A review: Enhanced recovery of natural gas hydrate reservoirs

Natural gas hydrate (NGH) is a highly efficient and clean energy, with huge reserves and widespread distribution in permafrost and marine areas. Researches all over the world are committed to developing an effective exploring technology for NGH reservoirs. In this paper, four conventional in-situ hydrate production methods, such as depressurization, thermal stimulation, inhibitor injection and CO₂ replacement, are briefly introduced. Due to the limitations of each method, there has been no significantly breakthrough in hydrate exploring technology. Inspired by the development of unconventional oil and gas fields, researchers have put forward some new hydrate production methods. We summarize the enhanced hydrate exploiting methods, such as CO₂/N₂–CH₄ replacement, CO₂/H₂–CH₄ replacement, hydraulic fracturing treatment, and solid exploration; and potential hydrate mining techniques, such as self-generating heat fluid injection, geothermal stimulation, the well pattern optimization of hydrate exploring. The importance of reservoir stimulation technology for hydrate exploitation is emphasized, and it is believed that hydrate reservoir modification technology is a key to open hydrate resources exploitation, and the major challenges in the process of hydrate exploitation are pointed out. The combination of multiple hydrate exploring technologies and their complementary advantages will be the development trend in the future so as to promote the process of hydrate industrialization.     

二氧化碳置换法模拟开采天然气水合物的研究进展

目前实验室模拟开采天然气水合物(NGH)的最主要的方法为外激法,通过注热、降压等方式使水合物分解释放出甲烷(CH₄),外激法最大的问题在于水合物的分解容易造成地层结构变化,导致地质斜坡灾害。利用二氧化碳(CO₂)在水合物相中置换开采CH₄,由于置换过程发生在水合物相中,不改变水合物相结构,因此可以降低地质灾害风险。本文全面介绍了利用CO₂在水合物相从NGH中置换CH₄的研究进展,从置换可行性、动力学模型、模拟研究、实验研究等方面对当前的研究进行了综述,并为进一步发展置换法开采CH₄技术指出了方向。     

Modeling of CO2‐Hydrate Formation at the Gas‐Water Interface in Sand Sediment

Sub‐seabed geological storage of CO₂ in the form of gas hydrate is attractive because clathrate hydrate stably exists at low temperature and high pressure, even if a fault occurs by diastrophism like a big earthquake. For the effective design of the storage system it is necessary to model the formation of CO₂‐hydrate. Here, it is assumed that the formation of gas hydrate on the interface between gas and water consists of two stages: gas diffusion through the CO₂‐hydrate film and consequent CO₂‐hydrate formation on the interface, between film and water. Also proposed is the presence of a fresh reaction interface, which is part of the interface between the gas and aqueous phases and not covered with CO₂‐hydrate. Parameters necessary to model the hydrate formation in sand sediment are derived by comparing the results of the present numerical simulations and the measurements in the literature.     

A Study on CO2 Absorption Kinetics by Aqueous Solutions of N,N‐Diethylethanolamine and N‐Ethylethanolamine

N‐Ethylethanolamine (EEA) and N,N‐diethylethanolamine (DEEA) represent promising candidate alkanolamines for CO₂ removal from gaseous streams, as they can be prepared from renewable resources. In this work, the reaction rate constant for the reaction between CO₂ and EEA was determined from the absorption rate measurements of CO₂ in a blend comprising DEEA, EEA and H₂O. A stirred‐cell reactor with a plane, horizontal gas‐liquid interface was used for the absorption studies. While the DEEA concentration in the formulated solution was varied in the range of 1.5–2.5 kmol/m³, the initial EEA concentration was 0.1 kmol/m³. A zwitterion mechanism for EEA and a base‐catalyzed hydration mechanism for DEEA were used to describe the reaction kinetics. At 303 K, the second‐order reaction rate constant for the CO₂ reaction with EEA was found to be 8041 m³/(kmol s). The liquid‐side mass transfer coefficient was also estimated, and its value (0.004 cm/s) is in line with those typical of stirred‐cell reactors.     

Untersuchung der CO2‐Absorptionskinetik in wässrigen Lösungen von N,N‐Diethylethanolanmin und N‐Ethylethanolamin

N‐Ethylethanolamine (EEA) and N,N‐diethylethanolamine (DEEA) represent promising alkanolamines for CO₂ removal from gaseous streams, as they can be prepared from renewable resources. In this work, the reaction rate constant for the reaction between CO₂ and EEA and the liquid‐side mass transfer coefficient were determined from the absorption rate measurements in a blend comprising DEEA, EEA and H₂O. A stirred‐cell reactor was applied for the absorption studies, whereas a zwitterion mechanism for EEA and a base‐catalyzed hydration mechanism for DEEA were used to describe the reaction kinetics.     

