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A clear understanding of two-phase fluid flow properties in porous media is of importance to CO 2 geological storage. The study visually measured the immiscible and miscible displacement of water by CO 2 using MRI (magnetic resonance imaging), and investigated the factor influencing the displacement process in porous media which were filled with quartz glass beads. For immiscible displacement at slow flow rates, the MR signal intensity of images increased because of CO 2 dissolution; before the dissolution phenomenon became inconspicuous at flow rate of 0.8mLmin -1 . For miscible displacement, the MR signal intensity decreased gradually independent of flow rates, because supercritical CO 2 and water became miscible in the beginning of CO 2 injection. CO 2 channeling or fingering phenomena were more obviously observed with lower permeable porous media. Capillary force decreases with increasing particle size, which would increase permeability and allow CO 2 and water to invade into small pore spaces more easily. The study also showed CO 2 flow patterns were dominated by dimensionless capillary number, changing from capillary finger to stable flow. The relative permeability curve was calculated using Brooks-Corey model, while the results showed the relative permeability of CO 2 slightly decreases with the increase of capillary number., (Copyright © 2016 Elsevier Inc. All rights reserved.)

In this study, magnetic resonance imaging (MRI) was used to dynamically visualize the diffusion process of CO2 in porous media saturated with liquid hydrocarbon. Based on the assumption of semi-infinite media, effective CO2 diffusivity was obtained directly by the nonlinear fitting of one MR profile during the diffusion process. These experimental findings obtained based on MRI method showed a close agreement with the conventional pressure-volume-temperature method. The novel MRI-based technique is a time-saving approach that can reduce the duration of CO2 diffusivity measurement more than 90%, and realize rapid and accurate measurement and estimation of CO2 diffusivity., (Copyright © 2015 Elsevier Inc. All rights reserved.)

The effect of fuel origin on synergy in coal/biomass blends during co-gasification has been assessed using a congruent-mass thermogravimetry analysis (TGA) method. Results revealed that synergy occurs when ash residuals are formed, followed by an almost complete gasification of biomass. Potassium species in biomass ash play a catalytic role in promoting gasification reactivity of coal char, which is a direct consequence of synergy during co-gasification. The SEM-EDS spectra provided conclusive evidence that the transfer of potassium from biomass to the surface of coal char occurs during co-pyrolysis/gasification. Biomass ash rich in silica eliminated synergy in coal/biomass blends but not to the extent of inhibiting the reaction rate of the blended chars to make it slower than that of separated ones. The best result in terms of synergy was concluded to be the combination of low-ash coal and K-rich biomass., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

The objective of this study was to understand fluid flow in porous media. Understanding of fluid flow process in porous media is important for the geological storage of CO2. The high-resolution magnetic resonance imaging (MRI) technique was used to measure fluid flow in a porous medium (glass beads BZ-02). First, the permeability was obtained from velocity images. Next, CO2-water immiscible displacement experiments using different flow rates were investigated. Three stages were obtained from the MR intensity plot. With increasing CO2 flow rate, a relatively uniform CO2 distribution and a uniform CO2 front were observed. Subsequently, the final water saturation decreased. Using core analysis methods, the CO2 velocities were obtained during the CO2-water immiscible displacement process, which were applied to evaluate the capillary dispersion rate, viscous dominated fractional flow, and gravity flow function. The capillary dispersion rate dominated the effects of capillary, which was largest at water saturations of 0.5 and 0.6. The viscous-dominant fractional flow function varied with the saturation of water. The gravity fractional flow reached peak values at the saturation of 0.6. The gravity forces played a positive role in the downward displacements because they thus tended to stabilize the displacement process, thereby producing increased breakthrough times and correspondingly high recoveries. Finally, the relative permeability was also reconstructed. The study provides useful data regarding the transport processes in the geological storage of CO2., (Crown Copyright © 2014. Published by Elsevier Inc. All rights reserved.)

The purpose of this study was to investigate the hydrate formation and dissociation with CO2 flowing through cooled porous media at different flow rates, pressures, temperatures, and flow directions. CO2 hydrate saturation was quantified using the mean intensity of water. The experimental results showed that the hydrate block appeared frequently, and it could be avoided by stopping CO2 flooding early. Hydrate formed rapidly as the temperature was set to 274.15 or 275.15 K, but the hydrate formation delayed when it was 276.15 K. The flow rate was an important parameter for hydrate formation; a too high or too low rate was not suitable for CO2 hydration formation. A low operating pressure was also unacceptable. The gravity made hydrate form easily in the vertically upward flow direction. The pore water of the second cycle converted to hydrate more completely than that of the first cycle, which was a proof of the hydrate "memory effect". When the pressure was equal to atmospheric pressure, hydrate did not dissociate rapidly and abundantly, and a long time or reduplicate depressurization should be used in industrial application.

