Natural gas recovery from hydrate compounds using CO2 replacement strategies: experimental study on thermal stimulation

CH4 hydrates could represent a sustainable energy source by coupling their extraction to a CO2 replacement process. This permits a permanent CO2 storage in a solid phase and the reduction of the seabed modification due to the CH4 extraction. Promising methodologies for CO2 replacement are depressurization, thermal stimulation (or a combination of both), or the use of chemical inhibitors. We have taken into account thermal stimulation. The CH4 hydrates were formed with a pore saturation of 10%, whereas different concentration of gaseous CO2 were fluxed inside the reactor. In a first set of experiments, the CO2 concentration was fixed at values never higher than 15%, while in a second set the gaseous CO2/CH4 ratio was inverted. Two tests for each condition were performed. In all tests, the temperature was increased at the end, to improve the replacement process. Results demonstrated that the CO2 fluxed leads to hydrates formation and, at the same time, hinder the CH4 hydrates dissociation. Two parameters were evaluated: i) the ratio between the stored CO2 and the amount of CO2 injected (ηcapture), ii) the ratio between both CO2 and CH4 hydrates moles after the replacement process, and the CH4 hydrates moles before the replacement (ηentrapped). The assumed ηcapture value are 0.42 and 0.44 in the first conditions, and 0.27 and 0.28 in the second one. By contrast, the second parameter shows an opposite behavior, being the two values 0.13 for the first conditions and 0.27 and 0.74 respectively for the second ones. This could be explained by the fact that injection of higher quantity of CO2 produces a greater thickness of CO2 hydrates in the external parts of pores, which prevent the CH4 to flowing