Replacement of CH4 in Hydrate in Porous Sediments with Liquid CO2 Injection

The dynamics of the replacement of CH₄ in hydrate in porous sediments with liquid CO₂ was investigated using a self‐developed experimental apparatus at different temperatures and initial pressures. The pressure increases steadily as the replacement reaction processes. The amount of the replaced CH₄ is almost the same as that of the CO₂ forming hydrate in the early stage and gradually becomes somewhat less in the later stage. The initial pressure has minor effects on the replacement rate, and temperature reduction causes a lower replacement rate. The experimental results suggest that the replacement rate is not related to the region of the temperature‐pressure conditions but is mainly affected by the fugacity differences of CH₄ hydrate decomposition and CO₂ hydrate formation.     

Hydrogen and carbon dioxide adsorption with tetra‐n‐butyl ammonium semi‐clathrate hydrates for gas separations

Gas adsorption rates of H₂, CO₂, and H₂‐CO₂ gas mixture (H₂/CO₂ = 3.4) with tetra‐n‐butyl ammonium salt (bromide, chloride, and fluoride) semi‐clathrate hydrate particles were measured at 269 K to assess their properties for gas separation. Equilibrium gas occupancies in the S‐cages of the particles were in order of (high to low) for hexagonal structure‐I, tetragonal structure‐I, and superlattice of cubic structure‐I structures with the maximum fractional occupancy by CO₂ being about 40%. The CO₂ diffusion rate depended on the anion size of the salt, which is attributed to distortion of the S‐cage that is close to the molecular size of CO₂. Simulations of semi‐clathrate hydrate particles with theory showed that H₂/CO₂ selectivities could be as high as 36 (3.0 mol% TBAF) and that selectivities for an ideal membrane (3.3 mol% TBAF) could be >100 (269 K, 0.3–4.5 MPa). Semi‐clathrate hydrates have wide application as separation media for gas mixtures. © 2014 American Institute of Chemical Engineers AIChE J, 61: 992–1003, 2015     

Effects of Graphene Oxide Nanosheets and Al2O3 Nanoparticles on CO2 Uptake in Semi‐clathrate Hydrates

Gas hydrate/clathrate hydrate formation is an innovative method to trap CO₂ into hydrate cages under appropriate thermodynamic and/or kinetic conditions. Due to their excellent surface properties, nanoparticles can be utilized as hydrate kinetic promoters. Here, the kinetics of the CO₂ + tetra‐n‐butyl ammonium bromide (TBAB) semi‐clathrate hydrates system in the presence of two distinct nanofluid suspensions containing graphene oxide (GO) nanosheets and Al₂O₃ nanoparticles is evaluated. The results reveal that the kinetics of hydrate formation is inhibited by increasing the weight fraction of TBAB in aqueous solution. GO and Al₂O₃ are the most effective kinetic promoters for hydrates of (CO₂ + TBAB). Furthermore, the aqueous solutions of TBAB + GO or Al₂O₃ noticeably increase the storage capacity compared to TBAB aqueous solution systems.     

Recovery and Recycling of CO2/N2‐Switchable Anionic Surfactants in Emulsions

To develop a mild, effective, and clean strategy for recovery and recycling of anionic surfactants in CO₂/N₂‐switchable emulsions, a CO₂/N₂‐switchable anionic surfactant, which is a combination of dodecyl seleninic acid (DSA) and N,N,N′,N′‐tetramethyl‐1,2‐ethylenediamine (TMEDA), here referred to as DSA–TMEDA, was used to stabilize an oil‐in‐water (O/W) emulsion. Upon stimulation with CO₂, DSA–TMEDA was switched off to form insoluble DSA and the water‐soluble TMEDA bicarbonate. Upon N₂ bubbling and heating, the OFF state of DSA–TMEDA was restored to the surfactant of DSA–TMEDA. In this manner, O/W emulsions stabilized by DSA–TMEDA can be switched reversibly between demulsification (phase separation) and re‐emulsification (recovered emulsion) by triggering with CO₂/N₂ over ten times. After breakage of the emulsion, nearly all of the OFF state surfactant could be separated conveniently away from the oil phase, thus facilitating recovery and recycling of the surfactant afterward in emulsifying oil. No obvious adverse changes in the dispersed oil particles size and the relative stability of the regenerated emulsions were observed over five cycles, and the surfactant loss can be neglected during the recycling.     

