One of the important mechanisms in the primary production of heavy oil reservoirs under solution gas drive is the foamy oil flow effect. This form of flow occurs only under certain combinations of capillary, viscous and gravity forces. Several studies have been carried out to examine this effect on the recovery process, but it remains poorly understood and difficult to model.
The objective of this study was to examine the roles of capillary, viscous, and gravity forces in foamy oil flow. Primary depletion experiments were conducted in a twometer long sand-pack holder to measure the oil and gas production at different depletion rates. The effect of gravitational forces was investigated by comparing the experiments conducted in horizontal and vertical orientations of the 2-meter long sand-pack. In the vertical orientation, the solution gas drive performance was evaluated with the production from the bottom end of the sand-pack, as well as with production from the top end. The experimental results are presented in terms of the oil and gas production behavior affected by the capillary number and gravity forces. The results indicate that the gravity forces do not significantly influence the ultimate oil recovery when the oil is produced under foamy oil flow. Even at the lowest depletion rate used in these tests, when oil flow and gravity act in the same direction, gas gravity segregation did not improve the oil recovery significantly. Recovery efficiency and the critical gas saturation at which free gas flow starts are primarily dependent on the capillary number.
Some of the heavy oil reservoirs in the world show anomalous primary production under solution gas drive. Smith1 was one of the first researchers who presented a detailed analysis of such unusual production behavior. Since then, it has been the subject of many studies, due to its importance in making reliable projections of future production and optimization of field operations. These studies can be divided into three categories. The first category focuses mainly on the geomechanical aspects.2,3 It is believed that the production of sand with oil improves the inflow performance by the creation of wormholes (high permeability channels). Experimental and theoretical studies3,4 have shown that the high pressure gradients present during cold production can overcome the cohesive strength of the sand matrix and this leads to the creation of wormholes. However, the mechanics of wormhole network development are not yet clear.
The second category focuses on multiphase flow behavior, and can be further divided into two subcategories. First are the conventional two-phase flow models5,6 which suggest that solution gas drive, whether in light oils or in heavy oils, is describable by the critical gas saturation and oil-gas relative permeability. In these, the high oil recovery is attributed mainly to low gas mobility. However, a good field performance match is not obtained by using low gas mobility; it also requires enhanced permeability far from the wellbore due to sand production7. The second subcategory of multiphase flowbased models is based on foamy oil flow.
