CO2 Hydrate Formation Research Articles

Evaluating liquid CO2 hydrate formation kinetics, morphology, and stability in oceanic sediments on a lab scale using top injection

Storing captured CO2 into the deep oceanic sediments as CO2 hydrates offers a potential approach to reducing atmospheric carbon emissions. However, two major challenges are slow CO2 hydrate formation kinetics and lack of an understanding of CO2 hydrate stability dynamics. To counter these challenges, the conversion of high-pressure liquid CO2 (LCO2) to CO2 hydrates has been experimentally explored [with and without a hydrate kinetic promoter L-tryptophan] in a lab-scale reactor embedded with an artificial seabed. The stability of these CO2 hydrates was also evaluated by submerging them in the aqueous phase (mimicking hydrostatic pressure of fluid) at thermodynamic conditions found at an oceanic depth of 1 km [3–4 °C, 10 MPa] for 14 days. In-situ high-pressure Raman spectroscopy was used to estimate the real-time solubility of CO2 in the aqueous medium, and an image processing thresholding technique was used for detecting CO2 hydrate formation/dissociation patterns in sediments during the stability testing.The lab-scale experimental results indicate that in comparison to gaseous CO2, the liquid CO2 forms hydrates much faster [60 times] in the sediments. These CO2 hydrates exhibited decent stability over 14 days when submerged in an aqueous medium [3–4 °C, 10 MPa]. At the end of the stability test, hydrates were dissociated by thermal stimulation to assess the CO2 hydrates inside the sediments qualitatively. A significant decrease in the height of the sand bed [6.0 cm to 5.5 cm] and the evolution of CO2 across the sediments confirmed the presence of a good number of hydrates inside the sediments.

Hydrate-Based Carbon Dioxide Capture Technology in the Ocean: Research Advances and Challenges

The urgent need for emission reduction due to climate change has spurred the development of carbon capture and storage (CCS) technologies, with carbon dioxide (CO2) hydrates emerging as a promising option due to their high storage capacity and cost-effectiveness. This paper provides an overview of the characteristics and formation mechanisms of CO2 hydrates, along with an exploration of mechanical and chemical methods to enhance CO2 hydrate formation. Thermodynamic additives are beneficial for reducing the formation pressure of CO2 hydrate, while mechanical methods and kinetic promoters play a positive role in improving kinetic parameters, especially when both are combined. To investigate the effectiveness of kinetic and thermodynamic promoters, this study explores the impact of various promoters on induction time, formation rate, phase equilibrium, and gas consumption. Additionally, the existing challenges of CO2 hydrate are discussed, and the future research directions are proposed. Based on this study, the hydrate method of CO2 capture technology is expected to assume a crucial role in marine carbon capture technology.

Effect of MgCl2 on CO2 sequestration as hydrates in marine environment: A thermodynamic and kinetic investigation with morphology insights

Oceanic hydrate-based CO2 sequestration (HBCS) holds great promise for achieving carbon neutrality. However, the presence of inorganic salts, particularly MgCl2 besides NaCl, in seawater can significantly impact the formation rate and the stability of CO2 hydrate. In this study, experimental investigations were conducted to examine the thermodynamics, kinetics, and the resulting morphological features of CO2 hydrate in the presence of MgCl2, covering mass fractions ranging from 0 to 5.0 wt%. The experimental findings reveal that MgCl2 exerts a thermodynamic inhibitory effect with its inhibitory capacity increasing with higher mass fractions. The solubility model of CO2 in MgCl2 solution was modified, demonstrating a gradual weakening of CO2 solubility as MgCl2 mass fraction increases. Additionally, the growth kinetics of CO2 hydrate decreases with increasing MgCl2 mass fraction. Regarding CO2 hydrate morphology, it was observed that at low mass fractions of MgCl2 (<1.0 wt%), a dense hydrate film rapidly formed at the gas-liquid interface after CO2 hydrate nucleation, hindering the further conversion of CO2 into hydrate. Conversely, at higher mass fractions (>3.0 wt%), CO2 hydrate exhibits a more porous and slurry-like structure, facilitating more gas-liquid contact and mass transfer, thereby enhancing the conversion of CO2 into hydrate. During hydrate dissociation, a salt-removal effect associated with CO2 hydrate formation was observed, leading to the accumulation of concentrated electrolyte (MgCl2) and facilitating CO2 hydrate dissociation. These findings have implications for understanding the CO2 hydrate formation and dissociation in the presence of MgCl2 relevant in the subsea environment and can contribute to the development of effective hydrate-based CO2 sequestration strategies.

