The wettability of porous media is of major interest in a broad range of natural and engineering applications. The wettability of a fluid on a solid surface is usually evaluated by the contact angle between them. While in situ local contact angle measurements are complicated by the topology of porous media, which can make it difficult to use traditional methods, recent advances in microfocused X-ray computed tomography (micro-CT) and image processing techniques have made it possible to measure contact angles on the scale of the pore sizes in such media. However, the effects of ionic strength, CO2 phase, and flow pattern (drainage or imbibition) on pore-scale contact angle distribution are still not clear and have not been reported in detail in previous studies. In this study, we employed a micro-CT scanner for in situ investigation of local contact angles in a CO2–brine–sand system under various conditions. The effects of ionic strength, CO2 phase, and flow pattern on the local contact-angle distribution were examined in detail. The results showed that the local contact angles vary over a wide range as a result of the interaction of surface contaminants, roughness, pore topology, and capillarity. The wettability of a porous surface could thus slowly weaken with increasing ionic strength, and the average contact angle could significantly increase when gaseous CO2 (gCO2) turns into supercritical CO2 (scCO2). Contact angle hysteresis also occurred between drainage and imbibition procedures, and the hysteresis was more significant under gCO2 condition.

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.

As a kind of potential new sources of energy, the dissociation processes of gas hydrates using the depressurization method has been investigated by experimental observations and numerical simulations. In this study, on the basis of summarizing the existing model, a one-dimensional mathematical model containing four phase (water phase, gas phase, hydrate phase, ice phase) and three constituents (water, gas, hydrate) using the finite difference method (FDM) was established for methane hydrates decomposition by depressurization in porous media. This paper focuses on the ice generation and distribution characteristics through changing the parameters of the relevant settings, and analyzes the effect of ice generation on the pressure, temperature, permeability, cumulative gas production and other parameters. The results show that, generation of ice increases gradually in the hydrates decomposition process, and occurred early near the side area because of the large pressure gradient. The absolute permeability and instantaneous gas generation rate at the early stage decline with ice generation, and the local pressure rise.

•A thermistor-based method combined with Micro-CT observation was employed.•The effective thermal conductivity of gas hydrate-bearing sediments was measured.•Various existing prediction models were evaluated with our experimental data.•A hybrid fitting model combining Parallel and Series models was proposed.As a potential alternative strategic energy source, and for their potential impact on global climate, natural gas hydrates have garnered worldwide attention. This study explored the measurement of effective thermal conductivity of methane hydrate-bearing sediments and evaluated existing models for the prediction of effective thermal conductivity. A thermistor-based method combined with Micro-CT observations was employed in the determination of the effective thermal conductivity of porous matrix materials with various hydrate and water saturation levels and physical characteristics. The effects of sample component characteristics, including the volume content of hydrate and water, phase conversion, and properties of porous materials, on the effective thermal conductivity of hydrate-bearing sediments were systematically evaluated. The effective thermal conductivity positively correlated with hydrate saturation, water content, and the thermal conductivity of porous media. In addition, the effective thermal conductivity slightly increased with hydrate dissociation, indicating an increasing heat transfer capacity during gas production. Existing prediction models were evaluated using our measured results, and a hybrid model combining the parallel and series models was proposed with expanded applicability to the scope of our research. The feasibility of the proposed model was also verified in a comparison with previous research. The results of this study are important for future investigation of the actual thermal properties of hydrate-bearing sediment and the understanding of the heat transfer mechanism during gas production. Furthermore, the results can provide guidance in the selection of an optimal technique for gas production from hydrate deposits at the field scale.

•The transport during brine injection into a gaseous CO2-filled packed bed was investigated.•An image analysis technique was employed to determine the interfacial area.•Local and overall mass transfer coefficients were calculated.A clear understanding of mass transfer properties during fluid injection in porous media is important for safe CO2 storage. In this study, dynamic experiments were conducted to elucidate transport during brine injection into a gaseous CO2-filled packed bed using X-ray CT technology. Homogenous glass beads were packed into the bed. Brine was injected downward at 0.005 and 0.03 mL/min. The CO2 distribution during brine injection was visualized along the flow direction, which reflected the mechanism transform between displacement and dissolution. The results showed that the distribution was related to the pore structure of the packed bed and flow rates. In addition, CO2 saturation and concentration profiles were obtained at various time points. CO2 concentrations predicted from saturation were orders of magnitude lower than equilibrium solubility. An image analysis technique was employed to determine the interfacial area, which was a function of saturation. Local and overall mass transfer coefficients which were correlated with local concentration and specific interfacial areas were calculated. This study provided a quantitative investigation of gas–liquid mass transfer in a packed bed, which contributed a calculated mass transfer rate during CO2 storage.

•The total number of moles of CO2 stored is unequal to CH4 recovered.•CO2 storage efficiency was first discussed in terms of CH4/CO2 replacement.•Two replacement methods using different experimental conditions were evaluated.•Effects of the freezing point on CH4 recovery and CO2 storage were investigated.•The combined method effectively improved CH4 recovery and energy efficiency.The replacement of CH4 by CO2 in methane hydrates is a promising method for simultaneously achieving CO2 storage and CH4 recovery for global warming mitigation and energy production, respectively. However, gas replacement is restricted to the slow diffusion-limited transport of CO2 caused by the formation of a mixed hydrate layer, and little attention has been paid to the storage of CO2. Therefore, this study proposed a combination of CH4/CO2 replacement and thermal stimulation to enhance CH4 recovery and CO2 storage. The effects of the methane hydrate saturation level, replacement zone, and freezing point on the replacement were analyzed. The CH4 replacement percentage and energy efficiency were obtained and compared using the replacement and combined methods. The results suggested that the combined method effectively improved CH4 recovery, with the CH4 replacement percentage exhibiting an upper limit of 64.63%. Moreover, In CH4/CO2 replacement, the total number of moles of CO2 stored is unequal to CH4 recovered, because the replacement is sensitive to the free water in the pores of the hydrate sediments. In addition, the CO2 storage efficiency was first discussed. The results proved that the CH4/CO2 replacement has obvious advantages in CO2 storage, and a maximum CO2 storage efficiency of 96.73% was achieved by combined method.Download high-res image (212KB)Download full-size image

