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Density‐driven convection mixing has been identified to enhance CO2 trapping in deep saline aquifers storage. In this study, the dynamics of fingering instability and associated mass transfer caused by convection mixing between the movements of miscible analogue fluid pairs are investigated by magnetic resonance imaging (MRI). Two kinds of homogeneous porous media and six kinds of heterogeneous porous media with varying permeability permutations were used at ambient pressure and temperature. In homogeneous porous media, the detail analyses on the data of fingering pattern, finger number and distribution identify that the large finger sizes and high sinking velocities can be developed when the permeability up to 3.3 × 10−11 m2. In heterogeneous porous media, the sand layer first touched by downward fingers plays a dominant role in finger evolution, channeling and fingering convection mechanisms were found among these cases. Moreover, the dissolution rate characterized by Sherwood number (Shm) (enhance or inhibit fluid mixing, depending on the layout of the sand layer) was found to be independent of the local convection dissolution, such as vortex flow and finger splitting at the sand boundary with mutation permeability (enhanced fluid mixing, promoting CO2 dissolution in carbon storage). Finally, the total CO2 dissolution rate Shm in all cases was compared, Shm are at a range of 970–5,079 in homogeneous porous media, while are 190–688 in heterogeneous porous media. Key Points: Heterogeneity plays a complex role in convection mixing, according to an analysis of the convection finger diameter and migration rateThe front fingers flow along the direction approximately orthogonal to the inclined boundary after entering a low‐permeability sand layerA boundary layer in heterogeneous porous media accelerates the local dynamic fracture‐matrix mass transfer [ABSTRACT FROM AUTHOR]

Highlights • The dissolution of brine injection into a CO 2 -filled packed bed was investigated. • An image analysis technique was developed to determine the interfacial area. • The effects of Re, gravity and heterogeneity on the interface were analysis. Abstract Explicit knowledge on the two-phase interface evolution during CO 2 dissolving in brine provides accurate predictions on the subsurface behavior of long-term CO 2 storage. In this research, the interfacial areas of CO 2 -unsaturated brine were dynamically measured during multiphase flow using 3D quantitative analyses. The two-phase interfaces during brine imbibition were divided into three terms based on their attributes, i.e., ganglia, cluster and singlet. The evolution terms of the interfaces were interesting, as their fates showed wide evolution patterns due to the diverging effects of the Reynolds number (fluid velocity × length scale/fluid viscosity) and gravity. The brine bypassed the CO 2 , and the interface evolved with the development of a priority path under a heterogeneity impact. Relying on the approach of the slice-averaged and volumetric measurement, the effects of forces and heterogeneity on the CO 2 -unsaturated brine interface were evaluated on different directions. Linear regression of the clouded data points exploited the validity of the power-law distribution from number of trapped cluster to frequency of interfacial area, and the max interfacial areas and variance decreased, while the mean interfacial area increased with brine saturation. Slice-averaged CO 2 -brine interfacial areas normalized by volume or geometric surface area decreased linearly with the brine saturation at different Reynolds numbers. [ABSTRACT FROM AUTHOR]

Highlights • Dynamic CO 2 injection with different flow rates was visually investigated into three stages. • The saturation gradient existed along the porous media and gradually vanished for low flow rates. • The apparent saturation at breakthrough and steady state were linearly related to the maximum saturation change rate. • Too low or high flow rate resulted in unstable front, relatively low saturation, against the utilization of pore space. • The effect of flow rate on storage and utilization of pore space can be applied to Sleipner CO 2 Storage Project. Abstract This study aimed to investigate dynamic CO 2 drainage using high-resolution magnetic resonance imaging (MRI) technology. Gaseous and supercritical CO 2 were injected downward in brine-saturated porous media at different flow rates at 40 °C/6 MPa and 40 °C/8 MPa. These flow rates (0.015, 0.03 and 0.1 mL/min, under 10 Mt/year), as reflected in, were chosen according to the distance (1 km–100 m) from the injection well. Three stages were found from the change of signal intensity during CO 2 drainage: before the CO 2 front reached the field of view (FOV), breakthrough, and steady state. Channelling or drainage fronts immediately established through the large pores, and CO 2 travelled vertically through these channels until breakthrough. The breakthrough time decreased with increasing flow rates and was longer for ScCO 2 than gCO 2 at the same flow rate, resulting in a longer residence time for ScCO 2 in the sample. At low flow rates, the fingers first established along the larger pore spaces (especially at 0.015 mL/min) and then gradually extended into adjacent regions, resulting in a relatively flat interface. However, at high flow rates, the front moved along the larger pore spaces until breakthrough. The flow patterns for ScCO 2 drainage were more uniform than those for gCO 2 drainage. The pore volume fraction occupied by CO 2 , as a quantitative parameter of the flow pattern, reflected that the sweep efficiency and pore space utilization were optimized at 0.03 mL/min (Ca = 4.35 × 10−9 for gCO 2 and Ca = 1.06 × 10-8 for ScCO 2). The effect of the flow rate on the CO 2 saturation and distribution was analysed. At low flow rates, the saturation gradient along the porous media gradually reduced, but trend to be stable at high flow rates. Additionally, the saturation at breakthrough and steady state were observed to be linearly related to the maximum rate of change in saturation during CO 2 injection. Overly fast drainage results in relatively low saturation and an inhomogeneous distribution. The results can be applied to provide information for enhancing pore space utilization and improving sweep efficiencies during field storage. [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]

