Foamy Oil Flow In Heavy Oil Prodution

Abstract

Abstract The term "foamy oil" is often used to describe certain heavy oils produced by solution gas drive which display obvious foaminess in wellhead samples. The primary production of heavy oil from several reservoirs in western Canada is in the form of an oil continuous foam. This foam resembles chocolate mousse in appearance and often persists in open vessels for several hours. Some of these reservoirs exhibit anomalously high production, in terms of both the production rate and the primary recovery factor. Smith(l) appears to be the first to publish a detailed analysis of such unusual production behaviour. He attributed the anomalous production behaviour to tlIe flow characteristics of heavy oil containing a large volume fraction of very small gas bubbles. Since then, the flow behaviour of such gas-oil dispersions has become a subject of several investigations(2–9) and considerable speculation, but it remains controversial and poorly understood. However, it is now accepted that solution gas drive in foamy oil reservoirs involves some unusual effects. Foamy Solution Gas Drive Table 1 compares the characteristics of foamy solution gas drive with those of conventional solution gas drive. Solution gas drive in foamy oil reservoirs is relatively more complex. Here the gas released from oil due to decline in the pressure tends to remain dispersed in oil in the form of very small bubbles. The dispersed gas eventually separates from the oil to form a free gas phase, but this separation takes a long time to occur. Thus at any given time the gas can exist in three forms:dissolved gas, which is thermodynamically a part of the liquid phase;dispersed gas, which is thermodynamically a separate phase but hydrodynamically a part of the liquid phase; and free gas, a part of which becomes trapped while the remaining part flows independently. The amount of gas which remains dispersed (and flows with the oil) can be a complex function of the pressure; time; pressure gradient; and the rate of change of pressure. The flow behaviour of such gas-in-oil dispersions is very complex. In many ways these dispersions are analogous to emulsions. Like emulsions, they are thermodynamically unstable and given sufficient time the phases will separate (in absence of re-dispersing forces). The size distribution of the dispersed bubbles is established by a dynamic equilibrium between coalescence and breakup. The dispersed bubbles can become trapped at pore throats much like emulsion droplets. However, there are some noteworthy differences. Compared to emulsion droplets, the gas bubbles are more compressible and they are more soluble in the continuous phase. Apparently these and other differences are more important in controlling the flow behaviour than the similarities between emulsions and bubbly dispersions. The rheological behaviour of such dispersions in porous media does not appear to follow any known emulsion theory. For example, it has been suggested in the literature that the apparent viscosity of gas-in-heavy oil dispersions may be lower than the viscosity of same heavy oil containing dissolved gas(7). Emulsion viscosity is never expected to be lower than the continuous phase viscosity.