Characterizing and assessing the transport and fate of hydrocarbons in association with methanogenesis in oil sands reclamation scenarios

Abstract

Alberta hosts one of the world’s largest oil reserves in the form of oil sands. Oil sands operators are actively developing effective strategies to manage produced tailings during the bitumen extraction and reclaim impacted landscapes to meet regulatory requirements. Syncrude’s Base Mine Lake (BML) is the first full-scale commercial demonstration of water-capped tailings technology (WCTT), which sequestered tailings by constructing an engineered pit lake. The overall objectives of BML are for the lake to develop into a self-sustaining ecosystem as a part of mine closure landscape, and obtaining a certificate of reclamation. Currently, an important component of BML is understanding the biogeochemical processes associated with microbial methane production (methanogenesis), because the produced methane can potentially impact the lake development through greenhouse gas emission, oxygen consumption within the water column, and transporting organics by ebullition. This thesis focuses on assessing the chemical compositions and distributions of hydrocarbons responsible for the methane production within BML fluid tailings, as well as characterizing and assessing the environmental behaviours of petroleum hydrocarbons present in the bitumen transported by methane ebullition in BML. The results of this research indicate that n-alkanes and monoaromatics derived from diluent naphtha are primary substrates fueling methanogenesis, as they are preferentially depleted within BML fluid tailings, and their concentrations are strongly correlated with measured methane production rate. With ongoing methanogenesis, volatile hydrocarbons are transported via direct partitioning into the methane bubble, and bitumen are transported through coating onto the bubble surface. Such transported bitumen is likely derived from small to large volume bitumen mats residing at the tailings-water interface of BML, rather than dispersed bitumen within the tailings. Moreover, the non-polar fraction of the transported bitumen is shown to highly adsorb to organic phase over aqueous and gas phase, so they are highly sorbed to residual bitumen along the shoreline or nearby soils without posing significant environmental risks to the surrounding ecosystems. Overall, this research provides important insights into methanogenesis and associated hydrocarbon dynamics within oil sands reclamation sites, which will aid in the management of BML towards achieving reclamation certification, inform plans for more than 30 pit lakes proposed in the oil sands region, and can be transferred to other organic rich aquatic environments where methanogenesis is present.