Key N-Solv™ Technology Principles
The use of solvent gas at its dew point to extract oil was proposed by Dr. Emil Nenniger in the 1970’s and patented in 1976. Though the process was 500 times faster than predicted by theory, scale-up calculations indicated that the production rate did not provide a high enough rate of return.
Two improvements to the original work have made the economics of solvent extraction favourable: horizontal drilling and the development of the N-Solv™ process. The advancement and use of horizontal drilling technology increases the contact surface area which, in turn, increases the amount and rate of oil recovered from a well pair. Improvements on the original process increased the production rate and lead to the development of the N-Solv™ process.
In solvent extraction, the production rate is limited by the rate that the solvent diffuses into the bitumen - a subtle but key difference with Vapex Theory. The penetration rate of solvent into bitumen is determined by the bitumen viscosity [pdf].
With Athabasca bitumen, a 25-30ºC temperature rise typically reduces the bitumen viscosity by a factor of 100. Thus, a substantial acceleration in the bitumen extraction rate is achieved with a very modest increase in temperature. This is the key principle of N-Solv™. Also, to achieve the desired temperature rise, it is necessary to have a solvent purity specification, as the condensation temperature is reduced by about 5ºC for every mole percent of methane contamination. Thus, even a small amount of methane contamination [pdf] in the gravity drainage chamber can greatly impair the ability of the solvent to deliver heat to the bitumen interface.
Although methane is naturally present in the in situ bitumen, the use of high purity condensing solvent at moderate temperatures provides a very efficient mechanism to remove the in situ methane from the chamber. Thermodynamic calculations show that N-Solv will be perhaps 20 times more efficient at removing the non-condensable gas from the chamber than steam extraction processes such as SAGD.
Conversely, unheated solvent extraction processes such as Vapex must add methane or another non-condensable gas to the solvent vapour in order to raise the dew point pressure to match the in situ pressure. However, the solvent is preferentially removed in the produced liquids, so the methane will accumulate in the chamber and eventually poison the mass transfer.
In N-Solv™, the condensed solvent and mobilized bitumen drain down the interface and form a pool at the bottom of the chamber, where they are removed via the production well. The solvent's purity specification helps ensure that the condensed solvent has maximum practical capacity to remove non-condensables from the chamber. Furthermore, N-Solv™ avoids reboiling, ensuring that the methane is efficiently removed from the chamber. This minimizes the risk of methane accumulation and consequent poisoning.
Benchmarking tests [pdf] using Athabasca bitumen at UTF in situ conditions have shown that N-Solv™ achieves extraction rates with 40-50 times faster than Vapex. These tests have also confirmed high yields, selective deasphalting, and that non-condensable gas contamination is a very effective poison.
The N-Solv™ Process
The N-Solv™ process can be described as the injection of a pure, heated solvent vapour into an oilsands reservoir where it condenses, delivering heat to the reservoir and subsequently dissolving the bitumen, with the resulting miscible liquids flowing by gravity to a production well. The process is run at moderate temperatures and pressures, and uses commercially proven horizontal well technology that has been developed for the oilsands. With Athabasca bitumen, a 25-30°C temperature rise typically reduces the bitumen viscosity by a factor of 100. Thus, a substantial acceleration in the bitumen extraction rate can be achieved with a modest increase in temperature.
As the solvent vapour is injected into a reservoir through the injection well, an extraction chamber is developed as the bitumen is dissolved and removed via the production well. The perimeter of the extraction chamber, or workface, is always cooler than the chamber itself, and this provides the mechanism for the solvent to convect and condense at the workface. With heat delivery to the workface, high rates of bitumen dissolution into the liquid solvent are achieved. Operating temperatures are 10° to 50°C higher than the reservoir temperature, and operating pressures are 50 to 1000 kPa higher than the reservoir pressure.
As the bitumen is dissolved into the solvent, a natural deasphalting of the bitumen occurs so that the valuable components of the bitumen are preferentially extracted while the coke-forming asphaltenes are safely and uniformly sequestered in the reservoir. Post-extraction core analyses show that the sequestered residue contains 60% to 70% asphaltenes as well as much of the sulphur, heavy metals (zinc, vanadium, iron) and carbon residue that is contained in the bitumen. This allows the N-Solv™ process to produce a partially upgraded 13° to 16° API oil, while typical bitumen has an API of about 8o.
To achieve high rates of bitumen extraction, N-Solv™ research has shown that in addition to heating the bitumen, it is important to control non-condensable gases, such as methane, in the extraction chamber. Non-condensable gases are released from the bitumen during the extraction process and will accumulate in the chamber if no measures are taken to remove them. Non-condensable gases will blanket the extraction interface and prevent the solvent from directly condensing on the bitumen at the workface. This reduces the dissolution effectiveness of the solvent, and thus reduces the extraction rate. With the right circulation rate of solvent, the N-Solv™ process provides the ability to minimise the concentration of non-condensable gases in the reservoir and thus sustain high production rates throughout the working life of the reservoir.
The N-Solv™ process uses solvent as a conveyor for fluids and energy into and out of the reservoir.
Step 1: Solvent vapour injection. Warm solvent vapour is injected into extraction chamber where it condenses and dissolves oil from the oilsand interface, draining downward to the production well as a solution containing solvent and oil. Naturally occurring water and non-condensable gases may also be carried along and out of the reservoir.
Step 2: Water separation. Upon being pumped up to surface, formation water is removed from the produced fluids. This is a straightforward process because of the lightweight oil/solvent solution (specific gravity 0.6) can easily be skimmed from the water (specific gravity 1.0).
Step 3: Oil Separation. After water is removed from the process, saleable oil is separated from the solvent by flashing the solvent to a separate process stream, allowing the oil product to be sent for further processing.
Step 4: Solvent purification. The N-Solv™ process requires a substantially pure solvent in order to function effectively; therefore the next step in the process is to distill the solvent, removing non-condensable gases such as methane, allowing high-purity solvent to be recycled back into the reservoir.
Step 5: Solvent make-up. For each barrel of oil removed from the reservoir, the volume must be replaced with solvent vapour. N-Solv™ estimates that a make-up quantity of 10-20% of the extracted oil must be added to the system as solvent. As the chamber grows, the make-up solvent is continually topped up. At the end of a well’s life, the solvent can be recovered and reused on another well or marketed.