Gas hydrate formation from hydrogen/carbon dioxide and nitrogen/carbon dioxide gas mixtures

Gas hydrates from CO₂/N₂ and CO₂/H₂ gas mixtures were formed in a semi-batch stirred vessel at constant pressure and temperature of 273.7 K. These mixtures are of interest to CO₂ separation and recovery from flue gas and fuel gas, respectively. During hydrate formation the gas uptake was determined and the composition changes in the gas phase were obtained by gas chromatography. The rate of hydrate growth from CO₂/H₂ mixtures was found to be the fastest. In both mixtures CO₂ was found to be preferentially incorporated into the hydrate phase. The observed fractionation effect is desirable and provides the basis for CO₂ capture from flue gas or fuel gas mixtures. The separation from fuel gas is also a source of H₂. The impact of tetrahydrofuran (THF) on hydrate formation from the CO₂/N₂ mixture was also observed. THF is known to substantially reduce the equilibrium formation conditions enabling hydrate formation at much lower pressures. THF was found to reduce the induction time and the rate of hydrate growth.     

A kinetic model of nano‐CaO reactions with CO2 in a sorption complex catalyst

This article describes the reactive kinetics of nano‐CaO with CO₂ in a sorption complex catalyst. Based on an observation of nano‐CaO reaction with CO₂ has a fast surface reaction regime and followed by a slow diffusion‐controlled regime, a criterion has been proposed to divide the fast surface reaction regime and the slow diffusion‐controlled reaction regime. The kinetics of the fast surface reaction was studied, and a new ion reaction mechanism was proposed. A surface reaction‐controlled kinetic model with a Boltzmann equation, X = X_u?X_u/[1+exp((t?t₀)k/X_u)], was developed. Experiments using nano‐CaO to react with CO₂ in a fast surface reaction regime within a sorption complex catalyst were performed using thermogravimetric analysis at 773–873 K under a N₂ atmosphere with 0.010–0.020 MPa CO₂. The activation energy of the kinetic model for carbonation is 30.2 kJ/mol, and the average relative deviation of the sorption ratio is less than 9.8%. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Non‐dispersive absorption of CO2 in parallel and cross‐flow membrane modules using EMISE

BACKGROUND: This paper reports an analysis of the mass transfer behaviour of CO₂ absorption in hollow fibre membrane modules in parallel and cross‐flow dispositions. The ionic liquid EMISE, 1‐ethyl‐3‐methylimidazolium ethylsulfate, is used to achieve a zero solvent emission process and the experimental results are compared with CO₂ permeation through the membrane, without solvent in the lumenside. RESULTS: Overall mass transfer coefficients K_{overall, CF} = (0.74 ± 0.02) × 10^?6 m s^?1 and K_{overall, PF} = (0.37 ± 0.018) × 10^?6 m s^?1 were obtained for cross‐flow and parallel flow, respectively. These values are one order of magnitude lower than the coefficient obtained in permeability experiments, K_{overall, PERM} = (6.16 ± 0.1) × 10^?6 m s^?1, indicating the influence of the absorption in the process. Including the specific surface and gas volume of each contactor in the analysis, a similar value of a first‐order kinetic rate constant, K_R = 2.7 × 10^?3 s^?1 is obtained, showing that the interfacial chemical reaction CO₂‐ionic liquid is the slow step in the absorption process. CONCLUSION: An interfacial chemical reaction rate constant K_R = 2.7 × 10^?3 s^?1, describes the behaviour of the CO₂ absorption in the ionic liquid EMISE using membrane contactors in parallel and cross‐flow dispositions. Copyright © 2012 Society of Chemical Industry     

Effects of aging on poly(1‐trimethylsilyl‐1‐propyne) membranes irradiated with vacuum ultraviolet radiation for gas separation

In this study, we investigated the effects of physical aging on the surface and gas‐transport properties of highly gas permeable poly(1‐trimethylsilyl‐1‐propyne) membranes irradiated with vacuum ultraviolet (VUV) radiation. VUV excimer lamp irradiation was performed on one side of the membrane for 6 or 60 min. The gas permeabilities for carbon dioxide (CO₂) and nitrogen (N₂) were determined through a volumetric measurement method at 23 °C. The gas permeabilities for CO₂ and N₂ increased temporarily at 7 days after 6 and 60 min of VUV irradiation of the membranes. The change in the gas permeability for N₂ was more remarkable than that for CO₂. These changes were related to the C?O or SiO_x ratio. The C?O ratio was related to the gas permeability of the membranes VUV‐irradiated for 6 min, whereas the SiO_x ratio was related to the gas permeability of the membranes VUV‐irradiated for 60 min. These changes affected the gas selectivities of the membranes. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45973.     