Adsorption behaviors for clathrate hydrates of CO2 with mixed gases

Here, the preference and adsorption capacities of guest molecules in the sI and sII hydrate of water cages when CO2 + H2S, CO2 + SO2 and CO2 + N2O gas mixture form hydrates are simulated by using grand canonical Monte Carlo and molecular dynamics hybrid simulations method. The results indicate that SO2 prefers to occupy large cages, H2S prefers small cages for CO2 + H2S and CO2 + SO2 and N2O exhibits little preference between small or large or cages of CO2 + N2O sI hydrates, respectively. The abundance ratio shows hydrate can effectively achieve the separation of flue gases. Moreover, the thermodynamics calculation results show that the impurity gases H2S, SO2 and N2O favors the formation of CO2 hydrate. The stability CO2 + H2S, CO2 + SO2 and CO2 + N2O sI hydrates is higher compared to the stability of sII hydrates. The phase equilibrium analysis provides clear evidence that enclathration of H2S and SO2 molecules into the water cages of the hydrate structure plays a crucial role in stabilizing the hydrate lattices. The findings of this study suggest that removal of certain flue gas impurities is not required during the replacement of CH4 in hydrate form with the CO2 gas. The current investigation provides theoretical background for executing replacement of CH4 in hydrate form with the CO2 gas.

Evaluation of amino acid L-leucine as a kinetic promoter for CO2 sequestration as hydrate: A kinetic and morphological study

Carbon capture and sequestration (CCS) are widely recognized as the most effective technology for reducing CO2 emissions and mitigating global climate change. Hydrate-based CO2 sequestration (HBCS) has emerged as a promising technology for long-term and stable CO2 storage. There are growing interests in employing hydrophobic amino acids to enhance CO2 hydrate formation kinetics recently. However, there is still a lack of understanding of how amino acids promote CO2 hydrate effectively. In this study, we systematically investigate an environmental-benign kinetic promoter (L-leucine, L-Leu) varying from 0.05 to 2.00 wt% to enhance CO2 hydrate formation at an initial pressure of 5.5 MPa and temperature of 275.2 K. The results show that varying L-Leu concentration has limited effect on tind. The kickoff concentration of L-Leu in promoting CO2 hydrate kinetics is 0.30 wt%. 2.00 wt% L-Leu yields the optimal CO2 gas uptake of 62.17 ± 0.78 mmol/mol with CO2 partition in the hydrate phase of 66 %. CO2 hydrate forms a layer at the gas-liquid interface and turns to be porous at CL-Leu above 0.30 wt% followed by a significant wall-climbing growth with rapid CO2 gas uptake. L-Leu precipitation occurs at CL-Leu above 1.00 wt% and leads to a decrease in final CO2 gas uptake in repeated experiments at CL-Leu = 2.00 wt%. A mechanism of L-Leu in promoting CO2 hydrate kinetics by adsorption on the surface at CL-Leu above 1.00 wt% is proposed. L-Leu is evaluated to be effective in improving CO2 hydrate kinetics. These findings provide insights for a novel environmental-benign CO2 hydrate kinetic promoter in HBCS application, which requires rapid CO2 hydrate formation and high CO2 gas uptake.

