| Foams are used in the petroleum industry to divert gas in enhanced oil recovery (EOR) processes and divert acid in matrix acidization treatments. These processes are exceedingly complex, but a model based on the "limiting capillary pressure" of foam greatly simplifies the quantitative description and design of foam processes. This dissertation describes this model and applies it to EOR and well stimulation.;It may be possible to bypass the daunting complexities of modeling the changes of foam texture and non-Newtonian foam rheology. By using the model of the "limiting capillary pressure", surprisingly simple and powerful conclusions can be drawn for foam plugging and flow in homogeneous reservoirs in radial geometry and in heterogeneous reservoirs with layers either isolated or in capillary equilibrium.;One application of the model is in improving sweep efficiency and oil recovery in gas-injection EOR processes. With fractional-flow theory, one can estimate the velocities of the front and rear edges of a foam bank with various initial conditions, surfactant retention levels, and chase fluids behind the foam. Moreover, one can estimate the mobility of each bank and the extent of flow diversion between layers, in linear or radial geometry. Fractional-flow analysis based on extrapolated laboratory data suggests that for continuous-injection foam processes, the best surfactant is one that drives water saturation to the lowest possible value at the injected foam quality and is sensitive to permeability. In contrast, for water-alternating-gas processes, the best foam is one that degrades over a range of water saturations.;Another application of the model is to predict foam diversion in matrix acidization of sandstones. This model predicts that foams can efficiently divert acid into low-permeability layers because capillary pressure makes foam less stable in these layers. The greatest diversion is obtained when foam is preceded by a surfactant preflush and followed by an acid slug that is compatible with the foam. The key to the success of such a process is the ability of a surfactant solution to immobilize the gas in previously injected foam. A process in which acid is injected continuously with foam can also achieve effective diversion. However, our model suggests that better performance is obtained by separating the foam and foam-compatible-acid slugs, and preceding the foam with a surfactant preflush. |
