Subscripts M , L, G = membrane, liquid, gas phase o = at entrance m = meanvalue a = atmospheric i = membrane-liquid interface
This paper describes experimental and theoretical studies of cation exchange in porous media with micellar fluids formulated using a broad-equivalent-weight (BEW) sulfonate. The sulfonates can be described as composed of two pseudocomponents a quasi-monosulfonate (the oil-moving pseudo components a quasi-monosulfonate (the oil-moving component) and a quasi-disulfonate (the sulfonate-solubilizing component). With this description and a mass-action model for cation exchange between the micelles, clays, and solution, a match between computer model predictions and results of laboratory single-phase flow tests in Berea sandstone was carried out. The assumptions required are reviewed and independent experimental results presented. With these assumptions and parameter values determined from the Berea history match, satisfactory predictions of divalent cation concentrations in field core experiments have been made. The good predictive capability of this model allows initial screening and development of micellar formulations for specific reservoir applications to be conducted at appropriate hardness levels. Introduction It is well known that the oil recovery performance of a micellar fluid is strongly affected by salinity and hardness (calcium and magnesium divalent cations). This is because they have strong effects on the phase behavior and interfacial tension (IFT) of the surfactant/oil/brine system. It is also well known that the hardness and salinity of the micellar fluid can change significantly as a result of cation exchange and dissolution as the micellar fluid propagates through the reservoir. Since a knowledge of the in-situ levels of salinity and hardness is of primary importance in the screening and development of micellar fluids for field applications, an adequate prediction is necessary. Cation exchange between a brine and the clays within a reservoir rock occurs if the injected fluids have a salinity and hardness different from that of the in-place fluids. Smith, Griffith, and Hill and Lake have studied this problem and have shown the significance of the cation-exchange capacity (CEC) of the clays and the selectivity of the cation species with the clays. The clay selectivity is a measure of the preference of the clay for monovalent vs. divalent cations; for a given brine, smaller values indicate a higher fraction of the clays complexed by the divalent cations. Further, Hill and Lake concluded that the law of mass action is the best model with which to describe the process. Smith, and Hill and Lake, also showed that calcium and magnesium ions have the same selectivity with the clays vs. sodium, and hence they can be treated as a single ionic species. Hill and Lake extended their study to systems containing surfactants. They found that cation exchange in the presence of a surfactant system was complicated by interaction between surfactant and divalent cations. To describe the levels of hardness measured in the presence of surfactant micelles, they postulated the formation of a divalent-cation/surfactant complex and modeled the phenomenon with a mass-action isotherm. Gupta provided phenomenon with a mass-action isotherm. Gupta provided additional data supporting the formation of such a complex. Hirasaki and Lawson proposed a Donnan equilibrium model to describe the association of sodium and calcium with the micelles, and they estimated selectivity values from the resulting expressions. Hirasaki has incorporated a mass-action model, surfactant adsorption, and electroneutrality conditions with the mass balances neglecting dispersion to obtain a description of cation exchange during single-phase-flow in porous media. He has solved the system of equations using a method-of-characteristics approach and has been able to describe the experiments of Hill and Lake and Gupta showing good agreement between experiment and theory. The model is limited to one surfactant species and two cations-one monovalent and one divalent. The surfactant system used by Gupta closely conforms to these limitations. The micellar system used by Hill and Lake, however, was composed of two petroleum sulfonates and sodium alkyl ethoxysulfate (Neodol 25–3S). Nevertheless, Hirasaki assumed the surfactant mixture to be acting as a single surfactant species. This paper deals with the cation exchange that occurs during the propagation of a micellar system containing BEW sulfonate. The objective is to history-match limited tests in Berea cores and then to use the understanding gained and parameter values obtained to predict hardness concentrations in field cores over a wide range of micellar compositions. To correlate the Berea data and to extrapolate to other conditions, the approach of Hirasaki is desirable because of its simplicity. However, the complex composition of the BEW sulfonate micellar system, as well as a desire to include an adsorption isotherm and dispersion-important for small slug processes-precluded the straightforward use of the equations and the solution that he put forward. SPEJ P. 580
Sulfonation studies were conducted on a series of monoisomerically pure di‐ and trialkylbenzenes in order to determine the regioselectivity of sulfonation and how the selectivity varies with structural changes. Sulfur trioxide, because of its commerical significance, was investigated as the primary sulfonating agent. Lewis base adducts of sulfur trioxide and other sulfonating agents were also investigated for comparison. The studies demonstrated, in a quantitative manner, the tendency of the sulfonation to occur para to alkyl substituents. In one class of the alkylbenzenes investigated, 1,3,4‐alkyl dimethylbenzene, the sulfonation which occurred at the position para to the 3‐methyl group ranged in selectivity from 87–99%, even though this position, which is ortho to the larger alkyl group, was much less favorable from steric considerations than the position ortho to the 4‐methyl group. The determination of the position of sulfonation by a combination of nuclear magnetic resonance and high pressure liquid chromatographic techniques is discussed.