Matrix effect of mixed‐matrix membrane containing CO2‐selective MOFs

Facilitated mixed‐matrix membranes (MMMs) containing Cu‐metal organic frameworks (Cu‐MOFs) with high CO₂ selectivity on an asymmetric polysulfone support were fabricated and examined the effect of gas separation performance using different matrices. An amorphous poly(2‐ethyl‐2‐oxazoline) (POZ) and semicrystalline poly(amide‐6‐b‐ethylene oxide) (PEBAX^®MH 1657) block copolymer were chosen as the polymeric matrix and the effect of the matrix on CO₂ separation for MMMs containing Cu‐MOFs was investigated. The interaction of CO₂ in different matrix was investigated theoretically using the density functional theory method, and it was found that the amide segment in PEBAX would contribute more to the CO₂ solubility than ether segment. The morphological changes were investigated by differential scanning calorimetry, field emission scanning electron microscope and X‐ray diffractometer. The ideal selectivity of CO₂/N₂ was enhanced significantly with the addition of a Cu‐MOF, and the values are higher in the Cu‐MOF/PEBAX MMM compared with that in a POZ based asymmetric MMM. Improvement in the CO₂/N₂ selectivity of a Cu‐MOF/PEBAX MMM was achieved via facilitated transport by the CO₂‐selective Cu‐MOFs due to both their high adsorption selectivity of CO₂ over N₂ and the decreased crystallinity of PEBAX due to the presence of the Cu‐MOFs, which would provide a synergic effect on the CO₂ separation. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 132, 42853.     

CO2 capture efficiency in post‐combustion using MWNT‐PAA in a packed bed column

The adsorption capacity of polyaspartamide (PAA) and multi‐wall carbon nanotubes with polyaspartamide (MWNT‐PAA) was investigated through a packed bed column with the flowing of flue gas composed of 15 % CO₂, 5 % O₂ and the balance N₂. The adsorption performed at 25 °C, 110 kPa and inlet gas flow rate of 60 mL/min resulted in high CO₂ adsorption capacity of 5.70 and 10.20 mmol‐CO₂/g for PAA and MWNT‐PAA, respectively. The adsorption kinetics was very high, so 7 min were enough for the effluent gas to reach the breakthrough after saturation. The consistency of adsorbents in recurring regeneration was successful through a continuous TSA system of 10 cycle adsorption‐desorption with temperatures of 25–100 °C. The evaluation of heat through differential scanning calorimetry (DSC) resulted in exothermic adsorption with heat release of 45.14 kJ/mol and 124.38 kJ/mol for PAA and MWNT‐PAA, respectively. The heat release was found favourable to promote the desorption as the temperature could rise after adsorption. This is an advantage for energy efficiency, as it depicts the potential of energy recovery. Thus, both adsorbent PAA and MWNT‐PAA were demonstrated to be promising for CO₂ adsorption capture in post‐combustion.     

CO2‐facilitated transport through poly(N‐vinyl‐γ‐sodium aminobutyrate‐co‐sodium acrylate)/polysulfone composite membranes

Poly(N‐vinyl‐γ‐sodium aminobutyrate‐co‐sodium acrylate) (VSA–SA)/polysulfone (PS) composite membranes were prepared for the separation of CO₂. VSA–SA contained secondary amines and carboxylate ions that could act as carriers for CO₂. At 20°C and 1.06 atm of feed pressure, a VSA–SA/PS composite membrane displayed a pure CO₂ permeation rate of 6.12 × 10^?6 cm³(STP)/cm² s cmHg and a CO₂/CH₄ ideal selectivity of 524.5. In experiments with a mixed gas of 50 vol % CO₂ and 50 vol % CH₄, at 20°C and 1.04 atm of feed pressure, the CO₂ permeation rate was 9.2 × 10^?6 cm³ (STP)/cm² s cmHg, and the selectivity of CO₂/CH₄ was 46.8. Crosslinkages with metal ions were effective for increasing the selectivity. Both the selectivity of CO₂ over CH₄ and the CO₂ permeation rate had a maximum against the carrier concentration. The high CO₂ permeation rate originated from the facilitated transport mechanism, which was confirmed by Fourier transform infrared with attenuated total reflectance techniques. The performance of the membranes prepared in this work had good stability. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 275–282, 2006     

Kinetic investigation of CO2 and N2 clathrate hydrate formation using cyclopentane: Application in desalination