•NGHs deposits with excess gas or water were both recoveried by depressurization.•Water saturation and depressurization range are key factors for NGHs dissociation.•Larger depressurization range accelerates NGHs dissociation for excess gas deposit.•The obvious water mobility in excess water deposits hinders methane gas output.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.

In preliminary analyses, the co-injection of CO2 with H2S, SO2 and N2 impurities has been shown to reduce total carbon capture and storage (CCS) cost. The multiphase flow properties of impurities in the CO2–brine system in porous media are the key to understanding the mechanisms and nature of geological CO2 sequestration projects. In this study experiments were performed on the multiphase flow process of CO2/N2/brine system at conditions similar to aquifer pressure and temperature using the X-ray CT technique. Experiments at various rates of CO2 injection that affect saturation and spatial distribution of injected gas were conducted in this experiment. The results indicate an strong relationship between gas saturation and porosity distribution in porous media, and the increasing capillary number leads to lower saturation in downward injection. Small capillary numbers and higher fractional flows in the gas phase both result in uniform saturation maps in the core. The CO2 clusters seem larger at high capillary numbers and the high CO2 collection regions extend based on the saturation distribution in the lower CO2 fraction as the flow pattern stays similar as the same capillary number. CO2 containing N2 tends to retain more correlative relationships at different gas injection rates compared with the pure CO2 stream. Even though both distribution and saturation are storage concerns, the N2 component has little effect on gas distribution, whereas it brings about an overall increase in the saturation for most experiments. Thus the N2 enhanced the storage performance of CO2.

Dispersion exists in many scientific and engineering applications, especially for CO2 enhanced gas recovery, which is a vital factor for controlling the contamination of remaining natural gas and gas recovery. In this paper, an in situ method for the dispersion coefficient measurement of liquid/supercritical CO2–CH4 in a sandpack using CT was proposed. The dispersion coefficient in the sandpack was obtained directly from the CO2 mole fraction profiles translated from a CT greyvalue image, which eliminate the deviation caused by the entry/exit effect. The finite difference method, Crank–Nicolson method, was applied to solve the advection dispersion equation for obtaining the dispersion coefficient. The breakthrough profile of the effluent gas was also analyzed and the apparent dispersion coefficient containing the entry/exit effect was measured using the dynamic column breakthrough method. The entry/exit effect enlarged the dispersion coefficient in the range of 14–23% under a water-free experiment according to the deviation between the two methods. And the dispersion coefficient with the sandpack containing residual water was smaller than that of the water-free condition, which was probably caused by the dissolution of CO2 in the displacing frontier into residual water. The dissolution stabilized the dispersion in the displacing frontier and resulted in the reduction of the dispersion coefficient.

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 CO2-enhanced oil recovery. In the present work, the dynamic processes of CO2 diffusion in bulk n-decane and n-decane saturated porous media were investigated using the micro-focus X-ray CT (micro-CT) scanning technique. CO2 diffusion was visually and quantitatively analyzed by interpreting the CO2 concentration with grayscale CT images. Next, local CO2 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 CO2 initial pressure leads to high CO2 diffusivity in oil. Experiment results in n-decane saturated porous media showed that the CO2 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 CO2 mass transfer and decrease the diffusion coefficient.

•MH dissociation characters were measured for ternary phases sediment.•Heat transfer is a control factor for MH dissociation pattern and rate.•There is a maximum depressurization range to make MH dissociation rate increase.•Methane gas seepage makes liquid water distribution changes.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.

Densities of compressed CO2 + undecane binary mixtures were measured using a magnetic suspension balance (MSB). Measurements were made at temperatures ranging from 313.15 to 353.15 K and pressures ranging from 8 to 19 MPa at CO2 mole fractions (x1) of 0, 0.2483, 0.4641, 0.6797, and 0.8874. The mixture densities were found to increase linearly with pressure and decrease with temperature; this behavior is similar to that of CO2 + decane or dodecane mixtures. The mixture densities show a crossover with composition when the CO2 mole fraction is high. The densities of alkanes increase almost linearly with carbon number, as do the densities of CO2 + alkane mixtures. With the of CO2 mole fraction, the mixture densities increase at first, then decrease. The increase of pressure makes the excess molar volume less negative, whereas the increase of temperature makes it more negative. Densities calculated from the perturbed hard chain equation of state (PHSC EOS) with improved parameters are in good agreement with the experimental densities. Adopting the constant binary interaction parameter kij for different alkanes in the PHSC EOS slightly enlarge the model deviation but simplifies the calculation.

Accurate temperature and velocity measurement of fluid is of vital importance for CO2 Capture and Storage (CCS). The aims of this study were to evaluate the application of several magnetic resonance imaging (MRI) temperature measurement techniques in CCS and to simultaneously measure velocity and temperature distribution for flow field. First, the relations between MRI parameters including apparent self-diffusion coefficient (D), longitudinal relaxation time (T1), longitudinal equilibrium magnetization (M0) and temperature for three different samples were investigated. The results show that in high magnetic strength field the linear relationship between D and temperature is better than M0–T and T1–T in terms of accuracy and sensitivity. Then we used inversion recovery tagging method to simultaneously measure temperature and velocity of water flowing through a heated vessel. Temperature measured by IR-tagging method is within a deviation of 2 °C from the numerical results obtained by computational fluid dynamics.

