Hydrate Morphology Research Articles

The Influence of Gas Hydrate Morphology on Reservoir Permeability and Geophysical Shear Wave Remote Sensing

AbstractWe show that direct estimates of the permeability of hydrate‐bearing geological formations are possible from remote measurements of shear wave velocity (Vs) and attenuation (Qs−1). We measured Vs, Qs−1 and electrical resistivity at time intervals during methane hydrate formation in Berea sandstone using a laboratory ultrasonic pulse‐echo system. We observed that Vs and Qs−1 both increase with hydrate saturation Sh, with two peaks in Qs−1 at hydrate saturations of around 6% and 20% that correspond to changes in gradient of Vs. We implemented changes in permeability with hydrate saturation into well‐known Biot‐type poro‐elastic models for two‐ and three‐phases for low (Sh < 12%) and high (Sh > 12%) hydrate saturations respectively. By accounting for changes in permeability linked to hydrate morphology, the models were able to describe the Vs and Qs−1 observations. We found that the first Qs−1 peak is caused by a reduction of permeability during hydrate formation associated with a transition from pore‐floating to pore‐bridging hydrate morphology; similarly, the second Qs−1 peak is caused by a permeability reduction associated with a transition from pore‐bridging hydrate morphology to an interlocking network of hydrate in the pores. We inverted for permeability using our poro‐elastic models from Vs and Qs−1. This inverted permeability agrees with permeability obtained independently from electrical resistivity. We demonstrate a good match of our models to shear wave data at 200 Hz and 2 kHz frequencies from the literature, indicating the general applicability of the models.

Study on the Growth Kinetics and Morphology of Methane Hydrate Film in a Porous Glass Microfluidic Device

Natural gas hydrates are widely considered one of the most promising green resources with large reserves. Most natural gas hydrates exist in deep-sea porous sediments. In order to achieve highly efficient exploration of natural gas hydrates, a fundamental understanding of hydrate growth becomes highly significant. Most hydrate film growth studies have been carried out on the surface of fluid droplets in in an open space, but some experimental visual works have been performed in a confined porous space. In this work, the growth behavior of methane hydrate film on pore interior surfaces was directly visualized and studied by using a transparent high-pressure glass microfluidic chip with a porous structure. The lateral growth kinetics of methane hydrate film was directly measured on the glass pore interior surface. The dimensionless parameter (−∆G/(RT)) presented by the Gibbs free energy change was used for the expression of driving force to explain the dependence of methane hydrate film growth kinetics and morphology on the driving force in confined pores. The thickening growth phenomenon of the methane hydrate film in micropores was also visualized. The results confirm that the film thickening growth process is mainly determined by water molecule diffusion in the methane hydrate film in glass-confined pores. The findings obtained in this work could help to develop a solid understanding on the formation and growth mechanisms of methane hydrate film in a confined porous space.

Investigation of crystal growth of CO2 hydrate in aqueous fructose solution for the potential application in carbonated solid foods

CO2 hydrate is applicable to solid carbonated foods. The hydrate crystal morphology, which represents the crystal size and shape, is an important characteristic that changes the texture of foods. We report an observational study of the crystal growth of CO2 hydrate in aqueous fructose solution. The difference between the phase equilibrium temperature and the experimental temperature, ΔTsub, is applied as an index of the driving force. Experiments were performed at ΔTsub range of 0.9 K to 5.4 K. At all ΔTsub, initial crystal formed at the gas-solution interface and grew along the interface. After covering the interface, the crystals grew in the liquid phase The individual crystals were identified as polyhedral with facets (ΔTsub = 0.9 K), skeletal crystals (ΔTsub = 2.0 K) and dendrites (ΔTsub = 3.0 K and 5.4 K). Based on these results, the potential effect of gas hydrate morphology on texture of foods has been discussed.

