Boilers produce the steam needed to recover bitumen located in underground reservoirs. This type of bitumen recovery is called ‘in-situ’ because the bitumen extraction occurs ‘in place’, or underground. Steam is injected through a well into the reservoir to heat the bitumen until it softens the fluid to a point where it will flow more easily. The bitumen is then brought to the surface along with the condensed steam, which has since cooled and turned back into liquid water while heating the reservoir. The water is recovered from the produced bitumen, treated and recycled through the boilers to create more steam and re-injected.
Around 80 percent of the water used by boilers is turned into steam. The remaining 20 percent becomes concentrated with salts and silica naturally found in the produced water from the reservoir. This stream is termed blowdown water and is largely recycled within the facility, and a portion is pumped to deep underground disposal wells or trucked offsite to approved disposal facilities. Through the Boiler Blowdown Reduction Technology (BBRT) project COSIA member companies are exploring new technologies to generate more steam from the boiler feed water thereby reducing the blowdown volume and the makeup water requirement.
Imperial and BP spearheaded the BBRT project. One of the options under investigation was the boilers-in-series configuration. Instead of putting all the boilers in a parallel configuration as they were in most in situ operations, the boilers-in-series concept placed one additional boiler downstream of the group of parallel boilers, its feed stream being the blowdown stream from the group of boilers arranged in parallel. This secondary boiler was then ‘in-series’ of the upstream boilers. Boilers-in-series feeds the blowdown from the primary steam generators into the secondary boiler, which produces more steam from the same amount of water and reduces the volume of blowdown water that must be disposed of.
Some companies have been testing various types of boiler-in-series technologies as well as other technologies such as zero liquid discharge and reduced liquid discharge, which use evaporators to further concentrate and reduce waste.
Another option being investigated is using “rifle tubes,” which are installed inside the boiler, replacing a portion of the smooth tubes in the radiant section of the unit to reduce scaling and increase heat transfer, potentially allowing for safe operation of steam production of up to 90 percent, with 10 percent blowdown. In 2014, Devon conducted a commercial-scale field pilot of rifle tubes.
Boiler-in-series technology is applicable to in situ operations that use Steam Assisted Gravity Drainage (SAGD) technology, which requires 100 percent quality steam, and Cyclic Steam Stimulation (CSS), that injects 80 percent quality steam. The applicability of the boiler-in-series technology for SAGD and/or CSS processes is based on the total dissolved solids (TDS) of the boiler inlet water stream as a boiler can only cycle TDS to such a limit as to not cause erosion/corrosion damage within the boiler.In SAGD, a pair of wells is drilled vertically to the reservoir and then turned horizontally to reach as much of the reservoir as possible while minimizing surface land disturbances. Steam is continuously injected into the top well, while the bottom well continuously gathers the bitumen that has ‘melted’ out of the reservoir and brings it to the surface. With CSS, steam is injected into one well (also drilled vertically and then horizontally for the same reasons as in-situ wells) over a period of months. Steam is halted and the bitumen is produced up the same well.
The BBRT technologies, boilers-in-series and rifle tubes, provide dual environmental benefits since they both increase the amount of steam produced from the same volume of water while reducing the amount of water to disposal. Both technologies target producing more steam from the same amount of boiler feedwater.
The boiler-in-series technology was patented by Imperial, who co-lead the BBRT Joint Industry Project (JIP) with BP Canada Energy Group. JIP participants include Canadian Natural, ConocoPhillips Canada, Nexen and Suncor.