After obtaining experimental data of CO(2) hydrate formation and dissociation in a porous medium using magnetic resonance imaging (MRI), the purpose of this study was to analyze the different dissociation rate of CO(2) hydrate using two heating rates. Images were obtained by using a fast spin-echo sequence, and the field of view was set to 40×40×40 mm. The vessel pressure was monitored during hydrate formation and dissociation, which was used to compare with MRI mean intensity. The result indicated that the MRI could visualize hydrate formation and dissociation, and the MRI mean intensity of water was in good agreement with the vessel pressure changes. The hydrate formation and dissociation rates were also quantified using the MRI mean intensity of water. The experimental results showed that the higher heating rate caused the rapid hydrate dissociation., (Copyright © 2011 Elsevier Inc. All rights reserved.)

Leakage prevention of carbon dioxide (CO 2) determines the safety of carbon marine geological storage. Hydrate caps have been proven to be one of the most effective structures for preventing CO 2 leakage, which has attracted worldwide attention. However, the structure and formation behavior of hydrate caps during CO 2 storage process are still unclear. In this study, a strategy of CO 2 -H 2 O co-injection was proposed to simulate the actual flow process and accelerate the hydrate cap formation rate. A series of CO 2 -H 2 O flow rates (0.25–10 mL/min) were employed. The results show that there will be four stages for the dynamic formation process of hydrate cap: fluid migration and diffusion, local hydrate formation, local area plugging, and hydrate cap formation. What's more, the water-gas flow ratio is inversely proportional to hydrate cap formation rate and positively proportional to the carbon storage efficiency. Meanwhile, the huge sealed capacity of hydrate cap was also proved in this work. The sequestration pressure of CO 2 under hydrate cap exceeds 12 MPa, and the maximum carbon storage efficiency increases from 53.12% to 93.30%. Moreover, the macrostructure of hydrate cap consists of two layers with a certain hydrate saturation: the zero-permeability layer and the low-permeability layer. The zero-permeability layer determined the sealed capacity of hydrate cap. These findings may provide a theoretical foundation for the prevention of CO 2 leakage in actual project of carbon marine geological storage. • Strategy of CO 2 -H 2 O collaborative injection for hydrate cap formation is proposed. • Dynamic evolution process of hydrate cap formation is investigated. • The maximal sequestration pressure of CO 2 hydrate cap reaches 12 MPa. • The carbon sequestration efficiency increases from 53.12% to 93.30%. [ABSTRACT FROM AUTHOR]

Summary: The recent increase in atmospheric CO2 concentration makes it necessary to investigate new ways to reduce CO2 emissions. Simultaneously, natural gas hydrate mining technology is developing rapidly. The use of depleted methane hydrate (MH) deposits as potential sites for CO2 storage is relatively safe and economical. This method can alleviate the shortage of hydrate displacement gas with CO2. The purpose of this study was to investigate CO2 hydrate formation characteristics during the seepage process—in reservoirs with excess water—and their effect on CO2 storage. The experimental process can be divided into 5 parts: MH formation, water injection, MH dissociation, CO2 hydrate formation, and CO2 hydrate dissociation. Magnetic resonance imaging was employed to monitor the distribution of liquid water, and the effects of different parameters on the formation and dissociation of CO2 hydrates were analyzed. It was found that a state of initial water saturation can effectively control hydrate saturation in artificial MH reservoirs for hydrate reservoirs with excess gas. In the process of CO2 flow, initial water saturation was not the main controlling factor for CO2 hydrate formation. Increasing the flow pressure and reducing the flow rate were beneficial for CO2 hydrate formation. This study is of great significance for advancing the science of CO2 geological storage in the form of deep‐sea hydrates. [ABSTRACT FROM AUTHOR]

This study investigated the dynamic displacement and dissolution of CO2 in porous media at 313 K and 6/8 MPa. Gaseous (gCO2) at 6 MPa and supercritical CO2(scCO2) at 8 MPa were injected downward into a glass bead pack at different flow rates, following upwards brine injection. The processes occurring during CO2 drainage and brine imbibition were visualized using magnetic resonance imaging. The drainage flow fronts were strongly influenced by the flow rates, resulting in different gas distributions. However, brine imbibition proceeded as a vertical compacted front due to the strong effect of gravity. Additionally, the effects of flow rate on distribution and saturation were analyzed. Then, the front movement of CO2 dissolution was visualized along different paths after imbibition. The determined CO2 concentrations implied that little scCO2 dissolved in brine after imbibition. The dissolution rate was from 10-8 to 10-9kgm-3s-1 and from 10-6 to 10-8kgm-3s-1 for gCO2 at 6 MPa and scCO2 at 8 MPa, respectively. The total time for the scCO2 dissolution was short, indicating fast mass transfer between the CO2 and brine. Injection of CO2 under supercritical conditions resulted in a quick establishment of a steady state with high storage safety. [ABSTRACT FROM AUTHOR]

