Synthetic Hydrate Research Articles

Effects of depressurization on gas production and water performance from excess-gas and excess-water methane hydrate accumulations

• Various water saturations simulate Classes I and II hydrate deposits in nature. • Ambient temperature controls periphery dissociation in excess-gas hydrate deposits. • Sensible heat dominates spatially uniform dissociation in excess-water deposits. • Secondary hydrate formation are prevented by adjusted gas production pressure. • Optimized depressurization is crucial to improve water production and CH 4 recovery. Depressurization is considered as the most promising technique for hydrate exploitation, as it achieves the highest energy profit ratio and is the most technologically feasible. However, the exploitation of excess-water hydrate accumulations generates high water production, leading to increased cost, poor energy efficiency, and problems with sand during operation. Thus, water management is crucial to gas recovery by the depressurization of different classes of hydrate accumulations, yet relevant studies remain limited. In this study, synthetic hydrate samples were prepared to simulate two types of natural methane hydrate sediments: Class 1 accumulations (excess-gas hydrate) and Class 2 accumulations (excess-water hydrate). Hydrate dissociation was conducted using a variety of depressurization approaches, and MRI imaging was employed to characterize water performance and methane recovery. Methane hydrate preferentially dissociated along the peripheries of the excess-gas samples due to more efficient heat dissipation. Methane hydrate dissociated more uniformly in the excess-water samples because the high specific heat capacity of water enabled the supply of extra heat. Furthermore, pressure histories, mean intensity change in MRI images, and water variations were monitored to analyze the characteristics of hydrate dissociation, changes in porosity, intrinsic permeability and reservoir heat, water and gas production rates, and possible secondary hydrate formation. The results of this study suggest that an optimized depressurization approach, such as stepwise depressurization, could improve methane recovery from Class 1 and Class 2 methane hydrate accumulations.

Phase Diagrams and Spectral Characteristics of Hydrofluorocarbon Hydrates: Insights from First-Principles Thermodynamics

Understanding the phase equilibrium conditions of hydrofluorocarbon (HFC) hydrates is significant to reduce greenhouse gas emissions and synthetic hydrates in experiments. Herein, we construct the ...

Dissociation and transport modeling of methane hydrate in core-scale sandy sediments: A comparative study

Better understanding of the dissociation and transport mechanism of methane hydrate in sandy sediments contributes to the future commercial extraction of hydrate. This paper presents a comparative study on the dissociation and transport modeling of methane hydrate in core-scale sandy sediments. The mathematical model is established considering the heat and mass transfer in the hydrate dissociation and the migration process of gas-water flow. The permeability model of the core is modified based on the lab test on the Berea sandstone, which is more feasible and physical than those by historical matching. The theoretical model is verified by comparing with experiments. The following improvements are achieved: 1) The distribution of the synthetic hydrate within the core in the lab is not uniform even when assuming the core is homogeneous. 2) Better agreements are acquired between the simulation and experimental data using the simulated initial distribution of the hydrate, water and methane in Masuda’s experiment. 3) As the first effort in literature, the mini peak of the far-field pressure and the temperature fluctuation in Masuda’s experiments are investigated by simulation, which reveals the physical reason for the phenomena. Moreover, this paper provides new insight into the modeling validation using Masuda’s experiment for numerical simulation benchmarking.

