OS6B-3:SPATIAL MODELING OF CO2-CH4-HYDRATE CONVERSION AND THE RESULTING METHANE RELEASE

Mehdi GHARASOO, Christian DEUSNER, Nikolaus BIGALKE, Elke KOSSEL, Matthias HAECKEL 
GEOMAR Helmholtz Centre for Ocean Research Kiel, GERMANY

    Injection of CO2 into methane hydrate bearing sediments and the subsequent replacement of CH4-hydrate by CO2-hydrate has been proposed as a technique for the overall emission-free production of natural gas from hydrates. Although this approach has been investigated in the lab and a recent field test, the underlying processes are still poorly understood and not well quantified.

    In the present study, we developed a spatial model in COMSOL Multiphysics® and simulated several lab-scale experiments of supercritical CO2 injection into a pressure vessel containing initially a water-saturated mixture of methane hydrate and quartz sand. The experiments were performed at different reservoir temperatures and pressures at which various efficiencies of methane production and CO2 retention were observed (Deusner et al., 2012). Using the model, we are able to explore (1) the propagation of CO2 and heat in the vessel, (2) the dissociation and dissolution rate of methane hydrate, (3) the formation rate of CO2-rich hydrates and re-formation of CH4-hydrates, and (4) the release and spread of methane at the different reservoir conditions. The model solves equations for two-phase fluid flow, heat transfer and gas hydrate related reactions in porous media.

    Currently, it is not possible to directly observe many of the processes involved in the CH4-CO2-hydrate conversion, e.g. the spatially-resolved (inhomogeneous) formation/dissociation of gas hydrates and their amount in the system, the temperature distribution resolved in time, or the development of preferential flow paths. The presented numerical model thus allows us to gain some insights into the processes and their scales in our experiments and therefore improves our understanding of the macroscopic mechanisms of hydrate conversion. Since the quality of the heat transfer is a potent factor dramatically influencing the entire system by controlling gas hydrate dissociation/formation and shaping the flow paths, the main focus of this study is on evaluating the spatial and temporal temperature distributions. This will allow us to improve the technical procedure of the CO2-injection approach in order to produce CH4 from hydrate-bearing sediments.