Summary

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).

Introduction

Typical steam/oil ratio values for the production of extra-heavy oil or bitumen using the SAGD method are between 2 and 4. This implies that the production of 1 bbl of bitumen requires the injection of 2 to 4 B/D of cold water equivalent of steam and that a similar quantity of hot water is produced together with the oil. In view of environmental regulations, it is likely that a partial or even total recycling of such produced water into steam will be required.

Most boilers used in enhanced oil production with steam injection are gas-powered OTSG boilers. These were specially developed for thermal flooding applications and are commonly used because they offer several advantages over utility boilers: OTSGs require less maintenance and can tolerate fairly hard water with a relatively high content in (soluble) solids, monovalent cations, anions, and silica, because only 80% of the feedwater is vaporized in a single pass. The vaporization rate is directly related to the acceptable concentration of silica in the water.

The risk of silica-salt deposition in the boiler tubes is high when the silica content in the feedwater exceeds 100 mg/L (OTSG specifications). Silica deposits increase the local thermal resistance and can lead to tube failure. Boiler-tube replacement resulting from silica plugging is costly. In SAGD operations, production water (after circulating in the ground) contains high concentrations of silica (up to 400 mg/L). Silica removal is expensive owing to the consumption of chemicals. Its cost represents 80% of water-treatment costs (which represent U.S. $ 0.7/bbl) and 30% of the investment costs. Silica removal also generates environmental problems because of the production of sludge (120 kg of sludge for 1 thousand BWPD). Use of silica-scale inhibitors as a replacement for silica removal could result in substantial savings in costs. Six inhibitors, never used before in the petroleum industry, were developed and tested for that purpose.

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