[Display omitted] • Hydrates form in water-containing sediments with flowing CO 2 under 277 K and 3 MPa. • The formed hydrates like a cap prevent the upward leakage of sequestrated CO 2. • Beneath the hydrate cap is the high-pressure CO 2 with a 7.0 MPa overpressure. • CO 2 storage capacity and safety may be improved benefiting from the hydrate cap. Sub-seabed sequestration of carbon dioxide (CO 2) is the most promising and feasible disposal of large-scale carbon storage, and the capacity and safety of CO 2 sequestration are vital considerations. However, the leakage prevention of injected CO 2 lacks necessary solutions and experimental investigations. In this study, the upward leakage process of sequestrated CO 2 through water-saturated sediments was simulated at constant 277.15 K and 3.0 MPa, and the dynamic water distributions were detected by magnetic resonance imaging (MRI) technique. Both the flux difference of CO 2 inflow and outflow and the MRI images reveal the formation of hydrates inside the sediments with seven water saturation (21.6 %∼67.8 %) that represent different stages of sequestrated CO 2 leakage process. When the initial water saturation exceeds 26.2 %, the CO 2 leakage will be discontinued after several minutes of hydrates formation, indicating the formation of hydrate cap in the CO 2 -displaced sediment aquifer. Then, the pressure of sequestrated CO 2 increases due to the plugging effect of hydrate cap, and the overpressure can reach at least 7.0 MPa. In hydrate cap zone, the overpressure of CO 2 greatly improves the storage capacity by at least 13.1 times, which may induce the increase of CO 2 sequestration capacity beneath the hydrate cap as well. These results contribute significantly toward safe CO 2 sequestration and provide fundamental data for sequestration site selection under sub-seabed areas. [ABSTRACT FROM AUTHOR]

A novel way to exploiting natural gas from methane hydrates combining carbon dioxide replacement and depressurization was experimentally studied in a high pressure vessel of 255.254 mL. Experiment results with or without the procedure of depressurization were compared to reveal the effects of depressurization on final replacement percent. Results show that making methane hydrates melt partially by depressurization will enhance replacement reaction which is restricted by the diffusion and transportation of carbon dioxide. The key factors affecting the replacement percent were investigated systematically for the first time, including: the saturation of methane hydrate, and the pressure and temperature condition. Ten runs of non-depressurization and five runs of depressurization replacement reaction were conducted with hydrate saturations ranging from 0.10 to 0.21, temperature and pressure condition above phase equilibrium line of carbon dioxide hydrates. Results show that higher saturation and temperature, and lower pressure enhance the methane exploiting and CO 2 sequestration. Pressure and temperature condition located between the phase equilibrium lines of methane hydrates and carbon dioxide hydrates is most effective with the optimum methane recovery and carbon dioxide sequestration. The combined method improves the replacement percent by about 10%, while, effects of depressurization on the stability of the sediments remains to be explored. It is supposed that methane hydrates melt once, which provides path for carbon dioxide penetrating into inner layer of hydrates. [ABSTRACT FROM AUTHOR]

The purpose of this study was to obtain the characteristics of CO 2 hydrate formation and dissociation by using different experimental modes and pressures with glass beads. From the experiments, it was found that hydrate forms rapidly in each cycle, especially at high pressure. Hydrate blockage rarely appeared during hydrate formation and dissociation when applying gas flow (Case 2); this approach was significantly better than the other two experimental modes. The maximum hydrate saturations for the three experimental modes were 28%, 24% and 67.5%. The overall hydrate saturation of Case 3 was significantly better than those of the other two cases. When a heating process was applied to induce hydrate dissociation, the rapid backpressure decrease led to migration of the pore solution during depressurization. The minimal residual water after hydrate formation resulted in consistent residual water saturation after hydrate dissociation for Case 2. Considering hydrate saturation and operating pressure, 5.00 MPa is a better choice for hydrate-based desalination and CO 2 capture. If the initial residual solution can be completely converted to hydrate, this technology has potential for industrial applications. [ABSTRACT FROM AUTHOR]

