grain-displacing hydrate in the laboratory and the current knowledge on the property of grain-displacing hydrate sediment are also introduced.Shortcomings in current theories and suggestions for future study are proposed.The work is hoped to provide valuable insights for the research into the hydrate accumulation,geophysics,and hydrate exploitation targeted at the grain-displacing hydrate in the marine sediments.

A covering liquid method to intensify self-preservation effect for safety of methane hydrate storage and transportation

In this work,experiments and comprehensive insights into the proposed covering liquid method to intensify self-preservation effect for methane(CH_(4))storage are presented.The CH_(4)hydrate decomposition percentage was 17.6%with the pressure of 0.61 MPa after 12 h at 266.0 K without a covering liquid,which can be reduced to 12.4%,13.8%,13.0%,and 8.3%with the pressure of 0.26 MPa,0.33 MPa,0.51 MPa,and 0.37 MPa by covering with tetrahydrofuran(THF),cyclopentane(CP),cyclohexane,and n-tetradecane,respectively.When the temperature for CH_(4)hydrate decompositionwas 274.2 K,covering with THF,CP,cyclohexane,and n-tetradecane failed to inhibit CH_(4)hydrate decomposition.The results suggested that the covering liquid may form a new solid layer(a hydrate layer or other solidified layer)around the CH_(4)hydrate,which inhibit CH_(4)transfer below the freezing point of water.However,the new solid layer cannot resist the fast transfer of CH_(4)from decomposed CH_(4)hydrate above the freezing point of water.The same phenomenon was also observed in a sodium dodecyl sulfonate(SDS)-dry solution CH_(4)hydrate formation system.Therefore,the covering method can only intensify the self-preservation effect below the freezing point of water,but cannot generate a self-preservation effect.

Methane hydrate formation in porous media:Overview and perspectives

Natural gas hydrate(NGH)has recently received more attention as a cleaner alternative energy source that not only reduces carbon emissions caused by the use of conventional fossil fuels but also plays a key role in global climate change.Furthermore,hydrate-based technologies,particularly hydrate-based carbon capture and storage,have enormous promise for decreasing global carbon emissions,and porous media play an important role in all hydrate-based technologies.Accordingly,this paper reviews the recent applications of porous media in the field of methane hydrate(MH)formation and analyzes the influence of porous media systems on MH phase equilibria and formation kinetics.This is because the efficiency of hydrate-based technologies is determined mainly by the phase equilibrium and formation kinetics of hydrates.The influence of the nature of the media on MH formation in porous media systems is comprehensively summarized to understand how porous media can efficiently enhance the kinetics of hydrate formation.Promoters are necessary for rapid hydrate formation,and the effect of various promoters on MH formation was also evaluated.Based on the aforementioned overview and understanding,the mechanisms for MH formation in various porous media systems are proposed.Finally,the future perspectives and challenges of hydrate-based technologies in tackling global climate change were discussed.This review provides a fundamental understanding of the application and development of porous media in rapid hydrate formation,a fair evaluation of the performance of various porous media systems,and critical insights into major research foci.

Investigation of the methane hydrate surface area during depressurization-induced dissociation in hydrate-bearing porous media

The surface area of hydrate during dissociation in porous media is essentially important for the kinetics of hydrate dissociation.In this study,the methane hydrate surface area was investigated by the comparison results of experiments and numerical simulations during hydrate decomposition in porous media.The experiments of methane hydrate depressurizationinduced dissociation were performed in a 1D high pressure cell filled with glass beads,an improved and valid 1D corescale numerical model was developed to simulate gas production.Two conceptual models for hydrate dissociation surface area were proposed based on the morphology of hydrate in porous media,which formed the functional form of the hydrate dissociation surface area with porosity,hydrate saturation and the average radius of sand sediment particles.With the establishment of numerical model for depressurizationinduced hydrate dissociation in porous media,the cumulative gas productions were modeling and compared with the experimental data at the different hydrate saturations.The results indicated that the proposed prediction equations are valid for the hydrate dissociation surface area,and the graincoating surface area model performs well at lower hydrate saturation for hydrate dissociation simulation,whereas at higher hydrate saturation,the hydrate dissociation simulation from the porefilling surface area model is more reasonable.Finally,the sensitivity analysis showed that the hydrate dissociation surface area has a significant impact on the cumulative gas production.

