Abstract

The use of foam to improve sweep efficiency during gas or steam injection has been studied since the end of the 50s. Foam reduces the mobility of gas or steam and this effect is amplified when the permeability increases, thereby correcting the effect of heterogeneity. However, although the consequences of the process are now generally recognized, it is still impossible to quantify them in terms of increase in oil production or reduction of quantities of gas/steam injected. The analysis of the field tests that have been performed in the past 20 years can help in the quantification and forecast of foam performance.

This paper presents conclusions resulting from a comprehensive analysis of steam-foam field tests. They show that, at the present time, steam-foam injection is most efficient in layered reservoirs. Injection of slugs of surfactant with steam in this type of reservoirs can yield an average 3.9bbl of incremental oil per kilogram of surfactant injected. The process is economically attractive even in these times of low oil prices, with a cost of $2 to $3 per barrel of incremental oil.

Continuous injection of surfactant with steam, more adapted to in-depth mobility control in non-stratified, heterogeneous reservoirs. It is a much less efficient method, with an average efficiency of 0.3 bbl of incremental oil per kilogram of surfactant injected, and is not economical.

Finally, guidelines for the design of steam-foam injection tests are provided, and injection pressure during steam-foam is related to steam quality and concentration of the surfactant.

Introduction

The reserves of heavy oils worldwide are enormous and they represent a significant part of the world production. To-date, thermal processes are often the only way to produce economically these heavy oils. Among them, steam injection and more specially steam flooding is the most used Improved Oil Recovery method. Unfortunately this process can suffer from poor sweep efficiency and premature steam breakthrough in the case of heterogeneous reservoirs or when gravity segregation can occur. In the past 20 years, foam has been used successfully several times in combination with steam to correct these negative effects. Foam is formed in the reservoir by adding a foaming agent (surfactant) to the steam. Its basic properties make it an ideal agent to correct injection profiles, reduce losses due to thief zones or more generally improve vertical and areal sweep efficiencies. First, foam reduces the mobility of steam (or more generally gas) thus improving the mobility ratio with the oil; second, this mobility reduction increases with the permeability, which helps smoothing the effects of heterogeneity; third, foam is also sensitive to oil, which means that it is more likely to be formed in the well flooded areas, where oil saturations are lower. thereby diverting the steam towards less swept zones.

Considering the potential benefits of foam injection, it is not surprising that an important effort of research has been made to understand the basic mechanisms of generation and destruction of foam as well as its rheological properties in the porous medium. However, even if the general understanding of foam mechanisms has improved tremendously over the past few years, yet it is still difficult to design simply a foam test in terms of surfactant concentration, volume to be injected, of injection schedule etc. according to the geological and petrophysical characteristics of the reservoir. It is also impossible to forecast the effect of foam in terms of incremental production and hence in terms of efficiency or profitability, and this may deter operators from using the process. In the absence of predictive simulation tools, there is much to gain from the analysis of past field experience. This is the aim of this paper.

The analysis of the field tests that have been conducted in the past 20 years can provide some guidelines and point out the pitfalls to be avoided to conduct successful field tests.

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