Hydrate-basedtechnology is a promising method for gas separationand seawater desalination. There is little information about the combinationof the two applications. The CO2hydrate formation anddissociation in saline water (3 wt % sodium chloride) and deionizedwater in a silica gel fitted vessel are experimentally investigatedby using a magnetic resonance imaging (MRI)-based pool measurementsystem. Three experimental cases were conducted with different procedures.MRI images and mean intensity (MI) were obtained using a spin echomultislice pulse sequence. From this study, it is found that the hydrateformation in saline solution is rapid compared to that in deionizedwater. It is caused by the “structure making” of ions.Hydrate is formed more rapidly in the flowing process than in thecooling process due to the additional mechanical effect. The so-called“memory effect” was identified for the hydrate dissociatedsolution, for which the nondissociated hydrate crystals exist. Itshows that the twice displacement is superior for experimental stability.Additionally, MR images show that the rapidly formed hydrate can causeblockages of the experimental loop. The sensitivity of the MRI systemis high when the temperature is above 284.15 K. This causes a rapiddecrease in the MI as the temperature increases. [ABSTRACT FROM AUTHOR]

Highlights: [•] CO2/N2 hydrate formation and dissociation were measured in porous media with the presence of additives mixture. [•] Increase of THF concentration decreased hydrate phase equilibrium pressure. [•] 3mol% THF was optimal choice for hydrate formation. [•] Improved thermodynamic model was proposed to predict hydrates mixture phase equilibrium. [ABSTRACT FROM AUTHOR]

Theseparation of CO2from fuel gas (CO2/H2) as hydrates was studied. In this investigation, the effectsand mechanism thereof of the additive mixture (1, 2, 3, and 4 mol% tetrahydrofuran (THF), with 1000 mg/L sodium dodecyl sulfate) onthe thermodynamic and kinetic properties of the hydrate in porousmedia were measured using an isochoric method, keeping the volumeconstant. The experimental results show that an increasing THF concentrationincreases the driving force for hydrate formation and decreases thehydrate induction time. The Langmuir constants of H2andCO2showed that H2may occupy the small cavitiesof s-II hydrate in the H2–CO2–THF–H2O system. The presence of THF results in a drastic decreaseof the hydrate phase equilibrium pressure. Higher THF concentrationscorrespond to lower hydrate phase equilibrium pressures, but the decreasein pressure with concentration slows when the THF concentration exceeds3 mol %. An improved thermodynamic model was used to predict the hydratephase equilibrium, and the calculations agreed well with the experimentaldata. [ABSTRACT FROM AUTHOR]

The study of formation and dissociation of CO hydrate in porous media was characterized by magnetic resonance imaging (MRI) system in in situ conditions. This work simulated porous media by using glass beads of uniform size. The growth and dissociation habit of CO hydrate was observed under different temperature and pressure conditions. The induction time and the hydrate saturation during the growth and dissociation process in different sizes of porous media were obtained by using the MRI signal intensity. The results indicate that hydrate growth rate and the induction time are affected by the size of porous media, pressure, and degree of supercooling. There are three hydrate growth stages, i.e., initial growth stage, rapid growth stage and steady stage. In this study, the CO hydrate forms preferentially at the surface of vessel and then gradually grows inward. The hydrate tends to cement the glass beads together and occupies the pore gradually. As the hydrate decomposes gradually, the dissociation rate increases to the maximum and then decreases to zero. [ABSTRACT FROM AUTHOR]

Abstract: The characteristics and stability conditions of carbon dioxide (CO2) hydrate formation are crucial for hydrate-based CO2 capture and storage. The effects of a mixture of additives (THF/SDS) on CO2 hydrate formation and dissociation in porous media have been investigated experimentally using a graphic method. The Gibbs phase rule is used to analyze the experimental p–T curves. Hydrate formation processes can be divided into two cases, depending on the CO2 initial state. The experimental results showed that 1000mg/L SDS is the best additive and concentration for CO2 hydrate formation among those studied in this investigation due to its shorter induction time and resultantly higher hydrate saturation than those of other concentrations. The presence of 3mol% THF dramatically decreased the hydrate phase equilibrium pressure. The hydrate equilibrium temperature is 291.55K in the aqueous phase with 3mol% THF and 0mg/L SDS, which is the highest equilibrium temperature at 3.04MPa observed in this investigation. The experimental results also showed that “pseudo-retrograde” behavior exists at nearly 3.00MPa with all SDS concentrations. An improved model is used to predict the phase equilibrium conditions for CO2 hydrates in glass beads in the presence of THF, in which the mechanical equilibrium of force between the interfaces in a hydrate–liquid–vapor system is considered. [Copyright &y& Elsevier]

Abstract: After obtaining experimental data of CO2 hydrate formation and dissociation in a porous medium using magnetic resonance imaging (MRI), the purpose of this study was to analyze the different dissociation rate of CO2 hydrate using two heating rates. Images were obtained by using a fast spin-echo sequence, and the field of view was set to 40×40×40 mm. The vessel pressure was monitored during hydrate formation and dissociation, which was used to compare with MRI mean intensity. The result indicated that the MRI could visualize hydrate formation and dissociation, and the MRI mean intensity of water was in good agreement with the vessel pressure changes. The hydrate formation and dissociation rates were also quantified using the MRI mean intensity of water. The experimental results showed that the higher heating rate caused the rapid hydrate dissociation. [Copyright &y& Elsevier]