The CO2-brine dissolution homogenizes the distribution of residual CO2 and reduces the leakage risk in the saline aquifer. As a key parameter to immobilize the free CO2, the dissolution rate of CO2-brine could be accelerated through mechanisms like diffusion and dispersion, which are affected by the subsurface condition, pore structure, and background hydrological flow. This study contributed the calculated dissolution rates of both gaseous and supercritical CO2 during brine imbibition at a pore-scale. The flow development and distribution in porous media during dynamic dissolution were imaged in two-dimensional visualization using X-ray microtomography. The fingerings branching and expansion resulted in greater dissolution rates of supercritical CO2 with high contact between phases, while the brine bypassed the clusters of gaseous CO2 with a slower dissolution and longer duration due to the isolated bubbles. The dissolution rate of supercritical CO2 was about two or three orders of magnitude greater than that of gaseous CO2, while the value distributions both spanned about four orders of magnitude. The dissolution rates of gaseous CO2 increased with porosity, but the relationship was the opposite for supercritical CO2. CO2 saturation and the Reynolds number were analyzed to characterize the different impacts on gaseous and supercritical CO2 at different dissolution periods. [ABSTRACT FROM AUTHOR]

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 0.8 mL min − 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. [ABSTRACT FROM AUTHOR]

The purpose of this work is to develop a permeability estimation method for porous media. This method is based on an improved capillary bundle model by introducing some pore geometries. We firstly carried out micro-CT scans to extract the 3D digital model of porous media. Then we applied a maximum ball extraction method to the digital model to obtain the topological and geometrical pore parameters such as the pore radius, the throat radius and length and the average coordination number. We also applied a random walker method to calculate the tortuosity factors of porous media. We improved the capillary bundle model by introducing the pore geometries and tortuosity factors. Finally, we calculated the absolute permeabilities of four kinds of porous media formed of glass beads and compared the results with experiments and several other models to verify the improved model. We found that the calculated permeabilities using this improved capillary bundle model show better agreement with the measured permeabilities than the other methods. [ABSTRACT FROM AUTHOR]

CO2/CH4 replacement in hydrate exploitation is considered a promising method to enhance natural gas production and mitigate the greenhouse effect. Since the gas concentration in mining hydrate sediment changes with the displacement process, it is crucial to investigate the hydrate formation and dissociation characteristics with the presence of different gas mixtures. To obtain fundamental thermodynamic data, CO2/CH4 gas mixture hydrate phase equilibrium conditions in porous media were experimentally investigated. The experiments were conducted at 276.55-284.85 K and 2.10-6.80 MPa using an isochoric method with different CO2/CH4 gas mixture compositions (CO2 mole fractions were 0.798, 0.499, and 0.199 mol/mol (79.8, 49.9, and 19.9 mol%)). The results showed that gas mixtures with higher CO2 mole fractions had lower hydrate equilibrium pressure ( p) under the same temperature ( T) according to the experimental data. The two different porous media used in the experiments had a slight influence on gas mixture hydrate phase equilibrium conditions. An improved thermodynamic model was proposed to calculate the CO2/CH4 hydrate phase equilibrium conditions in porous media, and the prediction results fitted closely to the experimental data. [ABSTRACT FROM AUTHOR]

The geological sequestration of carbon dioxide (CO2) is considered a promising technology for CO2 emissions. However, accurate estimation of the storage capacity for CO2 remains a challenge owing to the effect of capillary pressure variation. The CO2-salty aqueous contact angle and sub-core scale heterogeneity are the major factors that influence capillary pressure. This paper reports the results of CO2/brine two-phase flow experiments in porous media by using an X-ray computed tomography (CT) technique at 40°C and 8 Mpa and examines the strong influence of wettability and sub-core scale heterogeneities on the spatial distribution of CO2. Gas injection experiments were conducted at flow rates of 0.1 mL/min, 0.2 mL/min, and 0.5 mL/min, and the porous media were packed with feldspar, quartz, or dolomite. This study indicates that at the core scale, the wettability of porous media is a crucial factor in determining CO2 saturation. Capillary fingering and low CO2 saturation occurred with high water wettability. Sub-core scale heterogeneities controlled the CO2 distribution and saturation variation along the porous media, particular that with small pore sizes. © 2015 Society of Chemical Industry and John Wiley & Sons, Ltd [ABSTRACT FROM AUTHOR]

