Steam assisted gravity drainage, or SAGD, is the in situ method most commonly used to extract bitumen from oil sands that is too deep to mine. The technique involves drilling two horizontal wells into the reservoir, one on top of the other. Steam is injected into the reservoir through the top well, melting the bitumen and causing it to fall to the bottom of the reservoir, where it is produced to the surface through the lower well.
Water is required to produce the steam needed to extract the bitumen from the sand. Most boilers currently used in SAGD operations function at around 78 percent steam quality, requiring the disposal of 22 percent of water as “blowdown”. One of the key areas of focus for achieving the Water EPA’s In Situ Performance Goal is to improve the efficiency of steam generation. COSIA members are looking at boiler design and operations that could reduce overall water use owing to more efficient steam generation while also reducing greenhouse gas (GHG) intensity.
In 2014, Devon conducted a commercial-scale field pilot of a technology called rifle tubes, which had the potential to enable once through steam generator (OTSG) boilers to improve the quality of steam generated from 78 percent up to 90 percent steam quality, reducing water use and boiler blowdown, while increasing energy efficiency. The results of the pilot indicates that using rifle tubes could increase steam quality from 78 to 90 percent, reduce water use by up to 15 percent, reduce wastewater generation by 50 percent, and reduce GHG emissions between one and six percent.
OTSGs are the most common type of boiler used for steam generation in SAGD operations. These boilers use a series of pipes that run along the outside of a heat source. As the water passes from the inlet to the outlet of the pipe, heat is progressively added, transforming water into steam until at the outlet about 78 percent of the water gets converted to steam.
There are two main heating sections in an OTSG. The first is a convection section that preheats the water. The second is a radiant heat section, which uses a combustion chamber that heats the water to the point of evaporation (the point at which steam is produced) and keeps it hot to increase its ability to melt the bitumen when it is injected into the reservoir.
The radiant section of the boiler design used at Devon consists of four quadrants, each containing 25 pipe loops or lengths.
“As you start getting more steam and less water flowing through the pipe, there is less water available to cover the circumference within the pipe due to gravitation pooling of the water at the bottom of the pipe. This may lead to the development of hot spots at the top of the pipe at increased steam quality operation which, just like boiling a kettle dry, would cause the boiler to scale and foul,” says Jacob Denis, Senior Technical Advisor on Devon’s Technology Development team.
Rifle Tubes radiant sections
“In the usual SAGD process, scaling builds up on the inside of the pipes that produce steam. The scaling on the inside of the tubes increases the risk of tube failures and the frequency of boilers needing to be taken off-line and cleaned – both of which increase the environmental footprint and operational costs,” says Denis. “Steam output then drops and this directly impacts production. That’s where rifle tubes come in.”
Rifle Tubing technology involves using a “rifled” or “ribbed” tube instead of smooth tubes currently used in boilers that produce steam for SAGD. The internal ribbing of rifle tubes introduce centrifugal force in the tubes, not present in traditional, smooth tubes, helping to turn water into steam uniformly and more efficiently.
With this technology, steam generators can achieve higher steam production from less volume of water. This allows for reduced water use, which in turn, generates less wastewater, also known as boiler blowdown, consisting of naturally occurring minerals and contaminants that have been boiled out.
It’s a similar concept to the spiraled ridges on the inside of a gun barrel. It causes the bullet, or in this case water, to spin, creating a vortex that provides even distribution of water within the tube wall to avoid hot spots and scaling.
Devon completed a successful 10-month commercial-scale demonstration pilot at their Jackfish 2 in situ facility. The plant has six OTSG units, the last of which was retrofitted with 10 rifle tubes at the outlet of each quadrant of the radiant heat chamber. The unit was then run at 90 percent steam quality for 10 months and its performance was compared against 78 percent steam quality operation.
The Rifle Tubing pilot was completed and the results were positive. Most of the objectives of the pilot were met and all process parameters were verified during the first phase with one exception. There was a scaling tendency due to a failure of thermocouples, which were not related to the pilot. Later in 2015, the faulty thermocouples were replaced and destructive testing on the tubes was conducted which showed no adverse effect on the tubes during the elevated 90 percent steam quality operations.
Efforts are now focused on fully de-risking the technology. The next step proposed is to run the boiler at 90 percent steam quality for a few months in the first quarter of 2018 and observe for scaling. Once this is complete, rifle tubes will be evaluated for wider use both in new and existing operations.
The results of the commercial-scale demonstration pilot indicate that using rifle tubes can increase steam quality from 78 to 90 percent, reduce water use by up to 15 percent, reduce wastewater generation by 50 percent, and reduce GHG emissions between one and six percent.
Devon has contributed the results of their rifle tube pilot to the COSIA Water EPA. In 2013, Suncor also completed a separate rifle tube project at their MacKay River in situ facility. The two companies shared the results of their respective studies through the Water EPA and continue to collaborate on development of this technology with a view to further deployment in the future.
Rifle tubes are one of a number of technologies that have been explored through Imperial Oil and BP’s Boiler Blowdown Reduction Technologies study. Project participants also included Canadian Natural, ConocoPhillips Canada, Devon Canada, Nexen, Statoil Canada and Suncor.