Development of Surfactants for Chemical Flooding at Difficult Reservoir Conditions

Abstract

Abstract The production and properties of two families of anionic surfactants (internal olefin sulfonates and branched C16, 17 alcohol-based alkoxy sulfonates) are described for chemical flooding of oil reservoirs at high temperatures and/or high salinities. Surfactant properties measured include oil/water micro-emulsion phase behaviour obtained using new glassware-based procedures appropriate for higher reservoir temperatures. The results obtained relate to oil/water interfacial tension behaviour and give the "operating window" of the surfactants in terms of their optimal salinity and ability to solubilise oil in the micro-emulsion. The phase tests also give information on the quality of the micro-emulsions obtained where low viscosity and absence of gels is desirable. The surfactants described are promising for EOR and can be produced in commercial quantities. Different IOS products are available with different carbon chain cuts (with range C15 to C28) allowing matching of the IOS to the temperature, salinity and crude oil type of reservoirs. In addition, both IO carbon chain (degree of branching) and the degree of sulfonation influence the surfactant properties of the IOS mixture formed which provides a means for tailoring an IOS surfactant for optimal performance. 1. Introduction In chemically enhanced oil recovery (EOR) the mobilisation of residual oil saturation is achieved through surfactants that generate a sufficiently (ultra) low crude oil/water interfacial tension (IFT) to give a capillary number large enough to overcome capillary forces and allow the oil to flow1. However, reservoirs have different characteristics (crude oil type, temperature and water composition), and the structures of added surfactant(s) have to be tailored to these conditions to achieve an ultra low IFT. In addition, a promising surfactant must satisfy other important criteria including low rock retention, compatibility with the polymer to be used, compatibility with hard water (if present), thermal and hydrolytic stability, acceptable cost/performance balance and commercial availability in sufficient quantities. Because of the well-established relationship between the micro-emulsion phase behaviour and IFT2 it is common in the industry to screen surfactants and their formulations for low IFT through laboratory-based oil / water phase behaviour tests3, 4. This approach works well for tests carried out at room temperature and slightly higher, but at higher temperatures there may be safety issues. Specifically, conventional sealed glass tube test methods may be problematic from a laboratory safety standpoint at higher temperatures due to the vapour pressures from water and crude oil. This paper presents the results of the evaluation of surfactants using improved phase behaviour experimental methods for higher temperatures (up to 150°C). Optimal salinities and solubility parameters have been measured. Two surfactant families have been evaluated, both produced by Shell Chemicals: Internal olefin sulfonates (IOS) which are part of the ENORDET™ O series and proprietary, branched C16, 17 alcohol-based anionic surfactants which are part of the ENORDET™ A series. Both families are suitable for EOR because they have a reduced tendency to form ordered structures/liquid crystals that are undesirable in reservoirs4, IOS products because they are a complex mixture of surfactants of differing chain lengths and the branched C16, 17 alcohol based surfactants because of their randomly branched structures.