Study on acoustic properties of hydrate-bearing sediments with reconstructed CO2 hydrate in different layers during CH4 hydrate mining

Natural gas hydrate (NGH), a clean energy source with huge reserves in nature, and its safe and efficient exploitation fits perfectly with the UN Sustainable Development Goals (SDG-7). However, large-scale NGH decomposition frequently results in subsea landslides, reservoir subsidence, and collapse. In this work, in order to achieve safe and efficient exploitation of NGHs, the stability variation of different reservoir layers by depressurization/intermittent CO2/N2 injection (80:20 mol%, 50:50 mol%) was investigated using acoustic properties (P-wave velocity, elastic modulus), as well as reservoir subsidence under an overburden stress of 10 MPa. The P-wave velocity increased from 1282 m/s to 2778 m/s in the above-reservoir and from 1266 m/s to 2564 m/s in the below-reservoir, significantly increasing reservoir strength after CO2 hydrate formation. The P-wave velocity and elastic modulus in the top reconstructed reservoir were continually decreased by the shear damage of the overlying stress, while they remained stable in the bottom reconstructed reservoir during hydrate mining. However, due to superior pressure-bearing ability of the top CO2 hydrate reservoir, which was lacking in the bottom CO2 hydrate reservoir, the reservoir subsidence was relieved greatly. Despite the stiffness strength of reconstructed reservoir was ensured with CO2/N2 sweeping, the skeletal structure of CH4 hydrate reservoir was destroyed, and only the formation of CO2 hydrate could guarantee the stability of P-wave velocity and elastic modulus which was most beneficial to relieve reservoir subsidence. A large amount of CO2 was used in reservoir reconstruction and CH4 hydrate mining, which achieved the geological storage of CO2 (SDG-13). This work provided a new idea for safe and efficient NGHs mining in the future, and the application of acoustic properties served as a guide for the efficient construction of reconstructed reservoirs and offers credible technical assistance for safe exploitation of NGHs.

CO2 hydrates phase behaviour and onset nucleation temperatures in mixtures of H2O and D2O: Isotopic effects

In this work, we report the CO2 hydrate phase equilibria in water (H2O), heavy water (D2O), and their binary mixtures following the isochoric pressure search method using a rocking cell apparatus. The phase behaviour was mapped within the temperature and pressure range of 276.32 – 284.80 K and 1.59 – 3.78 MPa, respectively. It was found that there is a difference of ∼ 2 K in the equilibrium line of CO2 hydrates formed in H2O and in D2O, respectively. The hydrate dissociation enthalpies obtained using the Clausius-Clapeyron equation indicate almost similar values formed either in D2O, H2O or their mixtures. These shifts in this equilibrium temperature were compared with the triphasic equilibrium temperature variation estimation obtained using Molecular Dynamics Simulations and a very good agreement with the experimentally obtained values was observed. Further, a constant cooling method was used to obtain the onset temperature of hydrate nucleation for these systems at 3.6 MPa. It has been found that during the cooling ramps, the nucleation always occurred in the vicinity of the temperature of maximum density (TMD) of the systems where water still retains some structuredness. The nucleation experiments also give information about the metastable zone width (MSZW) of the studied systems. The results reported in this work indicate the magnitude of the isotopic effect on CO2 hydrate formation and dissociation that may have implications towards the application of hydrate technology for separation and purification processes.

Carbon dioxide sequestration as hydrates in clayey-sandy sediments: Experiments and modeling approach

Carbon capture and sequestration (CCS) is the key to a low carbon transition model applicable to attain the Net-Zero targets. In this direction, the hydrate-based CO2 sequestration (HBCS) approach has gained significant attention due to its high storage capacity and long-term stability. The current work evaluates the CO2 hydrate formation kinetics and storage capacity in clayey-sandy sediments and develops a chemical potential-based kinetic model for porous hydrate sediments. Experiments are performed in a high-pressure reactor mimicking the subsea sediment conditions. Multiple silica sand sediments with various clay content (2–8 wt%) are used as a porous media, and saline water of 3.5 wt% salinity is used as a substitute for seawater. For a successful deployment of HBCS technology, the effect of the following parameters is investigated for CO2 hydrate formation kinetics and storage capability: impact of clay content in sediment; salinity, sediment’s particle size; water saturation of the sediments; and hydrate promoter (L-Tryptophan). Besides the close prediction of the conversion kinetics of water-CO2 to CO2 hydrates in the aforementioned distinctive operating environments, the proposed kinetic model offers its generalized potential. This work offers important insights into the deployment of HBCS in subsea clayey-sandy sediments available in the Krishna-Godavari (KG) Basin of India.

