Hydration Pressure Research Articles

PFSA Membrane Thickness Impact on Chemical Degradation Rates

As the large-scale commercialization of proton exchange membrane fuel cells (PEMFC) for numerous applications, including heavy-duty trucks draws nearer, there is increasing interest in optimizing fuel cell membranes for both improved efficiency and durability. One of the major membrane attributes being considered for optimization is thickness. The current range of (perfluorosulfonic acid) PFSA membrane thicknesses employed for commercial applications range from 8 -25 µm. As with most adjustable materials parameters, there are a host of properties (cost, gas permeability, proton conductivity, etc.) that require co-optimization to provide the overall preferred position. For example, thin membranes will minimize proton resistance, facilitating high power densities, but will simultaneously allow increased gas crossover fluxes that can contribute to both reduced fuel efficiency and increased oxidative stress. On the other end of the spectrum, thick membranes will decrease gas crossover fluxes, offering improved fuel efficiency while simultaneously decreasing power density. A major question surrounding the tradeoffs of membrane thickness involves the chemical durability impact. Here, we report our investigations of PFSA membrane chemical durability as a function of thickness. For the studies reported here, a thickness series (8, 12 and 20 µm) of ePTFE reinforced Nafion®-like PFSA standalone membranes (SAMs) were prepared.A two-pronged approach was taken for this study. The first involves investigation of intrinsic chemical stability of the ionomer as a function of membrane thickness using an ex-situ H2O2 vapor test. The second investigative mode, which targets the oxidative stress generated by crossover gas flux, employs OCV durability tests of full MEAs. In addition to membrane thickness, gas crossover rates are known to be functions of membrane hydration and gas pressures.1,2 Accordingly, a 3(4-1) fractional factorial experimental design was executed to probe the role of membrane thickness as well as the other factors known to affect gas crossover. The fractional factorial design allows all main effects to be determined and further allows estimation of two factor interactions, if present.References T. Sakai, H. Tanenaka, E, Torikai, “Gas Diffusion in Dried and Hydrated Nations” J. Electrochem. Soc., 1986, 133, 88-92R. Jiang, T. Fuller, S. Brawn, C. Gittleman, “Perfluorocyclobutane and poly(vinylidene fluoride) blend membranes for fuel cells” Electrochimica, Acta, 2013, 110, 306-315

Imitating the effects of drilling fluid invasion on the strength behaviors of hydrate-bearing sediments: An experimental study

The drilling fluid invasion into hydrate-bearing sediments will change the geomechanical properties of the reservoir and may lead to uncontrolled geological disasters in the worst case. Native sediments are replaced with artificial samples in this study to investigate the effect of drilling fluid invasion on the strength behaviors of hydrate-bearing sediments. The triaxial test is used primarily to assess the strength behaviors of hydrate-bearing sediments at varying temperatures, pressures, and hydrate saturation levels. The process of water-based drilling fluid penetrating into hydrate-bearing sediments is then experimentally imitated under various experimental conditions, with reservoir temperature of 4°C and pore pressure of 10 MPa. The possible repercussions of drilling fluid invasion and hydrate phase transition are identified by testing the mechanical properties of sediments under various invasion times and temperatures. The findings reveal that when hydrate saturation rises from 5% to 35%, the gas hydrate sediments shift from strain hardening to strain softening, with the critical hydrate saturation value of transition is between 15% and 25%. Peak strength increases with increasing hydrate saturation and pressure, and decreases with increasing temperature, whether drilling fluid invasion is present or not. The peak strength, Young’s modulus, shear modulus, and secant modulus of hydrate sediments all decreased significantly after drilling fluid invasion, although the Poisson’s ratio rose. These mechanical parameters are related to temperature and pressure under the action of drilling fluid. Finally, engineering and research recommendations for reducing the risk of drilling fluid invasion and hydrate dissociation are made based on experimental findings and theoretical analysis. This study innovatively examine the geomechanical mechanical properties of drilling fluid invading hydrate reservoir, which is critical for avoiding production concerns.

