Identification and Evaluation of High-Performance EOR Surfactants

Abstract

Abstract We report results for a number of promising EOR surfactants based upon a fast, low-cost laboratory screening process that is highly effective in selecting the best surfactants to use with different crude oils. Initial selection of surfactants is based upon desirable surfactant structure. Phase behavior screening helps to quickly identify favorable surfactant formulations. Salinity scans are conducted to observe equilibration times, microemulsion viscosity, oil and water solubilization ratios and interfacial tension. Co-surfactants and co-solvents are included to minimize gels, liquid crystals and macroemulsions and to promote rapid equilibration to low-viscosity microemulsions. Branched alcohol propoxy sulfates, internal olefin sulfonates, and branched alpha olefin sulfonates have been identified as good EOR surfactants using this screening process. These surfactants are available at low cost and are compatible with both polymers and alkali such as sodium carbonate and thus are good candidates for both surfactant-polymer and alkali-surfactant-polymer EOR processes. One of the best formulations was tested in both sandstone and dolomite cores and found to give excellent oil recovery and low surfactant retention. Introduction Recent advances including the development of new synthetic surfactants and increased understanding of the structure-performance relationship of surfactants have made it possible to rapidly identify promising high-performance surfactants for EOR. This process involves laboratory screening using knowledge of the molecular structure and cost of the surfactants as well as pertinent reservoir-specific information (i.e. temperature, salinity and crude oil properties). This paper describes a process for identifying and evaluating potential EOR surfactants. The surfactant selection process starts with the screening of surfactants by phase behavior experiments and progresses to core floods with formulations that may incorporate co-surfactants, co-solvents, alkali, polymers and electrolytes as well. We illustrate the application of this approach to the selection of a surfactant formulation for use in both a sandstone outcrop and a West Texas dolomite reservoir, but focus mostly on the dolomite application because very few studies have been reported for carbonate[1] or dolomite reservoirs. These laboratory data were used in a parallel simulation study of the same reservoir and are described by Anderson et al.[2] in a companion paper. Background It is well known that the primary requirement needed to mobilize residual oil saturation is a sufficiently low interfacial tension (IFT) to give a capillary number large enough to overcome the capillary forces and allow the oil to flow.[3] Low IFT can be obtained with a wide variety of surfactants, but the best surfactant depends on the crude oil and reservoir conditions and must also satisfy several other stringent requirements. These requirements include low retention, compatibility with the electrolytes and polymer, thermal stability, aqueous stability, and low cost. Surfactant retention is due in part to adsorption on the rock surfaces, but other loss mechanisms including phase trapping can be just as important, or more so. There is a strong and well-established relationship between the microemulsion phase behavior and IFT.[4–10] This relationship can be used to great advantage to rapidly screen surfactants and predict which ones will likely perform best in the more difficult and expensive core floods, and this is the approach taken in this paper.