Methane hydrate (MH) is a clean energy source with significant potential. However, the hydrate formation/reformation characteristics during the natural gas hydrate (NGH) exploration process must be elucidated. In this study, methane hydrate was formed/reformed in three different modes to simulate the mining of NGH sediment. The experimental results indicated that when the hydrate forms in partly water saturated porous medium in a flow of methane gas, the methane gas flow rate does have a significant effect on hydrate saturation. Hydrate could be formed in larger amounts and faster by the methane displacement of the water-saturated porous medium than in any other experimental mode. However, the pressure curves fluctuated strongly in this mode. When the hydrate was reformed at a constant pressure in partly water-saturated porous medium, the hydrate saturation maxima increased. However, when the hydrate was reformed by flowing methane through the partly water-saturated porous medium, the hydrate saturation maxima decreased. [ABSTRACT FROM AUTHOR]

It is of great importance to investigate gas and water flow characteristics to better understand the geological CO2 sequestration process. The interfacial tension between CO2 and brine and the wettability of reservoir rocks are the most important parameters for two-phase flow in porous media that have a significant influence on the capacity of CO2 storage. In this paper, we present a set of interfacial tension data between CO2 and a sodium chloride (NaCl) solution and the contact angle data between CO2, NaCl and quartz by using a visual experimental method at multiple pressure, temperature and salinity conditions. We also performed simulations of CO2 and NaCl solution two-phase flow in quartz bead-packed beds by introducing interfacial tension and contact angle data into the pore network model to evaluate the relative permeability and capillary pressure. We found that the phase alternation of CO2 from a gas to its supercritical state has a significant influence on the wettability of pore surfaces and thus governs the capillary trapping mechanism. This indicates that CO2 sequestration efficiency in a saline aquifer will be greatly affected while CO2 is in its different phase states. The simulation results showed that a pore network model is available for the study of CO2 and brine two-phase flow characteristics in porous media. © 2015 American Institute of Chemical Engineers Environ Prog, 34: 1756-1762, 2015 [ABSTRACT FROM AUTHOR]

The study of immiscible fluid displacement between aqueous-phase liquids and non-aqueous-phase liquids in porous media is of great importance to oil recovery, groundwater contamination, and underground pollutant migration. Moreover, the attendant viscous, capillary, and gravitational forces are essential to describing the two-phase flows. In this study, magnetic resonance imaging was used to experimentally examine the detailed effects of the viscous, capillary, and gravitational forces on water–oil flows through a vertical straight capillary, bifurcate channel, and monolayered glass-bead pack. Water flooding experiments were performed at atmospheric pressure and 37.8 °C, and the evolution of the distribution and saturation of the oil as well as the characteristics of the two-phase flow were investigated and analyzed. The results showed that the flow paths, i.e., the fingers of the displacing phase, during the immiscible displacement in the porous medium were determined by the viscous, capillary, and gravitational forces as well as the sizes of the pores and throats. The experimental results afford a fundamental understanding of immiscible fluid displacement in a porous medium. [ABSTRACT FROM AUTHOR]

A clear understanding of two-phase flows in porous media is important for investigating CO geological storage. In this study, we conducted an experiment of CO/brine flow process in porous media under sequestration conditions using X-ray CT technique. The flow properties of relative permeability, porosity heterogeneity, and CO saturation were observed in this experiment. The porous media was packed with glass beads having a diameter of 0.2 mm. The porosity distribution along the flow direction is heterogeneous owing to the diameter and shape of glass beads along the flow direction. There is a relationship between CO saturation and porosity distribution, which changes with different flow rates and fractional flows. The heterogeneity of the porous media influences the distribution of CO; moreover, gravity, fractional flows, and flow rates influence CO distribution and saturation. The relative permeability curve was constructed using the steady-state method. The results agreed well with the relative permeability curve simulated using pore-network model. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]

CO2 flooding is one of the most popular secondary or tertiary recoveries for oil production. It is also significant for studying the mechanisms of the two-phase and multiphase flow in porous media. In this study, an experimental study was carried out by using magnetic resonance imaging technique to examine the detailed effects of pressure and rates on CO2/decane flow in a bead-pack porous media. The displacing processes were conducted under various pressures in a region near the minimum miscibility pressure (the system tuned from immiscible to miscible as pressure is increasing in this region) and the temperature of 37.8 °C at several CO2 injection volumetric rates of 0.05, 0.10, and 0.15 ml/min (or linear rates of 3.77, 7.54, and 11.3 ft/day). The evolution of the distribution of decane and the characteristics of the two phase flow were investigated and analyzed by considering the pressure and rate. The area and velocity of the transition zone between the two phases were calculated and analyzed to quantify mixing. The area of transition zone decreased with pressure at near miscible region and a certain injection rate and the velocity of the transition zone was always less than the 'volumetric velocity' due to mutual solution and diffusion of the two phases. Therefore, these experimental results give the fundamental understanding of tertiary recovery processes at near miscible condition. [ABSTRACT FROM AUTHOR]