Abstract: Experimental phase equilibrium conditions data for carbon dioxide (CO2) hydrate in porous medium with the presence of sodium chloride (NaCl) solution were investigated in this study. The experimental data were generated using graphic-method in presence of solutions contained (0, 0.2, 0.4, 0.6, and 0.8)mol/L NaCl. The results indicated the increase of NaCl concentration caused the enhancement in the equilibrium pressure of CO2 hydrate as the pore size and the temperature were kept the same. Effects of NaCl solutions on CO2 hydrate equilibrium conditions could be neglected when the temperature is lower than ice point. An improved model was used to predict CO2 hydrate equilibrium conditions, and the predictions showed good agreement with experimental measurements. [ABSTRACT FROM AUTHOR]

CO 2 hydrate caps can effectively reduce the leakage risk from subsea CO 2 sequestration. TBAB (tetrabutylammonium bromide) affects temperature and pressure required for the CO 2 hydrate formation, which improves hydrate cap application. This study analyzes hydrate formation and decomposition characteristics in TBAB + CO 2 system under different initial pressures and TBAB concentrations. The results show that the addition of TBAB effectively expand CO 2 hydrate stable area and prevent CO 2 leakage. 5 wt% TBAB not only provides crystal nucleus for hydrate formation and shortens the induction period, but also increases CO 2 hydrate cap temperature to 290.3 K. The hydrate decomposition process in TBAB system is divided into CO 2 ·nCO 2 and TBAB·26H 2 O·nCO 2 decomposition stage. Comparing with hydrate decomposition process in pure water system, it is found that TBAB mainly affects the second stage of hydrate decomposition process. In 5 wt% TBAB system, the hydrate decomposition process was affected by TBAB after more than 68 % hydrate decomposition. It is found that the hydrate decomposition rate with TBAB is almost 1/7 of that without TBAB. This study provides experimental support for expanding the application of CO 2 hydrate caps, and provides a basis for determining the geological storage area of CO 2 in sediments. • TBAB is employed to enlarge hydrate stable area to ensure CO 2 storage safety. • TBAB concentration is contributive to the expansion of thermodynamic stability. • CO 2 +TBAB hydrate forms without induction period due to the semi-structure. • The decomposition rate of CO 2 +TBAB hydrate is almost 1/7 of that in pure water. [ABSTRACT FROM AUTHOR]

The sequestration of CO 2 in natural gas hydrate reservoirs is an attractive idea for both yielding CH 4 and reducing CO 2 emissions. To investigate the kinetics of CO 2 –CH 4 gas mixture hydrate formation and dissociation, two types of CO 2 –CH 4 gas mixtures were applied to form hydrates in porous media with different porosities. The process of hydrate formation and dissociation was monitored in situ by magnetic resonance imaging (MRI) in this study. Hydrate saturation was calculated quantitatively using the mean intensity ( MI ) of each image. The results showed that the hydrates first formed in the centre of the porous media and then grew along the axis of the vessel. The hydrate saturation and the initial water saturation were linearly correlated when the initial water saturation changed from 5% to 40%. By comparing the formation process of two different CO 2 –CH 4 gas mixtures, the formation rate of the 79.8% CO 2 /20.2% CH 4 gas mixture hydrate was faster. The MRI data gave excellent information on the spatial distribution of water in the porous media during hydrate growth and dissociation. [ABSTRACT FROM AUTHOR]

[Display omitted] • The formation kinetics of CO 2 hydrate were dependent on the structure of nano-clay. • 2D nano-clay is more favored for CO 2 hydrate nucleation than 1D nano-clay. • Promotion of VNs on CO 2 hydrate formation was more than most reported nanoparticles. • Cage-like water near nano-clay significantly accelerated CO 2 hydrate nucleation. Gas hydrate can separate and capture enormous carbon dioxide (CO 2) only using water cages. Capturing CO 2 in hydrate form is viewed as a green and effective technology to reduce the global CO 2 emission. However, the sluggish kinetics and high cost of kinetics promotors of CO 2 hydrate formation limited the application of hydrate-based CO 2 capture technology. Here, we used the nano-clay prepared by cheap natural clay to promote the formation kinetics of CO 2 hydrate. At the low concentration range, the formation period of CO 2 hydrate in the two-dimensional nano-clay dispersion was significantly shorter than that of the one-dimensional nano-clay dispersion. The induction time (5 min) obtained in vermiculite nanoflakes dispersion was shorter than most solid promotors of CO 2 hydrate formation kinetics in reported works. Besides, we found that the water molecules near the negatively charged two-dimensional nano-clay surface were more likely to form the cage-like structure than those around the one-dimensional nano-clay surface. The similar hydrogen-bonded structure of the cage-like water with CO 2 hydrate made them favorable for the CO 2 hydrate formation. These results shed light on the design of the effective kinetics promotors of CO 2 hydrate and contribute to develop the hydrate-based CO 2 capture technologies. [ABSTRACT FROM AUTHOR]

