It is well known that the oil recovery efficiency of chemical EOR depends on microemulsion phase behavior and interfacial tension (IFT). The surfactants needed to obtain good phase behavior and ultra-low IFT vary greatly with oil characteristics and reservoir conditions. Hence, it is often necessary to test many surfactant formulations before finding a highly effective one. Based on both sound principles and extensive experience, one would expect to find a relationship between the optimum surfactant structure, the oil characteristics, the brine, and the temperature. Salager's equation (Salager et al., 1979, Anton et al., 2008 shows it is possible to correlate some of these variables to classical surfactant structure. We now have many new surfactants with widely different structures and many more good formulations with a wider range of oils, temperature and so forth. Thus, it becomes imperative to study the underlying trend and to identify the most important variables affecting the optimum surfactant structure. A new correlation has been developed using an extensive data set taking into account the effect of propylene oxide number (PON), ethylene oxide number (EON), temperature, brine salinity and the equivalent alkane carbon number (EACN) of the oil. The new correlation will help in identifying the most important variables and also to improve our understanding of the relationship among variables affecting optimum surfactant structure. In particular, the new equation can be used to predict the optimum carbon number of the surfactant hydrophobe. Results show that larger hydrophobes are needed as either the temperature or the equivalent alkane carbon number (EACN) of the oil increases. The surfactant formulations used for this study include mixtures of sulfate, sulfonate, carboxylate and non-ionic surfactants. This is a new and highly significant advance in the optimization of chemical EOR processes that will greatly reduce the time and cost of the effort required to develop a good formulation as well as to improve its performance.
Surfactant retention is one of the most important variables affecting the economics of chemical flooding and varies widely depending on the surfactant structure, mineralogy, salinity, pH, Eh, microemulsion viscosity, crude oil, co-solvent and mobility control among other variables. We have done a large number of dynamic surfactant retention measurements over a wide range of conditions using a variety of new-generation surfactants to recover crude oils from both sandstone and carbonate cores. Surfactant retention values for both surfactant-polymer (SP) and alkaline-surfactant-polymer (ASP) floods were measured and correlated with pH, total acid number (TAN) of the oil, temperature, co-solvent concentration, salinity of the polymer drive, mobility ratio, and molecular weight of the surfactant. Surfactant retention values ranged from about 0.01 to 0.37 mg/g of rock. SP and ASP formulations included mixtures of anionic and nonionic surfactants with and without co-solvents. The retention of anionic surfactants of all types was found to be similar on both sandstones and carbonate rocks.
The ability to select low-cost, high-performance surfactants for a wide range of crude oils under a wide range of reservoir conditions has improved dramatically in recent years. We have developed surfactant formulations (surfactant, co-surfactant, co-solvent, alkali, polymer, electrolyte) using a refined phase behavior approach. Such formulations nearly always result in more than 90% oil recovery in both outcrop and reservoir cores when good surfactants with good mobility control are used. Chemical flood residual oil saturations are typically less than 0.04 and surfactant retention between 0.01 and 0.1 mg/g with these formulations using as little as 0.2% surfactant concentration and 30% pore volume ASP slugs. We describe some of the advances that have improved the performance, reduced the cost, increased the robustness, and extended the range of reservoir conditions for these formulations. There are thousands of possible combinations of the chemicals that could be tested for each oil and each chemical combination requires many observations over a long time period at reservoir temperature for proper evaluation, so it would take too long, cost too much and in many cases not even be feasible to test all combinations. In practice we use our scientific understanding of how to match up the surfactant/co-surfactant/co-solvent characteristics with the oil characteristics, temperature, salinity, hardness and so forth. We have synthesized and tested new surfactants with much larger hydrophobes and more branching than previously available. We have tested new classes of co-solvents and cosurfactants with superior performance. These new developments have enabled us to develop good formulations for both oils that react with alkali to make soap and oils that do not. We have significantly lowered the chemical cost needed for waxy crudes with very high equivalent alkane carbon numbers. We have good results for oils with API gravities as low as 17, high temperature, high salinity, and high hardness brines. Many of these developments are synergistic and taken together represent a breakthrough in reducing the cost of chemical flooding and thus its commercial potential in both sandstone and carbonate reservoirs. SPE 129978ordered arrays such as gels or liquid crystals and decreases the reliance on alcohols or other co-solvents for rapid equilibration of microemulsions. Branched co-surfactants with different structures than the primary surfactant can be added to disrupt the orderly arrangement of surfactant molecules at interfaces (
The Guerbet reaction produces large, branched hydrophobes through the dimerization of linear alcohols. High-performance, low-cost enhanced oil recovery (EOR) surfactants are produced by carboxylation (carboxymethylation) of large Guerbet alkoxylates. Alkoxy groups such as propylene oxide (PO) and ethylene oxide (EO) are incorporated as extenders to the Guerbet alcohol, followed by carboxylation to make the anionic surfactant. Previously, the use of low-cost Guerbet alkoxy sulfate surfactants for EOR applications at high temperatures was established by enhancing their thermal stability when the pH is maintained at 10–11. Alternative thermally and chemically stable anionic surfactant structures are highly desired, especially for application under conditions where alkali usage is prohibitive. These novel large-hydrophobe carboxylate surfactants meet this need. In addition, the Guerbet alkoxy carboxylate structure can be tailored to fit the specific EOR requirements. These surfactants are stable at elevated temperatures both with and without alkali, and furthermore they can be used in environments of high salinity and high hardness (high concentration of divalent ions). Carboxylate surfactants have been used in formulations to produce ultra-low interfacial tensions with low microemulsion viscosities for a wide variety of crude oils under a large range of reservoir conditions. A parallel paper titled "Novel Large-Hydrophobe Alkoxy Carboxylate Surfactants for Enhanced Oil Recovery" will discuss the application of these surfactants. Thus, the advent of this class of cost-effective surfactants greatly broadens the application of chemical EOR.
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