Analysis of CO2 hydrate formation from flue gas mixtures in a bubble column reactor

Gigascale carbon capture and sequestration (CCS) is increasingly seen as essential to meeting the targets of the Paris Agreement. As an alternative to conventional CCS approaches, carbon dioxide (CO2) hydrates have received attention as materials which can enable new approaches to carbon capture as well as carbon sequestration. CO2 hydrates (ice-like materials of CO2 and water) form at medium pressures (<400 psi) and temperatures of >0 °C from a water-CO2 mixture. Bubble column reactors (BCR) have been studied as a preferred way of forming CO2 hydrates. This study uses an inhouse, recently-developed modeling framework to predict performance of a BCR for CO2 hydrate formation from flue gas (CO2/N2), and pure CO2 streams. We highlight and analyze specific aspects of hydrate formation that are important for CO2 sequestration, and for CO2 separation/capture. In particular, two performance parameters are analyzed: i) gas consumption rate for hydrate formation (normalized with reactor volume), and ii) fraction of CO2 that converts to CO2 hydrates in a single pass (conversion factor). The first metric quantifies the overall productivity of a BCR by obtaining the net CO2 that can be sequestered or separated from the flue gas stream. The second metric relates to the efficiency of the system by quantifying the need for recirculation and the quality of the exit stream after a single pass. Extensive parametric analysis is conducted to study the influence of pressure, temperature, CO2 mole fraction at inlet, gas flow rate and reactor geometry on hydrate formation. Across the range of simulations conducted in this study, the highest gas consumption rate per unit reactor volume was 28.9 ton/yr/m3 and the highest conversion factor was 67.8 %. Both parameters increase with increasing pressure, decreasing temperature and increasing inlet mole fraction of CO2. Increasing gas flow rate increases the gas consumption rate (i.e., hydrate formation rate) but reduces the conversion factor. This suggests that the overall productivity of BCRs increases with gas flow rate at the expense of its efficiency. Reduced efficiency increases recirculation-related costs and high flow rate increases compression and cooling costs. For flue gas, increasing the reactor volume by increasing the height or diameter increases conversion factor but significantly reduces the gas consumption rate per unit reactor volume. For pure CO2, increasing the reactor height increases the conversion factor without changing the volumetric gas consumption rate. Decreasing the diameter increases volumetric gas consumption rate without changing the conversion factor. These findings suggest that compact reactors are more suitable for CO2 hydrate slurry production (on a volumetric basis), while larger reactors are suitable for CO2 separation/capture applications. Overall, this study provides a basis for the design and operation of BCRs for CO2 hydrates-based CCS applications.

Detection of formation and dissociation of CO2 hydrates in fine-sands through acoustic waves

Natural gas hydrates in oceanic and permafrost sediments are considered a future energy resource. Injecting CO2 into these sediments is believed to aid in the safe and efficient extraction of CH4 and the geological sequestration of carbon. The effectiveness of this strategy can be ensured by assessing the formation of CO2 hydrates in the pores of these sediments at in-situ stress conditions by investigating their elastic properties (viz., compressional and shear wave velocities, elastic modulus, and shear modulus) by resorting to a suitable method such as seismic and/or acoustic logging. However, conducting such a study in field is seldom opted due to the requirement of expensive paraphernalia such as seismic equipment, pressure coring tools, logging while drilling tools, modular dynamic tester, etc. With this in view, efforts were made to synthesize CO2 hydrates in partially saturated fine-sands under triaxial conditions by employing the excess gas method. Ultrasonic wave transducers were employed in tandem to measure the compressional (VP) and shear wave (VS) velocities of the sample, both under formation and dissociation processes. These results have been utilized to propose hydrate formation and dissociation models which encompass three phases each. The rapid reduction in VP and VS observed during thermal dissociation indicates loss of cementation between the sand grains and hydrates, and commencement of hydrate dissociation at their interface. It has been demonstrated that the induction time for CO2 hydrate formation increases, and the water-to-hydrate conversion ratio decreases, with an increase in the initial volumetric water content. Further, the results generated in the present study were compared vis-à-vis those available in the literature for CH4 and CO2 hydrate-bearing sediments (HBS) and it has been found that both VP and VS of the CO2- and CH4- HBS are comparable for a given hydrate saturation.

