Abstract
Steam assisted steam drainage (SAGD) or steam and gas push (SAGP) is commercially utilized to recover bitumen from oil sands. It produces higher oil rate and ultimate recovery than the other thermal recovery methods. But for thin heavy oil reservoirs, the recovery is lower resulting from larger heat losses through cap-rock and poorer oil mobility under reservoir conditions. A new EOR method, ES-SAGP, is introduced to develop this kind of reservoir. That is to say, non-condensate gas and vaporizable solvent is injected with steam into steam chamber during SAGD. The scaled 3D physical simulation was employed to examine the effectiveness of ES-SAGP and analyze propagation mechanisms of steam chamber. Under the same experimental conditions, a contrast analysis about the expanding characteristics of steam chamber, the swept efficiency of steam chamber and the ultimate oil recovery were carried out between SAGP and ES-SAGP. The experimental results shows that the steam chamber gradually becomes an ellipse shape during SAGP; However, during ES-SAGP, non-condensate gas and vaporizable solvent gather at reservoir top to decrease heat losses, and oil viscosity near condensate layer of steam chamber is largely decreased both by hot steam and solvent, which makes the boundary of steam chamber vertical and gradually become a shape of similar rectangle. During ES-SAGP in thin reservoir, the expanding processes of steam chamber can be divided three stages including ascent stage, horizontal expanding stage and decline stage. During the ascent stage of steam chamber, the needed time is shorter than SAGP. The stages of horizontal expanding and decline consume more time during injecting nitrogen, solvent and steam to enlarge cross-section area of steam chamber bottom. When the instantaneous oil-steam ratio is lower than 0.1, the swept efficiency of steam chamber is over 75% and the ultimate oil recovery is 45.7%, which is 7.4% higher than SAGP. Therefore, during ES-SAGP, not only the volume of steam chamber is largely enlarged, but also the swept efficiency and the ultimate oil recovery are remarkably improved.
