Surfactant Selection for Chemical Flooding of Very Low Salinity Reservoirs – An Underrated Challenge

Abstract

Recently, research and development of EOR surfactants focused on so-called "harsh" conditions like e.g. high salinity reservoirs combined with high temperatures. It is well known and acknowledged that dealing with large amounts of dissolved electrolytes in the injection brine causes difficulties for most types of surfactants and other chemicals. Mostly overseen is the fact that, especially for surfactants, very low salinity conditions can also be quite challenging. We present a comprehensive phase behavior study based on the fundamentals of surfactant science. Surfactant flooding has highest efficiency when a surfactant is chosen whose optimum conditions match the reservoir parameters. The phase behavior of the injected surfactant slug and the crude oil depends strongly on temperature and water salinity, which directly influence the ability to reduce the interfacial tension. Near-fresh water injection brines do not have major precipitation issues, but they require surfactants of certain hydrophobicity. This is caused by the fact that decreasing salinity at constant temperature favors the interaction of the surfactant with the aqueous phase and increases its water solubility. In order to counterbalance the preference for the aqueous phase the hydrophobicity of the surfactant has to be increased to force the formation of a microemulsion middle phase. By this procedure the water – surfactant – crude oil interaction is adjusted, resulting in Winsor III type phase behavior and the desired ultra-low interfacial tension regime. However, increasing the surfactant hydrophobicity decreases its water solubility. This can disturb its transport within the injection water towards the crude oil in the formation. Our results provide a good overview of the challenges and advantages in the application of amphiphilic compounds at low salinity. The water solubility and the phase behavior of different classes of surfactants and their blends are shown. Several approaches are investigated to overcome these challenges or find ways to deal with them. The perils of blending surfactants are addressed and the question is posed how the appearance of the injection solution affects its performance. In the light of a growing interest in smart-water flooding technology the salinity in mature reservoirs is expected to drop significantly. This paper compares different approaches for the selection of appropriate surfactants when dealing with very low saline waters. To not impede the application of surfactants in a later stage of the reservoir life cycle and to achieve maximized outcome it has to be planned carefully if a smart-water flood shall be applied.