Formulating formation mechanism of natural gas hydrates

Hydrate Technologies for CO2 Capture and Sequestration: Status and Perspectives

CO2 capture and sequestration based on hydrate technology are considered supplementary approaches for reducing carbon emissions and mitigating the greenhouse effect. Direct CO2 hydrate formation and CH4 gas substitution in natural gas hydrates are two of the main methods used for the sequestration of CO2 in hydrates. In this Review, we introduce the crystal structures of CO2 hydrates and CO2-mixed gas hydrates and summarize the interactions between the CO2 molecules and clathrate hydrate/H2O frames. In particular, we focus on the role of diffraction techniques in analyzing hydrate structures. The kinetic and thermodynamic properties then are introduced from micro/macro perspectives. Furthermore, the replacement of natural gas with CO2/CO2-mixed gas is discussed comprehensively in terms of intermolecular interactions, influencing factors, and displacement efficiency. Based on the analysis of related costs, risks, and policies, the economics of CO2 capture and sequestration based on hydrate technology are explained. Moreover, the difficulties and challenges at this stage and the directions for future research are described. Finally, we investigate the status of recent research related to CO2 capture and sequestration based on hydrate technology, revealing its importance in carbon emission reduction.

Microscopic Molecular Insights into Hydrate Formation and Growth in Pure and Saline Water Environments

The growth dynamics of natural gas hydrates in saline water has been studied using copious experiments and spectroscopic observations; however, the microscopic evidences to the structural and molecular transformations that they have provided are poorly understood. In this view, we perform extensive molecular dynamics simulations to gain physical insights into the formation and growth mechanism of naturally occurring gas hydrates with a wide variation in the amount of methane (1:5 to 1:18 methane/water ratio) in pure and salt (0-5 wt %) water environments at 50 MPa and 260 K. A couple of new findings analyzed from the number of cages and F_4φ order parameter are as follows: (a) 1:6 (methane/water ratio) is an optimum ratio for the rapid growth of a properly ordered hydrate in pure water at which the hydrate growth retards with increasing salt concentration, (b) there is an inconsequential difference between methane hydrate dynamics in pure water and 0.8 and 1.5 wt % salt water at a ratio of 1:12 (methane/water), and (c) lower methane (1:18) and salt (0.8 wt %) concentrations promote hydrate growth. Besides, this study observes the structural coexistence of S-I and S-II methane hydrates as the large 5¹²6⁴ cages appear along with the small 5¹² and large 5¹²6² cages, in which the low methane concentration favors the S-II structure.

Nonmonotonous Lattice Distortion Model for Gas Hydrates

A gas hydrate forms when the hydrogen-bonded crystal structure of water entraps the small-sized gas molecules at a relatively low temperature and high pressure. Experimental and spectroscopic studies prove that the inclusion of a guest into an empty cavity leads to the distortion of the hydrate lattice via either the contraction or expansion of the cavity, which depends on the size and functional group of the guest. However, the existing lattice distortion theories represent only the expansion phenomena, and consequently, the degree of distortion is reported as a monotonous function of the size of the guest. Addressing this research gap, we propose the lattice distortion by using the statistical thermodynamics based model, in association with the modified Patel-Teja equation of state, and an ab initio quantum mechanical methodology for cavity potential calculations. To accurately capture the guest-host interactions, we propose the spin-component-scaled modification in the second order Møller-Plesset (SCS-MP2) perturbation theory applied with Dunning's basis set. The half-counterpoise method with the Pauling point correction factor is used to handle the basis set superposition (BSSE) and completeness (BSCE) errors. As an estimate of the degree of lattice distortion, the reference chemical potential difference (RCPD) is calculated by applying linear regression analysis to the experimental data of the hydrate phase equilibrium. We identify a nonmonotonous lattice distortion model, in which RCPD first decreases, and then increases, with the guest size. This result shows that the small guest contracts the cavity and that the larger guest expands the cavity during encapsulation. Therefore, for the first time, we report the RCPD (794.0913 J mol^-1) for the undistorted sII-type hydrate lattice as the minimum of the lattice distortion curve. The proposed model is validated with the phase equilibrium data of methane, nitrogen, oxygen, cyclopropane, propane, and isobutane hydrates that have a wide range of guest sizes.

Modeling recovery of natural gas from hydrate reservoirs with carbon dioxide sequestration: Validation with Iġnik Sikumi field data

Fundamental understanding of guest gas replacement in hydrate reservoirs is crucial for the enhanced recovery of natural gas and carbon dioxide (CO₂) sequestration. To gain physical insight into this exchange process, this work aims at developing and validating a clathrate hydrate model for gas replacement. Most of the practical concerns associated with naturally occurring gas hydrates, including hydrate formation and dissociation in interstitial pore space between distributed sand particles in the presence of salt ions and in irregular nanometer-sized pores of those particles, irregularity in size of particles and shape of their pores, interphase dynamics during hydrate formation and decay, and effect of surface tension, are addressed. An online parameter identification technique is devised for automatic tuning of model parameters in the field. This model is employed to predict the laboratory-scale data for methane hydrate formation and decomposition. Subsequently, the model is validated with the field data of the Prudhoe Bay Unit on the Alaska North Slope during 2011 and 2012. In this Iġnik Sikumi field experiment, mixed CO₂ (i.e., CO₂ + N₂) is used as a replacement agent for natural gas recovery. It is observed that the proposed formulation secures a promising performance with a maximum absolute average relative deviation (AARD) of about 2.83% for CH₄, which is even lower, 0.84% for CO₂ and 1.67% for N₂.

Fundamental of swapping phenomena in naturally occurring gas hydrates

Amount of natural gas contained in the gas hydrate accumulations is twice that of all fossil fuel reserves currently available worldwide. The conventional oil and gas recovery technologies are not really suitable to gas hydrates because of their serious repercussions on geo-mechanical stability and seabed ecosystem. To address this challenge, the concept of methane-carbon dioxide (CH₄-CO₂) swapping has appeared. It has the potential in achieving safe and efficient recovery of natural gas, and sequestration of CO₂. By this way, the energy generation from gas hydrates can become carbon neutral. This swapping phenomenon has not yet been elucidated at fundamental level. This work proposes a theoretical formulation to understand the physical insight into the transient swapping between natural gas and CO₂ occurred under deep seabed and in permafrost. Addressing several practical concerns makes the model formulation novel and generalized enough in explaining the swapping phenomena at diverse geological conditions.