In the present study, three types of experiments on immiscible CO2 flooding in porous media were conducted and high-resolution images of asphaltene precipitation obtained using X-ray micro-CT scanner. It was found that the effective oil mobility is reduced due to the adsorption of deposited asphaltene onto the rock which blocks the pore throats, whereby the formation wettability is changed and both the effective porosity and permeability are reduced. The deposited asphaltene cannot be redissolved or displaced by the reinjected crude oil, and the formation damage is irreversible. In addition, the porosity-based permeability model was applied to study the effective permeability reduction that results from porosity reduction. The porosity-based permeability was calculated based on the Kozeny–Carman equation and experimental data. The effective permeability variation rate obtained by the porosity-based permeability model agreed well with the results obtained by Darcy’s law, which demonstrates that the method is feasible in evaluating the effective permeability variation rate based on the porosity of the cores acquired from micro-CT images.

The densities of CO2 solution of the brine from Teikoku Oil Field located at Niigata Prefecture in Japan are measured by a magnetic suspension balance at temperatures from 303.15 to 323.15 K, pressures from 10 to 20 MPa, and CO2 mole fractions of 0, 0.0038, 0.0040, 0.0087, 0.0100, and 0.0160. Results show that the densities of CO2–brine solution increase to 0.86% from that of brine and linearly increases with pressure at a gradient of 0.411 kg·m–3·MPa–1 and with CO2 mole fraction at an average gradient of 514 kg·m–3·mol–1 at a temperature of 303.15 K. On the other hand, the density of CO2–brine solution decreases with increasing temperature at an average rate of −0.377 kg·m–3·K–1 under our experimental conditions. The ePC-PSAFT model is applied to predict the data obtained from this study and those from literature. It is demonstrated that the model works well with average relative deviation (ARD) of 0.27%. A correlation of density ratio of CO2–brine solution to brine is provided and validated by data used in the ePC-PSAFT model, which is convenient for engineering application in comparison with that by the ePC-PSAFT. The ARDs for density ratio predicted by ePC-PSAFT and correlation are 0.075% and 0.019% for this work, respectively.

Natural gas hydrates have aroused worldwide interest due to their energy potential and possible impact on climate. The occurrence of natural gas hydrates hosted in the pores of sediments governs the seismic exploration, resource assessment, stability of deposits, and gas production from natural gas hydrate reserves. In order to investigate the microstructure of natural gas hydrates occurring in pores, natural gas hydrate-bearing sediments were visualized using microfocus X-ray computed tomography (CT). Various types of sands with different grain sizes and wettability were used to study the effect of porous materials on the occurrence of natural gas hydrates. Spatial distributions of methane gas, natural gas hydrates, water, and sands were directly identified. This work indicates that natural gas hydrates tend to reside mainly within pore spaces and do not come in contact with adjacent sands. Such an occurring model of natural gas hydrates is termed the floating model. Furthermore, natural gas hydrates were observed to nucleate at gas–water interfaces as lens-shaped clusters. Smaller sand grain sizes contribute to higher hydrate saturation. The wetting behavior of various sands had little effect on the occurrence of natural gas hydrates within pores. Additionally, geometric properties of the sediments were collected through CT image reconstructions. These findings will be instructive for understanding the microstructure of natural gas hydrates within major global reserves and for future resource utilization of natural gas hydrates.

Depressurization has been considered an economic and practicable method for natural gas hydrate (NGH) exploitation. To obtain the kinetic data of methane hydrate (MH) dissociation under different backpressures, MH dissociation by depressurization in a porous medium was investigated in situ using magnetic resonance imaging (MRI). MH was dissociated under backpressures that were varied from 2.8 to 2.2 MPa, and the hydrate saturation variation during dissociation was analyzed. One experimental case was carried out with constant backpressure, and four cases of variable backpressure depressurization experiments were carried out. The radial dissociation pattern during depressurization was confirmed. During hydrate dissociation, free water was observed to move toward the outlet of the vessel and decreased the water saturation after the hydrate totally dissociated in the field of view (FOV). The MRI data provided excellent information on the spatial distribution of water in the porous media during hydrate dissociation.

Various saturated methane hydrates (MHs) formed in porous media were swapped with CO2 to investigate the principles of replacement reactions in different regions surrounded by three curves: (L–V)CO2, (H–V)CH4, and (H–V)CO2. The replacement percentage was analyzed to evaluate the productivity of MHs, with the replacement reactions in zone A [above (H–V)CO2, above (H–V)CH4, and above (L–V)CO2], zone B [above (H–V)CO2, above (H–V)CH4, and below (L–V)CO2], and zone C [above (H–V)CO2, below (H–V)CH4, and below (L–V)CO2]. Temperature conditions under the ice point caused restrictions in the replacement process. The two key factors affecting this phenomenon are the area of MHs for the replacement reactions and the diffusibility of CO2 in the three zones. Pressure signals of replacement procedures in the three zones were discussed to investigate the possible CO2–CH4 exchange kinetics. Two replacement stages were observed: surface replacement and inner layer replacement. Pressures in zone C decreased instead of recovering to the equilibrium line of the MHs. We also analyzed other affecting factors, such as the pressure and temperature. In zone C, replacement percentage decreased as the pressure increased at the same replacement temperature and MH saturation. In zone B, replacement percentage increased as the temperature increased with the same pressure and MH saturation, although this tendency was not obvious when the temperature was below the freezing point.

