Silica-Scale Inhibition for Steam Generation in OTSG Boiler

The production of extra-heavy oil or bitumen using the steam-assisted gravity-drainage (SAGD) method entails the generation and injection of a large quantity of steam into the reservoir. A similar quantity of hot water is produced together with the oil. Taking into consideration environmental regulations, it is probable that partial, or even total, recycling of the produced water into steam will be required. After circulating in the ground, production water may contain high concentrations of silica (up to 400 mg/L). Silica removal is very expensive in terms of investment and operating costs. The risk of silica-salt deposition in the boiler tubes becomes significant when the silica content in the feedwater exceeds 100 mg/L [once-through-steam-generation (OTSG) boiler-supplier specifications]. Deposition of silica increases local thermal resistance and can lead to tube failure. The silica-removal process is also an environmental issue owing to the production of sludge. This paper demonstrates that silica-scale inhibitors can be used in steam generation in an OTSG boiler. The original laboratory test program and results obtained are presented. Test results were corroborated on an industrial OTSG boiler in operating conditions. Results show that use of silica-scale inhibitors instead of silica removal could generate substantial savings in cost. The use of silica scale for steam generation in heavy oil exploitation has been incorporated into French patent 2,858,314-A1 (Gauthier et aL 2003).