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Experiments were run to investigate the effect of surfactant concentration, injection rate, foam quality, oil saturation, and rock absolute permeability on the formation and propagation of foams in Berea sandstones and Ottawa sandpacks.
First, constant pressure unsteady-state gas-liquid relative permeability experiments were conducted in Berea sandstone for permeability experiments were conducted in Berea sandstone for varying residual saturations of synthetic oil. It was found that high oil saturations can hinder the formation of foam (no substantial decrease in the gas phase relative permeability). In such cases, some of the oil must first be displaced from the core before a foam can form.
Other experiments were performed by flowing preformed foams into cores at various conditions. The foams were preformed at high rates representative of tubing or sandface rates. They were injected into the porous medium at lower rates representative of near wellbore conditions (1.5-5 m from the wellbore) but not of in-depth reservoir conditions. Results showed that the foam propagation rate was significantly affected by rock permeability and injected foam texture. In high permeability sandpacks (40-50 darcies), the foam propagated at the same rate as the liquid phase. Foam propagation rates decreased substantially in lower permeability propagation rates decreased substantially in lower permeability media.
Foams are currently used to improve steam sweep efficiency in both cyclic and drive operations. Two types of reservoir problems reduce the effectiveness of steam applications - gravity override and steam channeling. In both cases, the steam preferentially sweeps sections of the reservoir. A significant fraction of the initial oil-in-place remains bypassed either in the deepest regions of the reservoir (gravity override case) or in the lower permeability zones (channeling case).
Because of the high fuel cost to generate the injected steam, research efforts by the oil industry have recently been directed toward overcoming both gravity override and channeling effects. One of the most promising methods being investigated is the injection of surfactants with steam to form a resistive foam which can divert steam into bypassed zones.
When a mixture of surfactant, steam, and noncondensible gas is injected into a well, a foam is generated either in the tubing or at the sand face. In order to be effective in the diversion of steam, the foam must not only penetrate into the steamswept zones, but must also propagate away from the wellbore. This is necessary to prevent the follow-up steam from flowing back into the swept zone beyond the foam plug a few feet away from the wellbore.
Foams are gas-liquid emulsions that exhibit a viscous behavior in porous media. Several authors have proposed mechanisms or observed flow patterns pertinent to the viscous behavior of foams in porous media. Fried proposed that foam moves through pore spaces as a body. Holm proposed that gas flows as a pore spaces as a body. Holm proposed that gas flows as a discontinuous phase separated by liquid lamellae and that the lamellae break and reform as gas passes through pore channels. Hirasaki and Lawson concluded that a foam's flow regime and its viscous behavior in capillary tubes can be correlated to the ratio of bubble size to capillary radius. This ratio determines whether a bulk foam or a chain of bubbles (where each pair is separated by an individual lamella) is flowing in the pair is separated by an individual lamella) is flowing in the porous media. Holm also showed that the presence of oil in the porous media. Holm also showed that the presence of oil in the rock can be detrimental to the formation of a foam.
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