Alkali Surfactant Gas Injection - Attractive Laboratory Results in Carbonates under Harsh Salinity and High Temperature

(Alkali) Surfactant, Polymer injection is an attractive enhanced oil recovery technique that allows achieving almost zero residual oil saturation at the microscopic scale when properly designed. In this combination of chemicals, the role of polymer is to achieve the necessary mobility control of the microemulsion / oil fronts which are formed and propagated through the reservoir. Foam has been recently identified as an alternative to polymers to achieve such a mobility control. This paper describes the (A)SG laboratory results which have been obtained on carbonate core samples under harsh salinity and temperature conditions. The starting point was the development of a surfactants formulation to achieve ultra low interfacial tensions between the oil and injected solution, using the classical test tubes approach. The efficiency (in terms of oil recovery) of this chemical formulation in this particular S and T condition was demonstrated with SP core floods. The same chemical formulation was used for (A)SG; the polymer was replaced by nitrogen co-injected with surfactant to create foam. Although SP and ASP flooding has been studied and implemented for decades, there remain few applications in carbonates, and none in the particular case of high salinity and high temperature carbonate reservoirs. Even if adequate surfactants can be tailored to these conditions, the use of polymers for mobility control in low permeability formations, particularly at high temperatures and in the presence of calcium, is a significant challenge. These results demonstrate that (A)SG is an alternative that works at laboratory scale and might be the starting point of a potential future pilot implementation . Foam quality has to be optimized in order to achieve the best recovery, but keeping also in mind that it is possible that gas sourcing and recycling will be a predominant consideration at the field scale; low quality foam may be suitable. In some situations the same surfactants can contribute to ultra low interfacial tension and have good foaming properties. - ASG results are presented in high salinity and high temperature conditions. Replacement of polymers by foam can work and is particularly attractive if lower permeability and higher temperature are targeted - For the many carbonate fields where water injection is already implemented, (A)SG EOR is a new option to consider.