Modelling water reinjection after methane extraction in Lake Kivu

Abstract

The large amount of dissolved gases including carbon dioxide and methane in Lake Kivu poses a significant threat due to the potential for catastrophic gas eruption. To mitigate this risk, the implementation of the project to extract methane gas from the lake for electricity generation, and reduce the amount of dissolved gases in the lake has been initiated. However, the extraction imposes significant risks that could negatively impact the lake ecosystem and increase the probability of a gas eruption. This study aims at modelling water reinjection after methane extraction in Lake Kivu within 100 years by using the Kivu-Simstrat model, to identify the most effective and sustainable reinjection strategies for maintaining the stability and dissolved gas concentrations. Two cases are used, the first without methane extraction in the lake, and the other with 200 MW power production under the variation of extraction and reinjection depths to simulate methane, carbon dioxide, temperature, salinity, density and buoyancy frequency profiles. The results show that both extraction and reinjection depths have significant impacts on the physical and chemical properties of the lake, because they both affect the stability and concentration of dissolved gas. To mitigate these impacts, the results indicate the possible reinjection and extraction strategies: (1) reinjected water should be returned and restratified closely above the upper limit of the normal main chemocline at around 240 m depth to maintain a stable density stratification and optimal methane usage; (2) one should extract gas from the deepest layers of the lake to prevent the accumulation of dissolved gases and the formation of hazardous gas concentrations below the extraction depths.