Characterization and Removal of High Concentration Silica in Steam-Assisted Gravity Drainage (SAGD) Evaporator Blowdown Wastewater
Creators: name: Rao, Saheli
Publisher: name: Graduate Studies
Year: 2024
Subjects: subject: Wastewater treatment
subject: Disposal water treatment
subject: Silica removal
subject: Organics removal
subject: Evaporator blowdown
subject: Equilibrium modelling
subject: Steam-Assisted Gravity Drainage
subject: Engineering--Environmental
subject: Engineering--Chemical

description: Evaporator blowdown (EBD) wastewater is a highly alkaline stream generated from the evaporation-based treatment of produced water arising from steam-assisted gravity drainage operations. It contains elevated concentrations of dissolved solids, of which silica poses a major challenge in both the waste stream treatment and its disposal. Silica has a propensity for scaling and fouling the process equipment. Additionally, pH-neutralization of highly concentrated silica brines results in silica gelation. This makes it difficult to filter and dewater before its disposal via down-hole injection. Moreover, the interaction of silica and other chemicals present in the EBD with those in underground formation waters can result in the plugging of the injection wells due to mineral precipitation. In this research, the removal of silica from these concentrated waste streams (largely comprised of brine solution) is investigated, while also understanding the speciation of silica to mitigate challenges during deep well injections. A systematic experimental program was conducted to investigate the effects of SiO2 concentration, NaCl:SiO2 ratio, and pH on the residual silica concentration, percent silica removal, filtration rate, and filtration effectiveness. The objective was to prevent silica gelation. The results indicated that when the NaCl:SiO2 ratios were higher than 4.5, silica precipitation during pH reduction did not lead to the formation of gel or sol. Additionally, the optimal ranges of factors for achieving the maximum silica removal, the highest filtration rate, the best filtration effectiveness, and the minimum residual silica concentration were determined using 3D response surfaces. Further, an investigation into the characterization and treatment of evaporator blowdown was conducted. The impact of acid treatment to remove silica and organics by filtration was studied. Both HCl and H2SO4 gave similar silica and organic removal efficiencies. The HCl-treatment was investigated further with both the EBD and lab-prepared silica solutions for silica removal efficiency and filtration performance. The experiments conducted at ~20°C demonstrated higher silica removal efficiencies at pH of 10 and 8, whereas higher organic removal efficiencies were obtained at pH of 4 and 2. The experiments conducted at an elevated temperature (~80°C) showed enhanced silica removal at a pH of 8 and higher organic removal at a pH of 2. The precipitated solids were characterized using XRD and ATR-FTIR spectroscopy to identify the minerals and functional groups. The XRD patterns demonstrated that the solids generated were amorphous. This research was undertaken to investigate the speciation and saturation indexes of minerals under various process conditions using equilibrium geochemical modelling in EBD wastewater and mixed waters resulting from combining EBD and formation waters. Parameters such as pH, acid type, and temperature were assessed for their impact on the speciation of silica and other species, as well as mineral saturation indices in EBD. Results highlighted the significant influence of pH on the speciation of silica and other species. The addition of either HCl or H2SO4 had no significant effect on the saturation indexes of silicate minerals, or the concentrations of silica and carbonate species. Additionally, simulations were performed for the mixture of untreated EBD and treated EBD at alkaline, neutral, and acidic pH levels with two formation waters. The major species in these mixed waters were silicates, sulphates, and carbonates. Simulations conducted on these mixed waters suggested that disposing of neutral to slightly acidic evaporator blowdown, as opposed to the alkaline EBD, in deep wells is conducive to preventing plugging and clogging issues that may result from mineral precipitation.