OS5A-1:IN-SITU REPLACEMENT KINETICS OF (SII) ETHANE-METHANE HYDRATES EXPOSED TO CO2

Andrzej FALENTY1, Junfeng QIN1, Andrey SALAMATIN2 and Werner F. KUHS1
1. GZG Abt.Kristallographie, Universität Göttingen, GERMANY; 2. Department of Applied Mathematics, Kazan State University, RUSSIA

    Deposits of natural gas hydrates containing sII type structure are a potential candidate for recovering gas and concomitantly sequestering greenhouse gas CO2; an approach that offers a nearly emission-free technology without causing significant alterations to the original mechanical properties of the hydrate-bearing sediment. CO2 hydrate formed by gas injection, however, crystallizes always in a type sI structure and therefore a phase transition: sII -> sI must take a place. Though a number of studies have investigated the isostructural exchange of sI CH4 hydrates into sI CO2 hydrates, little is known about the swapping process involving the structural change. A few experimental studies using local techniques like Raman spectroscopy provided valuable local information at a diameter of a few microns but were not able to provide reliable, representative kinetic data for a modelling due to a strong heterogeneity of the process. We overcome this issue using time-resolved in-situ neutron diffraction delivering accurate space-averaged kinetic data on the exchange reaction between sII CH4-C2H6 clathrates and liquid CO2 as well as evolution of gas composition in pore spaces taken at conditions relevant to sedimentary matrixes of continental margins. We provide also quantitative cage occupancies of each gas in both, the newly formed sI and the initial sII hydrates as established by the Raman spectroscopic [1] and neutron diffraction. Analysis of the exchange products recovered at the end of the experiment indicates that in the newly formed sI hydrates both C2H6 and CO2 reside preferentially in the large cages. The replacement is seen as two stage processes: 1) a fast surface reaction involving a destruction of existing structure and reformation of sI mixed hydrates 2) followed by the sluggish, diffusion-controlled transport of gas molecules through the thickening sI hydrate shell described by “hole-in-the-cage” mechanism [2].

    [1] Qin J and Kuhs, WF (2013), "Quantitative analysis of gas hydrates using Raman spectroscopy", AIChE Journal, 59, No. 6, pp 2155-2167.
    [2] Falenty, A, Salamatin, AN, and Kuhs, WF (2013), "Kinetics of CO2-Hydrate Formation from Ice Powders: Data Summary and Modeling Extended to Low Temperatures", Journal of Physical Chemistry C, 117, No 16, pp 8443-8457.