Increased Oil Tolerance of Polymer-Enhanced Foams: Deep Chemistry or Just "Simple" Displacement Effects?

Abstract

Abstract In recent years, foam treatment has moved towards field qualification as a practical method for gas shut-off in oil wells producing at excessive gas/oil ratio (GOR). However, the time of gas blockage achieved in most field trials is limited to a few months. This may be too short for general industry acceptance of this economically very attractive niche technology. Laboratory and scattered field data show that addition of polymers can enhance foam performance in oil-reservoir rock. However, little is known on precisely how addition of polymer improves the application-critical properties of a foam system. This paper reports new experimental data for gas-blocking foams that appear to have superior oil tolerance caused by the presence of polymer. The results are discussed in a context of field experience with gas shutoff foams and thin-film research. Core tests were conducted at reservoir temperatures, with crude oils from fields where field trials were done or planned. Systems studied include AOS surfactants and polyacrylamide polymers, bulk chemicals used in the field. Foam performance was measured in gas-blockage mode and forced flow. At zero oil saturation in cores, good foam performance was obtained with or without polymer added and with no systematic trends. Film properties for the same systems similarly gave no clear correlation with polymer concentration. A new high-pressure cell built to allow direct visual observation of oil/foam interaction at reservoir conditions was used to see if polymer influenced the oil tolerance of AOS-based foams. No effect on the spreading/entering regime was found. With oil in the core, AOS foams gave positive results only if containing polymer. Foam performance correlated with the level of oil saturation. It was hypothesised that the apparent oil tolerance of AOS/polymer foam was caused by the lower oil saturation in the core during foam generation. This hypothesis was tested by pre-flushing a core with the polymer/surfactant solution, but then generating gas-blocking foam from a polymer-free foamer solution. The resulting performance was identical to that of the polymer-enhanced foam. Hence, no chemical interactions of polymer and surfactant, or any liquid-film stabilisation mechanism, is required to explain these results. The role of polymer in the systems studied was only to improve the foamer/oil displacement by causing more efficient displacement of oil before or during foam generation. If generally valid, these results have two major consequences: Product selection by optimising polymer-foam systems for oil tolerance at a given oil saturation level is bound to fail. Foam process design should emphasise the need to create and maintain low oil saturations in the treated zone.