Influence of the initial CH4-hydrate system properties on CO2 capture kinetics

Recovering methane from natural gas hydrate deposits using carbon dioxide injection is currently of great environmental and energetic interest as it shows potential for producing an energy resource while mitigating CO2 emissions through CO2 sequestration. This work investigates the exchange kinetic between CH4 and CO2 (or CO2-N2(v)) in synthetic hydrates, with an emphasis on the impact of CH4 hydrate formation conditions (e.g. driving force Δp) on the subsequent exchange reactions. Different driving forces Δp are utilized and show that the exchange kinetic is improved by a factor of ~3 when the exchange is performed with low Δp CH4 hydrates, for which there is a higher relative amount of free H2O(liq) (277 K); the kinetic is further improved when stirring is applied. Isobaric CH4 hydrates exhibit a fast primary hydrate dissociation and CH4 release, followed by a slower exchange kinetic, possibly limited by solid-state exchange diffusion or secondary CO2-rich hydrate formation within the stability field of CH4 hydrates. Upon exposure to a mixed CO2-N2(v) gas stream, secondary hydrate production is governed by the effective Δp remaining after dissolution of the gas mixture, and results in an even slower exchange reaction rate. These results may help optimizing recovery processes in field trial experiments, where both hydrates and liquid water coexist, and aid in predicting the risk of geo-hazards induced by unstable secondary hydrate formation. Furthermore, the exchange kinetic dependencies highlighted here are important as they affect the overall energy efficiency and energy cost of CH4 recovery processes in gas hydrate field trials.

Adsorption of synthetic and real Kinetic Hydrate Inhibitors (KHI) wastewaters on activated carbon: adsorption kinetics, isotherms, and optimized conditions

ABSTRACT The treatment of the KHI wastewater by using activated carbon was investigated. Taguchi method was applied to study the influences of temperature, KHI initial concentration, adsorbent dosage, and pH. The ANOVA results confirmed that initial concentration was the most significant factor and increasing the pH and adsorbent dosage led to an enhancement in the KHI removal. The highest KHI removal was reported at 68.75% and 22.8% at the initial concentrations of 50 and 250 mg/L, for synthetic and real wastewaters, respectively. The results showed that the highest adsorption capacity was 17.75 mg/g.

Influence of metakaolin on the conversion and compressive strength of quaternary phase paste

AbstractApplication of calcium aluminate cement in construction faces the challenges of high manufacturing cost and volumetric instability associated with hydrates conversion. This work introduces a newly developed high‐performance Ca20Al26Mg3Si3O68 (Q phase)‐metakaolin (MK) composite binder. The influence of MK on the conversion and strength development of Q phase paste cured at 40°C was investigated. The mechanism of MK on the stability of synthetic hydrate was studied by solution chemistry, XRD, and NMR. The pure Q phase paste experiences a significant strength reduction due to hydrates conversion, whereas the Q phase paste containing 15% MK exhibits a continuous increase in strength. MK promotes the formation of CAH10, contributing to the refinement of pore structure and enhanced mechanical property. The AlV and AlIV dissolved from MK increase the concentration in the pore solution, and then the solubility of CAH10 decreases due to the common‐ion effect, thus inhibiting the subsequent precipitation of C3AH6. In addition, the release of dissolved alumina from MK considerably impedes silica dissolution, and consequently, the formation of C2ASH8 is hindered at a higher content of MK.

Advances of experimental study on gas production from synthetic hydrate reservoir in China

China has entered the area of new normal economy which requires the harmonious development of energy consumption, environmental protection and economic development. Natural gas hydrate is a potential clean energy with tremendous reserve in China. The successful field test of marine hydrate exploitation in South China Sea created a new record of the longest continuous gas production from natural gas hydrate. However, the corresponding fundamental research is still urgently needed in order to narrow the gap between field test and commercial production. This paper reviewed the latest advances of experimental study on gas production from hydrate reservoir in China. The experimental apparatus for investigating the performance of hydrate dissociation in China has developed from one dimensional to two dimensional and three dimensional. In addition, well configuration developed from one tube to complicated multi-well networks to satisfy the demand of different production models. Besides, diverse testing methods have been established. The reviewed papers preliminary discussed the mechanical properties and the sediment deformation situation during the process of hydrate dissociation. However, most reported articles only consider the physical factor, the coupled mechanism of physical and chemical factor for the mechanical properties of the sediment and the sand production problem should be studied further.

