Surfactant-Enhanced Alkaline Flooding: Buffering at Intermediate Alkaline pH
- Rudin Jeff (Illinois Inst. of Technology) | Darsh T. Wasan (Illinois Inst. of Technology)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- November 1993
- Document Type
- Journal Paper
- 275 - 280
- 1993. Society of Petroleum Engineers
- 4.1.5 Processing Equipment, 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 5.2.1 Phase Behavior and PVT Measurements, 4.1.2 Separation and Treating, 5.8.7 Carbonate Reservoir, 5.4.1 Waterflooding, 2.5.2 Fracturing Materials (Fluids, Proppant), 4.3.3 Aspaltenes, 5.4.10 Microbial Methods
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An experimental investigation of buffered alkaline flooding system chemistry was undertaken to determine the influence of various species present on interfacial tension (IFT) as a function of pH and ionic strength. IFT was found to go through an ultralow minimum in certain pH ranges. This synergism results from simultaneous adsorption of unionized and ionized acid species on the interface.
The alkaline flooding process involves injecting alkaline agents into the reservoir to produce more oil than is produced through conventional waterflooding. The interaction of the alkali in the flood water with the naturally occurring acids in the reservoir oil results in in-situ formation of soaps, which are partially responsible for lowering IFT and improving oil recovery. The extent to which IFT is lowered depends on the specific oil and injection water properties. Numerous investigators1-20 have attempted to clarify the relationship between system chemical composition and IFT.
Low IFT is necessary for improved oil recovery, and many regard it as the primary mechanism in alkaline flooding. However, low IFT alone is not sufficient to improve recovery; other phenomena may enhance or hinder the recovery process. Such mechanisms as emulsification and entrainment,20 emulsification and entrapment,15,19 and emulsification and coalescence21,22 (all of which require low IFT) could facilitate oil recovery. Wettability reversal is another mechanism that is reported to enhance oil displacement.19 The presence of alkali can reduce surfactant losses to the rock surface23-25 and precipitate divalent ions,23,25,26 thus improving oil-bank propagation.
Currently, ionic strength is believed to be the governing mechanism for the minimum in IFT when alkali concentration is increased. An increase in pH produces surface-active soaps, which lower the IFT. A further increase in alkali increases the IFT because the alkali acts as an electrolyte, which increases the ionic strength and either partitions the salt of the ionized acid into the oil11 or precipitates it.3 Although IFT is influenced by. ionic strength, we will show that pH is equally or more important for producing the minimum because the un-ionized acid in the acidic oil contributes to the IFT in conjunction with the ionized acid and the extent of acid ionization is governed by pH, not ionic strength.
Most investigators1-24 use a single alkaline chemical that buffers at a specific pH (i.e., NaOH, NaHCO3, Na2CO3, Na4SiO4, etc.); however, this pH may not be the optimal pH to obtain ultralow IFT. To get the optimal pH, the alkaline solution must be formulated at low concentrations if the buffering pH of the alkali is above optimal, which is usually the case. The alkali concentration then is too low to survive transport through the reservoir owing to the influence of external factors, such as divalent ions, acids, rock, and dispersion. This paper describes a way to lessen the influence of these external factors. The alkalis should be mixed (i.e., NaHCO3 with NaOH or with Na2CO3) to obtain the optimal pH at alkali concentrations that can survive transport through the reservoir. Note that high alkali concentration does not mean high pH.
EOR literature seems to lack information on the use of mixed alkalis. Some investigators25-27 have used mixed alkalis; however, they used a fixed chemical composition, such as sodium sesquicarbonate (NaHCO3/Na2CO3 with a molar ratio of unity) and silicates with select SiO2/Na2O ratios at a fixed pH, which may not be optimal for the particular crude oil used. The patent literature28-32 also contains information. on the use of mixed alkalis to enhance oil recovery.
Despite numerous investigations in this area, we still do not have a basic understanding of the fundamental mechanisms. This work illustrates a new approach to enhancing oil recovery through in-situ-generated surfactants augmented by chemical injection.
Materials and Experiments
In this study, oil containing natural organic acids was put into contact with an alkaline-pH aqueous solution. Long Beach crude oil was obtained from THUMS Long Beach Co.; it has an acid number of 1.0 determined by American Soc. for Testing Materials (ASTM) Procedure D-664 and 9-g/cm3 gravity. The oil was centrifuged at 40,000 G for 30 minutes to remove water and clays. The oil viscosity after centrifuging was 52 mPa·s at 25°C, and IFT against deionized water was 24.5 mN/m.
The alkaline solutions were a mixture of NaOH, NaCl, and Trona (a 94% NaHCO3 industrial-grade chemical). All alkalis except Trona were obtained from Fisher Scientific Co.; Trona was obtained from Kerr-McGee Chemical Co.
A preformed surfactant, the petroleum sulfonate Petrostep B-105 (55% active), obtained from Stepan Chemical Co., was also added to the alkaline solution. Surfactant solutions were made on a 100%-active basis.
Pre-equilibrated samples were made by stirring equal volumes of aqueous and oil phases for at least 6 hours, then allowing them to equilibrate for 1 week or more. The spinning-drop technique was used to measure equilibrium and transient (nonequilibrated) IFT's. Equilibrium IFT readings were made after the IFT reached a steady value between 0.5 and 8.0 hours. The volumetric WOR in the spinning-drop tensiometer is ˜140. If all the acid in the oil drop went into the water phase during transient IFT measurements, the ionized acid concentration would be 0.12 mol/m3. Percent transmittance was measured with a Beckman spectrophotometer set at 600 nm, with deionized water as the reference. pH was measured with an Orion 901 microprocessor analyzer with a Ross combination electrode designed for low sodium error. All experiments were performed at 25±1°C.
The solutions were prepared by diluting an equimolar ratio of Trona/NaOH (the 20/20 mixture) with either the same molarity of Trona or NaOH plus sufficient NaCl to keep the total sodium constant. Changing the Trona/NaOH ratio changes the pH, so that adding Trona/NaCl solution to the 20/20 mixture lowers the pH or adding NaOH/NaCl solution to the 20120 mixture raises the pH. The buffering ability was investigated by diluting a constant molar ratio of Trona/NaOH solution containing 343 mol/m3 total sodium with 343 mol/m3 NaCl. Note that 343 mol/m3 total sodium is ˜2.0 wt% Trona/NaOH mixture.
Results and Discussion
Several investigators33-36 identified the carboxylic acids naturally present in some crude oils. Chong and McKay33 analyzed Green River oil shale and found that the extracted carboxylic acids consisted of 56% n-carboxylic, 20% dicarboxylic, 12% branched carboxylic, and 12% aromatic carboxylic acids, with carbon numbers from 11 to 34. These carboxylic acids have been proved to be responsible for the high interfacial activity observed.34-36 Other components, such as asphaltenes36 and phenols,34,35 do not lower IFT in the basic pH range.
In this study, acids present in Long Beach crude oil are expected to consist of varying chain lengths and types. When the acids are ionized into the alkaline solution, they will form mixed micelles, giving rise to an effective critical micelle concentration (CMC) for the system. In this paper, therefore, CMC refers to the effective system CMC, not the CMC of individual ionized acid molecules.
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