Jaehyoung Lee

Sequestering carbon dioxide into complex structures of naturally occurring gas hydrates

Large amounts of CH4 in the form of solid hydrates are stored on continental margins and in permafrost regions. If these CH4 hydrates could be converted into CO2 hydrates, they would serve double duty as CH4 sources and CO2 storage sites. We explore here the swapping phenomenon occurring in structure I (sI) and structure II (sII) CH4 hydrate deposits through spectroscopic analyses and its potential application to CO2 sequestration at the preliminary phase. The present 85% CH4 recovery rate in sI CH4 hydrate achieved by the direct use of binary N2 + CO2 guests is surprising when compared with the rate of 64% for a pure CO2 guest attained in the previous approach. The direct use of a mixture of N2 + CO2 eliminates the requirement of a CO2 separation/purification process. In addition, the simultaneously occurring dual mechanism of CO2 sequestration and CH4 recovery is expected to provide the physicochemical background required for developing a promising large-scale approach with economic feasibility. In the case of sII CH4 hydrates, we observe a spontaneous structure transition of sII to sI during the replacement and a cage-specific distribution of guest molecules. A significant change of the lattice dimension caused by structure transformation induces a relative number of small cage sites to reduce, resulting in the considerable increase of CH4 recovery rate. The mutually interactive pattern of targeted guest–cage conjugates possesses important implications for the diverse hydrate-based inclusion phenomena as illustrated in the swapping process between CO2 stream and complex CH4 hydrate structure.

Guest Gas Enclathration in Semiclathrates of Tetra-n-butylammonium Bromide: Stability Condition and Spectroscopic Analysis

In this study, guest gas enclathration behavior in semiclathrates of tetra-n-butylammonium bromide (TBAB) was closely investigated through phase equilibrium measurement and spectroscopic analysis. The three-phase equilibria of semiclathrate (H), liquid water (L(W)), and vapor (V) for the ternary CH(4) + TBAB + water and CO(2) + TBAB + water mixtures with various TBAB concentrations were experimentally measured to determine the stability conditions of the double TBAB semiclathrates. Equilibrium dissociation temperatures for pure TBAB semiclathrate were also measured at the same concentrations under atmospheric conditions. The dissociation temperature and dissociation enthalpy of pure TBAB semiclathrate were confirmed by differential scanning calorimetry. The experimental results showed that the double CH(4) (or CO(2)) + TBAB semiclathrates yielded greatly enhanced thermal stability when compared with pure CH(4) (or CO(2)) hydrate. The highest stabilization effect was observed at the stoichiometric concentration of pure TBAB semiclathrate, which is 3.7 mol%. From the NMR and Raman spectroscopic studies, it was found that the guest gases (CH(4) and CO(2)) were enclathrated in the double semiclathrates. In particular, from the cage-dependent (13)C NMR chemical shift, it was confirmed that CH(4) molecules were captured in the 5(12) cages of the double semiclathrates.

Experimental Verification of Methane–Carbon Dioxide Replacement in Natural Gas Hydrates Using a Differential Scanning Calorimeter

The methane (CH4) - carbon dioxide (CO2) swapping phenomenon in naturally occurring gas hydrates is regarded as an attractive method of CO2 sequestration and CH4 recovery. In this study, a high pressure microdifferential scanning calorimeter (HP μ-DSC) was used to monitor and quantify the CH4 - CO2 replacement in the gas hydrate structure. The HP μ-DSC provided reliable measurements of the hydrate dissociation equilibrium and hydrate heat of dissociation for the pure and mixed gas hydrates. The hydrate dissociation equilibrium data obtained from the endothermic thermograms of the replaced gas hydrates indicate that at least 60% of CH4 is recoverable after reaction with CO2, which is consistent with the result obtained via direct dissociation of the replaced gas hydrates. The heat of dissociation values of the CH4 + CO2 hydrates were between that of the pure CH4 hydrate and that of the pure CO2 hydrate, and the values increased as the CO2 compositions in the hydrate phase increased. By monitoring the heat flows from the HP μ-DSC, it was found that the noticeable dissociation or formation of a gas hydrate was not detected during the CH4 - CO2 replacement process, which indicates that a substantial portion of CH4 hydrate does not dissociate into liquid water or ice and then forms the CH4 + CO2 hydrate. This study provides the first experimental evidence using a DSC to reveal that the conversion of the CH4 hydrate to the CH4 + CO2 hydrate occurs without significant hydrate dissociation.

Effect of Interlayer Ions on Methane Hydrate Formation in Clay Sediments

Natural methane hydrates occurring in marine clay sediments exhibit heterogeneous phase behavior with high complexity, particularly in the negatively charged interlayer region. To date, the real clay interlayer effect on natural methane hydrate formation and stability remains still much unanswered, even though a few computer simulation and model studies are reported. We first examined the chemical shift difference of 27Al, 29Si, and 23Na between dry clay and clay containing intercalated methane hydrates (MH) in the interlayer. We also measured the solid-state 13C MAS NMR spectra of MH in Na-montmorillonite (MMT) and Ca-montmorillonite (MMT) to reveal abnormal methane popularity established in the course of intercalation and further performed cryo-TEM and XRD analyses to identify the morphology and layered structure of the intercalated methane hydrate. The present findings strongly suggest that the real methane amount contained in natural MH deposits should be reevaluated under consideration of the compositional, structural, and physical characteristics of clay-rich sediments. Furthermore, the intercalated methane hydrate structure should be seriously considered for developing the in situ production technologies of the deep-ocean methane hydrate.