Effects of carbon nanotube on the nucleation and growth of CO2 hydrate: Insights from the molecular dynamics simulations

As an essential technology for achieving carbon neutralization, hydrate technology shows broad prospects in CO2 capture and storage. Carbon nanotubes have been used to promote gas hydrate formation as a material with good physical and chemical properties. In this work, molecular dynamics simulations were performed at a pressure of 40 MPa and a temperature of 250 K by adding carbon nanotubes with different quantities and lengths. The simulation results showed that adding one carbon nanotube had little effect on the formation of CO2 hydrate. As the cage structure formed, the hydrate moved away from the carbon nanotubes and repelled them into the critical aqueous phase. When three carbon nanotubes were added, they appeared to agglomerate due to hydrophobicity, which inhibited the formation of CO2 hydrate. Only when two carbon nanotubes were added can they effectively promote the nucleation and growth of CO2 hydrate. At the same time, shorter carbon nanotubes can maximize their excellent thermal conductivity, thus promoting nucleation and stable growth of hydrates. Therefore, the promotion or inhibition effect of CO2 hydrate by carbon nanotubes depends on their size and amount.

Diffusion model of gas hydrate formation from ice considering the gas pressure drop

A kinetic model of the process of gas hydrate formation from ice that considers the gas pressure drop caused by this process is presented. This model is based on the diffusion equation. The issues during the formation of gas hydrate from ice powder and that from a flat ice layer are considered separately. For each of these problems, a solution in the framework of a quasi-stationary approximation is obtained, allowing for the calculation of the gas pressure drop in the gas phase. The calculated data are compared with available experimental data on the kinetics of CO2 hydrate formation from pure ice powder and ice powder with the addition of sodium dodecyl sulfate (SDS) at a mass concentration of 240 ppm: the calculated and experimental curves of CO2 pressure drop are compared with each other. From this comparison, the values of the diffusion coefficient of CO2 through a layer of CO2 hydrate formed from pure ice and ice with the addition of SDS at a mass concentration of 240 ppm were determined. Based on these values, it was found that SDS molecules present in the gas hydrate affect the rate of gas diffusion through the gas hydrate layer.

Crystallization-driven evolution of water occurrence states with implications on dewaterability improvement of waste-activated sludge

