A slug containing 2 percent sulfonate, polymer for mobility control, and sodium tripolyphosphate for multivalent ion control - the entire system driven by a polymer solution - recovered 85 percent of the residual oil from a waterflooded Berea core. Because the surfactant adsorption is low, it appears that the process may be economically feasible for tertiary recovery of oil. Introduction In 1927, Uren and Fahmy concluded that an inverse relationship exists between oil-water interfacial tension and the percentage of oil recovered by waterflooding. In that same year a patent was issued to Atkinson that proposed the patent was issued to Atkinson that proposed the use of aqueous solutions of soap or other materials to decrease the "surface tension" between oil and the flooding medium and thereby increase the recovery of oil. During the next 25 years a major part of the reported research on the use of surfactants to recover oil was carried out by a group at Pennsylvania State U. This group recognized that interfacial tension. wetting conditions (contact angle), and surfactant adsorption were important factors. Preston and Calhoun discussed chromatographic transport of surfactants through porous rocks. Ojeda et al., and Paez et al., correlated residual oil saturation in Paez et al., correlated residual oil saturation in cores after a surfactant flood with sigma/Delta p and with a pore geometry parameter (k phi)1/2. These correlations pore geometry parameter (k phi)1/2. These correlations indicated that residual oil saturation could go to zero at values of sigma/Delta p approaching zero. Excluding patent literature, published research results for the last two decades have discussed the screening of surfactants for oil recovery "efficiency", changing wettability to improve oil recovery, adsorption and chromatographic transport of surfactants, and the role of major factors known to affect oil recovery by aqueous surfactant flooding. A comprehensive study directed primarily toward the influence of interfacial tension was reported by Reisberg and Doscher in 1956. Using a system that combined an aqueous alkali with a surfactant, they measured interfacial tensions less than 0.01 dynes/cm. With this same chemical system they recovered 100 percent of the oil from sand packs and more than 90 percent from Torpedo sandstone cores. Wagner and Leach showed that oil recovery increases when interfacial tension is reduced to about 0.07 dynes/cm and that further small decreases in interfacial tension result in large increases in oil recovery. In 1968, Taber presented theoretical and experimental results that presented theoretical and experimental results that further clarify the relation between residual oil saturation and Delta p/L sigma. He recognized that this correlation group should include the contact angle, but because of difficulties in systematically varying or indeed in even measuring the contact angle inside rocks, he examined the effects of the other parameters only. For Berea cores Taber found that a significant quantity of discontinuous oil (residual) was displaced when the ratio of Delta p/L sigma reached a value of about 5 (psi/ft)/(dynes/cm). He designated this as the critical value of the ratio and noted that further increases in the value of this ratio invariably produce more residual oil. He concluded that nearly 100 percent of the residual oil can be displaced if very high values of Delta p/L sigma, can be obtained. Gogarty and Tosch have recently discussed the principles of micellar surfactant systems. principles of micellar surfactant systems. JPT P. 186
Studied was a novel technique for mobility control during chemical flooding, namely, the injection of alternate slugs of inert gas and surfactant solution. This method of reducing mobility offers a possible alternative to polymer thickened water. Performance was investigated in consolidated sandstone and carbonate. Sandstone included cores, and large slabs of rock oriented so as to maximize gravity segregation of gas. Reported data suggest that, for mobility control in laboratory rocks, performance of alternate slugs of gas and dilute surfactant compares favorably with water soluble polymers, without the many disadvantages of the latter. During experiments in large sandstone slabs, excessive foam drainage, and gravity segregation of gas, did not occur. Introduction Surfactant enhanced waterflooding (micellar flooding) is an oil recovery method for reservoirs depleted by waterflood. The preferred method of conducting micellar floods is to:inject a small (less than 1 Vp) slug of relatively concentrated surfactant, andfollow with a less expensive drive liquid. The surfactant slug lowers oil-water interfacial tension, promotes oil-water miscibility, and reduces So. If the surfactant slug is effective, a reconnected oil bank is generated, and driven, by the slug, which is, in turn, displaced by the drive. Since surfactant slugs are often as small as 0.1 Vp, or perhaps less, formation of viscous fingers can lead to bypassing, dilution, and process breakdown. For process efficiency, the process breakdown. For process efficiency, the mobility of the slug and drive must be less than the mobility of the displaced liquids. Mobility control in the surfactant slug is accomplished either by injecting the surfactant as an inherently viscous microemulsion or soluble oil, or by the addition of water soluble polymer. Drive is generally polymer thickened water. Dilute polymer solutions