Effect of biological gas generation on oil sand fine tailings
The Mildred Lake Settling Basin (MLSB) is the largest disposal site for mature fine tailings (MFT) at the Syncrude Canada Ltd. oil sands plant. Over the past years (since 1996), there has been a marked change in the densification behaviour of MFT in the MLSB. Methane-producing microorganisms, known as methanogens, have become very active, and large amounts of biogas (mainly methane) have been produced. In certain regions within the MLSB, gas bubbles are released to the water surface of the tailings pond. Continued field monitoring of the MLSB has provided convincing evidence of the rapid densification process (rapid water drainage from the tailings) at the area with intense microbial activity. This phenomenon contradicts the consolidation models for MFT developed over the past 20 years. This rapid densification has caused pumping challenges in the transfer of fine tailings from the Mildred Lake Settling Basin for the creation of composite tailings. It may also have potential positive effects in accelerating the reclamation of the oil sands fine tailings.
A field and laboratory research program was performed to study the mechanism leading to the rapid densification phenomenon. Systematic field investigations were performed to determine the distribution and characteristics of the rapidly densified MFT. A number of small-scale column tests were carried out to observe the gas evolution and to measure the changes of some geotechnical parameters under different microbial activities. Also, a series of gassy MFT densification tests were conducted to study the mechanism of the rapid densification of the MFT under microbiological activity. A review and discussion of the research program is given and some results of the field investigations and small-scale column tests are presented in the paper.
Field observations were used to map the gas bubble distribution on the water surface of the MLSB. Based on the presence and the relative number of the gas bubbles and the ongoing gas bubble release rate, two zones with different microbial activities were determined. It was found that the microbial activity at the southern part of the tailings pond was more active than that at the northern part. Also, steel plate penetration tests were used to investigate the densification properties of the MFT at the tailings pond. These field tests showed that rapid densification has progresed the most in the southern region of the pond. Five small-scale column tests were conducted to study the influences of microbial activity on MFT densification. Columns 1, 3, and 5 were incubated at 25ºC room temperature and with 0, 0.52 g, and 1.52 g sodium acetate amendments per litre MFT, respectively. Columns 2 and 4 were placed at 4°C room temperature with 0.52 g and 1.52 g sodium acetate amendments, respectively. For columns 1, 3, and 5, with the increase of acetate amendments, total gas generation volume increased. However, there was no visual gas generation in columns 2 and 4 even after different amounts of sodium acetate were added. The figure shows the solids content profiles in the columns (the initial solids contents were the same) at the end of testing. With the increases of microbial activity and biogas generation in columns 1, 3, and 5, the solids contents at the end of testing also increased. The field investigations and smallscale column tests demonstrated that microbial activity and gas generation and migration can help densification of the MFT. The in-depth mechanism of the rapid densification of the MFT under microbial activity was further studied by gassy MFT densification tests.