During the last 20 years, it has been a very large discussion about different production mechanisms to explain the anomalous production behavior observed in some heavy oil reservoirs especially in the Orinoco Belt in Venezuela and Lloydminster in Canada. This behavior has been characterized by high production rates and/or high primary oil recovery, besides good pressure maintenance. A special phenomenon associated to the solution-gas drive mechanism, known as "foamy" or "bubbly oil", seems to be responsible for the recovery of a much higher fraction of the original oil in place. However, the "foamy oil" behavior cannot clearly explain the mechanism of the observed oil mobility enhancement.
Considering the physical principles of multiphase flow through porous media, the momentum transfer or viscous coupling appears as a hidden driving mechanism to explain the improved oil mobility. A capillary model is used to assess the importance of lubrication in two-phase flow and to determine the effect of the viscous coupling on relative permeabilities. Different cross-sectional geometries are analyzed. The problem is solved analytically even for the unequal viscosity case. This is the first attempt known to these authors not only of using this analytical method of solution, but also of using the concept of contact angle for determining the fluids distribution inside a non-circular channel.
The model results show that the viscosity ratio does affect the relative permeabilities, especially in systems involving heavy oil.
Several heavy oil reservoirs in Canada and Venezuela show higher than expected production rates, low produced gasoil ratio, and high recovery factors during primary production. Typical heavy oil reservoirs are expected to exhibit 2 to 4% recoveries of the original oil in place, whereas these reservoirs exhibit recoveries from 10 to 15% of the original oil.[1] Production rates can be up to 50 to 100 times[2, 3] and as high as 300 times[4] higher than the rate predicted by Darcy's equation.
To explain the improved recovery factors, it is necessary to postulate a mechanism that traps a larger fraction of the released solution gas in the reservoir. This mechanism appears to be the so-called "foamy oil", "bubbly oil" flow or "enhanced solution-gas drive". It is a particular form of twophase flow in which at least a part of the released gas flows in the form of dispersed bubbles.[5]
On the other hand, to explain the high primary production rates, basically three potential mechanisms have been proposed: sand production; reduction in the oil phase viscosity, and gas lubrication.
Some believe that the sand production increases the fluid mobility in the near well zone by increasing the permeability in the affected zone. Continuing sand production generates a growing zone ("wormholes" and cavities) of enhanced permeability that could be considered a growing negative skin effect. However, Smith[6] concluded that the large sand cuts in the Lloydminster area are insufficient to account for the abnormal production rates. Since then, Claridge and Prats[7] have become aware of other reservoirs, in both North and South America, which exhibit large apparent mobilities and have produced essentially no sand.