•H2/CO2 is separated as flowing through porous media by hydrate formation.•Tetrahydrofuran and sodium dodecyl sulphate are used as additives simultaneously.•Hydrate forms firstly and dissociates lastly along the axial of the vessel.•Smaller glass beads (BZ-01) is suitable for hydrate-based H2/CO2 separation.•Porous media and temperature have remarkable impacts on gas compositions.Hydrate-based gas separation is considered a promising technology for H2 purification and CO2 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 (H2/CO2) 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 H2 concentration was 26.1 mol% for the hydrate dissociation process, whereas the highest value was 44.8 mol%. The overall H2 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 H2 concentrations in the hydrate dissociation process, whereas only porous media significantly affect hydrate saturation.

Natural gas hydrates are globally considered a potential alternative form of energy suitable for sustainable development. The microstructure of natural gas hydrates in sediments governs their seismic and acoustic exploration, stability of seafloors, and gas production from hydrate deposits. To investigate the microstructure and occurrence of natural gas hydrates in pores, natural gas hydrate-bearing porous media were directly observed using microfocus X-ray computed tomography (CT). The spatial distributions of free gas, natural gas hydrates, water, and grains were identified. The results indicated the preference of natural gas hydrates to form primarily within pore spaces and not to cement the adjacent grains, which was described by the floating model. Moreover, the migration of gas and water within pore spaces during hydrate formation appeared random, and natural gas hydrates were found to nucleate preferentially at the gas–water interface. The values of porosity and hydrate saturation obtained via reconstruction of CT images agreed well with the conventional methods, indicating that X-ray CT is effective in microstructural studies on natural gas hydrate-bearing sediments. These findings could have implications for both understanding of natural gas hydrate existence within deposits and future gas production from hydrate-bearing sediments.

To clarify the dissociation characteristics by depressurization with heat flow rate from over-underburden layers (Qov), the effects of different Qov levers on gas production by depressurization were analyzed with various initial hydrate saturations in a 5 L pressure vessel. The ratio of sensible heat of the hydrate sediments to hydrate dissociation latent heat (ΔHSen/ΔHL), the accumulated volume of gas production, the percentage of gas production, and the rate of gas production were obtained and compared. The effects of ΔHSen and Qov on gas production in the fast depressurization stage and the stable temperature stage were analyzed separately during the gas production process. A sharp increase of temperature and pressure was observed which was caused by the latent heat of ice formation during the fast depressurization stage. It is concluded that the Qov has a positive influence on gas production during the stable temperature stage after the total consumption of ΔHSen. The Qov effectively increased the production temperature, rate of gas production, and percentage of gas production under these experimental conditions. With increased Qov, the promotion effects are different depending on ΔHSen and Shi. High Qov had a remarkable influence on the rate of gas production and the percentage of gas production for the high Shi sample. In this experiment, va increased from 1.33 to 2.24 SL/M depending on the Qov, an increase of 68.42%. In addition, with high Qov, the upward migration of free water decreased the thermal conductivity of the hydrate sediments, which would decrease the rate heat flow from Qov.

The injection of CO2 into oil reservoirs (CO2 enhanced oil recovery, CO2-EOR) can result in higher production, and the use of CO2 as a mining resource can thus be an economic driver for oil production. The thermodynamic properties of CO2 mixtures are essential for the design and operation of CO2-EOR systems. This paper addresses the (p, ρ, T) properties of a CO2 + tetradecane solution. Experimental densities were measured on a magnetic suspension balance, and experiments were performed at pressures from 10 MPa to 19 MPa, temperatures from 313.15 K to 353.15 K, and CO2 mole fractions of x1 = 0, 0.2469, 0.5241, 0.7534, and 0.8773. Solution densities increased with pressure and decreased with temperature over the experimental range. Density versus the CO2 mole fraction increased at first and then decreased at higher temperatures and higher CO2 concentrations. The compositions intersect when plotted, and the pressure intersection increased with temperature. The excess molar volumes of the binary mixtures were negative over the entire range of composition, which increased with increasing pressure and became more negative with increasing temperature. The PC-SAFT and tPC-PSAFT equations of state were used to calculate the densities of the binary mixtures. New PC-SAFT parameters for tetradecane were obtained by fitting to experimental densities directly. In both PC-SAFT and tPC-PSAFT, the binary interaction parameter kij was fitted as a function of the CO2 mole fraction. The tPC-PSAFT combined with the correlation of kij gave the best predictions of the CO2 + tetradecane mixture densities.

Hydrate-based technology is a promising method for gas separation and seawater desalination. There is little information about the combination of the two applications. The CO2 hydrate formation and dissociation in saline water (3 wt % sodium chloride) and deionized water in a silica gel fitted vessel are experimentally investigated by using a magnetic resonance imaging (MRI)-based pool measurement system. Three experimental cases were conducted with different procedures. MRI images and mean intensity (MI) were obtained using a spin echo multislice pulse sequence. From this study, it is found that the hydrate formation in saline solution is rapid compared to that in deionized water. It is caused by the “structure making” of ions. Hydrate is formed more rapidly in the flowing process than in the cooling process due to the additional mechanical effect. The so-called “memory effect” was identified for the hydrate dissociated solution, for which the nondissociated hydrate crystals exist. It shows that the twice displacement is superior for experimental stability. Additionally, MR images show that the rapidly formed hydrate can cause blockages of the experimental loop. The sensitivity of the MRI system is high when the temperature is above 284.15 K. This causes a rapid decrease in the MI as the temperature increases.