Wettability of graphite under 2D confinement

The thermodynamics of solid/liquid interfaces under nanoconfinement has tremendous implications for liquid transport properties. Here using molecular dynamics, we investigate graphite nanoslits and study how the water/graphite interfacial tension changes with the degree of confinement. We found that, for nanochannel heights between 0.7 nm and 2.6 nm, graphite becomes more hydrophobic than in bulk, and that the value of the surface tension oscillates before eventually converging towards a constant value for larger slits. The value of the surface tension is correlated with the slip length of the fluid and explained in terms of the effective and interfacial density, hydration pressure and friction coefficient. The study clearly indicates that there is a critical channel height of 0.9 nm (achievable experimentally [1]) at which the surface tension reaches its highest value, but the water diffusion across the channel is at its minimum. The structural analysis shows that for this pore size a transition between a 2D and 3D hydrogen bond network is accompanied by an abrupt increase in configurational entropy. Our results show that the wettability of solid surfaces can change under nanoconfinement and the data can be used to interpret the experimental permeability data.

Four-layer model of hydrate slurry flow in pipes considering rheological properties

With the exploitation of deep-water hydrate, the limitations and worrying safety of traditional mining methods such as depressurization gradually pronounced. Therefore, multiphase transportation of slurry draw attention rapidly. Hence, it is necessary to clarify the gas–liquid–solid flow regularity in slurry pipelines and provide accurate guidance for engineering projects. Wherefore, a steady-state four-layer model was developed from Hernandez (2006). The novelties of this model are that the rheological properties of the hydrate slurries were introduced and the effects of concentration distribution (C(y)) both on the solid diffusivity (ε) and interference settling velocity (wm) were considered during the calculation of C(y) in the heterogeneous layer. After improvement, this model can effectively predict the flow state of gas and slurry in pipelines. A comparison with the previous model showed that the prediction accuracy of the proposed model is within 25% for the apparent viscosity and pressure drop gradient in the pipe, indicating that the rheological properties of the hydrate slurries have a non-negligible effect on the flow state. Moreover, the present model improves the prediction accuracy of the hydrate particle C(y) in the heterogeneous layer by considering the interaction between the ε, wm, and C(y). The prediction results of the proposed model for the C(y) and change rate near the interface between the heterogeneous layer and the moving bed layer were in good agreement with the experimental data. Finally, the change laws of the hydrate particle concentration distribution, bed height, and pressure drop gradient under different working conditions were analyzed. Compared with the previous model, there is no flow transition in the velocity range of 0.05–0.5 m/s, and an economic mean concentration exists among the different mean concentrations (CS), which can provide guidance for engineering projects.

Water infiltration and swelling pressure development in GMZ bentonite pellet mixtures with consideration of temperature effects

Bentonite pellet mixture (BPM) is considered as a potential buffer/backfill material for geological repository of radioactive waste. Once emplaced in a repository, the initially discrete BPM with heterogeneous porosity would be subjected to groundwater infiltrated from surrounding rock and decay heat released from nuclides, leading to temperature depended processes of water infiltration and swelling pressure development. In this study, Gaomiaozi (GMZ) BPMs were hydrated with distilled water under constant volume condition at temperatures 20, 40, 60 and 80 °C. The water content profiles, dry density profiles and swelling pressure were periodically measured and recorded, respectively. Results indicate that, both water infiltration and swelling pressure development were accelerated by increasing temperature. Due to local swelling formation that brining water ahead and prevailing vapour diffusion through the large interconnected inter-pellet voids, the water infiltration through the BPM was anomalous super-diffusion at non-Boltzmann scale. Accordingly, a non-Boltzmann variable based water infiltration model was developed and verified by the experimental data. The non-Boltzmann variable was inversely related to water content, dry density and montmorillonite content, whereas positively related to temperature. It was proved to be able to qualitatively reflect the mobility of water for different unsaturated soils with different water contents. In addition, a comprehensive explanation of temperature effects on swelling pressure was put forward, stating that the variation of swelling pressure with temperature depended on the competitions among the thermal-induced responses of hydration pressure, water pressure and osmotic pressure. The competitions could be controlled by the clay mineralogy, pore water properties, electrolyte properties and the clay-water-electrolyte interactions at micro scale, and by dry density, thermal mode and drainage condition at macro scale.