ИЗУЧЕНИЕ СВОЙСТВ ГЛИНИСТЫХ ГИДРАТСОДЕРЖАЩИХ ОБРАЗЦОВ С ИСПОЛЬЗОВАНИЕМ ТЯЖЕЛОЙ ВОДЫ ПО ДАННЫМ МЕТОДА ЯМР-РЕЛАКСОМЕТРИИ

A model of synthetic hydrates in a mixture of quartz pellets with kaolin clay is considered using by tetrahydrofuran solution based on distilled water and heavy water. The dissociation of tetrahydrofuran is investigated under atmospheric pressure. The analysis of the NMR-characteristics of tetrahydrofuran hydrate in a clayed sand is performed. The stages of hydrate dissociation excluding water contribution are established and it is shown that the NMR-relaxometry method is sensitive to the clay content. The obtained results allow us to simulate the processes of hydrate formation in real rocks and study their physical characteristics.

Изучение особенностей процессов образования и механохимической переработки синтетических гидратов природного газа

Изучение особенностей процессов образования и механохимической переработки синтетических гидратов природного газа

Rapid Gas Hydrate Formation Processes: Will They Work?

Researchers at DOE’s National Energy Technology Laboratory (NETL) have been investigating the formation of synthetic gas hydrates, with an emphasis on rapid and continuous hydrate formation techniques. The investigations focused on unconventional methods to reduce dissolution, induction, nucleation and crystallization times associated with natural and synthetic hydrates studies conducted in the laboratory. Numerous experiments were conducted with various high-pressure cells equipped with instrumentation to study rapid and continuous hydrate formation. The cells ranged in size from 100 mL for screening studies to proof-of-concept studies with NETL’s 15-Liter Hydrate Cell. Results from this work demonstrate that the rapid and continuous formation of methane hydrate is possible at predetermined temperatures and pressures within the stability zone of a Methane Hydrate Stability Curve (see Figure 1).

A natural hydrate dissolution experiment on complex multi-component hydrates on the sea floor

Dissolution of natural hydrate cores was measured using time-lapse photography on the seafloor at Barkley Canyon (850 m depth and 4.17 °C). Two types of hydrate fabrics in close contact with one another were studied: a “yellow” hydrate stained with condensate oil and a “white” hydrate. From thermogenic origins, both fabrics contained methane as well as heavier hydrocarbons. These multi-component hydrates were calculated to be well within p– T stability conditions (<200 m water depth needed at 4.17 °C). While stable in pressure and temperature, the hydrates were bathed in under-saturated seawater, which promoted dissolution. The flux of gas from the shrinking yellow hydrate core was 0.15 ± 0.01 mmol gas/m 2 s, while the white hydrate dissolved faster at 0.25 ± 0.02 mmol gas/m 2 s. To determine the controlling mechanism for the observed changes in the hydrate cores, experimental results were compared with an engineering correlation for convective mass transfer. Using water velocity as a fitting parameter, the correlation agreed well with results from a previous dissolution experiment on well-characterized synthetic hydrates. Even with a number of other unknowns, when applied to the natural hydrate, the mass transfer correlation predicted the dissolution rate within 20%. This seafloor-based experiment, along with visual observations of seafloor hydrate dissolution over a 3-day period, were used to further understand the fate of natural seafloor hydrates exposed on the seafloor. By showing that mass transfer is the rate-controlling mechanism for dissolution of these natural hydrate outcrops, proper hydrodynamic calculations can be employed to give a refined estimate on hydrate dissolution rates.

About this title - Sediment-Hosted Gas Hydrates: New Insights on Natural and Synthetic Systems

There is much interest in gas hydrates in relation to their potential role as an important driver for climate change and as a major new energy source; however, many questions remain, not least the size of the global hydrate budget. Much of the current uncertainty centres on how hydrates are physically stored in sediments at a range of scales. This volume details advances in our understanding of sediment-hosted hydrates, and contains papers covering a range of studies of real and artificial sediments containing both methane hydrates and CO 2 hydrates. The papers include an examination of the techniques used to locate, sample and characterize hydrates from natural, methane-rich systems, so as to understand them better. Other contributions consider the nature and stability of synthetic hydrates formed in the laboratory, which in turn improve our ability to make accurate predictive models.

