Abstract

Foams are used for mobility control in Enhanced Oil Recovery operations involving injection of gases or steam. The ability of foams to lower the mobility of the vapor phase, under certain conditions, helps in reducing gravity override and channeling leading to improved sweep efficiency and oil recovery. A series of foam-flood experiments were carried out for implementing the process in a carbonate reservoir currently under miscible flood. The present paper focuses on simulation of a few of these experiments using steady-state foam model with an objective of establishing parameters critical to field implementation.

The mobility reduction factor and the threshold water saturation at which the foam breaks down were studied using numerical models that simulated laboratory experiments. A field scale model validated by actual foam tests was employed to evaluate the effect of duration of Surfactant Alternating Gas (SAG) injection cycle, reservoir heterogeneity, and economic aspects of the process.

It was established that favorable conditions for creation and propagation of foam for combating gravity override may exist in certain heterogeneous reservoirs, where paths of least resistance for flow of surfactant solution (from injectors to producers), occur via upper pay intervals containing mobile gas.

Introduction

Mobilization and displacement of oil by injected gases (hydrocarbon, CO2, N2, air, or steam) dominates the Enhanced Oil Recovery (EOR) processes throughout the World1. However, due to higher mobility and lower density with respect to the displaced oil, these gases tend to bypass and override the oil-rich zones, resulting in poor sweep and recovery efficiency. Foams under certain reservoir settings can help improve the sweep efficiency by lowering the mobility of injected gas2. The key to success of the foam application, therefore, lies in identifying the conditions under which it could be utilized to economically improve the sweep and oil recovery in a gas injection process.

The nature of foam in porous media has been a contentious issue. There are two differing views3–5 concerning fluid movement in the presence of foam - flow of foam as a body or flow of water and gas as separate phases. The parameters used to define the characteristics of the foam are, therefore, related to the property of the ‘foam’ or to the property of water and gas phases. Foam quality or fractional flow to gas, compressibility, and apparent viscosity characterize the foam. Injection pressure, pressure-drop between injector and producer, injectivity index, mobility reduction factor, foam flow resistance, apparent foam viscosity, and reduced relative permeability to gas are the measures of foam effectiveness in reducing the mobility of injected gas, leading to improved sweep. These are common indicators of strength and stability of the foam in porous media.

Attempts have been made by various workers6–8 to identify the mechanisms involved in generation, propagation, and decay of foam in the porous media. The effect of presence of oil phase on propagation of foams in porous media has been extensively researched9–11. The retention of foam-forming surfactant on rock surfaces is a key element in determining the economic success of a foam project.

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