• The first observation of water drawing and migration induced by CO 2 -C 3 H 8 hydrate formation in glass sand by MRI. • The location of CO 2 -C 3 H 8 hydrate-induced water migration are uncertain, but growth features are similar. • Water can be drawn from the surrounding sands by the local formation of CO 2 -C 3 H 8 hydrates. • Continuous hydrate growth towards the gas phase induces further upward-climbing of water. Hydrate-induced water migration is an unusual behaviour that occurs in certain hydrate formation processes, such as in the formation of propane-containing hydrate. This study is the first to investigate the continuous, staged processes of constant-volume formation, constant-pressure reformation and exhaust dissociation of CO 2 -C 3 H 8 hydrates using the magnetic resonance imaging (MRI) technique. Real-time MRI images indicate that the formation of CO 2 -C 3 H 8 hydrates draw water from its surroundings, causing the local water saturation to increase. As the concentrated water can further form hydrates and extend to the gas phase, the water appears as though it climbs upward through the glass sand and hydrate zone towards the gas phase. Experiments under different formation pressures indicate that the locations of hydrate-induced water migration are uncertain and depend on hydrate nucleation. The evolution processes of hydrate-induced water migration under different initial pressures have good similarity. In addition, water migration will still occur during the reformation stage. The initial pressure has great effects on both the formation and dissociation of hydrate. In addition, faster dissociation may make the water move upward due to strong gas release. The findings in this study are significant to future research on propane-containing hydrate, especially for hydrate-based seawater desalination applications. [ABSTRACT FROM AUTHOR]

Abstract Carbon dioxide (CO 2) emission is one of the primary causes of global warming, while CO 2 storage is an effective way to solve this problem. Moreover, CO 2 storage as CO 2 hydrate is regarded as a viable and promising way. The development of natural gas hydrate mining technologies has provided a safe and appropriate method to store CO 2 in depleted gas hydrate reservoirs. In addition, different concentrations of N 2 are contained in the captured CO 2 from flue gas. Therefore, CO 2 storage with different N 2 concentrations in depleted natural gas hydrate reservoirs with excess water was investigated in this study. Three types of gas (pure CO 2 , 90 mol%CO 2 + 10 mol%N 2 and 80 mol%CO 2 + 20 mol%N 2) and three pressures (3.0 MPa, 3.2 MPa and 3.5 MPa) in the constant pressure process were investigated. Magnetic Resonance Imaging (MRI) was employed to monitor the entire experimental process. The experimental results showed that the hydrate was nucleated and generated in different areas independently during the storage process. N 2 was beneficial for quick hydrate formation; however, pure CO 2 was most efficient for hydrate formation from a long-term perspective. In addition, CO 2 was primarily stored as hydrate; a portion of CO 2 was stored in the form of gas; and a small amount of CO 2 was dissolved in the pore water. Moreover, the CO 2 storage density (226.56 kg/m3) was the highest at the conditions of 275.15 K and 3.5 MPa using 90 mol%CO 2 + 10 mol%N 2 in the experiments. The highest CO 2 storage density (229.99 kg/m3) calculated through water conversion occurred at the conditions of 275.15 K and 3.5 MPa using pure CO 2. Highlights • CO 2 captured from flue gas was stored in depleted methane hydrate reservoir. • CO 2 storage density in hydrate, as gas and dissolved in water were analyzed. • Water conversion to hydrate was employed to evaluate for CO 2 storage. [ABSTRACT FROM AUTHOR]

