Estimation of Foam Mobility in Heterogeneous Porous Media

Abstract

Abstract It has been shown experimentally, both here and elsewhere, that foaming gas is a promising technique for achieving mobility control and diverting injected fluid to low permeability strata within heterogeneous porous media. However, the factors most important for diversion have not been stated and explored definitively. Gas mobility in the presence of foam depends critically on foam bubble size; bubble size may vary with permeability, porosity, surfactant type and concentration, and the velocity of liquid and gas. This paper adopts a local-equilibrium, scaling perspective to describe quantitatively foamed gas mobility within heterogeneous porous media. Conventional and percolation network scaling ideas are employed. The equations developed indicate, for instance, that porosity plays an important role in setting gas mobility because it reflects the relative abundance of foam germination and termination sites per unit volume of porous media. Liquid velocity is also important because gas mobility is inversely proportional to this factor. Predictions are checked via comparison to new experimental results. The experimental system is composed of a 0. 395 porosity, 5. 35 µm2 synthetic sandstone and a 0. 244 porosity, 0. 686 µm2 natural sandstone. The cores are arranged in parallel and communicate through common injection and production conditions. Nitrogen is the gas phase and a classical alpha-olefin sulfonate (AOS 1416) in brine is the foamer. Three types of experiments were conducted. First, gas alone was injected into the system after presaturation with the foamer solution. Second, gas and foamer solution were coinjected at an overall gas fraction of 90% into cores presaturated with surfactant. Each core accepted a portion of the injected gas and liquid according to the mobility within the core. Lastly, gas and foamer solution were coinjected into the individual, isolated porous media in order to establish baseline behavior. The results show that in-situ generation of foam is an effective method for improving the sweep efficiency of lower permeability strata. The results also confirm predictions that foamed gas can be more mobile in lower permeability porous media. Introduction The presence of heterogeneity frustrates, to some extent, our ability to displace or remove fluids from porous media. Examples include the production of crude oil through the injection of gases such as steam and carbon dioxide and removal of nonaqueous phase contaminants from groundwater aquifers using aqueous-phase surfactants. In both of these cases, high permeability stratified zones preferentially carry most of the injected fluid. The physical explanation is simple. The mobility, ?, of a Newtonian injection fluid is generally largest in the most permeable zones and, hence, injected fluids prefer to channel and follow the path of highest mobility. This fact makes it difficult to establish uniform injection profiles around wells in heterogeneous formations and to sweep effectively regions of oil fields or aquifers that lie far from wells. One possibility to overcome the problem of channeling is to design a displacement so that injected fluid mobility adjusts as a function of the permeability of the medium and/or the velocity of the injected fluid. Thus, injected fluid mobility increases as porous medium permeability decreases. Some authors refer to this concept as selective mobility reduction1. Foam flow in heterogeneous porous media appears to exhibit these attributes (c. f. , ref. 2). Foam is a dispersion of gas within a surfactant-laden continuous liquid phase. The surfactant provides a measure of stability against coalescence of bubbles; effective foaming surfactants increase significantly the period of time that any individual bubble persists. Effective foams can be formed with the addition of surfactant chemical to brine at concentrations on the order of 0. 1 to 1 wt%.