Densities of binary mixtures of CO2 and dodecane were measured by a magnetic suspension balance (MSB) at temperatures ranging from 313.55 K to 353.55 K under pressures from (8 to 18) MPa at five different CO2 mole fractions: x1 = 0, 0.2497, 0.5094, 0.7576, and 0.8610. Densities of the binary mixture increase with increasing pressure and decrease with increasing temperature. A crossover phenomenon is observed at high CO2 concentrations, and the crossover pressure increases with increasing temperature. The experimental densities increase with the CO2 mole fraction at first and then decrease at high CO2 concentrations. The excess molar volumes are negative over the whole range of CO2 concentrations and are more negative with increasing temperature and less negative with increasing pressure. The Benedict–Webb–Rubin–Starling (BWRS) and the improved Perturbed Hard-Sphere Chain (PHSC) equations of state were used to calculate the densities of the CO2–dodecane mixtures. The improved PHSC model has fewer parameters and shows little loss of overall calculation accuracy compared to BWRS.

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.

The separation of CO2 from fuel gas (CO2/H2) as hydrates was studied. In this investigation, the effects and mechanism thereof of the additive mixture (1, 2, 3, and 4 mol % tetrahydrofuran (THF), with 1000 mg/L sodium dodecyl sulfate) on the thermodynamic and kinetic properties of the hydrate in porous media were measured using an isochoric method, keeping the volume constant. The experimental results show that an increasing THF concentration increases the driving force for hydrate formation and decreases the hydrate induction time. The Langmuir constants of H2 and CO2 showed that H2 may occupy the small cavities of s-II hydrate in the H2–CO2–THF–H2O system. The presence of THF results in a drastic decrease of the hydrate phase equilibrium pressure. Higher THF concentrations correspond to lower hydrate phase equilibrium pressures, but the decrease in pressure with concentration slows when the THF concentration exceeds 3 mol %. An improved thermodynamic model was used to predict the hydrate phase equilibrium, and the calculations agreed well with the experimental data.

To evaluate the potential for the commercially viable production of gas from hydrate reservoirs, several pieces of key information are needed to examine in hydrate dissociation process in porous media. In this study, a two dimensional (2D) axisymmetric finite-difference, fully implicit model was developed to investigate the gas production behavior of hydrate dissociation by depressurization in hydrate-bearing porous media. The simulation results indicated that the hydrates dissociate along the radial and longitudinal direction, and the dissociation in the radial direction is earlier than that of the longitudinal direction of the laboratory-scale hydrate sample. Moreover, a series of simulations was performed to study the effect of several parameters including initial hydrate saturation, permeability reduction index N, absolute/relative permeability, intrinsic porosity, and the assumption of stationary water phase on the gas production behavior from hydrate dissociation in hydrate-bearing porous media. The results of the sensitivity analysis showed that significant amelioration of gas production behavior is obtained with high initial hydrate saturation, low permeability reduction index, high gas relative permeability, and high intrinsic porosity. On the other hand, it can be found that the cumulative gas production is not affected by the absolute permeability and the assumption of stationary water phase with the condition of simulation scale length vs diameter L/d < 50; however, there would be some opposite results presented in gas production performance under a larger simulation scale L/d > 50. Finally, the simulation results also suggested that the reliable relative permeability model and the reasonable value of the permeability reduction index corresponding to different forms of hydrate occupying in the porous media should be very important to predict the gas production performance from hydrate dissociation by depressurization and improve the accuracy of numerical simulator.

The objective of this study is to understand the process of fluid flow in pipe and porous media with different pore structures. High-resolution Magnetic Resonance Imaging (MRI) technique was used to visualize the pore structure and measure fluid flow. The porous media was formed by packed bed of glass beads. Flow measurement was carried out by a modified spin echo sequence. The results show that the velocity distribution in pipe is annular and the linear relation between MRI velocity and actual velocity is found in pipe flow measurement. The flow distribution in porous media is rather heterogeneous, and it is consistent with heterogeneous pore structure. The flow through pores with the high volume flow rate is determined largely by geometrical effects such as pore size and cross-sectional area.

CO2 flooding is used extensively as a commercial process for enhanced oil recovery. In this study, the visualization of CO2 flooding in immiscible and miscible displacements in a high-pressure condition was studied using a 400 MHz MRI system. For CO2 immiscible displacement, the phenomenon of CO2 channelling or fingering was obviously due to the difference in fluid viscosities and densities. Thus, the sweep efficiency was small, and the final residual oil saturation was 37.2%. For CO2 miscible displacement, the results showed that pistonlike displacement occurred, and the phenomenon of the miscible regions and CO2 front was obvious. The viscous fingering and gravity override caused by the low viscosity and density of the gas were restrained effectively, and the velocity of the CO2 front was uniform. The sweep efficiency was high, and the final residual oil saturation was 13.5%, indicating that CO2 miscible displacement could recover more oil compared with CO2 immiscible displacement. Finally, the average velocity of the CO2 front was evaluated by analyzing the oil saturation profile. A special core analysis method was applied to in situ oil saturation data to directly evaluate the effect of viscosity, buoyancy, and capillary pressure on CO2 miscible displacement.

A triaxial system is designed with a temperature range from −20 °C to 25 °C and a pressure range from 0 MPa to 30 MPa in order to improve the understanding of the mechanical properties of gas hydrate-bearing sediments. The mechanical properties of synthetic gas hydrate-bearing sediments (gas hydrate-kaolin clay mixture) were measured by using current experimental apparatus. The results indicate that: (1) the failure strength of gas hydrate-bearing sediments strongly depends on the temperature. The sediment’s strength increases with the decreases of temperature. (2) The maximum deviator stress increases linearly with the confining pressure at a low-pressure stage. However, it fluctuates at a high-pressure stage. (3) Maximum deviator stress increases with increasing strain rate, whereas the strain-stress curve has no tremendous change until the axial strain reaches approximately 0.5%. (4) The internal friction angles of gas hydrate-bearing sediments are not sensitive to kaolin volume ratio. The cohesion shows a high kaolin volume ratio dependency.

