Scale Formation from Dissolved Organic Matter in SAGD Steam Generators─Part I: Acidic Species Characterized by FTICR-MS

Boiler feedwater in in situ oil production from Alberta (Canada) oil sands contains a complex mixture of organic and inorganic compounds, parts of it responsible for scale formation in steam generators, which challenges operation efficiency and increases greenhouse gas emissions. A series of heating experiments using a closed reactor system were performed to study the effect of high temperature (300 °C) and pressure (9 MPa) on the quality of the organic compounds. For this goal, a detailed molecular organic matter characterization of six boiler feedwaters before and after heating from two Alberta geographical areas was carried out. Organic species from water and heated water samples were analyzed with ESI-N FTICR-MS. Solid generation and a pH drop (even to pH 1.5) were the immediate impacts of heating BFWs. Our analysis showed that the formed solids mainly consisted of organic species. The analysis of DOM showed that the Ox class group is the most abundant in all water samples, followed by the SOx class group in the Athabasca region and NOx species in the Cold Lake area, respectively. The analysis of solids with FTICR-MS showed enrichment of compounds with higher carbon numbers and DBE and S-containing species compared to the BFW DOM, while sulfur-containing constituents in BFWs were depleted after heating. Based on the experimental results, we propose the following mechanisms for the observed phenomena. (1) Cleavage of weak C–S bonds in dissolved organic matter leading to countless reactions is likely one of the key initiators of precipitation. (2) pH decrease during heating facilitates solid precipitation. (3) Part of the existing large multifunctional species breaks down and produces smaller acidic species, contributing to a further pH decrease. (4) Bonding of some compounds with multiple functional groups forms larger molecules that precipitate out and assist in lowering the pH by reducing alkaline compounds in the water.