Gas Production from Offshore Methane Hydrate Layer and Seabed Subsidence by Depressurization Method

Numerical simulations on consolidation effects have been carried out for gas production from offshore methane hydrates (MH) layers and subsidence at seafloor. MH dissociation is affected by not only MH equilibrium line but also consolidation (mechanical compaction) depended on depressurization in the MH reservoir. Firstly, to confirm present model on consolidation with effective stress, the history matching on gas production and consolidation has been done to the experimental results using with synthetic sand MH core presented by Sakamoto et al. (2009). In addition, the comparisons of numerical simulation results of present and Kurihara et al. (2009) were carried out to check applicability of present models for gas production from MH reservoir in field scale by depressurization method. The delays of pressure propagation in the MH reservoir and elapsed time at peak gas production rate were predicted by considering consolidation effects by depressurization method. Finally, seabed subsidence during gas production from MH reservoirs was numerically simulated. The maximum seabed subsidence has been predicted to be roughly 0.5 to 2 m after 50 days of gas production from MH reservoirs that elastic modulus is 400 to 100 MPa at MH saturation = 0.

Experimental investigation in the permeability of methane hydratebearing fine quartz sands

The permeability is one of the intrinsic parameters that determines the fluid flow in the porous media.The permeability in hydrate-bearing sediments affects the gas recovery and production of hydrate reservoirs significantly.The irregular permeability characteristics are challenging for fine-grained hydratebearing sediments.In this study,a series of experiments was conducted using an one-dimensional pressure vessel to investigate the hydrate formation characteristics and the permeability in hydratebearing fine quartz sands(volume weighted mean diameter was 36.695 mm).Hydrate saturations(0 e26%in volume)were controlled and calculated precisely based on the amount of injected water and gas,the system pressure and temperature.The results indicated that the hydrate nucleation induction period was completed during gas injection,and the average time of hydrate formation was within 500 min.The permeability of methane hydrate-bearing fine quartz sands was investigated by steady gas volume flow.For hydrate saturation lower than 13.94%,the hydrate mostly formed in grain-coating,the permeability reduction exponent calculated by Parallel Capillary,Kozeny Grain Coats and Simple Cubic Filling models were 2.00,2.10 and 1.74 respectively,and Simple Cubic Filling model was in accordance with the experimental data best.However,for hydrate saturation ranged from 13.94%to 25.91%,the permeability increased due to the flocculation structure formation of fine quartz sands and hydrate,which caused the increase of effective porosity.A new relationship among hydrate saturations,effective porosity,the ratio of permeability in the presence and the absence of hydrate was developed.This study developed the mathematical models for predicting the permeability with hydrate saturation in fine quartz sands,which could be valuable for understanding the characteristics of hydrate-bearing finegrained sediments.

Efficient promotion of methane hydrate formation and elimination of foam generation using fluorinated surfactants

Methane hydrate preparation is an effective method to store and transport methane.In promoters to facilitate methane hydrate formation,homogeneous surfactant solutions,sodium dodecyl sulfate(SDS)in particular,are more favorable than heterogeneous particles,thanks to their faster reaction rate,more storage capacity,and higher stability.Foaming,however,could not be avoided during hydrate dissociation with the presence of SDS.This paper investigated the ability of five fluorinated surfactants:potassium perfluorobutane sulfonate(PBS),potassium perfluorohexyl sulfonate(PHS),potassium perfluorooctane sulfonate(POS),ammonium perfluorooctane sulfonate(AOS),and tetraethylammonium perfluorooctyl sulfonate(TOS)to promote methane hydrate formation.It was found that both PBS and PHS achieve a storage capacity of 150(V/V,the volume of methane that can be stored by one volume of water)within 30 min,more than that of SDS.Cationic ions and the carbon chain length were then discussed on their effects during the formation.It was concluded that PBS,PHS,and POS produced no foam during hydrate dissociation,making them promising promoters in large-scale application.

Three-dimensional DEM investigation of the stress-dilatancy relation of grain-cementing type methane hydrate-bearing sediment

In this study,the Discrete Element Method(DEM)was employed to investigate numerically the effects of hydrate cementation and intermediate principal stress on the stress-dilatancy relation of graincementing type methane hydrate-bearing sediment(MHBS)by conducting a series of conventional and true triaxial tests.A novel 3D thermo-hydro-mechanical-chemical(THMC)contact model for MHBS was employed.The numerical results show that with increasing hydrate saturation and back pressure,or decreasing confining pressure,temperature and salinity,the stress-dilation relation of grain-cementing type MHBS evolves from dilation-dominant to bond-dominant.For the clean sand samples,the relationship between the normalized stress ratio h/Mcr and the dilatancy rate d is close under different intermediate principal stress coefficients.However,for the MHBS samples,this relationship is still affected by the intermediate principal stress coefficient b,due to the effect of hydrate cementation.

