Multicomponent Solvent Co-injection with Steam in Heavy and Extra-Heavy Oil Reservoirs

Al-Murayri et al. (2016) provided an experimental study on performance of Expanding-Solvent Steam Assisted Gravity Drainage (ES-SAGD) relative to Steam Assisted Gravity Drainage (SAGD) based on typical Long Lake reservoir properties. The article published in Energy & Fuels, focused on the use of gas condensate as a solvent additive to steam to increase the mobility of highly viscous oils or bitumen relative to conventional Steam Assisted Gravity Drainage (SAGD) wherein only steam is injected.

 The solvent-steam mixtures before condensation should remain vaporized until reaching the boundary of the vapor chamber in the reservoir where steam releases its latent heat of vaporization upon condensation and then drains downward. Meanwhile the solvent vapor also condenses near the vapor chamber boundary facilitating dilution of oil resulting in reduction in oil viscosity. However, solvent vapor does not condense into  the liquid phase until the partial pressure of solvent in the vapor phase exceeds  its dew point pressure.

It has been proven mathematically that steam/solvent co-injection increases instability at the interface between the vapor chamber and the bitumen at the edge of the chamber compared to steam only injection. This led to increased mixing at the edge of the chamber resulting in observed higher rates of oil mobilization and production from solvent co-injection with steam.

Co-injected solvent ES-SAGD should remain in the vapor phase before condensing at or near the boundary of vapor chamber. If solvent condensation takes place earlier, co-injected solvent will be short-circuited without necessary contact with highly viscous bitumen-vapor chamber interface; likewise, if solvent does not condense, it is unable to be effective in mixing with bitumen and reducing its viscosity and thereby increasing bitumen production rate.

According to Al-Murayri, M.T. (Experimental Investigation of Expanding Solvent Steam Assisted Gravity Drainage using Multicomponent Solvents, Ph.D. Dissertation, University of Calgary, 2012)  relatively heavy solvents like cracked naphtha tend to be more soluble in bitumen and have greater potentialfor enhanced dilution if they can remain vaporized within vapor chamber and travel to the vapor chamber edge with the steam. Hence, the interrelationship between pressure, temperature and solvent concentration has to be taken into consideration.

Evaluation of performance of ES-SAGD in relation to SAGD was achieved by making use of typical Long Lake reservoir properties with operating conditions at different concentrations of gas condensate.of 5, 10, and 15vol% on a cold liquid equivalent basis for experiments 2, 3 and 4 respectively.

Results obtained from experiments showed that co-injection of gas condensate with steam causes more oil to be drained and produced using much lower amounts of steam relative to the baseline SAGD experiment which implies that early gas condensate with steam is extremely beneficial to accelerate the establishment of inter-well communication between SAGD injection and production wells.

Results from temperature profiles within the model of SAGD baseline and gas condensate ES-SAGD experiments showed that same amount of oil can be produced using ES-SAGD with gas condensate while having lower temperatures within the sand matrix due to combined benefits of heat and mass transfer.

Profile showing the cumulative steam-to-oil ratio (cSOR) of gas condensate ES-SAGD experiments relative to the baseline SAGD depicted lower values of cSOR in the ES-SAGD experiments.

Cumulative injected steam and cumulative produced water remained reasonably in balance throughout the life of both baseline SAGD and gas condensate ES-SAGD experiments. The depleted sand matrix for baseline SAGD and gas condensate ES-SAGD experiment revealed that residual oil saturation varies from one region to another based on extent of steam chamber growth.

Asphaltene content in oil extracted from depleted sand matrix showed an increase for ES-SAGD compared to the SAGD baseline. Asphaltene content in original oil used to saturate sand pack experiment 2,3,4 was 22.5%, 22.3% and 22.3% respectively while asphaltene content with depleted sand matrix for experiment 2,3 and 4 was 27.1%, 27.3% and 29.4% respectively.

Density and viscosity of the produced oil from experiment 4 were lower than those from experiments 1, 2, 3 until gas condensate co-injection was discontinued which is mainly due to higher concentration of the co-injected gas condensate at experiment 4.

Al-Murayri et al. (2016), evaluated performance of gas condensate ES-SAGD cases as being more energy efficient, having higher oil production rates and better post production water handling than baseline SAGD. 10 vol% gas condensate was chosen as the solvent additive concentration because it was deemed to be high enough to deliver improved process performance while also taking into consideration the importance of using the minimal amount of high-cost solvent..