Growth pattern of dispersed methane hydrates in brine-saturated unconsolidated sediments via joint velocity and resistivity analysis

Although the different morphologies of methane hydrates have been identified in pore spaces, their growth mechanism has not yet been fully resolved. In this study, we performed two sets of experiments using dissolved methane in brine-saturated unconsolidated sediments composed of 95.5 wt% quartz sand and 4.5 wt% kaolin to simulate the formation of dispersed marine gas hydrates. The in situ ultrasonic velocities (VP and VS), electrical resistivity, and methane hydrate saturation (Sh) were simultaneously measured and were jointly analyzed and compared to elastic and electrical rock physics models of various hydrate morphologies in pore spaces to quantify the invisible dynamic evolution of the hydrates’ morphologies. The results indicate that the hydrate growth pattern is characterized by distinct stages as Sh increases. During the first set of experiments, the frame-strengthening morphology was dominant in all of the stages, the contact-cementing morphology occurred infrequently when Sh > 8%, and the pore-filling morphology gradually became identifiable when Sh > 25%. It is proposed that the frame-strengthening hydrates primarily grow on grain surfaces when Sh < 25%, then they extend toward the center of the pore, and finally they begin to plug the pores when Sh > 35%. In the second set of experiment, the geophysical responses were significantly affected by the presence of free gas, and the observed hydrate growth behavior was differed from that observed in the first set of experiments.

Visualization of interactions between depressurization-induced hydrate decomposition and heat/mass transfer

Visual evidences to understand the interactions between hydrate decomposition and heat/mass transfer are currently lacking. This study proceeds from the hydrate morphology to visualize the interactions between depressurization-induced hydrate decomposition and heat/mass transfer from different scales. Reactor-scale hydrate distribution evolution shows that the dominant influencing factor of hydrate decomposition transforms from heat transfer to mass transfer. More importantly, pore-scale visual evidences suggest that the mass transfer of gas shows significant effects on hydrate morphology evolution. Specifically, the limited gas diffusion in liquid phase could lead to the hydrate morphology evolution from patchy pore-filling to “grain-bridging” during hydrate decomposition. The combination of grain-bridging hydrate together with the water layer that wraps the hydrate is termed as “hydrate bridge” in this work. It is also worth noting that the grain-bridging hydrate could accelerate fluid flow in pores according to our seepage simulation results. These findings provide visual evidences for variations in physical properties of hydrate-bearing sediments during hydrate decomposition. Since physical properties of hydrate-bearing sediments play important roles in hydrate decomposition, the hydrate morphology evolution characteristics analyzed here are valuable for hydrate exploitation in field tests.

Shear wave velocity prediction based on LSTM and its application for morphology identification and saturation inversion of gas hydrate

Compressional wave velocity (Vp) and shear wave velocity (Vs) are important geophysical parameters for studying gas hydrate systems. However, the Vs parameter is not commonly used in logging-while-drilling because of the high costs of acquiring it. To address this issue, we propose a long short-term memory (LSTM) neural network for predicting a complete shear wave velocity curve at wells in Alaminos Canyon Block 21 (AC 21) based on the limited measurement of Vs logs. The LSTM method performs better than the LSF method when comparing the Vs curves generated by them with the measured one. The measured Vp and predicted Vs curves were applied to determine the distribution of hydrates using the ratio of Vp increment to Vs increment, and we projected velocity samples into the cross plot of modelled Vp and Vs by assuming pore- and fracture-filling hydrate reservoirs to identify the hydrate morphologies. The pore- and fracture-filling gas-hydrate-bearing reservoirs were proved at 541–633 feet below sea floor (fbsf) and 1200–1500 fbsf in hole AC 21-A, respectively. However, the gas hydrate saturations estimated using the joint usage of Vp and Vs at shallow and deep depths are less than 10% (at most 30% locally) and 10%–20%, respectively. Because less Vs data is measured, a transfer learning technique is introduced to estimate a Vs curve at well AC 21-B. Comparing velocity samples with the theoretical velocity curves, the morphology of the hydrate in well AC 21-B is observed to be a horizontal fracture-filling type, and its gas hydrate saturation is approximately 10%–20% at the depth intervals of hydrate occurrence. According to our numerical calculations, the hydrate occurrence at well AC 21-B shows a better continuity than in well AC 21-A. • A LSTM model is developed to predict a complete VS curve in JIP wells. • Morphologies and distributions of gas hydrates are determined. • Gas hydrate saturation is estimated based on the jointly usage of Vp and Vs data. • transfer learning to predict a Vs curve and further analyse gas hydrates.