Hydrogen production from steam reforming of glycerol in a fluidized bed reactor has been simulated using a CFD method by an additional transport equation with a kinetic term. The Eulerian–Eulerian two-fluid approach was adopted to simulate hydrodynamics of fluidization, and chemical reactions were modelled by laminar finite-rate model. The bed expansion and pressure drop were predicted for different inlet gas velocities. The results showed that the flow system exhibited a more heterogeneous structure, and the core-annulus structure of gas–solid flow led to back-mixing and internal circulation behaviour, and thus gave a poor velocity distribution. This suggests the bed should be agitated to maintain satisfactory fluidizing conditions. Glycerol conversion and H2 production were decreased with increasing inlet gas velocity. The increase in the value of steam to carbon molar ratio increases the conversion of glycerol and H2 selectivity. H2 concentrations in the bed were uneven and increased downstream and high concentrations of H2 production were also found on walls. The model demonstrated a relationship between hydrodynamics and hydrogen production, implying that the residence time and steam to carbon molar ratio are important parameters. The CFD simulation will provide helpful data to design and operate a bench scale catalytic fluidized bed reactor.

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 Vf 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/μg. Moreover, the higher Bond number results in higher final displacing-phase saturation, whereas the capillary number has an opposite effect.

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.

Pore structure is one of important factors affecting the properties of porous media, but it is difficult to describe the complexity of pore structure exactly. Fractal theory is an effective and available method for quantifying the complex and irregular pore structure. In this paper, the fractal dimension calculated by box-counting method based on fractal theory was applied to characterize the pore structure of artificial cores. The microstructure or pore distribution in the porous material was obtained using the nuclear magnetic resonance imaging (MRI). Three classical fractals and one sand packed bed model were selected as the experimental material to investigate the influence of box sizes, threshold value, and the image resolution when performing fractal analysis. To avoid the influence of box sizes, a sequence of divisors of the image was proposed and compared with other two algorithms (geometric sequence and arithmetic sequence) with its performance of partitioning the image completely and bringing the least fitted error. Threshold value selected manually and automatically showed that it plays an important role during the image binary processing and the minimum-error method can be used to obtain an appropriate or reasonable one. Images obtained under different pixel matrices in MRI were used to analyze the influence of image resolution. Higher image resolution can detect more quantity of pore structure and increase its irregularity. With benefits of those influence factors, fractal analysis on four kinds of artificial cores showed the fractal dimension can be used to distinguish the different kinds of artificial cores and the relationship between fractal dimension and porosity or permeability can be expressed by the model of D = a − bln(x + c).Highlights► The proposed sequence of divisors in BC method can eliminate the border effect. ► Appropriate threshold value can be obtained using the minimum-error method. ► Higher image resolution brings more accurate estimation of fractal dimension. ► Fractal dimension can be used to distinguish the different kinds of artificial cores. ► Relationship between FD and porosity or permeability is expressed by D = a − bln(x + c).

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.

•The failure strength increases with the increase of CO2 hydrate volume ratio.•A comparison is made between mixed hydrate- and CH4/CO2 hydrate-bearing sediments.•CH4–CO2 replacement is conducive to the stability of the hydrate-bearing layers.CH4–CO2 replacement method for extracting methane from hydrate-bearing sediments has attracted more and more attention due to its twin advantages: CH4 recovery and CO2 geologic sequestration. In this study, a series of triaxial tests were conducted on sediments containing CH4 and CO2 hydrate mixtures to prove the mechanical stability of CH4–CO2 replacement in permafrost-associated methane hydrate-bearing sediments. And a comparative analysis was made between the mixed hydrate- and pure CH4/CO2 hydrate-bearing sediments containing ice. The stress–strain curves, failure strength, initial yield modulus, shear strength and the effects of CO2 hydrate volume ratio, confining pressure, temperature and strain rate on the mechanical properties of mixed hydrate-bearing sediments containing ice were investigated. The experimental results indicated that CH4–CO2 replacement was conducive to the stability of the methane hydrate-bearing layers during gas production.

In this work, magnetic resonance imaging (MRI) was employed to observe the in-situ formation and dissociation of methane hydrates in porous media. Methane hydrate was formed in a high-pressure cell with controlled temperature, and then the hydrate was dissociated by thermal injection. The process was photographed by the MRI, and the pressure was recorded. The images confirmed that the direct visual observation was achieved; these were then employed to provide detailed information of the nucleation, growth, and decomposition of the hydrate. Moreover, the saturation of methane hydrate during the dissociation was obtained from the MRI intensity data. Our results showed that the hydrate saturation initially decreased rapidly, and then slowed down; this finding is in line with predictions based only on pressure. The study clearly showed that MRI is a useful technique to investigate the process of methane hydrate formation and dissociation in porous media.

Tetrahydrofuran (THF) hydrate was formed in bulk as well as in glass beads pack with a mean diameter of 3.0 mm by controlling the temperature under ambient pressure. Images of THF hydrate formation procedure were obtained using the magnetic resonance imaging (MRI) technique. The experiment results showed that MRI is an effective method for the detection of hydrate formation. Saturation of hydrate formed both in bulk and glass beads can be confirmed by intensity integration of MRI images.