are used as drives because of economy, and the lack of suitable alternatives. The two classes of polymers used most frequently are polyacrylamides and Xanthan gums; neither is ideal. At elevated temperatures, both can degrade with a consequent loss of solution viscosity. Both can form gels with multivalent cations, and can flocculate finely divided solids. The result is injection well plugging. In addition, polyacrylamide is degraded by mechanical shear at polyacrylamide is degraded by mechanical shear at shear rates low enough to preclude injection through chokes, and, in many instances, through perforations. As marketed, Xanthan gums contain bacterial residue which must be removed by filteration. They are also a favorite diet of bacteria, and bacterial growth must be inhibited. It is not known for certain whether either class of polymer can be transported totally intact through a reservoir. In this paper, we present a different method for achieving mobility control during chemical flooding, namely, the injection of alternate slugs of gas and surfactant solution. The object is to generate, in porous media, a low mobility dispersion of gas in liquid. For brevity, we shall refer to the dispersion of gas in liquid as foam. Foam has been suggested as a means for providing mobility control for a variety of processes, providing mobility control for a variety of processes, e.g. flooding with CO2, steam, solvent, or even water. It offers a unique advantage, however, in the case of chemical flooding. The surfactant adsorption requirement of the reservoir rock is satisfied by the micellar slug.
JPT Forum articles are limited to 1,500 words including 250 words for each table and figure, or a maximum of two pages in JPT. A Forum article may present preliminary results or conclusions of an investigation that the present preliminary results or conclusions of an investigation that the author wishes to publish before completing a full study; it may impart general technical information that does not war. rant publication as a full-length paper. All Forum articles are subject to approval by an editorial committee. Letters to the editor are published under Dialogue, and may cover technical or nontechnical topics. SPE-AIME reserves the right to edit letters for style and content. The ionic composition of surfactant systems is an important determinant of system performance. This is reflected by the use of prefloods to displace formation waters considered too saline and/or too "hard" and by limitations placed on the ionic composition of the water used in the surfactant fluid. Recent literature reflects a growing concern about the role of the reservoir clay in determining the in-situ composition of the surfactant formulation. In this article we show a laboratory example of a low-salinity preflood that, because of cation exchange with clays, results in a threefold increase in divalent cation (Ca++ and Mg++) concentration in the front of the surfactant flood, We outline the basic concepts required to model the reversible changes in cationic composition that result from cation exchange and indicate the general direction of our research efforts on this important aspect of surfactant flooding. Disaggregated Tar Springs sandstone with a cation exchange capacity of 0.44 meq/100 gm was used to prepare a 1 -in.-diameter, 2-ft-long sand pack. We prepare a 1 -in.-diameter, 2-ft-long sand pack. We saturated the pack with a saline Tar Springs formation water (Fig. 1) and flowed 3.8 PV of this water through the pack at a frontal advance rate of 1 ft/D and a temperature of 85 deg. F. We then displaced the formation water with a "preflood" containing 7.5 percent formation water mixed with a fresh lake water. After 1.5 PV of preflood injection, steady-state composition was achieved. The flood was continued to a total preflood injection of 4.4 PV. After this pack equilibration period, 3.7 PV of a PV. After this pack equilibration period, 3.7 PV of a chemical slug prepared in lake water containing 5 percent formation water were injected. Steady-state composition was achieved after 1.5 PV. As shown in Fig. 1, the preflood "loaded" the clays with calcium and magnesium. The "denuded" chemical slug water and the front portion of the chemical slug "unloaded" these cations from the clays. The concentration of divalent cations in the front part of the chemical slug increased from 0.0153 to 0,051 meq/ml. Calculated as Ca++ only, this is equivalent to an increase from 306 to 1,020 ppm, an amount sufficient to radically alter interfacial and viscometric properties of the system. Various equations describing the relation between cation distribution on a clay surface and composition of the equilibrium solution have been given in the literature. For the experiment reported, simple mass-action equations adequately describe the equilibria between clays and brine solutions but, as used, do not adequately do scribe the equilibria with the surfactant system. These equations are (1) and (2) where C and C are the concentrations of the indicated cation associated with the clay and equilibrium solution, respectively, both expressed as milliequivalents per milliliter of pore volume. Applicable values of K and K may vary with the type of clay, an ionic strength parameter. and temperature. Thus, each reservoir may present a parameter. and temperature. Thus, each reservoir may present a new experimental evaluation problem. P. 1336
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