The Change in Geomechanical Properties of Gas Saturated Methane Hydrate‐Bearing Sand Resulting From Water Saturation

AbstractLaboratory tests were carried out on gas‐saturated hydrate‐bearing sand (HBS) specimens that were subsequently water saturated to determine the influence of water saturation on their small‐strain (stiffness) and large‐strain (strength) behavior. Hydrate formed using the “excess gas” method led to significant increases in stiffness and strength of the sand. The magnitude increase in stiffness was largest for the lowest hydrate saturation tests and was sensitive to the distribution of hydrate within the sand. In contrast, increases in strength were directly related to increases in hydrate saturation, and to a lesser extent, on the applied confining stresses. Subsequent water saturation of these gas‐saturated HBS (achieved by flushing up to three times the pore volume of water through the HBS specimen) led to ∼90% and ∼70% reduction in stiffness and strength, respectively, along with a 37% reduction in hydrate volume (initial hydrate pore saturation ∼21.5%). Specimens with higher initial hydrate saturations (∼48%) saw slightly smaller reductions in stiffness and strength associated with ∼17% reduction in hydrate saturation. Changes in behavior appear to be related to the reduction of “cementing” hydrate at particle contacts through dissociation/dissolution of the hydrate, with remaining hydrate being more “pore filling.” Processes that may induce fluid flow in a natural gas‐saturated HBS, such as during the depressurization of the HBS reservoir during hydrate production, may lead to changes in hydrate morphology and affect the geomechanical properties of the HBS even when the HBS are still under hydrate stability conditions.

Study on fluidizing the highly converted methane hydrate for gas storage and transportation

Fluidizing highly converted hydrate particles is a potential way for continuously producing gas hydrate in gas storage application. However, the fundamental problem is to prevent hydrate agglomeration while keeping a high formation rate. In this study, water–oil systems were used to investigate the feasibility of fluidizing the methane hydrate with fast formation rate. The formation kinetics and the slurry morphology of methane hydrate were systematically studied in the presence of single or commingled additives. Among the single additives, Span 20 provided the highest volumetric gas uptake of 177.7 V/Vw (standard volume of gas per volume of water) at water cut of 20 vol%; with an increase in water cut, the volumetric gas uptake decreased and more hydrate agglomerated. All the single additives could not prevent hydrate agglomeration perfectly, even for Span 20, the best-performing single additive, prevented hydrate agglomeration well only at low water cut of 5 vol% and 10 vol%. The commingled additives comprised 1.0 wt% (to water) Span 20 and 2.0 ~ 3.0 wt% co-additives. When these co-additives were used in coordination with Span 20, most of them inhibited hydrate formation. By decreasing the concentration of TBAB from 5.5 wt% to 0.025 wt%, the inhibiting effect of TBAB faded away, while the anti-agglomerating effect remained. Similar phenomena were also found with some other co-additives. The results showed that low water cut, high Span 20 concentration, particularly the commingled additives could be solutions for fluidizing highly converted hydrate particles.

Morphology identification of gas hydrate from pointwise Lipschitz regularity for P- and S-wave velocity

The accurate morphology identification of gas hydrate-bearing sediments (GHBS) has great significance in practical exploitation and subsequent resource evaluation. Previous studies have disclosed two main morphologies for gas hydrate in sediments: pore- and fracture-filling. However, the existing identification methods of gas hydrate’s morphology rarely consider their intrinsic differences in distribution characteristics. In this paper, a new method is proposed to identify the morphology of hydrate according to the scattered distribution of fracture dips for fracture-filling GHBS. Firstly, numerical simulations are performed to study the relationships between the morphology of hydrate and the sonic velocities. Considering the dip variation is within a certain range for fracture-filling hydrate, the theoretical curves show that the resulting mutation degrees between P- and S-wave velocities are inconsistent in fracture-filling GHBS, which is different from pore-filling GHBS. Then the modified estimation method for pointwise Lipschitz exponent α is introduced to capture their differences. The cross plots of Lipschitz exponent for P-wave velocity, α(Vp), and Lipschitz exponent for S-wave velocity, α(Vs), indicate that most of the dots representing pore-filling GHBS are evenly distributed near the line α(Vp)=α(Vs), while the dots representing fracture-filling GHBS are scattered outside the line α(Vp)=α(Vs). Based on these characteristics, a ratio method is put forward to differentiate the two types of hydrate. These hypotheses and methods are verified using the measured P- and S-wave velocities log data at different sites in Leg 204, Ocean Drilling Program (ODP), in the United States. Finally, the results of this new method agree closely with core data.