Comparative Analysis on the Evolution of Seepage Parameters in Methane Hydrate Production under Depressurization of Clayey Silt Reservoir and Sandy Reservoir

Gas hydrates are likely to become an important strategic resource with commercial development prospects. It is therefore of great significance to realize the long-term and efficient production of methane hydrate reservoirs. Previous studies have shown that the lithological characteristics of hydrate reservoirs have a significant impact on reservoir productivity by influencing the evolution of seepage parameters in the process of hydrate production. The porosity (Φ) and initial hydrate saturation (SH) affect the amount of hydrate decomposition and pressure transfer, and also indirectly affect the reservoir temperature field. The permeability (k) directly affects the rate of pressure-drop transmission and methane gas discharge. Due to the differences in seepage parameters caused by different reservoir lithology, a sandy hydrate reservoir (SHR) in Japan and a clayey silt hydrate reservoir (CHR) in China were found to have different gas production rates and the spatial evolution characteristics of the temperature and pressure fields varied in gas hydrate production tests. Therefore, to ensure the long-term and efficient production of the CHR in China, two models were established for a comparative analysis based on a numerical simulation. The two models were depressurizing models of the CHR of the W11 drilling site in the Shenhu Sea area of the South China Sea and the SHR of the AT1 drilling site in the Eastern Nankai Trough of Japan. Both models considered the heterogeneity of seepage parameters, and the TOUGH+HYDARATE (T+H) code was used in subsequent calculations. Four key results were obtained: (a) The order of the significance levels of the lithological parameters on productivity was k > SH > Φ in the CHR and SH > k > Φ in the SHR. (b) The heat conduction and heat convection in the CHR were weaker than in the SHR, which made it difficult to recover the low-temperature area caused by hydrate decomposition. (c) The exploitation of a high k hydrate reservoir should be given priority when the other initial conditions were the same in both the CHR and SHR. (d) The exploitation of both the CHR and SHR should not only rely on the hydrate content or seepage capacity to determine the reservoir exploitation potential, but the combined effect of the two parameters should be fully considered.

Pore-scale observations of natural hydrate-bearing sediments via pressure core sub-coring and micro-CT scanning

Both intra-pore hydrate morphology and inter-pore hydrate distribution influence the physical properties of hydrate-bearing sediments, yet there has been no pore-scale observations of hydrate habit under pressure in preserved pressure core samples so far. We present for the first time a pore-scale micro-CT study of natural hydrate-bearing cores that were acquired from Green Canyon Block 955 in UT-GOM2-1 Expedition and preserved within hydrate pressure–temperature stability conditions throughout sub-sampling and imaging processes. Measured hydrate saturation in the sub-samples, taken from units expected to have in-situ saturation of 80% or more, ranges from 3 ± 1% to 56 ± 11% as interpreted from micro-CT images. Pore-scale observations of gas hydrate in the sub-samples suggest that hydrate in silty sediments at the Gulf of Mexico is pore-invasive rather than particle displacive, and hydrate particles in these natural water-saturated samples are pore-filling with no evidence of grain-coating. Hydrate can form a connected 3D network and provide mechanical support for the sediments even without cementation. The technical breakthrough to directly visualize particle-level hydrate pore habits in natural sediments reported here sheds light on future investigations of pressure- and temperature-sensitive processes including hydrate-bearing sediments, dissolved gases, and other biochemical processes in the deep-sea environment.

Thermodynamic modeling and measurement of CO2 clathrate equilibrium conditions with a hydrophobic surface – An application in dry water hydrate

Dry water is a powdery liquid composed of tiny water droplets surrounded by super-hydrophobic nanosilica. We used a high-pressure cell to determine the equilibrium temperature and pressure of CO2 hydrate in the presence of dry water. The Bridging-Cluster model for the hydrophobic attraction force was used to evaluate the hydrophobic force effect on the excess properties of water in the liquid phase. We modified Ballard and Sloan's framework for the CO2 clathrate hydrates in the hydrate phase by introducing the Bridging-Cluster model. The distance of hydrate cells from the hydrophobic surface (δ) was calculated by optimizing the measured hydrate formation conditions. An absolute average relative deviation for pressure was 1.99% and 1.22% for silica 5 and 10 wt% dry water systems, respectively. The present model explains the promotion effect of the dry water hydrate system and the calculated equilibrium formation conditions with high accuracy.