Mechanical and electromagnetic properties of northern Gulf of Mexico sediments with and without THF hydrates

Using an oedometer cell instrumented to measure the evolution of electromagnetic properties, small strain stiffness, and temperature, we conducted consolidation tests on sediments recovered during drilling in the northern Gulf of Mexico at the Atwater Valley and Keathley Canyon sites as part of the 2005 Chevron Joint Industry Project on Methane Hydrates. The tested specimens include both unremolded specimens (as recovered from the original core liner) and remolded sediments both without gas hydrate and with pore fluid exchanged to attain 100% synthetic (tetrahydrofuran) hydrate saturation at any stage of loading. Test results demonstrate the extent to which the electromagnetic and mechanical properties of hydrate-bearing marine sediments are governed by the vertical effective stress, stress history, porosity, hydrate saturation, fabric, ionic concentration of the pore fluid, and temperature. We also show how permittivity and electrical conductivity data can be used to estimate the evolution of hydrate volume fraction during formation. The gradual evolution of geophysical properties during hydrate formation probably reflects the slow increase in ionic concentration in the pore fluid due to ion exclusion in closed systems and the gradual decrease in average pore size in which the hydrate forms. During hydrate formation, the increase in S-wave velocity is delayed with respect to the decrease in permittivity, consistent with hydrate formation on mineral surfaces and subsequent crystal growth toward the pore space. No significant decementation/debonding occurred in 100% THF hydrate-saturated sediments during unloading, hence the probability of sampling hydrate-bearing sediments without disturbing the original sediment fabric is greatest for samples in which the gas hydrate is primarily responsible for maintaining the sediment fabric and for which the time between core retrieval and restoration of in situ effective stress in the laboratory is minimized. In evaluating the impact of core retrieval on specimen properties, it is also important to consider how far removed hydrate-bearing samples are from hydrate stability conditions.

X‐ray synchrotron diffraction study of natural gas hydrates from African margin

Natural gas hydrates recovered from the Congo‐Angola basin and Nigerian margins are analyzed by synchrotron X‐ray powder diffraction. Biogenic methane is the most abundant gas trapped in the samples and others minor components (CO2, H2S) are co‐clathrated in a type I cubic lattice structure. The refinement for the type I structure gives lattice parameters of a = 11.8646 (39) Å and a = 11.8619 (23) Å for specimens from Congo‐Angola and Nigerian margins respectively at 90 K. These values, intermediate between the lattice constant of less pure methane specimens and pure artificial methane hydrates, indicate that lattice constants can be affected by the presence of encaged CO2, H2S and other gas molecules, even in small amounts. Thermal expansion is also presented for Congo‐Angola hydrate in the temperature range 90–200 K. The coefficients are comparable with values reported for synthetic hydrates at low temperature and tend to approach thermal expansion of ice at higher temperature.

Direct measurements of multi-component hydrates on the seafloor: Pathways to growth