The permeability in the methane hydrate reservoir is one of the key parameters in estimating the gas production performance and the flow behavior of gas and water during dissociation. In this paper, a three-dimensional cubic pore-network model based on invasion percolation is developed to study the effect of hydrate particle formation and growth habit on the permeability. The variation of permeability in porous media with different hydrate saturation is studied by solving the network problem. The simulation results are well consistent with the experimental data. The proposed model predicts that the permeability will reduce exponentially with the increase of hydrate saturation, which is crucial in developing a deeper understanding of the mechanism of hydrate formation and dissociation in porous media.

Methane production from hydrate reservoir may induce seabed slide and deformation of the hydrate-bearing strata. The research on mechanical properties of methane hydrate is considered to be important for developing an efficient methane exploitation technology. In this paper, a triaxial test system containing a pressure crystal device was developed with the conditions to stabilize the hydrate. A series of triaxial shear tests were carried out on artificial methane hydrate specimen. In addition, mechanical characteristics of methane hydrate were studied with the strain rates of 0.1 and 1.0 mm/min, respectively, under the conditions of different temperatures (T = −5, −10, and −20 °C) and confining pressures (P = 0, 5, 10, 15, and 20 MPa). The preliminary results show that when the confining pressure was less than 10 MPa, the increase of confining pressure leaded to the enhancement of shear strength. Furthermore, the decreasing temperature and the increasing strain rate both caused the increase in shear strength.

•Accurate densities of CO2–CH4 were measured at 300–308.15 K and 2–18 MPa by MSB.•Combined standard uncertainty of T, p  , and uM(ρ)uM(ρ) are 0.02 K, 0.001 MPa and 0.029 kg·m−3.•The measured density agrees well with GERG-2008 as x < 0.60 with deviation within 2.0%.•The max deviation with GERG-2008 appears as T–p close to critical point at high x.The accurate densities of CO2–CH4 binary mixtures with CO2 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 CO2–CH4 mixtures were around 0.30% with CO2 mole fraction lower than 0.60 while they increased up to 0.96% with CO2 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 CO2 mole fraction lower than 0.60 at 300 K as the relative deviations were generally within 2.0%. When CO2 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 CO2–CH4 binary mixtures with high CO2 mole fraction in the vicinity of the critical point accurately.

Over the years, clathrate hydrates have been investigated for its potential as an energy resource and other industrial applications. Nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI) are two powerful NMR technologies for both molecular level and microscopic measurement, which have been applied in gas hydrate research to provide fundamental and useful information. 1H and 13C NMR spectroscopy are the most commonly applied method to study cage occupancies of guest species and crystal structures. MRI technique, on the other hand, provides microscopic insights towards the gas hydrate formation and dissociation in porous media and the study of CH4/CO2 hydrate replacement. We also reviewed the state of the art application of NMR based technology in research on the gas-liquid multiphase flow and temperature mapping within porous media. Potential improvements in NMR technology to improve the fundamental understanding towards gas hydrates is also discussed in this review article.

In this study, a numerical model is developed to investigate the hydrate dissociation and gas production in porous media by depressurization. A series of simulation runs are conducted to study the impacts of permeability characteristics, including permeability reduction exponent, absolute permeability, hydrate accumulation habits and hydrate saturation, sand average grain size and irreducible water saturation. The effects of the distribution of hydrate in porous media are examined by adapting conceptual models of hydrate accumulation habits into simulations to govern the evolution of permeability with hydrate decomposition, which is also compared with the conventional reservoir permeability model, i.e. Corey model. The simulations show that the hydrate dissociation rate increases with the decrease of permeability reduction exponent, hydrate saturation and the sand average grain size. Compared with the conceptual models of hydrate accumulation habits, our simulations indicate that Corey model overpredicts the gas production and the performance of hydrate coating models is superior to that of hydrate filling models in gas production, which behavior does follow by the order of capillary coating>pore coating>pore filling>capillary filling. From the analysis of t1/2, some interesting results are suggested as follows: (1) there is a “switch” value (the “switch” absolute permeability) for laboratory-scale hydrate dissociation in porous media, the absolute permeability has almost no influence on the gas production behavior when the permeability exceeds the “switch” value. In this study, the “switch” value of absolute permeability can be estimated to be between 10 and 50 md. (2) An optimum value of initial effective water saturation Sw,e exists where hydrate dissociation rate reaches the maximum and the optimum value largely coincides with the value of irreducible water saturation Swr,e. For the case of Sw,eSwr,e, there are different control mechanisms dominating the process of hydrate dissociation and gas production.

Measurement of two phase flow in porous medium for sequestration was carried out using high-resolution magnetic resonance imaging (MRI) technique. The porous medium was a packed bed of glass beads. Spin echo multi sequence was used to measure the distribution of CO2 and water in the porous medium. The intensity images show that the fluid distribution is non-uniform due to its viscosity and pore structure of porous medium. The velocity distribution of fluids is calculated from the saturation of water and porosity of porous medium. The experimental results show that fluid velocities vary with time and position. The capillary dispersion rate donated the effects of capillary, which was largest at water saturations of 0.45. The displacement process is different between in BZ-02 and BZ-2. The final water residual saturation depends on permeability and porosity.

Minimum miscible pressure (MMP) of gas and oil system is a key parameter for the injection system design of CO2 miscible flooding. Some industrial standard approaches such as the experiment using a rising bubble apparatus (RBA), the slim tube tests (STT), the pressure–density diagram (PDD), etc. have been applied for decades to determine the MMP of gas and oil. Some theoretical or experiential calculations of the MMP were also applied to the gas–oil miscible system. In the present work, an improved technique based on our previous research for the estimation of the MMP by using magnetic resonance imaging (MRI) was proposed. This technique was then applied to the CO2 and n-alkane binary and ternary systems to observe the mixing procedure and to study the miscibility. MRI signal intensities, which represent the proton concentration of n-alkane in both the hydrocarbon rich phase and the CO2 rich phase, were plotted as a reference for determining the MMP. The accuracy of the MMP obtained by using this improved technique was enhanced comparing with the data obtained from our previous works. The results also show good agreement with other established techniques (such as the STT) in previous published works. It demonstrates increases of MMPs as the temperature rise from 20 °C to 37.8 °C. The MMPs of CO2 and n-alkane systems are also found to be proportional to the carbon number in the range of C10 to C14.