Pore-fracture network alteration during forced and spontaneous imbibition processes in shale formation

Imbibition controls mass transfer in the complex pore-fracture network in shale, which may change the pore-fracture network and lead to a low efficient flowback of fracturing fluid. Thus, it is necessary to accurately characterize the pore-fracture network alteration during the imbibition process. In this study, forced and spontaneous imbibition tests were conducted under the confining pressure on selected shale core samples with induced fractures, collected from Longmaxi Formation, Sichuan Basin, China. The low-field nuclear magnetic resonance (NMR) spectrometer was employed to monitor the variation of pores and micro-fractures in the shale core samples during the dynamic imbibition process. In addition, optimal surface relaxivities, ranging from 0.019 to 0.033 μm/ms, were determined by comparing the NMR T2 distributions with the pore size distributions (PSDs) measured via high-pressure mercury intrusion (HPMI) tests. Then, the measured dynamic T2 spectra with three distinct peaks were converted into the corresponding PSDs to quantitatively analyze the number, size, and connectivity changes of small pores, large pores, and micro-fractures in shale. Results show that the total porosities of the four shale core samples are increased by 3.5%, 10.2%, 32.9%, and 36.3% after the imbibition tests. The forced imbibition leads to more remarkable improvements in the pore volume of large pores with radius between 0.2 and 3.6 μm. In contrast, the spontaneous imbibition results in more significant increases in the number and size of small pores with radius between 0.0004 and 0.36 μm. It is also found that the total porosity increment is primarily an outcome of small pore alteration during the imbibition. Moreover, the enlarged pores and micro-fractures are mainly categorized as the capillary bounded fluid pores and movable fluid pores, which significantly affect the efficiency of oil and gas transfer in shale. The findings of our study demonstrate the comprehensive effects of capillary force, clay hydration, osmotic potential, confining and pore pressures, and creep deformation and failure on the pore-fracture network alteration in shale and advance the understanding of the mechanisms behind the forced and spontaneous imbibition processes.

Impact of physiological factors on longitudinal structural MRI measures of the brain

Longitudinal MRI is used in clinical research studies to examine illness progression, neurodevelopment, and the effect of medical interventions. Such studies typically report changes in brain volume of less than 5%. However, there is a concern that these findings could be obscured or confounded by small changes in brain volume estimates caused by physiological factors such as, dehydration, blood pressure, caffeine levels, and circadian rhythm. In this study, MRI scans using the ADNI-III protocol were acquired from 20 participants (11 female) at two time points (mean interval = 20.3 days). Hydration, systolic and diastolic blood pressure, caffeine intake, and time of day were recorded at both visits. Images were processed using FreeSurfer. Three a priori hypothesised brain regions (hippocampus, lateral ventricles, and total brain) were selected, and an exploratory analysis was conducted on FreeSurfer's auto-segmented brain regions. There was no significant effect of the physiological factors on changes in the hypothesised brain regions. We provide estimates for the maximum percentage change in regional brain volumes that could be expected to occur from normal variation in each of the physiological measures. In this study, normal variations in physiological parameters did not have a detectable effect on longitudinal changes in brain volume.

Wettability of Graphite Under 2d Confinement

The thermodynamics of solid/liquid interfaces under nanoconfinement has tremendous implications for liquid transport properties. Here using molecular dynamics, we investigate graphite nanoslits and study how the water/graphite interfacial tension changes with the degree of confinement. We found that, for nanochannel heights between 0.7nm and 2.6nm, graphite becomes more hydrophobic than in bulk, and that the value of the surface tension oscillates before eventually converging towards a constant value for larger slits. The value of the surface tension is correlated with the slip length of the fluid and explained in terms of the effective and interfacial density, hydration pressure and friction coefficient. The study clearly indicates that there is a critical channel height of 0.9nm (achievable experimentally1) at which the surface tension reaches its highest value, but the water diffusion across the channel is at its minimum. The structural analysis shows that for this pore size a transition between a 2D and 3D hydrogen bond network is accompanied by an abrupt increase in conformational entropy. Our results show that the wettability of solid surfaces can change under nanoconfinement and the data can be used to interpret the experimental permeability data.

