Self-Organized Colloids Thermodynamically Weaken the Effect of Salt on Methane Hydrate Formation

Natural gas hydrate deposits are typically rich in organic matter; they together with the host sediments play a crucial role in the nucleation and accumulation of gas hydrates, yet their potential thermodynamic effect on hydrate formation is rarely studied. In the present work, it was found that the deprotonated organic compounds could self-organize with the cations in the solution, forming organic colloids via coordination bonds. Elemental mapping showed that these organic colloids were filled with Ca2+ and Na+ ions as well as more concentrated organic matter surrounded by thin rings of Mg2+ and Cl– ions. This was found to result in a concentration decrease of Ca2+ and Na+ ions in the solution with the presence of the colloids; the resulting decreased electrostatic effect of salt ions on water molecules would subsequently relieve the energy barrier of hydrate formation, showing a shift of the phase equilibrium conditions to a milder situation. This finding would be more nontrivial in low-permeability hydrate layers where mass exchange with the abundant ions in the seawater could be rather sluggish. The results could be of help in determining the local thickness and location of the gas hydrate stability zone and thereby the estimated reserves.