A clear understanding of two-phase fluid flow properties in porous media is of importance to CO2 geological storage. The study visually measured the immiscible and miscible displacement of water by CO2 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 CO2 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 CO2 and water became miscible in the beginning of CO2 injection. CO2 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 CO2 and water to invade into small pore spaces more easily. The study also showed CO2 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 CO2 slightly decreases with the increase of capillary number.

It is of great importance to study the CO2-oil two-phase flow characteristic and displacement front behavior in porous media, for understanding the mechanisms of CO2 enhanced oil recovery. In this work, we carried out near miscible CO2 flooding experiments in decane saturated synthetic sandstone cores to investigate the displacement front characteristic by using magnetic resonance imaging technique. Experiments were done in three consolidated sandstone cores with the permeabilities ranging from 80 to 450 mD. The oil saturation maps and the overall oil saturation during CO2 injections were obtained from the intensity of magnetic resonance imaging. Finally the parameters of the piston-like displacement fronts, including the front velocity and the front geometry factor (the length to width ratio) were analyzed. Experimental results showed that the near miscible vertical upward displacement is instable above the minimum miscible pressure in the synthetic sandstone cores. However, low permeability can restrain the instability to some extent.

•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.•3 mol% THF was optimal choice for hydrate formation.•Improved thermodynamic model was proposed to predict hydrates mixture phase equilibrium.Hydrate-based gas separation is a promising technology for CO2 capture and storage. Hydrate phase equilibrium data are the most basic information for hydrate formation and dissociation. The effects and mechanism of the additive mixture (mole fraction 1%, 2%, 3%, 4% and 5% tetrahydrofuran, THF, with 1000 mg/L sodium dodecyl sulphate, SDS) on the hydrate phase equilibrium (THF + SDS + CO2 + N2 + H2O system) were investigated using an isochoric method. The experimental results showed that the presence of THF resulted in a substantial decrease of the hydrate phase equilibrium pressure. The rate of decrease of the hydrate phase equilibrium pressure with THF concentrations slowed down as the THF concentration exceeded a mole fraction of 3%. An improved model with the PR equation of state associated with a modified Huron–Vidal second-order model mixing rule and non-random two liquid model was proposed further to predict hydrate phase equilibrium. The predictions showed an acceptable agreement with the experimental data. When the hydrate phase equilibrium pressure was higher than 2.00 MPa, the absolute average deviations of the predicted results were obviously smaller.

•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.Hydrate-based CO2 capture is a promising technology. To obtain fundamental data for a flowing system, we measured the distribution of pore solution to analyse hydrate formation/dissociation and gas separation properties. An orthogonal experiment was carried out to investigate the effects of glass beads, flow rates, pressures and temperatures on it. Magnetic resonance imaging (MRI) images were obtained using a spin echo multi-slice pulse sequence. Hydrate saturations were calculated quantitatively using an MRI mean intensity. The results show that hydrate blockages were frequently present. During the hydrate formation and dissociation process, the movement of the solution occurred in cycles. However, the solution movement rarely occurred for residual solution saturations obtained with a high backpressure. The solution concentrate phenomenon occurred mostly in BZ-04. The highest hydrate saturation was 30.2%, and the lowest was 0.70%. Unlike that in BZ-01, there was no stability present in BZ-02 and BZ-04. The different CO2 concentrations for the three processes of each cycle verified hydrate formation during the gas flow process. The highest CO2 concentration was 38.8%, and the lowest one was 11.4%. To obtain high hydrate saturation and good separation effects, the values of 5.00 MPa, 1.0 ml min−1 and 280.00 K were chosen. For the gas flow process, only the pressure had a significant impact on gas composition, and all the factors had a significant impact on the gas composition of the depressurisation process. The temperature had a significant impact on the gas composition of the hydrate dissociation process. The flow rate did not have a significant impact on the composition of the depressurisation process.Application of hydrate based technology on carbon dioxide capture and storage (CCS).Download full-size image

Natural gas hydrates have aroused worldwide interest due to their energy potential and possible impact on climate. The occurrence of natural gas hydrates hosted in the pores of sediments governs the seismic exploration, resource assessment, stability of deposits, and gas production from natural gas hydrate reserves. In order to investigate the microstructure of natural gas hydrates occurring in pores, natural gas hydrate-bearing sediments were visualized using microfocus X-ray computed tomography (CT). Various types of sands with different grain sizes and wettability were used to study the effect of porous materials on the occurrence of natural gas hydrates. Spatial distributions of methane gas, natural gas hydrates, water, and sands were directly identified. This work indicates that natural gas hydrates tend to reside mainly within pore spaces and do not come in contact with adjacent sands. Such an occurring model of natural gas hydrates is termed the floating model. Furthermore, natural gas hydrates were observed to nucleate at gas–water interfaces as lens-shaped clusters. Smaller sand grain sizes contribute to higher hydrate saturation. The wetting behavior of various sands had little effect on the occurrence of natural gas hydrates within pores. Additionally, geometric properties of the sediments were collected through CT image reconstructions. These findings will be instructive for understanding the microstructure of natural gas hydrates within major global reserves and for future resource utilization of natural gas hydrates.