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
Published in Petroleum Transactions, Volume 207, 1956, pages 65–72. Abstract In quantitative interpretation of electrical logs the presence of clay minerals introduces an additional variable which further complicates an already complex problem. Although recognizing the difficulties introduced as a result of the heterogeneity of natural sediments and despite the present incomplete state of knowledge regarding electrochemical behavior of shades, disseminated clay minerals and concentrated electrolytes, it was felt that useful empirical correlations might be obtained from experimental investigation. Six typical sandstone formations, having a wide variety of petrophysical properties, were selected for the study. Approximately 45 samples from each formation were selected to satisfactorily represent the range of pore size distribution within the particular formation. As a matter of general interest, four limestone formations were also included in the investigation. Previously proposed equations relating to resistivity, SP and interrelationship of the two phenomena have, where possible, been tested with data obtained in this investigation. These equations do not satisfactorily describe experimental behavior of samples through all degrees of shaliness or throughout the range of brine solution resistivities normally encountered in logging practice. An empirical equation has been developed which quantitatively relates formation resistivity factor to saturating solution resistivity, porosity, and "effective clay content." This relation is indicated to be uniformly applicable to clean or shaly reservoir rocks. It is shown that both the SP and resistivity phenomena of shaly samples are related to the sample cation exchange capacity per unit pore volume. The independent chemical determination of this parameter is thus a means of determining the "effective clay content" of samples. Some implications regarding theory and electric log interpretation of shaly sands are discussed. Introduction The use of electrical resistivity logs as a means for estimating formation porosity is based upon the original work of Archie.
Mass-action equilibrium equations give a good description of cation-exchange effects in laboratory floods with solutions containing sodium, calcium, and magnesium cations. These equations can help when designing prefloods for surfactant and polymer processes. processes. Cation exchange in the presence of a surfactant system is found to be complicated significantly by interaction between surfactant and divalent cations. The evidence suggests that a divalent cation-surfactant "complex" may be a new exchangeable species. Both divalent cation and surfactant transport are described qualitatively by a model incorporating Langmuir chemisorption and a divalent cation-surfactant complex. Surfactant adsorption in Berea rock was reduced by a factor of one-fifth by reducing divalent-cation concentration in the surfactant from 300 ppm to zero, dissolving carbonate minerals from the core, and converting clays to the sodium form. Introduction Chemical flood performance is a relatively sensitive function of the ionic composition of a chemical system. Initial composition can be controlled, but control during traverse of the reservoir is extremely difficult. Recognized underground process that may alter ionic composition include mixing with in-situ waters, partition of dissolved gases and polar materials between the crude oil and the slug, dissolution of minerals, chromatographic lag of the surfactant, and cation exchange between the surfactant slug and reservoir clays. Wilson recently reviewed these and other in-situ processes affecting the performance of polymer, caustic, and surfactant floods. performance of polymer, caustic, and surfactant floods. Melrose et al. described a complex model involving six equilibria that explained substantial increases in the concentration of NaHCO3 observed during a pilot polymer flood. Partition, ionization, solubility, and cation-exchange processes were included in the model and, after due consideration of the additional effects of reservoir heterogeneity and fluid mixing, the authors estimated that in-situ viscosity of the polymer solution was only about 25% that of the injected solution. More recently, Smith discussed a model describing cation exchange during preflooding. For one planned project, he showed that 2 PV preflush were required before effluent divalent-cation concentration reached the injected level. He concluded that calcium and magnesium ions could be treated as a single specie and that idealized estimations of preflush efficiency could be made from the model, if adequate experimental information for the reservoir and pertinent waters were available. Predictive techniques pertinent waters were available. Predictive techniques resulting in a continuous description of the ionic environment of the chemical slug as it traverses the reservoir would allow more accurate performance prediction and provide improved slug-design criteria. This paper is a progress report on experimental efforts to understand and predict cation exchange and other interrelated equilibria. Companion papers describe the application of chromatographic principles to the problem and a simulator that, given the principles to the problem and a simulator that, given the correct equilibria description, can predict ionic environment. CATION-EXCHANGE ISOTHERMS Two-Cation Exchange Cation-exchange equilibria have been described by empirically and theoretically derived equations. Using double layer theory, Bolt derived a relation for sodium-calcium exchange that simplifies into (1) where C1 and C3 are the concentration of calcium and sodium in the equilibrium solution, respectively. C1 and C3 are corresponding concentrations on the clays, and the subscript g indicates Gapon equilibria. In this simplified form, the expression was derived much earlier by Gapon. Magistad et al. reported additional data supporting the applicability of the Gapon equation. SPEJ p. 445
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
A new approach to stabilizing clays has been developed that uses anoil-soluble surfactant to coat the clays with a tenacious film of oil.Two alternative formulations of the oil-coating treatment an emulsion of diesel oil and an aqueous system consisting of a dispersion of the surfactant in brine have been developed. Introduction In some waterflood operations it is necessary to injectfresh water when a suitable brine is not available or is toocostly. If the reservoir rock contains interstitial clay thatswells and disperses in fresh water, permeability may beimpaired and injection rates may be lowered. Many methods of stabilizing clays are based on ioniccomposition of the water or the use of hydrolyzed metalliccations. This paper describes a new approach to stabilizingclays that uses an oil-soluble surfactant to cause the clays to be coated with a tenacious film of oil.Following laboratory development, the oil-coatingtechnique was field tested. It is an effective andinexpensive method giving protection from fresh-waterimpairment. Two alternative formulations of the oil-coatingtreatment have been developed:an emulsion of diesel oil, containing the oil-wetting surfactant, in brine(referred to as the OC emulsion), andan aqueous systemconsisting of a dispersion of the surfactant in brine(referred to as the OC dispersion). The second method, which uses the residual oil in the formation to coat theclays, is less expensive and is generally preferable.However, it is not effective with certain heavy crudes, forwhich the first method must be used. Both methods areapplied as a batch treatment that affects only a small radiusaround the wellbore. Since the pressure drop decreasesunder conditions of radial flow with the log of the distancefrom the wellbore, a treated zone of about 10 ft isgenerally adequate. Usually, the treatment is preceded by an acid job so that the formation may be cleaned and broughtto maximum permeability before being stabilized. It is recommended that laboratory tests of the treatmentbe made before applying it in a new waterflood. This isbest done by measuring the relative permeability to freshwater of preserved cores from the reservoir at residual oilbefore and after treating with the oil-coating process. Theeffect and comparability of acid and acid additives alsoshould be checked. In principle, the optimum volume oftreatment may be determined by assuming various radii oftreated zones, substituting into the radial flow equationtreated and untreated permeabilities as determined from the cores, and comparing the resultant injection ratesand costs. Laboratory Evaluation of the Oil-CoatingSystems Oil-Coating Emulsion Testing The OC emulsion was formulated with two considerationsin mind:that it should be effective in stabilizingthe clays, andthat it should be fine enough to beinjected easily into the formation. The composition ofthe emulsion is given in Table 1 and the function of eachingredient is as follows. The agent that causes the sandgrains to become oil wet is Redicote 75 TXO, a cationicsurfactant. Another cationic agent, E(11), is added tostabilize the emulsion. A nonionic agent, E(12L), controlsthe fineness of the emulsion. The diesel oil or tolueneprovides a low-viscosity oil to coat the sand grains. JPT P. 1053^
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