Abnormal methane occupancy of natural gas hydrates in deep sea floor sediments

Natural gas hydrates were recovered from near-seafloor sediments and analysed to compare two distinctive methane inclusion phenomena. We document the first observation of abnormal methane occupancy in sediment-rich NGH deposits.

Modeling of solute transport in a single fracture using streamline simulation and experimental validation

Abstract Streamline simulations have been extensively used in petroleum engineering due to its computational speed and the freedom from numerical dispersion. This study applies streamline simulation to the modeling of solute transport in a single fracture and verifies the streamline method with experimental data. In order to model dispersive transport, a new term, the advection–dispersion ratio is employed, which is defined as the relative extent of advection to dispersion along streamlines. It is observed that the tracer breakthrough curves from the simulation match well with those from the experiments. In addition, the tracer displacement profiles from the simulation also show resemblance to those from the experiments. Simulations with various link transmissivity types result in no serious disparities. The distributions of time of flight and tracer breakthrough curves from the simulations using different link transmissivity types are much alike. Transport simulation is performed by allocating different advection–dispersion ratios along streamlines. Afterwards, the results are compared with the simulation result using single representative advection–dispersion ratio over the flow domain. Although streamlines actually have different advection–dispersion ratios, its effect is found to be not severe. Therefore, a representative advection–dispersion ratio can be used for modeling transport through the whole streamlines in a single fracture.

Experimental Study on Two-phase Flow in Artificial Hydrate-bearing Sediments

ABSTRACT It is important to investigate gas-water multiphase flow in methane hydrate-bearing sediment. In this work, we measured relative permeability of gas and water in artificial methane hydrate bearing sediment by the unsteady state method, and determined the effects of hydrate saturation on relative permeability curves. The results show that the relative permeability shifted left with increasing hydrate saturation.

Oil Recovery in a Fracture of Variable Aperture With Countercurrent Imbibition: Experimental Analysis

A parallel plate model is widely used to simulate flow in a fracture due to simplicity of idealizing the fracture. However, a real fracture has characteristics of variable aperture in nature. To assess the effect of variable aperture in water flooding, we performed water injection tests through fractured samples initially saturated with oil. A casting method is employed to obtain aperture distribution of real fractures. From these results, we represented the morphology of the fractures using statistical parameters such as mean, coefficient of variation (Cv), skewness (S), and an anisotropy ratio. Water is injected into unfractured sandstone samples with constant rate and into artificially fractured sandstones with an increasing flow rate. We interpreted cumulative oil recovery curves from the fractured rocks using corresponding statistical parameters in the results. The results show that morphology of a fracture has a significant effect on oil recovery as well as water injection rate. As Cv and S increase, overall efficiency of oil recovery decreases. Anisotropy ratio affects the breakthrough time of injected water and overall oil recovery as significant as Cv. At lower anisotropy ratio such as less than one, early breakthrough and poor oil recovery have occurred. The effect of the statistical parameters is pronounced as injection rate increases. We observed a point where imbibition rate shows identity regardless of fracture morphology, if both flow rate of injected water and matrix characteristics are the same.

SWAPPING CARBON DIOXIDE FOR COMPLEX GAS HYDRATE STRUCTURES

Large amounts of CH4 in the form of solid hydrates are stored on continental margins and in permafrost regions. If these CH4 hydrates could be converted into CO2 hydrates, they would serve double duty as CH4 sources and CO2 storage sites. Herein, we report the swapping phenomena between global warming gas and various structures of natural gas hydrate including sI, sII, and sH through C solid-state nuclear magnetic resonance, and FT-Raman spectrometer. The present outcome of 85% CH4 recovery rate in sI CH4 hydrate achieved by the direct use of binary N2 + CO2 guests is quite surprising when compared with the rate of 64 % for a pure CO2 guest attained in the previous approach. The direct use of a mixture of N2 + CO2 eliminates the requirement of a CO2 separation/purification process. In addition, the simultaneously-occurring dual mechanism of CO2 sequestration and CH4 recovery is expected to provide the physicochemical background required for developing a promising large-scale approach with economic feasibility. In the case of sII and sH CH4 hydrates, we observe a spontaneous structure transition to sI during the replacement and a cage-specific distribution of guest molecules. A significant change of the lattice dimension due to structure transformation induces a relative number of small cage sites to reduce, resulting in the considerable increase of CH4 recovery rate. The mutually interactive pattern of targeted guest-cage conjugates possesses important implications on the diverse hydratebased inclusion phenomena as clearly illustrated in the swapping process between CO2 stream and complex CH4 hydrate structure.

Phase and kinetic behavior of the mixed methane and carbon dioxide hydrates

Large amounts of CH4 are stored as hydrates on continental margins and permafrost regions. If the CH4 hydrates could be converted into CO2 hydrate, they would serve double duty as CH4 sources and CO2 storage sites in the deep ocean sediments. As preliminary investigations, both the phase behavior of CH4 hydrates and kinetic behavior of CO2 hydrate were measured at versatile conditions that can simulate actual marine sediments. When measuring three-phase equilibria (H-LW-V) containing CH4 hydrate, we also closely examined pore and electrolyte effects of clay and NaCl on hydrate formation. These two effects inhibited hydrate nucleation and thus made the hydrate equilibrium line shift to a higher pressure region. In addition, the kinetic data of CO2 hydrate in the mixtures containing clay and NaCl were determined at 2.0 MPa and 274.15 K. Clay mineral accelerated an initial formation rate of CO2 hydrate by inducing nucleation as initiator, but total amount of formed CO2, of course, decreased due to the capillary effect of clay pores. Also, the addition of NaCl in sample mixtures made both initial formation rate and total amount of CO2 consumption decrease.