Abstract: CO2/water displacement process in porous media under sequestration conditions was observed using high-resolution Magnetic Resonance Imaging (MRI) technique. The porous media was a packed bed filled with glass beads. Fast spin echo multi slice sequence (FSEM) was used to measure the distribution of CO2 and water in the porous media. For immiscible displacement experiments, three stages were obtained from the MR signal intensity profile. The CO2 channeling or fingering phenomena was obviously for the difference of permeability during the second stage. The final water residual saturation depended on permeability, and lower permeability always leads to larger final saturation. For miscible displacement, the displacement process was also divided into three stages from MR signal intensity profile. The MR signal intensity decreased gradually for the resolving of CO2 into water in the first stage. A piston-like miscible front moved uniformly from the top to the bottom of the FOV (field of view) during the second stage, and the MR signal intensity decreased sharply. The displacement efficiency in miscible displacement is larger than that in immiscible displacement process. It is clear that water displacement efficiency or average CO2 saturation depends on flow rate. With the CO2 flow rate increasing, the relatively uniform CO2 distribution and the uniform CO2 front occurred. Additionally, final water saturation decreased. With the core analysis methods, the CO2 velocities were obtained, which were applied to evaluate the capillary dispersion rate, viscous dominated fraction flow and gravity flow function. The capillary dispersion rate donated the effects of capillary, which was largest at water saturations of 0.6 and 0.7. At a high flow rate, the viscous force was dominated over the porous media. When at a low flow rate, the gravity force became important and played a positive role in downward displacements. [Copyright &y& Elsevier]

CO 2 flooding has been proven to be an effective method to improve oil recovery, and the interface characteristics of the displacement front directly affect the oil recovery and CO 2 utilization efficiency in reservoirs. In this study, we aimed to investigate the in situ interfacial behavior during CO 2 flooding using high-resolution magnetic resonance imaging (MRI). A series of CO 2 flooding tests were carried out using upward and downward injection methods consisting of different rates (28, 56 and 112 pore volume (PV)/day) in porous media. The unstable CO 2 front is indicated to significantly impact the gas sweep efficiency, which is not favorable for oil recovery and leads to low energy utilization efficiency. For downward injection, subordinate displacement of CO 2 was observed after breakthrough, which can improve oil recovery. The optimum oil recovery and CO 2 utilization factor were obtained under upward injection at a low rate in the immiscible state, which was controlled by the stable interface behavior. From the perspective of CO 2 storage, miscible flooding improved the storage volume by more than 25% compared to immiscible flooding. For high permeability oil reservoirs, the oil recovery and gas sequestration can be synthetically optimized by adjusting the injection rates and phase state. Image 1 • MRI data can directly reflect the fluid interfacial behavior in porous media. • A stable displacement interface benefits oil recovery and CO 2 utilization factor. • A secondary flooding phenomenon was observed for the first time. • Correlation of CO 2 -oil mixing layer and oil recovery was proposed. [ABSTRACT FROM AUTHOR]

Natural gas hydrates are abundant and occur globally. The storage and transportation of natural gas in the form of hydrates is considered to be an efficient method limited by low formation rates. In this study, the kinetic effects of SiO 2 nanofluids and porous medium on hydrate formation were studied by using magnetic resonance imaging (MRI) technology. The SiO 2 nanofluid concentration was varied from 0.1 to 0.4 wt% to analyze its influence. The results showed that hydrates formed near the wall and grew into pores with sufficient water. The concentration variation study revealed a significant decrease in induction time and increase in gas consumption during hydrate formation. Among the various concentrations studied, 0.2 wt% had the strongest promoting effect, and the induction time was shortened by 75.7%, while the gas consumption was 67.7% higher than that in pure water at an initial water saturation of 30%. Concentrations of 0.1–0.3 wt% positively influenced the hydrate formation rate, but a concentration of 0.4 wt% significantly inhibited hydrate formation. In addition, hydrate formation was affected by the initial water saturation of the porous medium. The lower the initial water saturation was, the more notable the promotion effect was. This study provides theoretical support for the storage and transportation of natural gas by combining porous medium and nanofluids. • The kinetic effects of porous medium and nanofluids on hydrate formation were first investigated by using MRI technology. • The combination of porous medium and SiO 2 nanofluids shortens the induction time of hydrate formation. • The combination of porous medium and SiO 2 nanofluid promotes hydrate formation rate and gas consumption. • Low initial water saturation accelerates the formation of hydrates. • The interaction of nanofluids with porous medium can be used in natural gas transportation. [ABSTRACT FROM AUTHOR]

