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Abstract

The guest molecule exchange of methane (CH4) by carbon dioxide (CO2) in natural gas hydrate reservoirs is considered a desirable possibility to produce CH4 and at the same time sequester CO2. So far process evaluation is commonly based on CH4-CO2 exchange yields and rates from small- or medium-scale experiments in partly water-saturated sediments, both of which does not represent natural conditions. The experiments are presented in detail in a currently submitted manuscript by Heeschen et al. (2019). The presented data originate from two large-scale experiments (210 L) investigating the efficiency of the CH4-CO2 exchange under fully water-saturated natural reservoir conditions. For details on the equipment and the methods used see: Priegnitz et al., 2013; Schicks et al., 2011; Spangenberg et al., 2014. The reservoir conditions were 13 MPa and 8 °C, and the gas hydrate saturation in the sand (Sh) was 50% of the pore space. The gas hydrate was formed from dissolved CH4 only. About 50 kg heated CO2 was injected 1) discontinuously with intermediate soaking periods (E1) and 2) continuously (E2). In both cases, the CO2 injection periods were followed by a discontinuous depressurization of the reservoir. The experiments demonstrate the importance of fluid migration patterns, heat transport, sample inhomogeneity, reaction kinetics, and secondary gas hydrate formation in water-saturated sediments. Methane production yields of 5% were small in both experiments during the injection periods, whereas controlled depressurization following the injection of CO2 into a CH4 hydrate reservoir could be a possible approach for the production of CH4 from a gas hydrate reservoir. However, the success of this method strongly depends on the distribution of CO2, and the availability and distribution of residual pore water.