Turning gas hydrate nucleation with oxygen-containing groups on size-selected graphene oxide flakes

Gas hydrate is a promising alternative for gas capture and storage due to its high gas storage capacity achieved with only structured water molecules.Nucleation is the critical controlling step in gas hydrate formation.Adding an alien solid surface is an effective approach to regulate gas hydrate nucleation.However,how the solid surface compositions control the gas hydrate nucleation remains unclear.Benefiting from the fact that the surface compositions of graphene oxide(GO)can be finely tuned,we report the effect of functional groups of size-selected GO flakes on methane hydrate nucleation.The carbonyl and carboxyl of GO flakes showed a more prominent promotion for methane hydrate nucleation than the hydroxyl of GO flakes.Surface energy,zeta potential,Raman spectra,and molecular dynamics simulation analysis were used to reveal the regulation mechanism of the functional groups of size-selected GO flakes on methane hydrate nucleation.The GO flakes with abundant carbonyl and carboxyl exhibited higher charge density than those enriched in hydroxyl.The negatively charged GO flakes can induce water molecules to form an ordered hydrogen-bonded arrangement via charge-dipole interactions.Therefore,the water molecules surrounding the carboxyl and carbonyl showed a more ordered hydrogen-bonded structure than those around the hydroxyl of GO flakes.The ordered water arrangement,similar to methane hydrate cages,significantly accelerated methane hydrate nucleation.Our study shows how the surface chemistry of solids control gas hydrate nucleation and sheds light on the design of effective heterogeneous nucleators for gas hydrate.

Pore pressure built-up in hydrate-bearing sediments during phase transition: A poromechanical approach

Due to the density contrast between the hydrate and methane gas,the pore pressure is accumulated in the sediment during the decomposition process of methane hydrate.This accumulation of pore pressure decreases the magnitude of effective stress,further triggering potential geological disasters such as landslide.This paper establishes a theoretical framework to investigate the evolution of fluid pressure in the hydrate-bearing sediments during the decomposition process.This model consists of two parts:an unsaturated thermo-poromechanical constitutive law as well as a phase equilibrium equation.Compared with the existing studies,the present work incorporates the effect of pore volume change into the pressure built-up model.In addition,the capillary effect is considered,which plays a nontrivial role in fine-grained sediments.Based on this model,the evolution of fluid pressure is investigated in undrained conditions.It is shown that four mechanisms mainly contribute to the pressure built-up:the density contrast between decomposing hydrate and producing fluid,the variation of pore volume,the compaction of hydrate due to variation of capillary pressure,and the thermal deformation of pore constituents induced by temperature change.Among these mechanisms,the density contrast dominates the pore pressure accumulation.Under the combined effect of these contributions,the evolution of fluid pressure exhibits a strong nonlinearity during the decomposition process and can reach up to dozens of mega Pascal.Nevertheless,this high-level pressure built-up results in a significant tensile strain,yielding potential fracturing of the sediment.

Formation of nanobubbles generated by hydrate decomposition:A molecular dynamics study

Natural gas hydrate is estimated to have huge reserves. Its exploitation can solve the global oil and gas shortage problem. Hydrates decompose into water and methane, and methane molecules are supersaturated to form nanobubbles.Methane nanobubbles can affect the decomposition efficiency of hydrates. They can provide abundant methane sources for the re-nucleation of hydrates. Molecular dynamics is employed in this study to investigate the decomposition process of type I methane hydrate and the formation of methane nanobubbles generated during decomposition under different methane mole fraction, pressures, and temperatures. The results indicate that external pressure inhibits the diffusion of methane molecules, thereby preventing the formation of nanobubbles. A higher mole fraction of methane molecules in the system requires a higher external pressure to generate stable nanobubbles after the decomposition of the hydrate structure.At 330 K, it is easy to form a nanobubble structure. Results of this study can help provide ideas for the study of efficient extraction and secondary nucleation of hydrates.

Molecular Dynamics Simulation of Hydrate Decomposition Under Action of Alcohol Inhibitors

The molecular dynamics method is used to investigate decomposition of methane hydrate at different temperatures,pressures and concentrations of inhibitor.By analyzing the parameters of system conformation,mean square displacement and radial distribution function,the decomposition of hydrate in the presence of alcohol inhibitors ethylene glycol and glycerol is explored.The results show that the hydroxyl groups in alcohol molecules can destroy the cage structure of hydrate,and form hydrogen bonds with nearby water molecules to effectively prevent the reformation of hydrate.Therefore,ethylene glycol and glycerol serve as inhibitors of methane hydrate,furthermore,in terms of inhibition effect,glycerol is better than ethylene glycol by comparing rate of hydrate decomposition.