Natural gas hydrates (NGHs) are new and clean energy resources with significant potential. Many studies have investigated NGHs in an attempt to recover natural gas from NGHs deposits. Additional investigations are still needed to clarify the dissociation characteristics of NGHs to develop safe and efficient recovery methods. In this study, two types of NGH deposits were simulated by forming methane hydrates (MHs) in porous media: the first type was formed with excess gas, and the other type was formed with excess water. The formed MHs were dissociated by depressurization methods. Magnetic resonance imaging (MRI) was used to monitor the liquid water distribution and quantify the MH amounts during formation and dissociation. The results showed that a larger depressurization range enhanced the average rate of MH dissociation and gas production for excess gas conditions. For excess water conditions, the mobility of liquid water was dominant during MH dissociation and hindered methane gas output. Furthermore, a larger depressurization range accelerated MH dissociation. When MH dissociations were compared for various gas-water saturated porous media, liquid water saturation and depressurization range were identified as two key factors affecting MH dissociation. [ABSTRACT FROM AUTHOR]

Gravity drainage characteristics are important to improve our understanding of gas–liquid or liquid–liquid two-phase flow in porous media. Stable or unstable displacement fronts that controlled by the capillary force, viscous force, gravitational force, etc., are relevant features of immiscible two-phase flow. In this paper, three dimensionless parameters, namely, the gravity number, the capillary number and the Bond number, were used to describe the effect of the above mentioned forces on two-phase drainage features, including the displacement front and final displacing-phase saturation. A series of experiments on the downward displacement of a viscous fluid by a less viscous fluid in a vertical vessel that is filled with quartz beads are performed by using magnetic resonance imaging (MRI). The experimental results indicate that the wetting properties at both high and low capillary numbers exert remarkable control on the fluid displacement. When the contact angle is lower than 90°, i.e., the displaced phase is the wetting phase, the average velocity V f of the interface of the two phases (displacement front velocity) is observably lower than when the displaced phase is the non-wetting phase (contact angle higher than 90°). The results show that a fingering phenomenon occurs when the gravity number G is less than the critical gravity number G ’ = Δμ /μ g . Moreover, the higher Bond number results in higher final displacing-phase saturation, whereas the capillary number has an opposite effect. [ABSTRACT FROM AUTHOR]

Marine CO 2 storage by hydrate method is a promising technology expected to achieve permanent CO 2 storage. Here, this study focuses on the formation kinetics and morphological characteristics of CO 2 hydrate under two typical scenes with seawater residual zone (50 % porewater saturation) and seawater saturated zone (100 % porewater saturation). Four kinds of porous media were selected to simulate sediments with different permeability. The results indicate that in the seawater residual zone, hydrates are mostly flaky and form granular cementation, while in the seawater saturated zone, hydrates form from the pore center and are distributed as granular filling. Poor pore connectivity and gas-liquid contact in the seawater saturated zone hinder further hydrate formation, resulting in final hydrate saturation being at least 25 % lower than that in the seawater residual zone. Additionally, the final gas consumption and water-to-hydrate conversion decreased with increasing particle size. The maximum gas consumption was 118 mmol CO 2 /mol H 2 O, with only 28 % of the water converting into hydrate. These findings are significant for marine CO 2 storage under different porewater saturation and particle size conditions. [Display omitted] • The formation kinetics and morphology of CO 2 hydrate in different sediments are evaluated. • Seawater residual zone is more conducive to marine CO 2 storage by hydrate method. • In the same area, the hydrate saturation in seawater residual zone is about 25 % higher than that in seawater saturated zone. [ABSTRACT FROM AUTHOR]

Hydrate-based gas separation is considered a promising technology for H 2 purification and CO 2 capture. To investigate the behaviour of this separation technology in a flowing system for industrial application, an orthogonal experiment was conducted to explore the effects of glass beads, flow rate, pressure and temperature on a mobile-phase (H 2 /CO 2 ) separation system in cooled porous media. The distribution of the pore solution was measured using magnetic resonance imaging (MRI) to analyse hydrate formation/dissociation. The experimental results demonstrated that solution movement scarcely occurred in BZ-01. The lowest H 2 concentration was 26.1 mol% for the hydrate dissociation process, whereas the highest value was 44.8 mol%. The overall H 2 concentration for hydrate dissociation decreased with an increase in pressure, and the hydrate saturation increased with pressure. Suitable parameters for gas separation were determined to be 1.0 mL min −1 and 281.0 K in this investigation. Variance analysis indicated that porous media and temperature significantly affect the H 2 concentrations in the hydrate dissociation process, whereas only porous media significantly affect hydrate saturation. [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]

Highlights: [•] Hydrate-based CO2–N2 separation data was obtained for flow in porous media. [•] Tetrahydrofuran and sodium dodecyl sulphate are used as additives simultaneously. [•] Solution movement rarely occurs when residual solution saturations are low. [•] Bothe of pressure and temperature have remarkable impacts on gas compositions. [•] A suitable operation parameter choice is proposed for hydrate-based CO2 capture. [ABSTRACT FROM AUTHOR]

