Detection of formation and dissociation of CO2 hydrates in fine-sands through acoustic waves

Natural gas hydrates in oceanic and permafrost sediments are considered a future energy resource. Injecting CO2 into these sediments is believed to aid in the safe and efficient extraction of CH4 and the geological sequestration of carbon. The effectiveness of this strategy can be ensured by assessing the formation of CO2 hydrates in the pores of these sediments at in-situ stress conditions by investigating their elastic properties (viz., compressional and shear wave velocities, elastic modulus, and shear modulus) by resorting to a suitable method such as seismic and/or acoustic logging. However, conducting such a study in field is seldom opted due to the requirement of expensive paraphernalia such as seismic equipment, pressure coring tools, logging while drilling tools, modular dynamic tester, etc. With this in view, efforts were made to synthesize CO2 hydrates in partially saturated finesands under triaxial conditions by employing the excess gas method. Ultrasonic wave transducers were employed in tandem to measure the compressional (VP) and shear wave (VS) velocities of the sample, both under formation and dissociation processes. These results have been utilized to propose hydrate formation and dissociation models which encompass three phases each. The rapid reduction in VP and VS observed during thermal dissociation indicates loss of cementation between the sand grains and hydrates, and commencement of hydrate dissociation at their interface. It has been demonstrated that the induction time for CO2 hydrate formation increases, and the water-to-hydrate conversion ratio decreases, with an increase in the initial volumetric water content. Further, the results generated in the present study were compared vis-`a-vis those available in the literature for CH4 and CO2 hydrate-bearing sediments (HBS) and it has been found that both VP and VS of the CO2- and CH4- HBS are comparable for a given hydrate saturation.