This study proposed to improve the dewaterability of waste-activated sludge (WAS) through crystallization-driven evolution of water occurrence states. Primarily, the feasibility of clathrate hydrate (i.e., CO2 hydrate) formation in WAS was examined. The thermodynamic analysis indicated that the CO2 hydrate formation with the excessive water in WAS followed pseudo-first-order kinetics, and fit of the data yielded a kobs value of 3.905 × 10−5 L∙mol−1∙s−1 for 274.15 K. With the water conversion efficiency of 100%, the crystallization-dissociation process of CO2 hydrate significantly improved the dewaterability of WAS in term of capillary suction time (CST) decreasing from 251.5 s to 57.4 s. Also, the relief of gas pressure can induce the hydrate dissociation, which creates a novel way to recycle CO2 gas and save the consumption of chemicals required by sludge dewatering process. Regarding the mechanism of hydrates-based sludge dewatering, the evolution of water occurrence state was investigated. The in-situ synchrotron X-ray computed microtomography visually analyzed the micro-scale porosity and interstitial water of WAS flocs. The model of three-dimensional pore structure was established and the porosity parameters of solid aggregates were determined. It was found that the volume of connected pores and the total pore volume fraction of solid compositions increased. But the mean volume and mean area of isolated pores simultaneously decreased by 14.6% and 12.4%, respectively, which meant that the steric hindrance caused by isolated pores was weakened due to the reduced solid-water contact area. In addition, the crystallization of water caused the reformation of conformation arrangement of vicinal water and solid molecules, which highly organized the water molecules into the crystal structure. Accordingly, an estimation method for vicinal water layer thickness was developed based on atom force microscope. The thickness of vicinal water layer was found to be reduced by 77.4% and the hydration repulsion among solid compositions was correspondingly weakened, which facilitated the aggregation of solid compositions, and the relatively separated hydrate phase and solid phase could be formed. All the above results open up a novel strategy for enhanced water-solid separation of WAS through the crystallization-driven evolution of water occurrence states. As distinguished from the conventional approaches, the hydrates-based sludge dewatering enhances the water-solid separation only with regulating the spatial arrangement of water-solid molecules, but without altering the chemical compositions. Thus, more chances can be created to increase the environmentally friendly attributes related to WAS dewatering.

Synergistic Effect of Water-Soluble Hydroxylated Multi-Wall Carbon Nanotubes and Graphene Nanoribbons Coupled with Tetra Butyl Ammonium Bromide on Kinetics of Carbon Dioxide Hydrate Formation

In this work, the thermodynamics and kinetics of hydrate formation in 9.01 wt% tetra butyl ammonium bromide (TBAB) mixed with water-soluble hydroxylated multi-wall carbon nanotube (MWCNTol) systems were characterized by measuring hydrate formation conditions, induction time, and final gas consumption. The results showed that MWCNTols had little effect on the phase equilibrium of CO2 hydrate formation. Nanoparticles (graphene nanoribbons (GNs) and MWCNTols) could significantly shorten the induction time. When the concentration was ≤0.06 wt%, MWCNTols had a better effect on the induction time than the GN system, and the maximum reduction in induction time reached 44.22%. The large surface area of MWCNTols could provide sites for heterogeneous nucleation, thus shortening the induction time of hydrate formation. Furthermore, adding different concentrations of nanoparticles to the 9.01 wt% TBAB solution effectively increased the final gas consumption, and the maximum increase was 10.44% of the 9.01 wt% TBAB + 0.08 wt% GN system. Meanwhile, the suitable initial pressure and experimental temperature could also promote the hydrate formation and increase the motivation in hydrate formation. The 9.01 wt% TBAB + 0.02 wt% MWCNTol system had the best effect at 3.5 MPa and 277.15 K. The induction time was reduced by 66.67% and the final gas consumption was increased by 284.11% compared to those of the same system but at a different initial pressure and experimental temperature. This work helps to promote the industrial application of hydrate technology in CO2 capture and storage.

Experimental study and chemical affinity model on the inhibition of CO2 gas hydrate formation

Natural gas and water can form complex cage crystals in oil and gas pipelines under certain conditions. The natural gas hydrate formed after crystal clustering can block the pipeline and affect the operation efficiency of the pipeline, and even cause equipment damage, bringing economic and security problems to enterprises. In the present study, we experimentally investigated the inhibition of CO2 formation hydrate using attapulgite (ATP) and glucose (GLC). The kinetic influence on the formation of CO2 hydrate by the complex pairing of kinetic inhibitor ATP is conducted under the working conditions of 277.15 K and 3.5 MPa. The effect of compound inhibitors on the temperature and pressure conditions, growth process and gas consumption of hydrate formation are revealed. The chemical affinity model of CO2 hydrate formation under a compound inhibitor system is derived and established based on experimental studies. The results show that the combination of GLC and ATP can inhibit the formation of CO2 hydrate to varying degrees, prolong the induction time of hydrate, and reduce the consumption of CO2. The optimized experimental studies demonstrate that the best inhibitory compound system is 15 mg/mL GLC + 1.00 mg/mL ATP. Compared with the pure water system and single 15 mg/mL GLC system, the induction time is extended by 122.61% and 122.23%, while the gas consumption is reduced by 23.72% and 3.41%. The results provide new ideas and methods for the prediction of hydrate formation in the compound inhibitor system for the safe operation of oil and gas pipelines.

