Experimental Study on a Novel Way of Methane Hydrates Recovery: Combining CO2 Replacement and Depressurization

A novel way to exploiting natural gas from methane hydrates combining carbon dioxide replacement and depressurization was experimentally studied in a high pressure vessel of 255.254 mL. Experiment results with or without the procedure of depressurization were compared to reveal the effects of depressurization on final replacement percent. Results show that making methane hydrates melt partially by depressurization will enhance replacement reaction which is restricted by the diffusion and transportation of carbon dioxide. The key factors affecting the replacement percent were investigated systematically for the first time, including: the saturation of methane hydrate, and the pressure and temperature condition. Ten runs of non-depressurization and five runs of depressurization replacement reaction were conducted with hydrate saturations ranging from 0.10 to 0.21, temperature and pressure condition above phase equilibrium line of carbon dioxide hydrates. Results show that higher saturation and temperature, and lower pressure enhance the methane exploiting and CO2 sequestration. Pressure and temperature condition located between the phase equilibrium lines of methane hydrates and carbon dioxide hydrates is most effective with the optimum methane recovery and carbon dioxide sequestration. The combined method improves the replacement percent by about 10%, while, effects of depressurization on the stability of the sediments remains to be explored. It is supposed that methane hydrates melt once, which provides path for carbon dioxide penetrating into inner layer of hydrates.