Experimental Studies of N2 - and CO2-Foam Properties in Relation to Enhanced Oil Recovery Applications
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Foams can do more than soften a beard or extinguish a fire. Foam also offers the oil industry better mobility control. The presence of a foaming agent in porous rocks can reduce the mobility of gas and water, stabilize the gas injection front and prevent unwanted production of gas and water. These unique properties can assist the reservoir engineer in different optimization processes to enhance oil recovery (EOR) and improve the economics of mature oil fields.
A number of factors influence the properties of foam, such as the foaming agent, gas type, rock properties, interactions with oil, injection strategies, and temperature and pressure conditions. A change in one or several of these parameters may affect the performance of the foam and, consequently, the success of the intended foam application. For that reason, it is important to understand foam on a broad experimental scale.
This thesis presents experimental studies of foam in bulk and porous media.
The studies in porous media investigated: I) CO2-foam properties compared with those of N2- foams and II) the impact of rock material on foam generation performance and mobility control. The experiments were performed in oil-free outcrop sandstone core samples in the range of 30-280 bar and 50-100°C using alpha-olefin sulfonate (AOSC14-C16) surfactant.
The studies in bulk evaluated a set of foaming agents relative to: I) various experimental methods (bulk tests, core flooding), II) different gas types (CO2, N2, air) and III) the absence and presence of oils (crude oils, alkanes). A new bulk test was designed in the thesis to allow foams with gases other than air to be studied under low pressure. The combination of several experimental approaches was introduced to improve the evaluation and screening of surfactants.
The experimental results obtained in this thesis show that the presence of different gas types (CO2, N2) strongly influences the properties of foam in bulk and in porous media.
The CO2-foams were inherently weaker than the N2-foams. Possible reasons for the apparent weakness of the CO2-foam compared with the N2-foam were investigated more closely. A good correlation between the CO2-density and the CO2-foam strength was found; conditions where the density of CO2 is low improved the CO2-foam strength. Also, new foam experiments with pre-equilibrated fluids were conducted. These experiments suggested that the kinetics of the mass transfer between CO2 and the surfactant solution could not be the main cause why the CO2-foams were weaker than the N2-foams. However, the use of preequilibrated fluids significantly improved the water-blocking capabilities of the CO2-foams, indicating that gas dissolution into the injected water is one of the predominant mechanisms that weaken the CO2-foams during liquid injection following generation.
N2-foam experiments in various outcrop sandstone core samples showed that the rock material is one of the main parameters controlling the in-situ foam generation performance. The results demonstrated that foam was able to be generated and reduce mobility in all the sandstone cores used under all the conditions listed above. However, large variations in foam strength and mobility control were obtained between the different core samples. The presence of low permeability laminated heterogeneities, detected through various types of core analysis, appeared to be one of the parameters affecting the foam generation performance. The detailed interactions between the rock surface properties and the thin liquid films were beyond the scope of this thesis, but are suggested to be of central importance to in-situ foam generation performance.
The combination of several experimental techniques, including the new bulk test, was shown to be valuable for improving the evaluation and screening of foamers in the absence and presence of oil. Although certain similarities and interesting trends were observed between the experiments in bulk and porous media, the bulk foam properties of this work did not generally correlate with the foam properties in porous media. It seems difficult to predict foam properties and performance separate from the porous media by means of simpler experimental methods.
It is hoped that the laboratory-derived results presented in this dissertation will contribute to generate new insights and ideas within the field of foam, provide valuable input to reservoir models and simulations, and suggest practical considerations towards the scaling of foam processes for different EOR applications.