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.