Primary deposition and early diagenetic effects on the high saturation accumulation of gas hydrate in a silt dominated reservoir in the Gulf of Mexico

On continental margins, high saturation gas hydrate systems (>60% pore volume) are common in canyon and channel environments within the gas hydrate stability zone, where reservoirs are dominated by coarse-grained, high porosity sand deposits. Recent studies, including the results presented here, suggest that rapidly deposited, silt-dominated channel-levee environments can also host high saturation gas hydrate accumulations. Here we present several sedimentological data sets, including sediment composition, biostratigraphic age from calcareous nannofossils, grain size, total organic carbon (TOC), C/N elemental ratio, δ13C-TOC, CaCO3, total sulfur (TS), and δ34S-TS from sediments collected with pressure cores from a gas hydrate rich, turbidite channel-levee system in the Gulf of Mexico during the 2017 UT-GOM2-1 Hydrate Pressure Coring Expedition. Our results indicate the reservoir is composed of three main lithofacies, which have distinct sediment grain size distributions (type A-silty clay to clayey silt, type B-clayey silt, and type C-sandy silt to silty sand) that are characteristic of variable turbidity current energy regimes within a Pleistocene (< 0.91 Ma) channel-levee environment. We document that the TOC in the sediments of the reservoir is terrestrial in origin and contained within the fine fraction of each lithofacies, while the CaCO3 fraction is composed of primarily reworked grains, including Cretaceous calcareous nannofossils, and part of the detrital load. The lack of biogenic grains within the finest grained sediment intervals throughout the reservoir suggests interevent hemipelagic sediments are not preserved, resulting in a reservoir sequence of silt dominated, stacked turbidites. We observe two zones of enhanced TS at the top and bottom of the reservoir that correspond with enriched bulk sediment δ34S, indicating stalled or slowly advancing paleo-sulfate-methane transition zone (SMTZ) positions likely driven by relative decreases in sedimentation rate. Despite these two diagenetic zones, the low abundance of diagenetic precipitates throughout the reservoir allowed the primary porosity to remain largely intact, thus better preserving primary porosity for subsequent pore-filling gas hydrate. In canyon, channel, and levee environments, early diagenesis may be regulated via sedimentation rates, where high rates result in rapid progression through the SMTZ and minimal diagenetic mineralization and low rates result in the stalling of the SMTZ, enhancing diagenetic mineralization. Here, we observed some enhanced pyritization to implicate potential sedimentation rate changes, but not enough to consume primary porosity, resulting in a high saturation gas hydrate reservoir. These results emphasize the important implications of sedimentary processes, sedimentation rates, and early diagenesis on the distribution of gas hydrate in marine sediments along continental margins.

Spatial differences in pressure and heat transfer characteristics of CO2 hydrate with dissociation for geological CO2 storage

The submarine CO2 hydrate-based geological storage is an effective method for carbon capture and storage, which is significant for mitigating the greenhouse effect. The pressure and temperature variation can affect the storage stability of CO2 hydrate. In order to investigate the spatial differences in pressure and heat transfer characteristics of CO2 hydrate mixed system and the morphological evolvement process with dissociation during depressurization stage, a 100 L high-pressure reactor is used to take comprehensively experimental study. Furthermore, the effects of environmental heat flux and hydrate volume fraction on dissociation behavior and hydrate stability are also analyzed. The results indicate that the energy of hydrate mixed system is spatially inhomogeneous during dissociation and the heat flux flows to the region where the sensible heat is greatly consumed. And the bottom temperature is approximately 1.4-fold versus upper temperature. The depressurization dissociation process can be divided into dissociation stable stage (DSS) and dissociation weakening stage (DWS). The duration of DSS at depressurization stage of 1.5–1.2 MPa is around 3-fold versus stage of 2.5–2.0 MPa.

