A new series of nonionic surfactants with double hydrocarbon tails and a single polyoxyethylene (POE) chain 1,3dialkyl glyceryl ether ethoxylates with an alkyl length between diC 8 and diC 12 and ethylene oxide (EO) number between 3.9 and 25.6 were synthesized and characterized. The correlation of their surfactant−polymer (SP) flooding behavior with the alkyl length and EO number were studied. Although the individual surfactants can be made highly surface-active by matching the alkyl lengths with an appropriate EO number as reflected by the low critical concentration range of 6 × 10 −7 to 9 × 10 −5 mol/L and γ cmc as low as 26.5−30.5 mN/m at 25 °C, they are not good surfactants for SP flooding when used alone since ultralow interfacial tension (IFT) between Daqing crude oil and connate water can only be achieved by a few members with short POE chains or hydrophile−lipophile balance (HLB) numbers near 7−8, which are poorly soluble in water. However, those with long POE chains exhibit excellent synergism in reducing Daqing crude oil/connate water IFT by mixing with a highly hydrophobic surfactant, didodecylmethylhydroxylpropyl sulfobetaine (diC 12 HSB). Ultralow IFT between 10 −3 and 10 −4 mN/m can be achieved in a wide range of total surfactant concentrations (0.3−10 mM) at 45 °C. The binary mixed systems also possess good resistance against adsorption by Daqing sandstone and the ability to maintain negatively charged surfaces water-wet over a wide concentration range, as well as a tendency to form crude oil-in-water (O/W) emulsions, which are all beneficial for high oil recovery in SP flooding. This synergism between nonionic surfactants and the sulfobetaine surfactant may be responsible for the excellent behavior of binary mixtures and the double alkyl chains in nonionic surfactants, which enhance the interaction between the surfactant monolayer and crude oil, making them superior to the conventional nonionic surfactants with a monoalkyl tail. Although the nonionic surfactants with long POE chains and varying alkyl lengths (diC 8 −diC 12 ) behave similarly, those with diC 8 alkyls are generally superior due to their excellent properties and relatively low molar mass (less dosage).

Enhanced oil recovery by chemical flooding has been a main measure for postponing the overall decline of crude oil output in China, and surfactant-polymer (SP) flooding may replace alkali-surfactant-polymer flooding in the future for avoiding the undesired effects of using caustic alkali. In this paper a series of double alkyl nonionic surfactants, 1,3-dioctyl glyceryl ether ethoxylates (diC 8 GE-E n , n=4.6-15.8), were synthesized and characterized and the effects of EO number on their properties were evaluated. The results show that diC 8 GE-E n are highly surface active as reflected by their low cmc (<2×10 -5 mol/L) and low γ cmc (<30 mN/m).Specifically the member with shortest EO chain (diC 8 GE-E 4.6 ), which gives a saturated adsorption (5.81×10 -10 mol cm -2 ) nearly double that of others (2.94~2.63×10 -10 mol cm -2 ) at air/water interface, can reduce Daqing crude oil/connate water interfacial tension (IFT) to ultralow (<0.01 mNm -1 ) solely at 5 mM at 45°C.The members with larger EO numbers, although can not reduce Daqing crude oil/connate water IFT to ultra low solely, are very good hydrophilic components superior to typical monoalkyl nonionic surfactant, such as polyoxyethylene (10) monododecyl ether (C 12 E 10 ), when mixed with a hydrophobic surfactant, didodecyl methyl hydroxylpropyl sulfobetaine (diC 12 HSB), for SP flooding free of alkali. At optimum formulation the molar fraction of diC 8 GE-E n can be decreased from 0.85 to 0.45 with increasing n from 8.0 to 15.8, and ultra low IFT can be easily achieved at wide effective concentration range (0.3~10 mM), thanks to the larger interaction of the double alkyl chains in the molecules with the oil. Derived from commercially available materials and produced by commercially realizable technology these surfactants are therefore practically significant for SP flooding.

Two series of double-chain single-head nonionic surfactants, Guerbet alcohol ethoxylates (C m GA–E n ; m = 16 and 20, n = 5.7–24.0), were synthesized, and their performances as surfactants for surfactant–polymer (SP) flooding free of alkali were evaluated. The products have relatively low critical micelle concentrations [(6.8 × 10–7)–(1.0 × 10–5 mol/L)] and a wide range of surface tensions at the critical micelle concentration (γcmc) values (30.3–38.0 mN/m) at 25 °C, depending on both the alkyl-chain length and the number of ethylene oxide (EO) units. When used alone, these nonionic surfactants are not efficient for reducing crude oil/connate water interfacial tension (IFT), but their binary mixtures with a highly hydrophobic zwitterionic surfactant, didodecylmethyl hydroxylpropyl sulfobetaine (diC12HSB), are very efficient in reducing Daqing crude oil/connate water IFTs. Ultralow IFTs can be achieved in a wide total concentration range (0.3–10 mM) at 45 °C, and the low limit (0.3 mM) can be further reduced to 0.15 mM by adding an anionic surfactant, α-olefin sulfonate (AOS), to the binary mixtures at a small mole fraction (0.1). Both the binary and ternary mixtures display good resistance against adsorption by Daqing sandstone, and when dissolved in connate water, they prefer to keep the negatively charged rock surface water-wet and produce a crude-oil-in-water emulsion. In addition, the incorporation of AOS into the binary mixtures was found to be beneficial for preventing crystallization of zwitterionic surfactant and flocculation of nonionic surfactants, as reported previously. Comparatively, for SP flooding free of alkali, the Guerbet alcohol ethoxylates are as excellent as the glyceryl ether ethoxylates, nonionic surfactants with a similar double-alkyl single-head structure, but are far superior to the conventional nonionic surfactants with monoalkyl single-head structures.

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A new series of sulfobetaine surfactants with double‐chain single‐head structure were derived from 1,3‐dialkyl glyceryl ethers and their performances in reducing Daqing crude oil/connate water interfacial tension (IFT) in the absence of alkali were studied. With a large hydrophilic head group and double hydrophobic chains, these surfactants are efficient at reducing crude oil/connate water IFT. Those with didecyl and dioctyl are good hydrophobic surfactants that can reduce Daqing crude oil/connate water to ultra‐low IFT by mixing with a small molar fraction of various conventional single‐chain hydrophilic surfactants, such as α‐olefin sulfonates, dodecyl polyoxyethylene (10) ether, and cetyl dimethyl hydroxypropyl sulfobetaine. The asymmetric double‐chain sulfobetaine derived from 1‐decyl‐3‐hexyl glyceryl ether can reduce Daqing crude oil/connate water IFT to ultra‐low solely over a wide concentration range (0.03–10 mM or 0.0017–0.58 wt.%), which allows for use of an individual surfactant instead of mixed surfactants to avoid chromatographic separation in the reservoir. In addition, formulations rich in sulfobetaine surfactants show low adsorption on sandstone, keeping the negatively charged solid surface water‐wet, and forming crude oil‐in‐water emulsions. These new sulfobetaine surfactants are, therefore, good candidates for surfactant‐polymer flooding free of alkali.