The phase behavior of multi-component hydrate systems formed in the ocean has been studied using a sea-going in situ Raman spectrometer. Results from a field study using a synthetic hydrate sample and preliminary measurements of naturally occurring thermogenic hydrates at Barkley Canyon are reported and compared. This work follows early field trials using simple and binary synthetic hydrates and an expedition to Hydrate Ridge, studying natural hydrates primarily composed of methane. This refined Raman technique was combined with visual observations utilizing remotely operated vehicles as experimental platforms in an oceanic laboratory. Traditional laboratory Raman analysis was also performed on hydrate samples following recovery. A synthetic hydrate was first studied in Monterey Bay to characterize the formation, evolution, and dissociation of multi-component gas hydrates formed in the deep ocean as a guide to understanding complex behavior of natural hydrate systems. The gas mixture (92.5% CH 4, 3.8% C 2H 6, 1.9% C 3H 8, 0.4% i-C 4H 10, 0.4% n-C 4H 10, 0.1% i-C 5H 12, 0.1% n-C 5H 12, and 0.1% neo-C 5H 12) was chosen to simulate a typical natural gas composition. Regions of sII and mixed sI–sII hydrate were measured in situ after 65 days of formation. The presence of a heterogeneous inter-mixed sI–sII hydrate suggests a formation mechanism where hydrate growth proceeds from localized pockets of isolated gas, such as gas bubbles separated from the bulk vapor phase by hydrate membranes, where sI formation follows the depletion of sII-forming gases within these individual hydrate reactors. Unstable conditions during recovery led to significant hydrate dissociation, leaving only ice and a compositionally homogenous sII hydrate phase, with no evidence of the sI that had been observed at the seafloor. To investigate whether this process may operate in natural systems, a Raman investigation of the natural thermogenic hydrates at Barkley Canyon was carried out. Massive seafloor hydrate outcrops varied in appearance from yellow, due to oil staining, to white. While the yellow hydrate had significant interference due to fluorescence, detailed analysis of the in situ Raman data was possible for the white hydrate. This white hydrate was mainly sII, but the presence of sI was also detected. The heterogeneity in both structure and composition of these natural hydrates was similar to the synthetic hydrate formed in Monterey Bay. These similarities suggest that a controlled seafloor experiment was able to simulate a natural thermogenic hydrate with reasonable accuracy. Further development of theory and testing is needed to determine if the ‘individual hydrate reactor’ mechanism was indeed the pathway to growth in this natural system.

A comparative Raman spectroscopic study of natural gas hydrates collected at different geological sites

Intact natural gas hydrates recovered on the West African margin in the South Atlantic Ocean (ZaiAngo and Neris II projects) and from the Norwegian Sea (Hakon Mosby Mud Volcano) are investigated by micro-Raman spectroscopy at ambient pressure and low temperature. The gas hydrates collected at different geological sites contain a high methane concentration relative to other minor components that are slightly dispersed in the samples. They crystallize in a type I cubic lattice structure as also confirmed by our preliminary synchrotron diffraction results obtained on the ZaiAngo specimen. However, detailed analysis of selected microscopic areas reveals a variation in the gas distribution among the different specimens. Trace amounts of CO 2 and H 2S can be identified by their characteristic vibrational signatures in the 1000–3800 cm − 1 spectral range. They are found to be co-clathrated with methane. Their presence produces a compositional effect on the relative cage occupancy of CH 4, as determined from the integrated band intensity ratio corresponding to the molecular stretching modes of methane in the hydrate. The comparative Raman analysis of synthetic hydrates of H 2S, CH 4 and CH 4-deuterohydrates allows the unambiguous assignment of weak band overtones of trapped methane and co-clathrated H 2S molecular vibrations.

Mechanical properties of sand, silt, and clay containing tetrahydrofuran hydrate

The mechanical behavior of hydrate‐bearing sediments subjected to large strains has relevance for the stability of the seafloor and submarine slopes, drilling and coring operations, and the analysis of certain small‐strain properties of these sediments (for example, seismic velocities). This study reports on the results of comprehensive axial compression triaxial tests conducted at up to 1 MPa confining pressure on sand, crushed silt, precipitated silt, and clay specimens with closely controlled concentrations of synthetic hydrate. The results show that the stress‐strain behavior of hydrate‐bearing sediments is a complex function of particle size, confining pressure, and hydrate concentration. The mechanical properties of hydrate‐bearing sediments at low hydrate concentration (probably &lt; 40% of pore space) appear to be determined by stress‐dependent soil stiffness and strength. At high hydrate concentrations (&gt;50% of pore space), the behavior becomes more independent of stress because the hydrates control both stiffness and strength and possibly the dilative tendency of sediments by effectively increasing interparticle coordination, cementing particles together, and filling the pore space. The cementation contribution to the shear strength of hydrate‐bearing sediments decreases with increasing specific surface of soil minerals. The lower the effective confining stress, the greater the impact of hydrate formation on normalized strength.