Experimental study on cement sheath integrity based on fiber Bragg grating technology

Fiber Bragg grating (FBG) sensing technology is a new structure monitoring technology. Aiming at the influence of hydration and cyclic casing pressure on cement sheath integrity in real wellbore conditions, firstly, the temperature and strain variation regularities of cement sheath in setting stage are investigated by FBG and a test device for evaluating cement sheath integrity. Then the circumferential strains of cement sheath and casing under cyclic casing pressure are studied. Furthermore, the applicability of FBG technology is verified. The results show that the temperature and strain of cement slurry change rapidly in 5 hours after pouring. In cyclic casing pressure stage, circumferential strains of cement sheath and casing are both tensile strains. In addition, circumferential strain of cement sheath and casing increases with the increase of casing pressure. The FBG sensors have high sensitivity that can reflect the change of casing pressure in time. The FBG technology is a scientific temperature and strain monitoring method for cement sheath. The results of this study are of great significance to the application of FBG technology for cement sheath integrity.

Fluid production behavior from water-saturated hydrate-bearing sediments below the quadruple point of CH4 + H2O

Driven by the large resource volume, extraction of CH4 from hydrate reservoirs has received worldwide attention. Depressurization is a feasible method and has been widely used in past field tests. However, fluid production behavior from water-saturated hydrate-bearing sediments below the quadruple point of CH4 + H2O has not been well understood and warrants investigation. In this study, we designed a series of experiments using ultra-deep depressurization with bottom hole pressure (BHP) of 1.0 MPa, 1.5 MPa and 2.1 MPa in water-saturated hydrate-bearing sediments to investigate the fluid production behavior, particularly in the region with potential ice or secondary hydrate formation. The effect of hydrate saturations and pressure drawdown rates on fluid production below the quadruple point were systematically examined. Our experimental results show that ice formation was likely based on the temperature response, however, the self-preservation effect by ice formation prevents a further drop in temperature to its equilibrium temperature at the corresponding BHP. The gas and water recovery ratios increased by 6.5% and 49.1%, respectively for BHP below the quadruple point. No significant benefit of water trapping due to ice formation was observed. Faster pressure drawdown rate results in a more substantial temperature drop, which could trigger ice formation. Still, interestingly gas production was continuous and not affected in the phase diagram of ice + CH4. Higher hydrate phase saturation improves gas production rate and final gas recovery ratio, yielding a lower water-gas ratio for more economic-viable CH4 recovery. Our results indicate that ultra-deep depressurization could be one technical feasible option for increasing both gas production rate and final recovery ratio. The findings provide valuable information for designing an ultra-deep depressurization scheme based on SH of interest in future production tests.

Simulation of the hydrate blockage process in a water-dominated system via the CFD-DEM method

With oil and gas exploitation developing towards deep-water fields, the extreme, low-temperature, and high-pressure environments are ideal for hydrate formation in subsea pipelines. Hydrate formation, growth, aggregation, and deposition in the flowlines substantially increase the hydrate blockage risk, necessitating the assessment and clarification of this process. This work employed computational fluid dynamics (CFD) and the discrete element method (DEM) to perform several numerical simulations using Fluent and EDEM to study the hydrate blockage process caused by aggregation and deposition. A three-dimensional model of a solid-liquid pipeline containing hydrate particles was established, which considered the cohesion force between the hydrate particles and the adhesion force between the hydrate particles and pipe wall. Two simulation strategies were implemented. The first simulated the hydrate blockage process using a throttling element, which assumed hydrate depositions on the pipeline wall. The second studied the impact of the collision between upstream and downstream hydrate aggregates. The critical hydrate blockage state was roughly defined based on the hydrate volume fraction and pressure drop changes during the hydrate aggregate collision process. These results represented the initial work for simulating the hydrate blockage process via combined CFD-DEM by considering the bidirectional interaction between the continuous water phase and discrete hydrate particles. These preliminary findings provided theoretical support for studying the hydrate blockage mechanism to solve the hydrate flow assurance issue in the subsea pipelines.

