Microsecond molecular dynamics of methane\u2013carbon dioxide swapping in pure and saline water environment

Avinash V Palodkar,Harshal J Dongre,Amiya K Jana,Niraj Thakre

doi:10.1038/s41598-022-06583-z

Abstract

This work aims at proposing the nondestructive methane-carbon dioxide (CH4–CO2) replacement mechanism as an ecofriendly energy production technique from the natural gas hydrate reserves in seafloor and permanently frozen grounds. Although the experimental data is widely available in literature, this replacement mechanism has not been elucidated at molecular level. In this contribution, we perform the microsecond level molecular dynamic simulations to evaluate two different CH4–CO2 replacement mechanisms: (i) direct CH4 displacement from hydrate structure, and (ii) dissociation of existing methane hydrate followed by a reformation of mixed CH4–CO2 hydrates. For this, we analyze CH4–CO2 replacement in three different modes i.e., CO2 as a replacing agent in (i) absence of free water molecules, (ii) presence of free water molecules, and (iii) presence of salt ions and free water molecules. Despite slow kinetics in the first mode, pure CO2 is observed to replace the methane more efficiently, while in the second mode, CO2 forms a new mixed hydrate layer on the existing seed crystal. However, in the third mode, salt ions help in destabilizing the methane hydrate and allow CO2 to form the hydrates. This proves that salt ions are favorable for CH4–CO2 replacement.

Highlights

This work aims at proposing the nondestructive methane-carbon dioxide (CH4–CO2) replacement mechanism as an ecofriendly energy production technique from the natural gas hydrate reserves in seafloor and permanently frozen grounds
We introduce free water molecules in the simulation box with C O2 to investigate the ‘dissociation of C H4 hydrate followed by the formation of CH4–CO2 hydrate’ type of replacement mechanism
To investigate the different mechanisms involved in the CH4–CO2 replacement, we perform the molecular dynamics simulation by using C O2 as a replacing agent in (i) absence of free water molecules, (ii) presence of free water molecules, and (iii) presence of free water molecules and salt ions

Summary

Introduction

This work aims at proposing the nondestructive methane-carbon dioxide (CH4–CO2) replacement mechanism as an ecofriendly energy production technique from the natural gas hydrate reserves in seafloor and permanently frozen grounds. As NGH are responsible for maintaining the stratum of the seafloor, the use of direct-destructive methods can invite geological disasters such as earthquakes, submarine landslides, etc These potential disasters can be overcome by employing a non-destructive mining mechanism for natural gas recovery from their hydrate r eservoirs[4]. The feasibility of the replacement reaction is confirmed through the experimental observations and theoretical models reported in the literature[8] In this regard, Ohgaki et al.[9] have conducted a preliminary feasibility study and evaluated the distribution coefficients of C H4 and C O2 between the gas and hydrate phases. On confirming the replaceability of CH4 with CO2 in the NGH, it is crucial to understand and evaluate this substitution mechanism In this light, Yoon et al.[14] performed in situ Raman spectroscopy on the coexisting hydrate and bulk water phase of mixed CH4–CO2 system/gases/formers. This is ascribed to the physical and thermodynamic properties of CO2 emulsion, which advantageously enhance the heat and mass transport inside the existing CH4 hydrates

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