Determination of synthetic hydrate content in sand specimens using dielectrics

Gas hydrates are solid crystalline compounds (clathrates) that encage gas molecules inside the lattices of hydrogen bonded water molecules within a specific temperature–pressure stability zone. It is imperative that reliable detection and quantification modi operandi are developed, as proposed in this research, to identify hydrate-laden strata and determine economic viability of this potential energy yield. This paper presents the experimental analysis of synthetic refrigerant (R-11) hydrates in 20/30 Ottawa sand using dielectric principles to determine specific hydrate content. Hydrate specimens were constructed via moist tamped Ottawa sand, purged with carbon dioxide (CO2), saturated with de-aired water, and mixed with a known amount of R-11 to produce precise hydrate contents. The specimen's bulk dielectric constant was measured using a ThetaProbe by applying the principles of time domain reflectometry (TDR). A distinct relationship between hydrate content and the bulk dielectric constant of sand specimens is determined, as well; volumetric expansion associated with hydrate formation is also portrayed.Key words: gas hydrates, synthetic hydrates, dielectrics, Ottawa sand, laboratory testing.

Raman spectroscopic measurements of synthetic gas hydrates in the ocean

A Raman spectrometer extensively modified for deep ocean use was used to measure synthetic hydrates formed in an ocean environment. This was the first time hydrates formed in the ocean have been measured in situ using Raman spectroscopy. Gas hydrates were formed in situ in the Monterey Bay by pressurizing a Pyrex cell with various gas mixtures. Raman spectra were obtained for sI methane hydrate and sII methane + ethane hydrate. Gas occlusion resulting from rapid gas growth of methane hydrate was measured immediately after formation. The Raman shift for methane free gas was coincident with that of methane in the small 5 12 hydrate cage. The methane Raman peak widths were used to discriminate between methane in the free gas and hydrate phase. Methane + ethane sII hydrate was formed for 43 days on the seafloor. In this case, gas occlusion was not measured when the gas hydrates were allowed to form over an extended time period. Equivalent Raman spectra were obtained for the in situ and laboratory-formed sII methane + ethane hydrates, under similar p, T, and x conditions. With the Raman spectrometer operating in the ocean, seawater contributes to the Raman spectra obtained. Both the Raman bands for the sulfate ion and water were used to qualitatively determine the distribution of water phases measured (hydrate, seawater) in the Raman spectra.

Occurrence and structural characterization of gas hydrates associated with a cold vent field, offshore Vancouver Island

Gas hydrate samples recovered from a cold vent field offshore Vancouver Island were studied in detail both by macroscopic observations and instrumental methods (powder X‐ray diffraction method (PXRD), nuclear magnetic resonance (NMR), and Raman spectroscopy). It was found that gas hydrates were massive from 2.64 to 2.94 m below seafloor (mbsf), elongated, nodular and tabular from 4.60 to 4.81 mbsf, and vein‐like from 5.48 to 5.68 mbsf, showing a trend of decreasing hydrate content with increasing depth. All samples were determined to be structure I hydrate from PXRD, NMR, and Raman spectroscopies. The hydration numbers were estimated to be 6.1 ± 0.2 on average as determined from the methane distribution over the cage sites from NMR and Raman analytical results. Estimates of conversion levels indicated that ∼78% of the water in the massive samples was hydrate, down to a low value of ∼0.4% for the pore hydrate samples. The results are compared with measurements on synthetic hydrates and samples recovered from below the permafrost on the Mallik site. Differences in methane content and lattice parameters for synthetic and natural samples are relatively minor. Additional work is needed to address the presence of minor gas components and the heterogeneity of natural hydrate samples.