Mechanical Properties of a Structure I CO₂–CH₄ Heteroclathrate Hydrate: Insight from Molecular Dynamics Simulations

Abstract

Mechanical stability of the CO 2 −CH 4 heteroclathrate hydrate dominates the geomechanical stability of natural gas hydrate deposits when CO 2 replaces CH 4 from gas hydrate reservoirs. Here, molecular dynamics simulations were employed to investigate the strain-induced fracture behaviors of the CO 2 − CH 4 heteroclathrate hydrate under mechanical loadings at various temperatures, pressures, and CO 2 saturations. Results show that all crystals exhibit brittle fracture behavior, and a crack first develops in the location where hydrogen bonds (HBs) in the hexagonal rings of the large 5 12 6 2 cages are parallel to the tensile direction. Increasing the temperature or CO 2 saturation leads to the decrease in Young's modulus and fracture strength of the CO 2 −CH 4 heteroclathrate hydrate. Particularly, abnormal mechanical strengthening of hydrates is observed when the CO 2 saturation is around 0.75, mainly attributed to the coupling of the lattice distortion with the host−guest interaction. HBs are the key factors to dominate the deformation of the CO 2 −CH 4 heteroclathrate hydrate. The slow decrease, rapid decrease, and abrupt increase in HB dynamics are corresponding to the elastic deformation, elastic−plastic deformation, and brittle fracture of the CO 2 −CH 4 heteroclathrate hydrate, respectively. With the further stretching after the brittle fracture, the water bridge made up of water molecules released by broken cages at high temperatures leads to different plasticity than at low temperatures and causes a further reduction of HBs. This work advances the understanding of mechanical stability of the gas hydrate, which is believed to be useful in the risk assessment of CO 2 replacing CH 4 from natural gas hydrates and the storage of CO 2 in gas hydrate reservoirs.

Mechanical Destabilization and Cage Transformations in Water Vacancy-Contained CO2 Hydrates

“…Recently, CCS technologies based on clathrate hydrate have been proposed as a feasible measure for large-scale CCS operations. ,, For example, CO 2 gases injected into natural gas hydrate-bearing sediments in the seabed can simultaneously recover methane energy and achieve CO 2 geological storage due to the replacement of CH 4 by CO 2 − within the hydrate zone, suggesting that methane replacement by CO 2 in hydrate systems is a promising technology for simultaneous exploitation of methane and storage of CO 2 . − Therefore, CO 2 hydrates find significant applications in the technologies of green/sustainable chemistry and engineering such as energy recovery, gas separation, and CO 2 capture, transport, and storage …”

“…As is known, clathrate hydrate is a kind of nonstoichiometric crystalline structure consisting of water and small molecules, where the small guest molecules are physically stored in the polyhedral cages formed by the hydrogen-bonded water molecules under low temperatures and high pressures. ,,− To date, it is identified that there are a variety of organic molecules that are molecularly sized and appropriate for being guests in the polyhedral cages of clathrate hydrates, varying from small molecules like H 2 , N 2 , CH 4 , CO 2 , C 2 H 6 , C 2 H 8 , and C 4 H 10 to heavier compounds such as cyclopentane and cyclobutanone. ,, Among these molecules, methane is the most common guest molecular species in nature; ,,, thereby, its corresponding clathrate hydrates are named natural gas hydrates (NGHs) that widely exist in the submarine continental margin and permafrost area. ,,,,, As a result, many studies on the mechanical stability of methane hydrates have been reported. ,− For example, Cao et al reported via classic molecular dynamics (MD) simulations that factors such as strain rate, temperature, and occupancy of guest molecules in 5 12 6 2 cages have a great effect on the mechanical properties and failure strain of monocrystalline structural I (sI) methane hydrate. In contrast, the crystal orientation has a negligible influence on the tensile response of monocrystalline methane hydrate, for example, along the [110] and [100] directions.…”

“…17,22,25 Among these molecules, methane is the most common guest molecular species in nature; 18,22,29,30 thereby, its corresponding clathrate hydrates are named natural gas hydrates (NGHs) that widely exist in the submarine continental margin and permafrost area. 10,12,18,22,29,30 As a result, many studies on the mechanical stability of methane hydrates have been reported. 18,31−36 For example, Cao et al 37 reported via classic molecular dynamics (MD) simulations that factors such as strain rate, temperature, and occupancy of guest molecules in 5 12 cages have a great effect on the mechanical properties and failure strain of monocrystalline structural I (sI) methane hydrate.…”

“…10,12,18,22,29,30 As a result, many studies on the mechanical stability of methane hydrates have been reported. 18,31−36 For example, Cao et al 37 reported via classic molecular dynamics (MD) simulations that factors such as strain rate, temperature, and occupancy of guest molecules in 5 12 cages have a great effect on the mechanical properties and failure strain of monocrystalline structural I (sI) methane hydrate. In contrast, the crystal orientation has a negligible influence on the tensile response of monocrystalline methane hydrate, for example, along the [110] and [100] directions.…”

“…CO 2 hydrate, namely, CO 2 •nH 2 O (n ≥ 5.75), is a family of nonstoichiometric compounds, where the H-bonded water molecules form an ice-like framework. 11,36,54 Typically, CO 2 hydrate crystallizes as an sI-type hydrate form, with a unit cell consisting of 8 CO 2 molecules and up to 46 H 2 O molecules occupying both small polyhedral cages (5 12 ) and large polyhedral cages (5 12 6 2 ) in a ratio of 1:3. 54 In this study, the molecular structure of sI CO 2 hydrate was generated as follows.…”