Abstract: Hierarchically porous carbons (HPCs) have been prepared by sol–gel self-assembly technology with nickel oxide and surfactant as the dual template. The porous carbons are further activated by nitric acid. The electrochemical behaviors of supercapacitors using HPCs as electrode material in different aqueous electrolytes, e.g., (NH4)2SO4, Na2SO4, H2SO4 and KOH are studied by cyclic voltametry, galvanostatic charge/discharge, cyclic life, leakage current, self-discharge and electrochemical impedance spectroscopy. The results demonstrate that the supercapacitors in various electrolytes perform definitely capacitive behaviors; especially in 6 M KOH electrolyte the supercapacitor represents the best electrochemical performance, the shortest relaxation time, and nearly ideal polarisability. The energy density of 8.42 Wh kg−1 and power density of 17.22 kW kg−1 are obtained at the operated voltage window of 1.0 V. Especially, the energy density of 11.54 Wh kg−1 and power density of 10.58 kW kg−1 can be achieved when the voltage is up to 1.2 V. [Copyright &y& Elsevier]

• Visualization of the convection mixing process using analogue fluid pairs. • A systematic quantitative analysis of three mass transfer processes. • Compared with pure diffusion, convection mixing greatly promotes mass transfer. CO 2 geologic storage in deep saline aquifers is a promising technology to reduce greenhouse gas emissions and maintain the pace of economic development. The dissolution and convection of CO 2 injected into brines are the primary trapping mechanisms to enhance storage efficiency and provide long-term storage security. The spatial mass transfer characteristics of convection in porous media remain poorly understood. In this study, two kinds of analogous fluid pairs were used as the equivalents of CO 2 -saturated brine and in situ brine. Density-driven convection experiments using magnetic resonance imaging (MRI) technology in heterogeneous porous media were performed at various stages, including the diffusion stage, the unstable trigger stage and the convection development stage. Combined with the spatial evolution of the fingers and the change in MRI mean intensity, the local fluid concentration and overall mass transfer behavior could be quantified. Physical quantities associated with mass transfer by convection, such as the mixing layer thickness and scalar dissipation rate at the diffusion stages, the onset time at the unstable trigger stage, the average velocity of the front finger, the finger number, the transverse concentration gradient of the finger, and the sweep area of the finger at the convection development stage, were also analyzed systematically. We found that convective mixing substantially promotes mass transfer over pure diffusion, and the dissolution rate can be further enhanced by improving the injection method at each stage. The range of Ra in this study covers most CO 2 capture and storage (CCS) sites and can provide basic data for engineering applications. [ABSTRACT FROM AUTHOR]

• Density-driven convection experiments of miscible fluid pairs with viscosity contrast in porous media were conducted using MRI. • Two types of fingers form vertically symmetrical structures in density-driven convection. Convection occurring in porous media depends not only on whether the mixture has reached the critical Rayleigh number but also on the domain aspect ratio. • An increase in the viscosity contrast causes the increase of the transition zone viscosity, hindering the density-driven convection process and retarding its completion. • The dimensionless mass transfer coefficient Sherwood number shows a power law relationship with Rayleigh number. CO 2 dissolution in saline water during aquifer sequestration causes natural convection because of the density difference between CO 2 -saturated brine and native brine. In this study, the convective fingers behavior during convection in porous media was visualized by magnetic resonance imaging (MRI). Four kinds of miscible fluid pairs were used as the equivalents of CO 2 -saturated brine and native brine. Images of finger development were obtained. There are two types of convective fingers that form vertically symmetrical shapes: downward fingers of the dense fluid and upward fingers of the light fluid. The typical stages of natural convection, finger appearance, finger propagation, new finger generation, and finger coalescence were observed in this study. We analyzed the effects of viscosity contrast variation on convective onset. It was found that the viscosity contrast affects not only the convection time but also the finger growth velocity. The dimensionless mass transfer coefficient is characterized by the Sherwood number. The Sherwood number has a power law relationship with the Rayleigh number, with the exponent being larger than 1 for fluid pairs with apparent viscosity contrast. Experimental results showed that a higher viscosity contrast of the fluid pair causes a viscosity increase in the transition zone, resulting in lower mobility of the fluids and retarding convective mixing. For fluid pairs with similar densities, a high viscosity contrast is not conducive to efficient mass transfer in the density-driven convection process. [ABSTRACT FROM AUTHOR]