Impacts of Space Restriction on the Microstructure of Calcium Silicate Hydrate.

The effect of hydration space on cement hydration is essential. After a few days, space restriction affects the hydration kinetics which dominate the expansion, shrinkage and creep of cement materials. The influence of space restriction on the hydration products of tricalcium silicate was studied in this paper. The microstructure, morphology and composition of calcium silicate hydrate (C-S-H) were explored from the perspective of a specific single micropore. A combination of Raman spectra, Fourier transform infrared spectra, scanning electron microscopy and energy dispersive X-ray spectroscopy were employed. The results show that space restriction affects the structure of the hydration products. The C-S-H formed in the micropores was mainly composed of Q3 silicate tetrahedra with a high degree of polymerization. The C-S-H formed under standard conditions with a water to cement ratio of 0.5 mostly existed as Q2 units. Space restriction during hydration is conducive to the formation of C-S-H with silica tetrahedra of a high polymerization degree, while the amount of water filling the micropore plays no obvious role on the polymeric structure of C-S-H during hydration.

Microscope insights into gas hydrate formation and dissociation in sediments by using microfluidics

Natural gas hydrates (NGHs) have tremendous potential and abundant reserves worldwide. Both the hydrate distribution and the potential inverse formation are related to the efficient exploitation of NGHs; however, studies of the micro-mechanism of hydrate morphology and the evolution of phase transition processes are still lacking. In this study, hydrate formation and dissociation were investigated at the microscale using a microfluidics device. Methane hydrate (MH) was formed at a system pressure of 5 MPa and temperature of 274.15 K and then was dissociated by using the depressurization method. Based on different gas–water contact areas, two kinds of stable crystal structures and two kinds of unstable crystal structures of the micromorphology in the formation stage were identified. During the dissociation process, the direct proof of the induced local re-formation of the hydrate by microbubble aggregation was given. First, the distribution of the CH4 bubbles (from 5 μm to 140 μm) inside the pores and throats was quantified. Normal bubble distributions were found, and the largest percentage of bubble diameters ranged from 10 μm to 30 μm. The average diameter of the bubbles increased with time, and the total number of bubbles decreased with time. The large density distribution of microbubbles with smaller diameters in liquids impeded the pressure propagation and heat transfer, which are keys to restraining the rate of hydrate dissociation. These findings are beneficial for understanding the microscale mechanisms of the hydrate phase transition and MH dissociation efficiency, which may be helpful for the selection and design of NGH exploitation schemes.

Crystal growth of clathrate hydrate formed with H2\u2009+\u2009CO2 mixed gas and tetrahydropyran

Hydrate-based gas separation technology is applicable to the CO2 capture and storage from synthesis gas mixture generated through gasification of fuel sources including biomass. This paper reports visual observations of crystal growth dynamics and crystal morphology of hydrate formed in the H2 + CO2 + tetrahydropyran (THP) + water system with a target for developing the hydrate-based CO2 separation process design. Experiments were conducted at a temperature range of 279.5–284.9 K under the pressure of 4.9–5.3 MPa. To simulate the synthesis gas, gas composition in the gas phase was maintained around H2:CO2 = 0.6:0.4 in mole fraction. Hydrate crystals were formed and extended along the THP/water interface. After the complete coverage of the interface to shape a polycrystalline shell, hydrate crystals continued to grow further into the bulk of liquid water. The individual crystals were identified as hexagonal, tetragonal and other polygonal-shaped formations. The crystal growth rate and the crystal size varied depending on thermodynamic conditions. Implications from the obtained results for the arrangement of operating conditions at the hydrate formation-, transportation-, and dissociation processes are discussed.

Crystallization Behavior of the Hydrogen Sulfide Hydrate Formed in Microcapillaries.

