Gas Extraction from Lake Kivu: The Dynamics of the Degassed Water Plume

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. Such an event occurred with Lake Nyos in Cameroon in 1986, asphyxiating 1,700 people. A floating gas extraction facility is under development 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 conveyed by pipeline to an electricity-generating facility. One important question regarding the safety of the extraction facility is whether the action of withdrawing and reinjecting water could cause a disruption of the lake stratification and result in a gas eruption. The vertical density profile in Lake Kivu is characterized by several mixed regions with nearly uniform density and several strong density gradients between the mixed regions. Computational fluid dynamics simulations of various gas-extraction scenarios show that the degassed water plume ultimately stratifies within a density gradient regardless of the discharge location. Thus discharging within a density gradient minimizes the downward movement of the plume and minimizes the uncertainty in the stratification depth.