A comprehensive review of the influence of particle size and pore distribution on the kinetics of CO2 hydrate formation in porous media

AbstractAs an essential greenhouse gas, CO2 is the leading cause of global warming and environmental problems. An efficient strategy to lower CO2 emissions is the hydrate‐based method of CO2 geological storage. The stability and formation process of hydrate is the premise and foundation of the hydrate method of CO2 geological storage. However, the formation rule of CO2 hydrate has a significant impact on the formation characteristics of CO2 hydrate. This paper thoroughly examines the formation properties of CO2 hydrate in porous media systems. The quantitative impacts and laws of many parameters on the CO2 hydrate production process are thoroughly examined. On this basis, the internal mechanism of particle size, pore distribution, and critical size of particles in porous media systems on the kinetics of CO2 hydrate formation are detailed. Finally, the shortcomings of the studies on CO2 hydrate formation kinetics in porous media systems and the main directions in the future are pointed out. The influence of pore distribution in porous media on the CO2 hydrate formation process still needs further study. The relative results will be useful in the future for CO2 capture and sequestration in sediments. © 2023 Society of Chemical Industry and John Wiley &amp; Sons, Ltd.

Unusual promotion/inhibition effect of polyethylenimine on CO2/CH4 hydrate formation: A potential green chemical additive for CH4-CO2 replacement process

The unusual behavior of a new class of compounds that selectively function as either promoters or inhibitors of gas hydrate formation depending on the process conditions has made their exploration increasingly appealing. In this study, polyethylenimine (PEI) exhibits desirable functionality that may be a candidate for environmentally benign compounds. The present work investigated PEI's thermodynamic and kinetic influence on the formation of CH4 and CO2 hydrates for two PEI concentrations (0.5 and 5.0 mass%) and two PEI molecular weights (25,000 and 750,000 g/mol). The isochoric method was applied to measure the hydrate-liquid water–vapor-three-phase-coexisting (H-LW-V-3PC) phase boundary. PEI is a thermodynamic inhibitor regardless of CH4 or CO2 as a hydrate former, and 0.5 mass% PEI system exhibits the highest average suppression temperatures of 0.25 and 0.62 K, respectively. Kinetic findings, such as induction time, initial hydrate formation rate, and nucleation rate, on the other hand, revealed that PEI could act differently in response to specific hydrate formers. PEI kinetically inhibits CH4 hydrates formation but promotes the formation of CO2 hydrates. This promotion effect is attributed to PEI's high CO2 absorption ability to enhance the solubility of CO2. This unique behavior of PEI presents appealing advantages in the natural gas hydrate exploitation field due to their potential for greater operational flexibility and minimal environmental impact.

Injectivity assessment for CCS field-scale projects with considerations of salt deposition, mineral dissolution, fines migration, hydrate formation, and non-Darcy flow

