Phase equilibrium for clathrate hydrates formed in the (methane, carbon dioxide or ethane) plus water plus ammonium chloride system

Methane hydrates are one of unconventional energy resources. There are vast depositions of methane hydrates on the sea floor and under permafrost. The methane hydrates are stable under high pressure and low temperature conditions. Depressurization method is being developed for methane gas production from deep sea methane hydrate layers. In this method, the methane hydrates are depressurized by a pump, and dissociated to be methane gas and water. There is a risk of plugging in the production well by reformed methane hydrates such as at stagnation points and during an operation stop. To manage the plugging risk, inhibitors for methane hydrate formation are necessary. Among several chemical inhibitors, thermodynamic hydrate inhibitors (THIS) are a unique technique that can dissociate methane hydrates. Ammonium salts are one of THIs which naturally occur in oceans and vastly available in the deep sea area. In this study, inhibition effects of ammonium chloride on gas hydrates of methane, carbon dioxide and ethane are investigated. Three phase (gas hydrate aqueous) equilibrium conditions are measured by a temperature search method. The equilibrium data show that ammonium chloride suppresses equilibrium temperatures of these gas hydrates until it reaches solubility limit in water. The comparison with molecular inhibitors showed that ammonium chloride inhibits methane hydrate formation more than urea, but less than methanol. The present data clarified the inhibition effect of ammonium chloride on subsea natural gas hydrates and CO2 injection and sequestration techniques. (C) 2018 Elsevier B.V. All rights reserved.