Hydrate-based desalination could be a promising technique for producing fresh water from saline water, as it is an eco-friendly process and suitable for large-scale implementation. To make the hydrate-based desalination technology easily scalable, we looked at using air (or N₂) or CO₂ as a hydrate former, along with cyclopentane (CP). Hydrate former CP helps to reduce the operating conditions, as CP forms hydrate at ambient pressure. However, hydrate formation kinetics due to water-insoluble CP is slow. In this work, the kinetics of hydrate formation in saline water were investigated and compared to identify the utility of CO₂ and N₂ as hydrate formers for desalination work. The addition of CP as a hydrate former should transform the structure of CO₂ hydrate from structure I (sI) to structure II (sII), as CP occupies the large cages (5¹²6⁴) in the gas hydrate. A set of three similar reactors were used for this study to collect data quickly. Furthermore, the triple reactor setup is a unique reactor design mounted on a shaker, and a set of SS-316 balls present inside the horizontal reactor imparts the mixing. Experiments with the CO₂-CP mixture and N₂-CP mixture have been studied in the presence or absence of 3 wt.% NaCl at 274 K and 3 MPa pressure. The gas uptake kinetics, water recovery, and separation efficiency have been investigated.     

Preparation of poly(ether‐block‐amide)/poly(amide‐co‐poly(propylene glycol)) random copolymer blend membranes for CO2/N2 separation

A series of poly(amide‐co‐poly(propylene glycol)) (PA‐PPG) random copolymers with different content of PPG were designed by polycondensation reaction. These random copolymers were blended up to 60% with commercially available Pebax 2533. The blend membranes were characterized by Fourier‐transform infrared (FTIR) spectroscopy, X‐ray diffraction (XRD), scanning electron microscope (SEM). Gas permeation properties of these blend membranes were investigated using five single‐gases (CO₂, H₂, O₂, CH₄, and N₂) at different temperature of 25–55°C and 1.0 atm. The impacts of content of PA‐PPG with different PPG content and operating temperature on CO₂ separation properties of Pebax/PA‐PPG blend membranes were studied. The results showed that CO₂ permeability gradually increased with the increasing operating temperature, whereas CO₂ permeability gradually decreased with the increase in content of PA‐PPG. CO₂/N₂ selectivity gradually increased with the increase in content of PA‐PPG. In particular, Pebax/PA‐PPG (50)–60% displayed excellent CO₂ and O₂ separation properties (P_CO2 = 79.7 Barrer and P_O2 = 13.6 Barrer, CO₂/N₂ = 34.7 and O₂/N₂ = 5.9) at 25°C and 1.0 atm. POLYM. ENG. SCI., 59:E14–E23, 2019. © 2018 Society of Plastics Engineers     

Metal‐organic framework membrane process for high purity CO2 production

Gas separation by metal‐organic framework (MOF) membranes is an emerging research field. Their commercial application potential is, however, still rarely explored due in part to unsatisfied separation characteristics and difficulty in finding suitable applications. Herein, we report “sharp molecular sieving” properties of high quality isoreticular MOF‐1 (IRMOF‐1) membrane for CO₂ separation from dry, CO₂ enriched CO₂/CH₄, and CO₂/N₂ mixtures. The IRMOF‐1 membranes exhibit CO₂/CH₄ and CO₂/N₂ separation factors of 328 and 410 with CO₂ permeance of 2.55 × 10^?7 and 2.06 × 10^?7 mol m^?2 s^?1 Pa^?1 at feed pressure of 505 kPa and 298 K, respectively. High grade CO₂ is efficiently produced from the industrial or lower grade CO₂ feed gas by this MOF membrane separation process. The demonstrated “sharp molecular sieving” properties of the MOF membranes and their potential application in production of value‐added high purity CO₂ should bring new research and development interest in this field. © 2016 American Institute of Chemical Engineers AIChE J, 62: 3836–3841, 2016     

Gas‐separation properties of amine‐crosslinked polyimide membranes modified by amine vapor

The modification of a polyimide (PI) membrane by aromatic amine vapor was performed in this work to increase the crosslinking of the membrane and to study the effect on gas permeability and the corresponding selectivity. The single‐gas permeability of the membranes at 35 °C was probed for H₂, O₂, N₂, CO₂, and CH₄. From the relationship between the combinations of gases and ideal permselectivities, this study showed that amine‐crosslinked PI membranes tended to increase gas permselectivities exponentially with the increasing difference in gas kinetic diameter. Moreover, this study illustrated that the permeability of the membranes was influenced by the formation rate of amine‐crosslinked networks or chemical structures after the reaction. The membranes had the highest level of permselectivities among crosslinked PI membranes for O₂/N₂, and the H₂/CH₄ permselectivity increased 26 times after vapor modification. Furthermore, the modification method that used aromatic amine vapor produced thin and strongly modified layers. These findings indicate that modification is an advantageous technique for improving gas‐separation performance, even considering thinning. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44569.