This paper presents a novel method of characterizing the broad equivalent-weight (BEW) sulfonates composed of two pseudosulfonate fractions that behave as the oil-moving and the sulfonate-solubilizing components. The terms quasi-monosulfonate and quasi-disulfonate are used to characterize these oil-moving and solubilizing components, respectively. Such a description of BEW sulfonates allows determination of sulfonate concentration in the flowing phases as well as the quantity adsorbed on the porous medium, and permits modeling of the sulfonate retention and transport behavior. Furthermore, the paper shows that the performance of a BEW sulfonate system in which a lower-phase microemulsion environment is predominant can be predicted by using independently measured input data, which include interfacial tension (IFT), fluid viscosity, and sulfonate retention isotherm. Introduction The BEW sulfonates commonly have a broad range of equivalent weights and a significant percentage of di- and polysulfonated components, and exhibit unique properties and phase behavior compared with the relatively narrow-equivalent-weight or pure sulfonates. They usually are fractionated during the oil displacement process. This paper introduces a method to calibrate the fractionated sulfonates by using two pseudosulfonate fractions obtained from a polarity partitioning technique. This technique, in conjunction with the high-performance liquid chromatography (HPLC) analysis, allows an artificial separation of BEW sulfonates into two major fractions, which simulate the oil-moving and the solubilizing components of sulfonate. The terms quasi-monosulfonate and quasi-disulfonate are used to characterize these oil-moving and sulfonate-solubilizing components, respectively. This calibration method was used successfully to determine sulfonate concentrations in the oil and aqueous phase effluents as well as the sulfonate retained on the rock surfaces. This has been valuable for interpreting the sulfonate retention and transport behavior. The retention measurements of quasi-monosulfonate were found to depend on micellar slug size, core length, and contact time. These results suggest that the effect of contact time may become significant in laboratory short-core tests with a small slug and should be an important consideration when interpreting the data. This paper also discusses the sulfonate propagation and displacement behavior of micellar systems with a BEW sulfonate in which the lower-phase microemulsion environment is predominant. It is shown that such a system can give effective oil displacement through the generation of low IFT, favorable phase behavior, and good mobility control. Furthermore, the performance of such a lower-phase system can be simulated easily by using independently measured input data, which include IFT, fluid viscosity, and sulfonate retention isotherm. This simulation provides a means to estimate sulfonate retention and also to optimize sulfonate use in micellar flooding. Micellar Fluid Systems and Experimental Details The primary surfactant used in all the formulations is a BEW vacuum gas oil (VGO) sulfonate. Table 1 lists the major components of the bulk sulfonate. The equivalent-weight distributions of VGO sulfonate components in this study range between 300 and 700. The cosurfactant was either isopropyl alcohol or an ethoxylated hexanol. A micellar formulation containing a VGO sulfonate and isopropyl alcohol is given in Table 2. The oil was a 4-cp [4-MPa's] field crude, and the in-place brine consisted of 0.25 N NaCl solution. Xanflood biopolymer was used as the mobility control agent. All tests were conducted at 110 degrees F [43.3 degrees C]. Oil displacement tests for the sulfonate propagation and retention studies were conducted in 2- and 6-ft [0.61- and 1.83-m] Berea cores at 2.3-ft/D [0.70-m/D] frontal advance rate. Sulfonate retention measurements in the absence of crude oil were conducted in 8-in. [20-cm] Berea cores. Details of these tests are given in Tables 3 through 5. Analyses of the core effluent components were accomplished by using HPLC, gas chromatography, and gel permeation chromatography. The fluid viscosity was measured by using a Brookfield viscometer with a UL adaptor. The IFT's were measured by using a spinning-drop interfacial tensiometer. SPEJ P. 435^
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