This paper aims to review the main mechanisms identified in the lab scale that impact CO2 injectivity during CCS projects in saline aquifers and evaluate their relevance in field scale through numerical models. They may include water vaporization, salt precipitation in the dry-out zone, mineral dissolution, which can result in fines migration in porous media, CO2 hydrate formation, and effects of non-Darcy flow. All those mechanisms are observed from lab experiments and significantly impact reservoir petrophysical properties such as porosity and permeability and consequently on injectivity in field operations. This study includes these mechanisms in numerical models to quantify their impacts on the CO2 injectivity well index in synthetic and real field cases. In addition, a sensitivity analysis was performed for a range of brine compositions of seawater and higher salinities. This analysis shows an additional injectivity reduction as the salinity increases due to the halite scale, in agreement with the lab data from the literature. Another dependence evaluated in this study is when the ratio between capillary and viscous forces is decreased, reducing the injectivity. Additionally, when mineral reaction dissolution is considered in our modeling, it is possible to observe two occurring competitive phenomena, where the mineral dissolution can compensate for part of the injectivity loss due to salt deposition in the dry-out zone. That is especially relevant when the reservoir rock matrix is rich in reactive minerals, such as calcite and dolomite. However, mineral dissolution may lead to fines, and the risk of fines migration can lower the injectivity by at least 10% within five years of CO2 injection, according to our results. The mineral dissolution is also considered in these calculations, and the loss in injectivity is the net result of dissolution and fines migration. The gain in porosity–permeability as the results of mineral dissolution was modeled using the Kozeny-Carman equation. The results show the importance of considering the uncertainty of injectivity loss in CCS projects, where the amount of CO2 captured for storage must be injected often with no other alternatives, highlighting the need to provide flexibility in project planning to deal with such risks.

Characteristics of continuous CO2 hydrate formation process using a NetMIX reactor

A new continuous process to obtain CO2 hydrates, which is scalable, without losing its heat removing capacity is presented. Hydrate formation is highly exothermic, and jacketed stirred tank reactors configurations have limitations in heat removal. Experiments to obtain CO2 hydrates (P = 27 bar, T = 2.7, 3.8 and 4.8 °C, CO2/water mass flow ratios of 0.02, 0.10 and 0.18) were perfomed. Hydrate formation in the NetMIX is very fast (seconds). The outstanding performance is due to the high gas–liquid contact and the capability in removing the heat released at the rate it is formed. The 0.18 CO2/water mass flow ratio formed hydrates at a higher concentration. Results suggest the gas uptake in the hydrate formed in this process is higher than those produced in batch systems. The process presented in this work is a promising alternative for the sequestration of CO2 due to the advantages of this device both in heat removal and enhanced mass transfer.

Gas Production Potential in Geothermal-Energy-Enhanced CH4–CO2 Swapping Processes

Carbon capture and storage has become a practice to reduce the greenhouse effect of carbon dioxide (CO2) on the global climate. Recent studies have generated increasing concerns about CO2 leakage from underground structures. This has called for more research on CH4–CO2 swapping in natural gas hydrate (NGH) reservoirs to lock CO2 in a solid state in underground structures. Because the CH4–CO2 swapping is too slow to be efficient, this study proposes to use geothermal energy to accelerate the process. This paper presents a technical feasibility analysis of using geothermal energy to assist CH4–CO2 swapping for simultaneously storing CO2 in NGH reservoirs and producing the dissociated natural gas. Mathematical models were developed to compute heat transfer from geothermal zones to NGH reservoirs. A case study was carried out using the data from an NGH reservoir in the Shenhu area, Northern South China Sea. The result of the case study indicates that heat conduction dictates the heat transfer process when the heat convection flow rate is less than 0.01 m3/s over a heat-releasing borehole length of 2,000 m. Heat convection can significantly accelerate the heat transfer inside the gas hydrate reservoir. The 15°C (designed gas hydrate dissociation temperature in the studied case) heat front will propagate to the upper and lower boundaries of the gas hydrate reservoir (39 ft or 12 m) in 220 days by heat conduction only. This time can be shortened to 140 days with the aid of a fluid convection rate of 0.005 m3/s. Geothermal heating can significantly increase the initial productivity of wells in heated gas hydrate reservoirs in CO2 swapping processes. When the gas hydrate reservoir is heated from 6 to 16°C, the fold of increase is expected to exceed five in the studied case. This study shows that CH4–CO2 swapping process using geothermal stimulation is a promising method for producing natural gas and locking CO2 permanently in NGH reservoirs. Further studies should first focus on investigations of the effect of CO2-hydrate formation on the CO2 mass transfer inside reservoirs.