There are no reports on the hydrogen sulfide hydrate growth process and morphology in micropores due to the toxicity of hydrogen sulfide. In this study, the experimental measurements and dissociation enthalpies were provided to assess the effect of the microcapillary silica tube size on hydrogen sulfide hydrate dissociation conditions. To simulate micropore sediments, the H2S hydrate growth processes and morphologies at different supercooling temperatures were observed in this study. The dissociation temperature depression of the hydrate crystal in the microcapillary was less than 0.001 °C, which shows that the stability of the hydrate is less affected by the microcapillary pore used in this study. The mass transfer from the gas phase to the liquid phase is easily blocked when the hydrogen sulfide hydrate shell covers the gas–water meniscus, causing the growth of the gas hydrate to be inhibited. The hydrate crystal morphology can be divided into fibrous, needle-like crystals and dendritic crystals when ΔTsub > 12.7; the hydrate crystal morphology can be categorized as dendritic crystals and columnar crystals when ΔTsub = 7.9–8.9, and the hydrate crystals can form polyhedral crystals when ΔTsub = 7.9–8.9. Additionally, a new “bridging effect” that a hollow crystal which was filled with the gas phase can connect with two separated gas phases was found at low supercooling temperature.

Methane Hydrate Crystallization on Sessile Water Droplets

This paper describes a method to form methane hydrate shells on water droplets. In addition, it provides blueprints for a pressure cell rated to 10 MPa working pressure, containing a stage for sessile droplets, a sapphire window for visualization, and temperature and pressure transducers. A pressure pump connected to a methane gas cylinder is used to pressurize the cell to 5 MPa. The cooling system is a 10 gallon (37.85 L) tank containing a 50% ethanol solution cooled via ethylene glycol through copper coils. This setup enables the observation of the temperature change associated with hydrate formation and dissociation during cooling and depressurization, respectively, as well as visualization and photography of the morphologic changes of the droplet. With this method, rapid hydrate shell formation was observed at ~-6 °C to -9 °C. During depressurization, a 0.2 °C to 0.5 °C temperature drop was observed at the pressure/temperature (P/T) stability curve due to exothermic hydrate dissociation, confirmed by visual observation of melting at the start of the temperature drop. The memory effect was observed after repressurizing to 5 MPa from 2 MPa. This experimental design allows the monitoring of pressure, temperature, and morphology of the droplet over time, making this a suitable method for testing various additives and substrates on hydrate morphology.

Multi-scale characterization of shell thickness and effective volume fraction during gas hydrates formation: A kinetic study

Gas hydrates have gained increasing attention in energy and environmental fields and have potential applications in gas storage and transport, carbon dioxide sequestration, and flow assurance. Nevertheless, the kinetics of hydrate formation are still not well understood. In situ high-resolution X-ray computed tomography measurements were performed to monitor xenon hydrate formation on water droplets and ice spheres. For the first time, three-dimensional thickness meshes were used to quantify and visualize the kinetics of hydrate formation. The evolution of the hydrate morphology was investigated, and the time-dependent kinetic parameters were obtained, including the hydrate shell thickness and inner and outer diameters and the effective volume fraction of hydrate particles. The results indicate that the formation of gas hydrates undergoes an initial reaction-controlled stage followed by a mass-transfer-limited growth stage. For hydrate formation from a water droplet with an initial diameter of 1.66 mm, the hydrate shell thickness was approximately 30 μm and the effective volume fraction of hydrate particles was approximately 11% at 12 h after hydrate formation began. The standard deviation of the shell thickness, which indicates the surface roughness of the hydrate shell, increased with time for hydrate formation from water droplets. The results presented in this study renew our knowledge on the kinetics of hydrate formation, which is essential for their use in flow assurance and other hydrate-related technologies.

Characterization of stress–dilatancy behavior for methane hydrate-bearing sediments

To evaluate the stability of seabed ground during gas hydrate exploitation, a constitutive model that can appropriately estimate the mechanical properties of methane hydrate-bearing sediments must be established. A better understanding of the stress–dilatancy characteristics is crucial in modeling the stress–strain behavior of methane hydrate-bearing sediments under complex loading paths. This study presents a compressive analysis of the effects of several factors on the stress–dilatancy behavior of methane hydrate-bearing sediments. The dilatancy behavior of methane hydrate-bearing specimens synthetized in the laboratory and natural hydrate-bearing samples acquired in the field are compared. Results show that the hydrate saturation, temperature, porosity, and hydrate formation pattern affect the peak stress ratio, dilatancy rate, and critical state stress ratio of methane hydrate-bearing sediments. However, the effect of effective confining pressure on the stress–dilatancy curve is minimal. Different hydrate formation methods yield different hydrate morphologies; consequently, the dilatancy behavior of methane hydrate-bearing sediments is altered significantly. Differences observed between synthetized specimens and natural samples may originate from various hydrate morphologies. The stress–dilatancy relationship of methane hydrate-bearing sediments is interpreted and described using Rowe's theory. Furthermore, the dilatancy behavior of methane hydrate-bearing sediments and cemented sands under similar test conditions is compared.

