Molecular simulation of imperfect structure I CO₂ hydrate growth in brine

Abstract

In order to investigate the viability of carbon dioxide (CO2) storage in seawater, molecular dynamics techniques were employed to study the dynamic evolution of CO2 hydrate in saline water. The simulation was conducted under specific conditions: a temperature of 275 K, a pressure of 10 MPa and a simulated marine environment achieved using a 3.4 wt% sodium chloride (NaCl) solution. The total simulation time was 1000 ns. The results of the simulation indicate that the pre-existence of CO2 hydrate crystals as seeds leads to rapid growth of CO2 hydrate. However, analysis of the F3 and F4 order parameters reveals that the hydrate does not meet the standard values of the perfect structure I (sI) type, confirming the existence of an imperfect structure during the simulation. Additionally, the changes in the number of different phase states of water molecules during the hydrate growth process shows that there are always some liquid water molecules, which means some water molecules fail to form solid water cages. Further investigation suggests that the presence of Na+ and Cl− hampers the hydrogen bonds between water molecules, resulting in incomplete cage structures. By analyzing the density variations in the system, it is observed that CO2 hydrate, with a density of around 1.133 g cm−3, forms rapidly, surpassing the average density of seawater. This density increase facilitates the efficient and swift containment of CO2 on the seabed, thereby supporting the feasibility of the CO2 storage theory.

Comprehensive Review and Perspectives of CO2 Hydrate Storage in Subseafloor Saline Sediments: Formation, Stability, and Optimization Strategies

“…250 The mean F4 order parameter values for water molecules in hydrate, liquid water, and ice are recorded as 0.7, − 0.04, and −0.4, respectively. 250,259 The positive value for hydrate indicates a stronger correlation between water molecules than liquid water, while the negative values for liquid water and ice suggest weaker correlations. 250,260 These values align with the expected behavior of water molecules in different phases.…”

“…CO 2 hydrate structural analyses, including examining F4 order parameters, mean square displacement (MSD), and hydrate cages, provide detailed insights into the system’s molecular structure and dynamics changes in electrical heating systems. , The F4 order parameter represents alterations in the structural characteristics among water molecules during hydrate formation, offering insights into the proximity of the hydrogen bonding network to the crystal structure. , This parameter can be determined analytically by computing the dihedral angle formed by two neighboring water molecules within the system, as specified in eq . The mean F4 order parameter values for water molecules in hydrate, liquid water, and ice are recorded as 0.7, – 0.04, and −0.4, respectively. , The positive value for hydrate indicates a stronger correlation between water molecules than liquid water, while the negative values for liquid water and ice suggest weaker correlations. , These values align with the expected behavior of water molecules in different phases. It is important to note that these values are approximate and can vary based on specific conditions and experimental methods.…”