The wettability of porous media strongly affects the trapping mechanism and storage capacity during CO 2 geological storage. Various novel measuring methods have been proposed recently with the development of imaging techniques. This review describes the state-of-the-art wettability characterization at different scales in three aspects: conventional laboratory-based contact angle characterization to analyze the porous external surface properties, pore-scale contact angle analysis to summarize the wettability of the porous inner surface based on micromodels and X-ray microtomography, and overall wettability evaluation through nuclear magnetic resonance and thermodynamic contact angle and topology analysis. The impact of the pore structure, mineral composition, and surface roughness on pore-scale wettability is described, including contact angle hysteresis and mixed wettability. The wettability in various pore events and wettability alternation with various storage conditions and fluid displacement patterns provide a description of reservoir properties. This review also illustrates the feasibility of multiscale wettability descriptions. Future studies should focus on the interrelation among the contact angles at different scales. How the contact angle of a porous surface, especially at the pore scale, changes under reservoir conditions and the mechanism of wettability alternation still need to be further investigated. Additionally, the impact of rock wettability on storage capacity in practical applications is concluded. Future research will focus on methods to control wettability in actual storage sites to enhance economic benefits and ensure safety. [Display omitted] • Comprehensive wettability measurements at different scales are summarized. • Pore events and wetting phenomena at the pore scale are analyzed. • Wettability alternation with various components and fluid displacement patterns is revealed. [ABSTRACT FROM AUTHOR]

3D porous nitrogen-doped graphene nanosheets (PNGs) are facilely prepared via a simple one-step segmented thermal annealing method, using graphene oxide (GO) as carbon source, urea as nitrogen source and pore generator. During the pyrolysis process at 300 °C, urea and oxygenous groups on GO sheets can release large amounts of gas such as NH 3 , HCNO and H 2 O, which act as pore generators to tear and strike GO nanosheets to form 3D porous nanostructured material. When the temperature is increased up to 800 °C, a large amount of nitrogen-containing gas can react with the functional groups on GO. Consequently, the pore forming and nitrogen doping processes of GO are successfully realized and 3D PNGs are prepared. The research results show that the PNGs with an optimal N-doping content of 6.9% display a high surface area of 490.2 m 2  g −1 and excellent electrochemical capacitive performance. The outstanding electrical double-layer capacitance and the increasing pseudocapacitance are attributed to the large specific surface area and the high nitrogen content, respectively. In 1 M H 2 SO 4 electrolyte, the PNGs exhibit a specific capacitance of 337.0 F g −1 and a low capacitance loss of 2.4% after 5000 cycles. Moreover, a symmetric supercapacitor with a cell voltage of 1.3 V is assembled, which displays both high energy density and high power density of 19.9 W h kg −1 and 650 W kg −1 , respectively. The all-in-one material with excellent electrical double-layer capacitance and pseudocapacitance is promising in supercapacitor. [ABSTRACT FROM AUTHOR]

[Display omitted] • The performance of CO 2 microbubbles imposed significant effects on the tiny pore space. • Considering dissolution trapping, the total CO 2 storage efficiency reached 26.3%. • Maximum CO 2 storage efficiency and oil recovery were increased by 5.1% and 9.0%. CO 2 capture and storage technology have the potential to help reach net zero emissions. CO 2 -enhanced oil recovery (CO 2 EOR) is a favourable method. Limited pilot testing has provided preliminary evidence that the unique physical properties of gas microbubbles (MBs) can be used to improve pore utilization efficiency. In this study, supercritical CO 2 injection at 0.05 mL/min into sand samples with different grain sizes under reservoir conditions was investigated by nuclear magnetic resonance (NMR). The spatial distribution of the CO 2 and oil and the dynamic pore occupancy mechanism were determined for the first time, and the results from normal bubbles (NBs) and MBs were compared. The results show a more significant left shift of the transverse relaxation time in the case of MB injection, which means that CO 2 can easily occupy a wide range of large pore spaces. Dynamic changes in the oil volume in the four types of pores classified by the transverse relaxation time indicate that the oil in large pores is flushed earlier when MB is injected and that the CO 2 saturation in minimal pores (T 2 relaxation time range from 5 ms to 100 ms) and small pores (T 2 relaxation time range from 100 ms to 500 ms) increases. The cumulative oil volume fraction decreases by 9.0% when injecting MBs. From the perspective of carbon storage, the gas storage potential is significantly enhanced within low-permeability sand when injected with MBs. The maximum volume of free CO 2 was enhanced by 16.9% in this case. In addition, the storage efficiency is enhanced up to 5.1% for low-permeability sand and 1.6% for medium-permeability sand. However, bubble flow is not as advantageous in high-permeability sand. Considering dissolution trapping, the storage efficiency reaches 26.3%. [ABSTRACT FROM AUTHOR]

