Plume Modeling in Lake Kivu, Rwanda for a Gas Extraction Facility

Abstract

Abstract Lake Kivu, located between Rwanda and the Democratic Republic of Congo, is unique among several East African Rift lakes with high concentrations of dissolved carbon dioxide in that it also contains methane, kept in solution by the pressure of the deep water. The gas poses a potential hazard as a vertical disruption of the lake water (due to a landslide or volcanic activity) could cause gas-laden water to be displaced to shallower depths (and lower pressures), allowing the gas to bubble out of solution, triggering a gas eruption. A floating gas extraction facility has been constructed to extract gas-laden water from deep in the lake, separate the methane and some of the carbon dioxide and reinject the degassed water, thus increasing the safety of the lake and simultaneously providing methane, which will be used to generate electricity. This paper explores the computational fluid dynamics technique used to analyze the degassed water discharge plume from that facility and describes the dynamics of the plume. We used a Reynolds-Averaged Navier Stokes (RANS) model to analyze the dynamics of the degassed water plume. The density of the water, and hence the plume dynamics, is heavily affected by the water temperature and the concentration of methane, carbon dioxide, and salt. Thus the computational method tracked these quantities throughout the simulation. The background conditions in the lake (including mixed zones separated by density gradients) were also included in the model. Lake Kivu is unique in that the water temperature increases with depth. The lake remains stratified because the salt concentration also increases with depth and more than compensates for the effect of water temperature. Two main concerns were raised with the degassed water plume: (1) could there be recirculation between the water intake risers and the degassed water discharge points? (2) could the dynamics of the plume lead to an overturn of the lake and a catastrophic gas eruption? Our simulations showed that the degassed water plume ultimately stratifies within a density gradient, that recirculation does not occur, and that the discharge plume does not result in uplift or overturn of the lake for the conditions evaluated.