Abstract
In steam drives and certain gas injection projects, the use of foamers to reduce steam and gas mobility has become a common way of improving sweep efficiency. Although the process is widely used, no effective means have been available to optimize it because there is no convenient flow model that properly describes the displacement characteristics of foam flow the way Buckley-Leverett theory describes simple two phase flow.
To attempt to develop a useful model for foam displacement, a number of experiments were run using surfactant laden water displaced by air at a constant rate in linear sandpacks. To decouple the displacement problem from the heat transfer mechanisms, no steam was used in these experiments. The saturations were measured in situ using the Cat Scanner; and, in addition, production and pressure histories were measured for the overall system.
First, water containing various concentrations of surfactant was displaced by air. As expected, these displacements follow Buckley-Leverett theory with saturation distributions growing linearly with volume injected. Also, as expected, the effective mobility of the air was a strong function of surfactant concentration as well as the air/water saturations. In addition, irreducible water saturations were found to be a strong function of surfactant concentration. Analytic expressions were developed to describe the relative permeability relationships as a function of saturations and surfactant concentration.
Next, experiments were run where surfactant laden water and air were injected simultaneously into a porous medium filled with pure water. This process more closely resembles field applications. The displacement fronts were spread more than when the water in place contained surfactant. When multiple fronts of different concentrations were used to match the displacements, the calculations predicted the greater spread observed experimentally, but did not properly ape the effect of distance moved. These calculations, for example, predicted that the saturation dissipation length would double as twice as much fluid was injected. The experiments showed less growth than this.
This slower growth rate could be the result of a dispersion term in the displacement. An equation was developed which included such a term and its predictions were found to match the experiments. The in situ saturation distributions were matched with distance and time as well as the overall recovery histories of the displacements. Under the conditions of these experiments, about 20% of the total spread of the displacements was caused by dispersion, and the rest was caused by Buckley-Leverett spreading.
These modeling efforts have already resulted in insight for practical application. They predict that a large slug of surfactant solution at low concentration injected ahead of the steam will have a greater mobility control effect than will the same amount of surfactant injected simultaneously with steam.
