Abstract

Foam is widely used to divert acid or abandon the high permeable layers. In this type of application foam should considerably reduce gas mobility. The nature of the gas and the surfactant may influence foaming behavior and thus the efficiency of the foam. In this paper an experimental study of the behavior of CO2 and N2 foams in granular porous media using X-ray Computed Tomography is reported. In the experiments gas is forced through natural porous media initially saturated with a surfactant solution, a process known as Surfactant Alternating Gas (SAG). The CO2 was either under sub- or super-critical conditions whereas N2 remained under subcritical conditions in all experiments. Alpha Olefin Sulfonate (AOS) surfactant was used as foaming agent. We found that injection of gas following a slug of surfactant can considerably reduce gas mobility and promote higher liquid recovery at the experimental conditions investigated. Foaming of CO2 builds-up a lower pressure drop over the core at both low and high pressures than N2. Both gases require a certain penetration depth to develop into foam. This length is longer for N2 (larger entrance effect) and increases with growing gas velocity. Moreover, the ultimate liquid recovery by CO2 foam is always lower than by N2 foam. The possible mechanisms explaining the observed differences in foaming behavior of the two gases are discussed in detail.

1. INTRODUCTION

Foam in porous media is a gas-liquid mixture with a continuous liquid phase wetting the rock whereas a part or all of the gas is made discontinuous by thin liquid films called lamellae [1–3]. Foam has been widely used to block the high-permeability layers and divert the injected fluid, for instance acid, to the damaged/unswept layers [4–7]. In these applications foam reduces the gas mobility by trapping a large part of the gas. Moreover, the foam reduces the liquid mobility by reducing the gas mobility in flowing fraction of foam [8]. Foams have been also used to improve the oil recovery of some fields [9–11]. The success of a foam application relies on the properties of rock and the foam itself. These include parameters such as surfactant concentration, surfactant adsorption on rock, foam propagation in porous media (foam strength/stability) and reservoir heterogeneity (and wettability). The physico-chemical properties of the injected gas can also play an important role in efficiency of foam in porous media.

The growing concern about the global warming and shortage of energy supply has increased the interest in combined geological CO2 storage and Enhanced Oil Recovery applications [12–14]. Although the geological storage of CO2 is considered as an attractive solution for global warming, the efficiency (or even feasibility) of the process is not yet established [15]. One major problem is the leakage of the injected CO2 through the walls of abandoned wells or through the cap rock [16]. In this case the foaming of CO2 may temporarily hamper the leakage while other actions are considered.

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