The heavy oils from Venezuela and Canada are being considered with a growing interest due to some promising large reserves. From many case histories, the implementation of a cold production scenario has been able to produce this oil at a reasonable cost due to an improved ultimate oil recovery higher than expected. A foamy oil behaviour has been postulated to partially explain the phenomenon.

A series of 5 experiments has been performed to carefully assess the hypothetical foamy oil effect on the ultimate recovery during the natural depletion process. Two parameters were studied: the gravity force impact and the depletion rate in order to upscale these data to the field application. A highly permeable long core was used with heavy reservoir oil (9°API). The porous media was set at initial reservoir conditions (60 bar(g), 49°C) with irreducible water saturation for each of these experiments. The cold production process by natural depletion was implemented at constant depletion rate with an accurate monitoring of the back-pressure regulator.

Comparing laboratory experiments conducted in horizontal or vertical flow direction at low depletion rate, gas gravity segregation effect seems to already improve the oil recovery at the laboratory scale although it is difficult to demonstrate the presence of a continuous gas phase. These results could indicate that the nucleation of the gas bubbles to create a gas phase is a very slow process due to the oil viscosity effect, above 1500 cP at reservoir condition in the presented case. The gas bubbles remain partly trapped within the oil and move with the liquid phase at least up to the threshold saturation. At field scale, corresponding to much lower depletion rate, this effect may be more significant particularly in unconsolidated sands where the vertical permeability is very high.

The ultimate recovery defined at laboratory scale appears to be significantly dependent on the depletion rate as expected and already reported in the literature. It has however to be noted that the mechanism, developed at high rate at the laboratory scale, is not applicable to field production. At low depletion (2.5 bar/d), about 19% IOIP were recovered whereas 42% IOIP were recovered at 12 bar/d and 45% IOIP at 25 bar/d.

The critical pressure at which free gas production occurred was estimated for each experiment. A pseudo 4.5% critical gas saturation was determined for the low depletion rate whereas a high depletion rate yielded 25.5% critical gas saturation.

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