Molecular diffusion has been considered to be an underlying mechanism for many oil recovery processes. Reliable estimation of the molecular diffusion coefficient as a transport property is therefore important for CO 2 -enhanced oil recovery. In the present work, the dynamic processes of CO 2 diffusion in bulk n-decane and n-decane saturated porous media were investigated using the micro-focus X-ray CT (micro-CT) scanning technique. CO 2 diffusion was visually and quantitatively analyzed by interpreting the CO 2 concentration with grayscale CT images. Next, local CO 2 diffusion coefficients, varying with time and position, were calculated from concentration profiles based on Fick's second law. The results showed that the local diffusion coefficients in bulk n-decane demonstrate an exponential function of diffusion distance and time. The total diffusion coefficients in bulk oil under pressure from 1 to 6 MPa and temperature at 29 °C and 35 °C were calculated. The results showed that the initial pressure has a strong influence on the diffusion coefficient, i.e., high CO 2 initial pressure leads to high CO 2 diffusivity in oil. Experiment results in n-decane saturated porous media showed that the CO 2 local diffusion coefficient decreases gradually along the diffusion path with time until reaching a stable state. The total diffusion coefficients in n-decane saturated porous media were smaller than those in bulk oil under the same pressure and temperature conditions because the diffusion path is more complicated than in bulk oil. It is demonstrated that the pathways of porous media impede CO 2 mass transfer and decrease the diffusion coefficient. [ABSTRACT FROM AUTHOR]

Natural gas hydrates (NGHs) are a promising energy source with huge reserves. The dissociation characteristics of NGHs need to be clarified further for developing safe and efficient technology for its recovery. In this study, Classes 1 and 2 NGH deposits were simulated by forming methane hydrate (MH) in porous media, and MH dissociation induced by depressurization was investigated using magnetic resonance imaging (MRI). MRI showed the liquid water distribution, which was used to analyze MH formation and dissociation. The vessel pressure was also measured during the experiments, which was compared with the MRI mean intensity of liquid water. MH dissociation processes were measured and analyzed under different backpressures, from 2.2 to 2.8 MPa. It was observed that liquid water hindered methane gas output during gas production; hydrate dissociation caused the movement of some liquid water, which usually led to fluctuations in MRI signal intensity. The experimental results also indicated that the MH dissociation pattern was affected by heat transfer; although a larger depressurization range led to faster dissociation, the average dissociation rate was controlled by heat transfer. [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]

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: The hierarchically porous carbons (HPCs) are prepared by sol–gel self-assembly technology at various surfactant concentrations (from 0 to 0.55molL−1). The influences of the surfactant (CTAB) concentration on the physical and electrochemical properties of the activated HPCs are investigated by nitrogen adsorption–desorption isotherm, cyclic voltammetry, galvanostatic charge–discharge, cycle life, electrochemical impedance spectroscopy (EIS) and self-discharging in 6M KOH electrolyte. The results demonstrate that the concentration of surfactant template is a crucial factor impacting the characteristics of HPCs. With the change of surfactant concentration, the activated HPCs at C CTAB =0.27molL−1 (HPCs-3) shows the largest specific surface area of 689m2 g−1. In addition, the activated HPCs show excellent supercapacitive behaviors and three sections self-discharge mechanism. Especially, the activated HPCs-3 appears superior electrochemical stability, low IR drop of 0.01V, and the maximum specific capacitance of 272Fg−1 obtained from cyclic voltammetry at the scan rate of 1mVs−1. Moreover, the supercapacitor used the activated HPCs-3 as electrode active material exhibits a specific capacitance of 61Fg−1 at the current density of 1Ag−1, and keeps a specific capacitance of 60Fg−1 after 5000 consecutive charge/discharge cycles. [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]

• The water conversion rate in large pores of porous media was higher than that in small pores. • Gas injection from top to bottom was beneficial to the hydrate-based gas storage and transportation. • The kinetics of methane hydrate formation in size-varied porous media was explored. • The contribution of 2nd hydrate growth to total gas consumption exceeds 33.7%. Using porous media to store and transport natural gas by the hydrate method can significantly improve gas-liquid contact and increase the hydrate formation rate. Therefore, it is particularly important to understand the characteristics of hydrate formation in porous media. In this study, the formation characteristics of hydrate in porous media with different particle sizes were quantitatively investigated by low-field 1H NMR from the microscopic scale to macroscopic characteristics. The experimental results show that hydrate always tends to grow in the larger pore space area caused by the heterogeneity of the porous media, and the water conversion rate in large pores is significantly higher than that in small pores. Due to the water accumulation under the action of gravity, the distribution of hydrate shows obvious spatial heterogeneity, and the lower part is more prone to clogging. Analysis of the macroscopic characteristics of hydrate formation shows that the induction time of hydrate has no obvious relationship with the particle size of the porous media, while the formation rate and gas consumption of hydrate increase with the decrease of the particle size of porous media. The gas consumption and formation rate of hydrate in BZ01 increase by 16.85% and 174.7% respectively compared with those in BZ02/04. In addition, due to gas diffusion – hydrate film rupture – capillary force, hydrate growth has the characteristics of 2nd growth, resulting in more than 33.7% of the final gas consumption. [ABSTRACT FROM AUTHOR]