Enhanced hydrate formation by natural-like hydrophobic side chain amino acids at ambient temperature: A kinetics and morphology investigation

Application of amino acids to copromote methane hydrate formation with tetrahydrofuran (THF) and their effects on the dissociation were investigated at room temperature. Hydrophobic side chain amino acids (valine, leucine, and methionine) were experimented together with 5.56 mol% THF on the hydrate formation kinetics and morphology at 293.2 K and 8 MPa in an unstirred reactor. Results show that all hydrophobic side chain amino acids affected the hydrate formation kinetics by increasing the formation rate at the same extent as surfactants. In addition, the presence of amino acids resulted in almost five times higher formation rate than with only 5.56 mol% THF. However, there were no significant difference hydrate formation rate with different amino acid concentrations. Surprisingly, the formation with certain hydrophobic side chain amino acids, leucine and methionine, showed multiple-step hydrate growth profiles, which resulted in the high methane uptake as compared to 5.56 mol% THF. The hydrate formation morphology in the presence of hydrophobic side chain amino acids illustrated different hydrate growth patterns from the THF solution and was totally different for each of the amino acid. Moreover, the presence of hydrophobic side chain amino acid did not affect the methane hydrate dissociation.

A Novel Gas Hydrate Morphology: Massive Hollow Fiber Growth on a Porous Substrate

Video microscopy reveals a novel morphology and growth process of a gas hydrate, promoted here by a porous activated carbon substrate astride a guest–host interface. Surface pores of the substrate ...

An Analytical Model for the Permeability in Hydrate‐Bearing Sediments Considering the Dynamic Evolution of Hydrate Saturation and Pore Morphology

AbstractHydrate‐saturation‐dependent permeability in sediments is largely affected by the pore morphology of hydrate. Hydrate pore morphology evolves from a single pore habit, which depends on its formation conditions, toward a sophisticated hybrid habit with increased hydrate saturation. Resulted permeability reduction curves in hydrate‐bearing sediments are diverse and lack universal models. By using a two‐parameter logistic function to link microscale hydrate pore habit evolution with macroscale permeability variations, this study establishes a pore‐morphology‐weighted permeability model that well captures the permeability evolution in hydrate‐bearing sediments at various conditions. The universality of this model is validated and the values of the two parameters in the model are calibrated using published laboratory and pressure core data. This newly proposed model offers a mechanistic understanding of the permeability in hydrate‐bearing sediments and an elegant expression that can be implemented in reservoir simulators for field‐scale gas production estimation.

Review of NMR Studies for Oilwell Cements and Their Importance

This paper summarizes experimental studies using Nuclear Magnetic Resonance (NMR) to evaluate cement porosity, pore size distribution, and other characteristics such as Calcium Silicate Hydrate (CSH) gel structure and morphology. The first known paper on NMR experiments to investigate cement pastes was published in 1978. Two main NMR parameters, the so-called longitudinal T1 and transverse T2 relaxation times, are commonly measured and analyzed, representing the water response which is trapped in the cement. The hydration process reported in this paper was found to be monitored from as low as 10 min to longer than 365 days. Other studies conducted experiments by using NMR, especially during the 1980s. These studies employed variations in methodologies and frequencies, making data comparison difficult. Additionally, different spectrometers and NMR concepts, as well as operating characteristics, were used. Therefore, it is challenging to reconcile results from previous NMR studies on cement. Other significant hurdles are different cement types, water/cement ratio, and curing conditions. One notable observation is that there has not been any comprehensive laboratory work related to NMR on oilfield cement types, including porosity and hydration. Two recent studies have presented NMR measurements on class G and class H cements.