Surfactants for EOR: Olefin Sulfonate Behavior at High Temperature and Hardness

Abstract

Summary. The surfactant properties usually required for EOR are investigated with alpha-olefin sulfonates (AOS's), particularly at high temperature, salinity, and hardness, together with their solubility in brine, chemical stability, phase behavior, and adsorption. The use of a cosolvent enables aqueous solutions to be prepared with concentrated brine, even at high divalent cation levels. But the chemical stability of some solutions can be affected by their sensitivity to the oxidation of unsaturated components, resulting in a decrease of the pH. Precautionary measures to stabilize the solutions are stressed-i.e., anaerobic environment, maintenance of an alkaline pH, or addition of alcohol. As already shown, these surfactants provide low interfacial tensions (IFT's) and high solubilization parameters at high salinity and divalent cation content. Properties of optimal formulations have been investigated as a function of Properties of optimal formulations have been investigated as a function of surfactant and cosolvent molecular weight and brine composition. Adsorption data on Na- and Ca- kaolinite are presented. In NaCl solutions, the amount of sulfonate adsorbed increases slightly with salinity. Preliminary measurements in hard water are shown to bring out the specific effect of calcium ions. According to the results concerning properties validly considered as screening criteria, we conclude that this family of sulfonates appears to be a potential candidate for EOR. potential candidate for EOR. Introduction Petroleum and synthetic aromatic sulfonates were the first Petroleum and synthetic aromatic sulfonates were the first surfactant families selected for EOR by micellar flooding because of their availability and relatively low cost. Their performances decrease as water salinity and divalent cation concentration increase, however, inducing poor brine solubility and low interfacial effectiveness. In many fields, the electrolyte content is higher than 50 g/L'; therefore, the micellar flooding process would increase to a great extent if the salinity is no longer a limiting factor. For this purpose, other classes of surfactants have been suggested-e.g., purpose, other classes of surfactants have been suggested-e.g., sulfated or sulfonated ethoxylated fatty alcohols or alkylphenols. which display both anionic and nonionic character in the same molecular structure, and AOS'S. These last compounds, dealt with in this paper, are relatively easy to manufacture and are of moderate price. paper, are relatively easy to manufacture and are of moderate price. They have been suggested for EOR, and recent papers have reported quite favorable behavior at high salinity from an interfacial standpoint: solubilization parameters and IFT'S. For a surfactant to be considered as a candidate for EOR, the main requirement is its ability to give low IFT's in multiphase systems formed in porous media The relationship between IFT's and phase behavior in the surfactant/oil/brine systems is well known, but it is imperative to take other properties into account, such assolubility in hard water (because of the possibility that the process is implemented by injection of aqueous surfactant solutions instead of oil-containing microemulsions);long-term chemical stability;sensitivity of phase behavior-e.g., to the variations in salinity that may arise from interactions with the rock surface, especially clay minerals, and to the variations in surfactant concentration; andadsorption onto the reservoir rock. Extending the evaluation of AOS's to these properties is the purpose of this paper. Encouraging results previously reported are confirmed, but some features are also pointed out related to solubility, pH conditions, and adsorption behavior that should be considered pH conditions, and adsorption behavior that should be considered in any investigation for implementing the process. Experimental Materials. C16, C18, and C20 through C14 AOS's were manufactured with sulfur trioxide and the falling-film technique. The AOS consisted of a mixture of alkane sulfonate R-CH = CH-(CH2), -S03Na, hydroxy sulfonate R-CHOH-(CH2),-SO3Na, and disulfonate R-CH(SO3Na)-(CH2),-SO3Na (less than 1 wt%), with n = 1 to 3. Specific synthesis of octadecene sulfonate was performed to provide a standard molecule for the analysis and to performed to provide a standard molecule for the analysis and to characterize its behavior with regard to AOS stability. The main characteristics of the surfactants used are given in Table 1. All the other chemicals mentioned in this paper (alcohols, dodecane, and inorganic salts) were of reagent grade. Twice-distilled water was used. A Charentes kaolinite was used for adsorption ex-periments. The surface area, determined by the BET method, wafound to be 26.8 m2/g [909 in.2/lbm]. Analytic Procedures. Sulfonates were titrated by turbidimet with Hyamine 1662 TM as a chemical reagent. Water and hydrocarbon (dodecane) concentrations were occasionally determined by gas chromatography with thermal conductivity and flame ionization detectors, respectively. The iodine index was measured according to Intl. Standards Organization Standard No. 3961 for determining alkene group concentration. Methods. Sulfonate solubility was studied by preparing series of mixtures having various compositions. After the mixtures were mixed and allowed to settle for a minimum of 1 week at regulated temperature, the boundaries between single and multiphase systems were constructed by visual examination. Critical micelle concentrations (CMC's) were determined by conductivity. Before infrared spectroscopy analysis, the samples of sulfonate were dehydrated by lyophilization. Phase behavior studies were performed in tightly closed, graduated test tubes. The mixtures, consisting of brine, alcohol, AOS, and hydrocarbon, were equilibrated by being shaken several times daily and then resting at least 1 week in an oven. Interfacial properties were determined mostly from solubilization parameter properties were determined mostly from solubilization parameter measurements. The solubilization parameter, 0, is the amount of water or hydrocarbon solubilized in the surfactant-rich phase per unit amount of surfactant assumed to be in that phase. By definition, conditions (salinity for instance) are said to be optimals when the amounts of water and hydrocarbon in the surfactant phase are equal. IFT's, y, were measured by the spinning-drop method.