An Experimental Study on the Interaction between Hydrate Formation and Wax Precipitation in Waxy Oil-in-Water Emulsions

The coupled formation of wax crystals and hydrates is a critical issue for the safety of deep-sea oil and gas exploration and subsea transport pipeline flow.Therefore,this paper conducts an experimental study on the characteristics of methane hydrate formation in a water-in-oil(W/O)system with different wax crystal contents and explores the influence of different initial experimental pressures on the induction period and maximum rate of hydrate formation.The wavelet function was introduced to process the reaction rate and calculate the maximum speed of hydrate formation.Notably,the higher the pressure,the smaller the maximum rate of hydrate formation.We observed that wax crystal precipitation increases the viscosity of the emulsion,which limits the diffusion of gas in the liquid phase during hydrate nucleation and thus delays the hydrate nucleation.The methane gas precipitation also affects the remaining fraction’s wax content and therefore affects the wax precipitation.Secondary hydrate formation was observed several times during the experiment,increasing the risk of pipeline blockage.Overall,this work provides insights into the effect of wax crystal precipitation on hydrate behaviour that could facilitate flow assurance applications in subsea multiphase pipelines and inform the safe transportation of oil and gas pipelines.

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.

Velocity-porosity relationships in hydrate-bearing sediments measured from pressure cores,Shenhu Area,South China Sea

Evaluating velocity-porosity relationships of hydrate-bearing marine sediments is essential for characterizing natural gas hydrates below seafloor as either a potential energy resource or geohazards risks.Four sites had cored using pressure and non-pressure methods during the gas hydrates drilling project(GMGS4)expedition at Shenhu Area,north slope of the South China Sea.Sediments were cored above,below,and through the gas-hydrate-bearing zone guided with logging-while-drilling analysis results.Gamma density and P-wave velocity were measured in each pressure core before subsampling.Methane hydrates volumes in total 62 samples were calculated from the moles of excess methane collected during depressurization experiments.The concentration of methane hydrates ranged from 0.3%to 32.3%.The concentrations of pore fluid(25.44%to 68.82%)and sediments(23.63%to 54.28%)were calculated from the gamma density.The regression models of P-wave velocity were derived and compared with a global empirical equation derived from shallow,unconsolidated sediments data.The results were close to the global trend when the fluid concentration is larger than the critical porosity.It is concluded that the dominant factor of P-wave velocity in hydrate-bearing marine sediments is the presence of the hydrate.Methane hydrates can reduce the fluid concentration by discharging the pore fluid and occupying the original pore space of sediments after its formation.

Molecular simulation studies on natural gas hydrates nucleation and growth:A review

How natural gas hydrates nucleate and grow is a crucial scientific question.The research on it will help solve practical problems encountered in hydrate accumulation,development,and utilization of hydrate related technology.Due to its limitations on both spatial and temporal dimensions,experiment cannot fully explain this issue on a micro-scale.With the development of computer technology,molecular simulation has been widely used in the study of hydrate formation because it can observe the nucleation and growth process of hydrates at the molecular level.This review will assess the recent progresses in molecular dynamics simulation of hydrate nucleation and growth,as well as the enlightening significance of these developments in hydrate applications.At the same time,combined with the problems encountered in recent hydrate trial mining and applications,some potential directions for molecular simulation in the research of hydrate nucleation and growth are proposed,and the future of molecular simulation research on hydrate nucleation and growth is prospected.

Nuclear magnetic resonance study of the formation and dissociation process of nature gas hydrate in sandstone

In this work,the authors monitored the formation and dissociation process of methane hydrate in four different rock core samples through nuclear magnetic resonance(NMR)relaxation time(T_(2))and 2D imaging measurement.The result shows that the intensity of T_(2) spectra and magnetic resonance imaging(MRI)signals gradually decreases in the hydrate formation process,and at the same time,the T_(2) spectra move toward the left domain as the growth of hydrate in the pores of the sample accelerates the decay rate.The hydrate grows and dissociates preferentially in the purer sandstone samples with larger pore size and higher porosity.Significantly,for the sample with lower porosity and higher argillaceous content,the intensity of the T_(2) spectra also shows a trend of a great decrease in the hydrate formation process,which means that high-saturation gas hydrate can also be formed in the sample with higher argillaceous content.The changes in MRI of the sample in the process show that the formation and dissociation of methane hydrate can reshape the distribution of water in the pores.