Summary

A laboratory study was designed to improve fluid displacement efficiency in porous media by an in-situ foaming process and to determine the effect of mixed surfactant chain length on surface properties of foaming solutions, bubble size, breakthrough time, and fluid displacement in porous media. We screened various mixed surfactant systems, such as sodium dodecyl sulfate and alkyl alcohols. Maximum breakthrough time and fluid displacement efficiency were observed when both components of the mixed foaming system possessed the same chain length. Results were compared with data obtained using water, brine, and sodium dodecyl sulfate alone. The microscopic studies revealed that the order of bubble size measured outside the porous medium for various mixed surfactants was indeed maintained in a micromodel. The increase in the porous medium length improved breakthrough time and fluid displacement efficiency in sandpacks and in Berea cores. Mixed surfactant systems showed a correlation among surface properties of foaming solutions, bubble size, breakthrough time, and fluid displacement efficiency in a porous medium. Maximum foaminess, minimum bubble size, minimum surface tension, maximum surface viscosity, maximum breakthrough time, and maximum fluid displacement efficiency were observed when the two components of the surfactant system had the same chain length.

Introduction

In 1958, Bond and Holbrook proposed that the oil-recovery agent may be a mixture of gas and an aqueous solution of a surface-active agent (e.g., foam instead of surfactant solution). The foam process to improve oil recovery has since been studied by various investigators. In their experiments, a water-soluble surfactant with foam-producing characteristics was injected into an underground formation as an aqueous slug. This slug was followed by gas to produce in-situ foam. It is well known that when two immiscible phases (e.g., liquid and gas) flow through a porous medium, each phase may be considered to follow separate paths or phase may be considered to follow separate paths or channels. As the saturation of the immiscible phases changes, the number of channels available to each phase also changes. The effective permeability of each phase is influenced by the percent saturation of that phase. In the presence of foam, the effective permeability of the porous presence of foam, the effective permeability of the porous medium to each phase is considerably reduced as compared to the permeability measured in the absence of foam. Foam is a material with properties that are considerably different from those of its components; for example, the viscosity of a foam is greater than either of its components (i.e., gas or surfactant solution). In addition, foam is a relatively low-density material that can easily overcome gravitational effects and can pass through most regions of a heterogeneous petroleum reservoir. Deming has reported that the high foaming ability and decrease in plasticity of foam favor high fluid displacement efficiency in porous media. Surface plasticity is manifested when the surface viscosity is non-Newtonian, but the bulk viscosity of the solution is Newtonian. An ideal solution has a shearing stress that is directly proportional to the rate of shear. A plastic substance, however, proportional to the rate of shear. A plastic substance, however, will not flow at all at low rates of shear until a certain critical value of shear (true yield) has been reached. Thereafter, the plastic flows at a rate proportional to the total stress minus the yield stress. Surface plasticity in some surface-active solutions was proposed to explain the foam stability by Wilson and Ries. They found that the plastic solutions gave erroneously high surface tension plastic solutions gave erroneously high surface tension because the yield value of the plastic film was added to the true surface tension. Fried stated that foam moves as a body when a stable foam is present in porous media, while Holm suggested that gas and liquid flow separately through porous media in the presence of foam and that foam does not flow through porous media as a body, even when the liquid and gas are combined outside the system and injected as foam.

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