Nucleation Mechanisms of CO2 Hydrate Reflected by Gas Solubility

Abstract

The concentration of gas has been confirmed as a key factor dominating hydrate nucleation. In this study, CO2 hydrates were formed in pure water and a sodium dodecyl sulphate (SDS) solution using a temperature reduction method under constant pressure at different temperatures. The dissolving properties of CO2 throughout the whole induction period were investigated in detail. The experimental results showed that the ‘memory effect’ of hydrate might not be attributed to residual water structures after hydrate dissociation. Instead, residual gas molecules in the aqueous phase should receive more attention. Hydrate nucleation was confirmed to be a type of chain reaction. Low temperature was a significant factor that promoted hydrate nucleation. As a result, these two factors enhanced the stochastic features of the CO2 hydrate nucleation reaction. Even under the same conditions, critical gas concentrations beyond the threshold that hydrates can spontaneously nucleate were not fixed, but they still exhibited linear relations regarding a set temperature. Taking the significant influences of temperature into account, a new nucleation mechanism for CO2 hydrates was established based on the potential of the reaction system. Therefore, this study sheds new light when explaining the reason for the formation of gas hydrates in natural reservoirs.

Simulation Study on the Influence of Gas Mole Fraction and Aqueous Activity under Phase Equilibrium

“…The gas mole fraction threshold value for maintaining the three-phase equilibrium state is different to the critical gas concentration. The gas mole fraction threshold value calculated in this work does not contradict the critical gas concentration proposed by Zhang et al [49]. They pointed out that there is a critical gas concentration in aqueous phase that can spontaneously nucleate in the induction period, and the critical gas concentration is calculated by the total amount of carbon dioxide consumed in vapor phase until hydrate nucleation.…”

Promoted Disappearance of CO2 Hydrate Self-Preservation Effect by Surfactant SDS

“…Undoubtedly, within these interfaces between the hydrate and ice structures, there are some nonenclathrated liquid water [53] and gas molecules dissolved inside (Figure 11C). SDS is one kind of macromolecular organic matter that can improve water molecule activities and thereafter the dissolution capabilities of gas molecules [54]. During the hydrate formation processes, SDS cannot enter into the clathrate structures and is only dissolved in the nonenclathrated liquid water outside the solid hydrate crystals (Figure 11B).…”

Promoted disappearance of CO2 hydrate self-preservation effect by surfactant SDS

“…In addition, owing to the promotion function of SDS in gas dissolution and the similar function of capillary force at low temperatures, it is very di cult for these dissolved gas molecules to desorb from the solution. 40 As a result, comparing with in the PW, the IDR are signi cantly lowered in the other experimental media (Fig. 5).…”

“…As known, SDS is one kind of macromolecular organic matter and can improve the activities of water molecules and then the dissolution capabilities of gas molecules. 40 During the formation processes of hydrate, SDS can not enter into the clathrate structures and is only dissolved in the nonenclathrated liquid water outside the solid hydrate crystals (Fig. 9 B).…”