Effect of HFE254 or Cyclopentanone on Phase Equilibrium Dissociation Conditions for Carbon Dioxide Hydrate

Hydrate equilibrium conditions of the CO2 + HFE254 or cyclopentanone + water system are determined by the isometric method. The presence of cyclopentanone promotes CO2 hydrate formation, while HFE254 does not affect the equilibrium pressure of CO2 hydrate. The hydrate dissociation temperature of the CO2 + cyclopentanone system increases as the pressure increases. However, the hydrate equilibrium temperature gradually decreases when pressure is above 3.2 MPa and a mixed hydrate of structure I and structure II forms. It can be seen that cyclopentanone is a thermodynamic accelerator that can promote the formation of CO2 hydrate.

Effects of temperature holding on methane hydrate decomposition process by thermal stimulation

Methane hydrate is an important clean energy source. In this study, the decomposition curve of methane hydrate in the thermal stimulation process with the heating rate of (0.9, 1.8, and 2.7) K·min−1 is found and summarized. When the temperature holding process occurs during the thermal stimulation process, the hydrate decomposition cannot be stopped immediately. The pressure of the reactor deviates from the thermal decomposition curve of methane hydrate without temperature holding process due to the impact of temperature fluctuation. It was found that when the holding temperature increased from 277.56 K to 281.15 K at the heating rate of 1.8 K·min−1, the maximum temperature fluctuation increased from 0.46 K to 1.25 K. Thus, the decomposition pressure of methane hydrate with a temperature holding process is higher than that of methane hydrate without a temperature holding process. After the temperature fluctuation process, the methane hydrate equilibrium curve under the equilibrium state reached by the hydrate at the holding temperature is summarized. When the temperature holding process is over, the temperature of the reactor continues to rise, leading to the decomposition of methane hydrate. The heat in the reactor is slowly transferred again. Then, the methane hydrate decomposition curve with and without temperature holding process coincide. It was found that when the holding temperature increases from 277.56 K to 281.15 K at the heating rate of 1.8 K·min−1, the percentage of pressure difference between the equilibrium curve and the decomposition curve of methane hydrate increases from 4.34% to 8.81%. The initial heating temperature and heating step have no effect on the decomposition characteristics of methane hydrate with a temperature holding process. The heating rates can affect the slope of the hydrate decomposition curve. This study is of great significance for the safe application of methane hydrate thermal stimulation process.

In Situ Determination of Solid Fraction from the Measured Hydrate Slurry Flow Rate and Pressure Drop across Orifice

Two-phase flow is encountered in various engineering areas, including the pharmaceutical, chemical, and food industries, desalination facilities, and thermal energy storage systems. Cost-effective and non-invasive monitoring of the solid volume fraction, which governs the thermos-physical properties of two-phase medium, is important for flow assurance. The flow loop having an inner diameter of 21.5 mm and length of about 12.2 m was equipped with square-edged orifice and slash plate pump. Tetrafluroethane (R134a) hydrate slurry of the specified solid volume fraction could be formed within the flow loop by removing an appropriate amount of water, and simultaneously injecting the pertinent amount of R134a while chilled at 275 K. The uncertainty in the thus-obtained solid volume fraction was smaller than 9%, with the largest contribution originating from the uncertain hydration number. The near power-law relationship between the orifice pressure loss coefficient and Metzner–Reed Reynolds number was recognized. However, the nonlinear nature of the Reynolds number with respect to the solid volume fraction inevitably makes the solution procedure iterative. The short span pressure differences across the orifice were regressed to yield empirical correlation, with which the solid volume fraction of R134a slurry could be determined from the measured pressure drop across the orifice and the flow rate. The uncertainty was less than 12% of the thus determined solid volume fraction.

A state-dependent hypoplastic model for methane hydrate-bearing sands

The mechanical behaviors of methane hydrate-bearing sands (MHBS) are largely affected by the presence of methane hydrate, temperature, and pore pressure. In this study, we present a simple hypoplastic model for MHBS. Methane hydrate saturation is included as a state parameter affecting the mechanical behaviors of MHBS. A new phase parameter is introduced to account for the coupled effects of temperature and pore pressure on the mechanical behaviors of MHBS. The phase parameter can be determined by a simple function of temperature and pore pressure. Comparison of the predictions with experiments shows that the model is able to capture the salient behaviors of MHBS.