Equilibrium Modelling of Silica Species and Minerals in Blowdown and Formation Waters
Creators: name: Murugan, Sivanesh
Publisher: name: Schulich School of Engineering
Year: 2022
Subjects: subject: Equilibrium Modelling
subject: Geochemical Modelling
subject: High silica brines
subject: Evaporator Blowdown water
subject: Disposal water treatment.
subject: Engineering--Chemical
subject: Engineering--General
subject: Geochemistry
subject: FOS: Earth and related environmental sciences

description: High concentrations of silica and other inorganic impurities present in the evaporator blowdown waters, resulting from steam generation in the Steam Assisted Gravity Drainage (SAGD) facilities for extracting heavy oil, cause significant issues in their dewatering and disposal in deep wells. The primary issue is the scaling and fouling due to silica when the water is treated with acids to neutralize its pH prior to disposal. There are also concerns with the reaction of silica and other impurities in the blowdown water interacting with the formation waters in deep wells resulting in minerals' precipitation, which leads to plugging issues. The focus of this study was to understand the silica speciation and mineral saturation in evaporator blowdown, formation waters and their mixture using geochemical modelling and simulation. The effects of essential parameters, such as silica concentration, temperature, pH, and the addition of HCl or H2SO4, were evaluated. In addition, the effects of NaCl, Mg2+ and Al3+ ions were studied on the mineral species. Mineral saturation indices were also studied and reported, along with the speciation of silica. Simulations were run for the mixture of two different formation waters with the evaporator blowdown to understand the silica speciation and mineral saturation for different mixing ratios. The predictions indicated that the concentration of silica species was significantly affected by the pH, whereas the addition of NaCl resulted in the formation of NaHSiO3 species. The addition of either HCl or H2SO4 had no significant effect on silica speciation, but the saturation indices of sulphate and chloride minerals increased when the respective acids were added. The presence of Mg2+ and Al3+ had significant effects on the minerals formed and the saturation indices of the new minerals. Antigorite was formed in the presence of Mg2+ in the evaporator blowdown, with the mineral being in the supersaturated state at alkaline pH. The major compounds that are formed in mixed waters are amorphous silica, magnesium silicates, calcium silicates, calcium and magnesium sulphates and carbonates. When the formation water was mixed with evaporator blowdown water, the speciation of silica is determined by the pH of the mixture. The saturation state of most minerals is below zero when the pH of the mixture is below 6. So, maintaining acidic conditions is preferable to prevent the precipitation of minerals in mixed waters. The two formation waters from the two sources have no differences in the composition, though their concentrations are different. Due to significant quantities of sulphate and carbonates in EBD, the mixed waters show no substantial difference in the species concentration and mineral saturation values of the water for a range of pH values.