Geological sequestration of carbon dioxide (CO 2) has been considered one of the most effective strategies against global warming. The greatest concern on the stored CO 2 in sub-seabed sediments is leakage risk and can be solved by the plugging effect of CO 2 hydrate cap, which is derived from the capillary force change by hydrate crystal formation inside pores. This study experimentally simulated CO 2 upward leakage process in water-containing sediments and investigated the plugging characteristics of formed hydrate cap via magnetic resonance imaging (MRI) and flow characteristic analysis. Different CO 2 flow rates (0.3–4.0 ml/min) and initial pressures (1.8–3.0 MPa) were employed for experimental conditions, and the hydrate cap appeared with no CO 2 efflux any longer after hydrate formation for several minutes. It is found that both slow flow of CO 2 and high pressure are beneficial for the formation of hydrate cap, and the strength of hydrate caps formed in all cases is confirmed by 10.0 MPa pressure test without any CO 2 leakage. In addition, the spatial water distribution and the hydrate cap location inside the sediments are analyzed by multi-level MRI images and pressure evolution calculation, respectively. Ultimately, this study conducted a CO 2 -water flow case and found that the strength of hydrate cap increases with the continuous formation of hydrates. Approximately 27.8% of hydrate saturation is a watershed of the plugging strength of CO 2 hydrate cap. This study provides experimental evidences for the plugging effect of hydrate cap on terminating CO 2 leakage and is of great significance for the scheme design and risk assessment of CO 2 geological sequestration. [Display omitted] • MRI visualization confirm the formation of hydrate cap during CO 2 leakage. • Hydrate cap strength passes the 10.0 MPa pressure test without any CO 2 leakage. • Slow flow and high pressure of CO 2 are beneficial to hydrate cap formation. • Leakage termination of CO 2 hydrate cap requires a over 27.8% hydrate saturation. [ABSTRACT FROM AUTHOR]

• Torrefaction condensate (TC) was used for acidification of CaO sorbent. • TC-modified sorbents exhibited excellent cyclic stability over cycles. • Carbon deposition was introduced to sorbents during calcination under N 2 atmosphere. • The introduced carbon deposition acted as overlay and hindered CaO grain sintering. • Increasing torrefaction temperature and TC dosage leaded to more carbon deposition. In this work, the organic acid-rich torrefaction condensate (TC) from biomass torrefaction process was attempted to replace commercial acetic acid (AC) for acidification treatment of CaO sorbent to improve its CO 2 capture performance during calcium looping cycles. The results showed that TC-modified sorbents exhibited a much slower carbonation reactivity decay than the AC-modified one over the cycles. According to the results of characterizations (thermal decomposition, morphology, phase composition, and N 2 adsorption), the enhanced cyclic stability was probably due to the presence of carbon deposition from the decomposition of heavy tar in TC, which might act as a spacer and protective overlay to slow down the sintering of CaO particles. In addition, more carbon deposition could be dispersed in-situ into the TC-modified sorbents by increasing torrefaction temperature and TC addition amount, resulting in more thorough carbon deposition overlay and better sintering resistance. However, excessive carbon deposition (>50 wt%) leaded to slightly lower carbonation conversions and rates in the first two or three cycles. But this deficiency would disappear in subsequent cycles, as some carbon deposition was consumed by the carbon dioxide released during calcination. And the carbonation conversion of TC-modified sorbent with a carbon deposition content of 69.1 wt% was still as high as 81.5% after 10 cycles. [ABSTRACT FROM AUTHOR]

The accurate densities of CO 2 –CH 4 binary mixtures with CO 2 mole fraction of 0.0998, 0.2017, 0.3997, 0.6015, 0.7985 and 0.8988 at temperatures from (300 to 308.15) K and pressures from (2 to 18) MPa were measured using a high-precision magnetic suspension balance. The combined standard uncertainties of temperature and pressure were estimated as 0.02 K and 0.001 MPa, respectively. The density measurement uncertainty of MSB was estimated as 0.029 kg·m −3 . Taking the effect of composition uncertainty and sorption into account, the combined standard uncertainties in the density measurement of CO 2 –CH 4 mixtures were around 0.30% with CO 2 mole fraction lower than 0.60 while they increased up to 0.96% with CO 2 mole fraction higher than 0.80 at pressure of 9.0–11.0 MPa. The measured densities were compared with the calculation from GERG-2008 EOS. It showed relatively good agreements between the GERG-2008 EOS and the measured densities with CO 2 mole fraction lower than 0.60 at 300 K as the relative deviations were generally within 2.0%. When CO 2 mole fraction was higher than 0.80 at 300 K, the relative deviations firstly increased to the maximum 3.73% in the vicinity of critical pressure and then decreased close to zero with the increasing pressure. The experimental data at other temperatures have the similar variation trends. Thus it’s difficult to apply GERG-2008 EOS to predict the density characteristics of CO 2 –CH 4 binary mixtures with high CO 2 mole fraction in the vicinity of the critical point accurately. [ABSTRACT FROM AUTHOR]

