The objective of this research was to develop a model to predict the optimum phase behavior of chemical formulations for a given oil based on the molecular structure of the surfactants and co-solvents. The model is sufficiently accurate to provide a useful guide to an experimental testing program for the development of chemical EOR formulations. There are thousands of combinations of surfactants and co-solvents that could be tested for each oil, so even approximate predictions are very useful in terms of reducing the time and effort required for testing and for prioritizing the chemical combinations to test that are most likely to yield ultra-low IFT at reservoir conditions. The effects of changing molecular structures (e.g. swapping head groups, swapping hydrophobes, increasing the length of hydrophobes, increasing the number of PO and EO groups, adjusting the ratios of surfactants) are shown. The variables with the greatest impact on the optimum salinity and solubilization ratio were identified, and methods are proposed to shift the optimum salinity and the optimum solubilization ratios in any desired direction. The structure-property model was developed and tested using a large dataset consisting of 684 microemulsion phase behavior experiments using 24 oils. The chemical formulations used 85 surfactants and 18 co-solvents in various combinations. Both optimum salinity and optimum solubilization ratio (and thus IFT) are modeled whereas other models have focused almost exclusively on the optimum salinity. Predicting the optimum solubilization ratio is actually of more value because of its relationship to IFT. The models include the effects of co-solvent partitioning, soap formation and the molecular structure of both the surfactants and co-solvents.
Favorable microemulsion rheology is required for achieving low surfactant retention and economic viability of chemical EOR. Co-solvents play a pivotal role in obtaining favorable microemulsion rheology as well as many other aspects of chemical EOR. We measured the partitioning of co-solvents between phases to better understand their behavior and how to select the best co-solvent for chemical EOR. There is an optimal co-solvent partition coefficient for microemulsion systems. Commercial co-solvents used for chemical EOR are actually mixtures of different components. We used HPLC to measure the partitioning of the constitutive components of phenol ethoxylate co-solvents between oil and water phases and between microemulsion and excess oil and water phases. These measurements show that the components partition independently and the partitioning of individual components is often different from the average. The co-solvent partition coefficients between oil and water were systematically evaluated as functions of the number of ethylene oxide groups, number of propylene oxide groups, temperature, salinity, and the equivalent alkane carbon number (EACN) of the oil. Novel alkoxylate co-solvents were also evaluated for chemical EOR. The novel alkoxylate co-solvents can be more effectively tailored to match the characteristics of different crude oils. Coreflood experiments were conducted to investigate co-solvent transport and retention. Co-solvents were identified that showed excellent performance and low retention.
Jang, Sung Hyun (The University of Texas at Austin) | Liyanage, Pathma Jith (The University of Texas at Austin) | Tagavifar, Mohsen (The University of Texas at Austin) | Chang, Leonard (The University of Texas at Austin) | Upamali, Karasinghe A. N. (The University of Texas at Austin) | Lansakara-P, Dharmika (The University of Texas at Austin) | Weerasooriya, Upali (The University of Texas at Austin) | Pope, Gary A. (The University of Texas at Austin)
The chemical cost to recover an incremental barrel of oil is directly proportional to the surfactant retention, so the single most effective way to reduce the cost is to reduce surfactant retention. The main objective of this research was to demonstrate how surfactant retention could be reduced to almost zero by careful optimization of the chemical formulations for different crude oils. Although surfactant retention has been studied for many years over a wide range of reservoir conditions, its dependence on the rheological behavior of the microemulsion that forms in-situ has not been adequately studied. Thus, in this paper we emphasize the importance of microemulsion rheology and demonstrate how to develop and test formulations with properties that give very low surfactant retention. Novel co-solvents (iso-butanol (IBA) alkoxylates and phenol alkoxylates) were tested in some of the formulations with excellent results. Unlike classical co-solvents used to optimize chemical formulations, the new co-solvents cause only a slight increase in the interfacial tension. A series of ASP corefloods were performed in sandstone cores with and without oil to measure surfactant and co-solvent retention and to elucidate the effects of microemulsion viscosity, salinity gradient, clay content, surfactant concentration and other variables. Dynamic adsorption was measured in cores with the same mineralogy and compared with the retention from oil recovery corefloods to determine the component of the retention due to phase trapping.
A. Nadeeka Upamali, Karasinghe (The University of Texas at Austin) | Liyanage, Pathma Jith (The University of Texas at Austin) | Cai, Jiajia (The University of Texas at Austin) | Lu, Jun (The University of Texas at Austin) | Jang, Sung Hyun (The University of Texas at Austin) | Weerasooriya, Upali P. (The University of Texas at Austin) | Pope, Gary A. (The University of Texas at Austin)
The ability to develop high performance, low cost chemical formulations for chemical EOR involves the use of not only highly efficient surfactants tailored to specific crude oil and reservoir conditions, but also the technical know-how for combining the surfactants and other chemicals to create the best formulation as a complete package. Scientific understanding of how the molecular structures of surfactants and co-solvents affect microemulsion properties greatly speeds up the process of arriving at optimal formulations for enhanced recovery of a specific crude oil in a specific oil reservoir. With the main emphasis on reducing the chemical cost of the formulations, a new slate of novel chemicals, both surfactants and co-solvents, has been developed and shown to have superior performance. We have synthesized and tested new classes of surfactants with different hydrophobe sizes and structures varying from large-medium-short-ultrashort in order to meet the needs of a variety of crude oil requirements. We have also developed ultra-short hydrophobe surfactants (with 2-ethylhexanol hydrophobe) possessing dual surfactant / co-solvent properties. Such duality in performance helps, in some cases, to minimize or altogether offset the use of co-solvents while maintaining low microemulsion viscosities, faster equilibration, and other desirable behavior. Thus, 2-ethylhexanol-propoxy-sulfate was developed as a surfactant that also encompasses co-solvent properties. The novel Gemini surfactants have also been incorporated in formulations and core flood experiments with excellent results. The new co-solvents offer advantages such as short equilibration time for the microemulsion formation and lower microemulsion viscosity. Systematic studies using these new surfactants and co-solvents clearly show that we now have the capability of developing highly robust formulations to meet the needs of a variety of reservoirs, resulting in high oil recoveries with low surfactant retention, which is the key to lowering the chemical costs and improving the economics of chemical enhanced oil recovery.