The Potential for Carbon Dioxide Sequestration in Oil Sands Processing Streams

Abstract

Abstract The consolidated tailings (CT) process involves chemical amendments to combine the clays and fines in oil sands mature fine tailings or thickened tailings with the coarser sand components to create a nonsegregating tailings (NST) mixture that will rapidly consolidate. Over the years, several amendment chemicals have proved useful in controlling the fluid tailings properties so that they may support sand loading and remain non-segregating. Suncor has several years of commercial-scale operating experience with gypsum as the CT process aid and in the years leading up to the commercialization of the CT process at Suncor, carbon dioxide was also investigated as a CT process aid. With the concerns over carbon dioxide related to the Kyoto Protocol, the extent to which carbon dioxide is trapped and chemically sequestered in the CT process has been investigated. The mechanism by which carbon dioxide addition affects the strength of the mature fine tailings or fluid tailings componentas been investigated, and the potential for carbon dioxide sequestration has been quantified. Depending upon the availability of gypsum as a CT or NST additive, carbon dioxide could beuseful alternative. Introduction Water-based extraction of bitumen from the Athabasca oil sands deposit results in the generation of a large amount of waste tailings. The tailings comprise slow settling fine clays with release water that is recycled for bitumen extraction, and a sand component that is generally used to create containment for the fluid fine ailings waste streams. The accumulated slowly settling fine claysare termed mature fine tailings (MFT) and settle to 30 to 45 wt﹪ after several years. Since approximately one barrel of fine tailings is generated from the production of 1 barrel of crude oil equivalent, over 1B m3 of MFT are currently impounded in containment ponds. Government regulations mandate that the containment ponds eventually be reclaimed to a natural landscape. The poor settling behaviour of fine tailings is a consequence of high concentrations of bicarbonate ion in the water, residual bitumen, and fine clays. Viable reclamation options that have been investigated in the industry involve some form of chemical manipulation using coagulants or polymeric flocculants to increase the dewatering rate, leaving behind a geotechnically stable deposit(1). The most successful strategy to date is the consolidated tailings (CT) process, which has been implemented by Suncor Energy Inc. using gypsum (CaSO4.2H2O) as a coagulant. The CT process can not only help deal with the accumulated MFT but, with thickeners to create an MFT analog at the end of pipe, it can also be used to prevent accumulation of fluid fine tailings or MFT. The making of a suitable CT mixture involves creation of a nonsegregating mixture of sand, clay, and water; rapid initial settling (water release) of the mixture; and ultimate consolidation of the mixture. Extensive studies by Scott et al.(1) have demonstrated that there is a wide range of sand-to-fines ratios, solids contents, and gypsum addition levels where these criteria are met.