Abstract
SAGD(Steam Assisted Gravity Drainage) is commercially adopted as a main thermal recovery method for heavy oil reservoirs. How to improve the recovery effect and the heat efficiency of SAGD process draws the attention of all the heavy oil reservoir engineers, especially under low oil price circumstance. This paper aims to utilize gas as additives to reduce the steam demand of SAGD projects, hence to improve the recovery efficiency of this method.
In this paper, study on improving recovery efficiency of steam injection process with gas additives in reservoir with great depth was carried out. Laboratory tests were carried out to quantify the properties of different gas additives with heavy oil. Also flooding experiment were carried out to compare the relative permeability curve before and after gas additives were included. Then simulation were done to evaluate the influence of gas additives on SAGD process. SAGD processes with different gas additives were compared and the influences of different gas additives on recovery process were analyzed in detail.
The study reveals that the SAGD recovery process can be greatly influenced by gas additives. All of gas additives can sharply reduce the steam demand for SAGD process. It was shown that CO2 can improve the production effect of SAGD because of their great solubility and viscosity reduction effect. In the CO2 case the drainage threshold temperature was decreased by 17.1°C in comparison to the pure steam SAGD process with the help of viscosity reduction. Addition of CH4 will impair SAGD process because CH4 overrides faster than steam and hinders the development of steam chamber. N2 can accelerate the oil recovery rate but has no effect on improving recovery factor. On the contrary the viscosity reduction effect of CH4 and N2 is much less than CO2. With the results of this paper, the recovery factor of SAGD process can be improved by 8% and SOR can be improved by 14.7% when CO2 was co-injected with steam.
The results of this paper are of great significance in that it combines the advantage of steam injection process and gas injection process. The heat assumption in SAGD process can be sharply decreased and the recovery factor can be enhanced with the help of soluble gas additives. Also the greenhouse gas emission can be lowered down.
