From Microscale (400l) to Macroscale (425L): Experimental Investigations of the CO2/N2-CH4 Exchange in Gas Hydrates Simulating the Ignik Sikumi Field Trial

In 2012 the production of CH4 from hydrate-bearing sediments via CO2 injection was conducted in the framework of the Ignik Sikumi Field Trial in Alaska, USA. In order to preserve the injectivity by avoiding a formation of CO2 hydrate in the near-well region, a mixture containing 77mol% N-2 and 23mol% CO2 was chosen. The interpretation of the complex test results was difficult, and the nature of the interaction between the N-2-CO2 mixture and the initial CH4 hydrate could not be clarified. In this study we present the results of our experimental investigations simulating the Ignik Sikumi Field Trial at different scales. We conducted (1) in situ Raman spectroscopic investigations to study the exchange process of the guest molecules in the hydrate phase on a molecular level in a flow-through pressure cell with a volume of 0.393ml, (2) batch experiments with pure hydrates and hydrate-bearing sediments in pressure cells with volumes of 420ml, and (3) the injection of a CO2-N-2 mixture into a hydrate-bearing sediment in a large-scale reservoir simulator with a total volume of 425L. The results indicate a dissociation of the initial CH4 hydrate rather than an exchange reaction. The formation of a secondary mixed hydrate phase may occur, but this process strongly depends on the local composition of the gas phase and the pressure at given temperature.