Selection of a suitable foaming agent for mobility control in steam floods requires screening a large number of available surfactants. Many different screening strategies have been reported in the literature for this purpose. Almost all of them rely on some type of foam stability measurement for initial screening. However, the relationship between such foam stability measurements and the flow behavior of foams in porous media under steam flood conditions has not been thoroughly investigated. The objective of this study was to determine whether or not a correlation exists between the foam stability index measured in static tests and the mobility reduction characteristics measured in a porous medium.
Foam stability tests were performed under ambient conditions and under conditions of high temperature (200 deg. C) and pressure (6.9 MPa). Foams were generated by injecting nitrogen gas through a gas sparger submerged under the test solution in pressure vessels equipped with glass windows. Subsequent drainage of liquid from the foam as well as rate of decay of the foam volume were then monitored. The mobility control characteristics were measured at 200 deg. C and 6.9 MPa by monitoring the pressure drop across a 30 cm long and 5.7 cm diameter core composed of unconsolidated sand during steady-state flow of foam through the core. Various foams were compared in terms of their mobility reduction factor which represents the ratio of pressure drop across the core at given flow rates of pressure drop across the core at given flow rates of the foaming liquid and gas to the pressure drop obtained at identical flow rates with pure water and gas.
The results show that foam stability measurements made under ambient conditions do not provide an indication of how the foam will perform at high temperature. However, foam stability measurements performed at high temperature and pressure are performed at high temperature and pressure are generally a good indicator of the mobility reduction factor. When comparing different concentrations of the same surfactant, the concentration which provided the maximum foam stability also gave the highest value of mobility reduction factor. When comparing different surfactants, a fairly good correlation was found between the foam stability index and the mobility reduction factor.
Gravity segregation and channeling are inherent parts of steam drive operations. Due to the large parts of steam drive operations. Due to the large density difference, the injected steam tends to migrate to the upper portions of the reservoir under the influence of gravity. Channeling can occur due to the presence of a high permeability zone, a large fracture, or a zone of high water saturation. In reservoirs containing very viscous oil, it is often necessary to artificially create a high mobility channel to achieve adequate steam injectivity in order to heat and mobilize the oil. As the steam drive develops and the oil depleted zone expands, the pressure gradient between the injection and production pressure gradient between the injection and production wells decreases due to availablility of a much larger area of high mobility. This decreases the oil-steam ratio because an increasing part of the injected steam merely passes through the reservoir without its heat content being utilized. In this situation it becomes desirable to reduce the mobility of the steam.
It has been known for more than 25 years that the mobility of gas can be reduced by generating a foam. It has been suggested that, if steam is injected as part of the gas phase in a foam, it will partially plug oil depleted zones and high permeability plug oil depleted zones and high permeability channels. A number of laboratory and field studies have confirmed the potential benefits of foam in steam drives.
Although the steam foam process is conceptually simple, its application on a field scale is not straightforward. It requires careful evaluation of the potential benefits in relation to the cost of injected chemicals. For best performance, the foaming agent should possess a number of desirable characteristics.