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.
© 2006. Society of Petroleum Engineers
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History
- Original manuscript received:
14 June 2004
- Revised manuscript received:
4 May 2005
- Manuscript approved:
30 May 2005
- Version of record:
20 February 2006
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