• The optimal occupancy of the 51264 cages for sII CO 2 hydrate is 2 CO 2 molecules. • Pressure is the key factor in improving the stability of double occupied CO 2 hydrate. • The sII CO 2 hydrate can maximum increase the storage efficiency by 36.5 wt%. • A sub-stable structure occurred when 3 CO 2 molecules occupied in the 51264 cages. The hydrate method is an important technology that has been widely studied for carbon dioxide (CO 2) capture, utilization and storage (CCUS). Improving the gas reserves per unit volume is the key goal for this technology. Therefore, the occurrence behaviors of large CO 2 hydrate cages, the structure II, occupied with 1–5 gas molecules were investigated via molecular dynamics (MD) in this study. The occupancy rate, stability and decomposition laws of CO 2 hydrate were systematically analyzed. The results indicate that the large hydrate cages occupied with 4–5 molecules were directly broken due to excessively large intermolecular repulsion. In contrast, the large hydrate cages occupied with 3 molecules showed an obvious four-rupture-stage: (1) a small part of cages rupturing; (2) metastable cages formation along with the escape of CO 2 molecules, and the cages do not continue to break until all the CO 2 molecules escape; (3) pure residual cages rupturing, with no CO 2 gas existing in cages in this stage; and (4) complete decomposition of the CO 2 hydrate. It is interesting that the large hydrate cages occupied with 2 gas molecules are stable, and 2 is the maximum occupancy number. The pressure is more important for improving the stability of the double-occupancy hydrate, and a higher pressure (20 MPa) strengthens the stability of the CO 2 hydrate structure. The findings in this study indicate that the sII CO 2 hydrate under sufficiently high pressure can maximally increase the storage efficiency by 36.5 wt%. [ABSTRACT FROM AUTHOR]

[Display omitted] • Delamination and the electric field induced by clay particle shortens induction time. • Delayed growth kinetics observed in clay suspension with increasing mass fraction. • Water migration and clay agglomeration yielded the stratification of CO 2 hydrate and clay layer. • Heterogeneous spatial distribution of CO 2 hydrate observed in formation and dissociation. Hydrate-based CO 2 sequestration in marine sediments has emerged as a novel sustainable technology for long-term and stable CO 2 sequestration. However, the role of clay minerals in CO 2 hydrate formation and dissociation in clay-rich sediments, which are widely distributed in the South China Sea, remains controversial due to a lack of experimental evidence. This study investigates the effect of a representative clay mineral, sodium montmorillonite (Na-MMT), on the nucleation and growth kinetics of CO 2 hydrate in suspensions with a msass fraction below 20.0 wt%. The kinetic experiments reveal that Na-MMT significantly reduces the induction time due to the additional nucleation sites provided by the delamination of clay particles and the induced surface electric field. While the average growth rate of CO 2 hydrate is reduced by ∼72 % for Na-MMT mass fraction above 5.0 wt%. Morphologically, hydrate-clay stratification and gas-tunneling behaviors were observed, providing explanations for the retarded kinetics. The mass transfer of CO 2 from the gas phase to the liquid phase is impeded by the high viscosity of the suspension and the clay-induced strongly-polarized water layer, which retards the overall kinetics of CO 2 hydrate formation. Upon thermal stimulation, partial CO 2 hydrate dissociation was observed within the pure CO 2 hydrate stability region for 10.0 wt% Na-MMT suspension, indicating a possible impact of Na-MMT on CO 2 hydrate thermodynamics. Moreover, the addition of Na-MMT clay promotes CO 2 hydrate dissociation kinetics significantly. This study provides fundamental insights into the interaction between Na-MMT clay and CO 2 hydrate, offering new perspectives for designing effective strategies for CO 2 sequestration in abundant clay-rich marine sediments. [ABSTRACT FROM AUTHOR]

Abstract: The CH4–CO2 replacement method to recover methane from hydrate-bearing sediments has received great attention because it enables the long term storage of CO2 and is expected to maintain the stability of gas hydrate-bearing sediments. In this paper, the mechanical properties of CO2 hydrate-bearing sediments were measured by a low-temperature and high-pressure triaxial compression apparatus. The strength differences between the CO2 and CH4 hydrate-bearing sediments were then analyzed to evaluate the safety of the CH4–CO2 replacement method. The strength of the CO2 hydrate-bearing sediments was found to increase as the temperature and porosity decreased and as the strain rate increased. When the confining pressure was less than 5 MPa, the strength of the CO2 hydrate-bearing sediments also increased as the confining pressure increased. However, owing to pore-ice melting and particle breakage, the strength of the CO2 hydrate-bearing sediments decreased as the confining pressure increased for confining pressures exceeding 5 MPa. The strength of the CO2 hydrate-bearing sediments was found to be larger than that of the CH4 hydrate-bearing sediments, with the strengths of the CH4 and CO2 hydrate-bearing sediments varying with the influence factors in a nearly identical fashion. The results indicate that the stability of gas hydrate-bearing sediments could be maintained using the CH4–CO2 replacement method to recover methane from these sediments. [Copyright &y& Elsevier]