Rheology and microstructure of zwitterionic-anionic surfactant for enhanced oil recovery

Locally Synthesized Zwitterionic Surfactants as EOR Chemicals in Sandstone and Carbonate

Zwitterionic surfactants are found to be highly effective in reducing the IFT and changing the wettability. This work studied the solubility and wettability alteration performance of locally synthesized zwitterionic surfactants in Berea sandstone and Indiana limestone. Contact angle measurements were conducted to study the wettability under different conditions. SEM images and TGA results were combined to reflect on the wettability alteration mechanism. The zeta potential test was adopted to study the surface charge of the Indian limestone powder. Results showed that five of the six surfactants dissolved in deionized water to form 1.0 wt % solution, indicating efficient solubility for EOR purposes. Although its wettability alteration performance on oil-aged Berea sandstone is weak to moderate, the performance of ZW6 on Indiana limestone is excellent. ZW6 can change the strongly oil-wet (162°) rock back to water-wet (62.9°) conditions. Increasing its concentration from 0.01 to 0.5 wt % continuously enhanced the performance. The addition of NaCl to 150000 ppm did not affect the wettability alteration. However, the addition of CaCl2 largely suppressed the wettability alteration, while Na2SO4 and MgCl2 both enhanced the performance. With the same headgroup, a more hydrophobic tail group impairs the wettability alteration. The quite different wettability alteration performance of MgCl2 and CaCl2 cases (which had approximately the same amount of calcite dissolution), and the comparable wettability alteration performance of Na2SO4 and MgCl2 (which had very different calcite dissolution amounts) indicate that calcite dissolution is unrelated to wettability alteration.

Study on the relationships between the oil HLB value and emulsion stabilization

In order to study the relationship between the HLB value of oil and emulsion stabilization, the optimal formation of emulsification system was determined, and then, the properties of emulsion, such as particle size, stability, interfacial tension and zeta potential, were tested by laser particle analyzer, stability analyzer, and interfacial tensiometer. Experimental results showed that the optimal ratio of emulsification was Tween 80 : Span 80 = 5 : 5. Meanwhile, when the HLB value of the emulsification system was close to that of oil, the emulsion exhibited the best stability. This phenomenon is due to the fact that when the HLB values are close, the surfactant molecules are arranged more closely on the oil-water interface, leading to smaller sized emulsion droplet, which is conducive to emulsion stability. This study provides new insights into the effective adjustment of emulsion stability.

Gradual transformation of anionic/zwitterionic wormlike micelles from viscous to elastic domains: Unravelling the effect of anionic surfactant chain length

HYPOTHESIS

Ultra-long tailed zwitterionic surfactants often form aqueous wormlike elastic micelles, whereas the shorter ones mainly exhibit spherical viscous micelles. Anionic surfactants are widely used to tune the micellar morphology from spherical into wormlike. Systematic investigations in the molecular level are insightful to understand the viscoelasticity regulative mechanism.

EXPERIMENTS

Anionic/zwitterionic hybrid wormlike micelles are composed of sodium alkylsulfate (SAS) homologues and dodecyl dimethyl amidopropyl hydroxyl sulfobetaine (DHSB). The formation of wormlike micelles was studied by employing rheometer, cryogenic transmission electron microscopy (cryo-TEM) and small angle X-ray scattering (SAXS) techniques. The effects of surfactant concentration, molar ratio, anionic surfactant chain length and temperature were investigated systematically.

FINDINGS

SAS promoted the formation of SAS/DHSB hybrid wormlike micelles. The increase in both chain length and molar ratio (x_SAS) of SAS are advantageous in the enhancement of viscosity. Interestingly, sodium hexadecylsulfate (SHS) endowed elastic wormlike micelles with thermally insensitive viscosity below its Krafft temperature (T_k), which was distinguished from the viscous ones formed by sodium octylsulfate (SOS). SAXS results showed that the size of SAS/DHSB wormlike micelles was primarily determinate by surfactants with longer hydrophobic tails.

Effect of Rheological Properties of Polymer Solution on Polymer Flooding Characteristics

Polymer flooding is an appropriate enhanced oil recovery (EOR) process that can increase macroscopic sweep efficiency. We examined two polymeric superpushers at different salinities (10,000 and 42,000 ppm of NaCl and 18,000 ppm of CaCl₂) and temperatures (30 to 75 °C) as polymer-flooding agents for the EOR process. Rheological and thixotropic tests were attempted to find shear viscosity change when the polymer solutions were introduced under different salinity and temperatures, followed by describing the rheological behavior with the two most common rheological models used for polymer solutions, and then a quadratic model with Design-Expert to detect the effective parameters. Core flooding tests were conducted afterward to determine the final proposed fluid. An increase in the concentration of monovalent ions and the addition of divalent ions adversely affected both types of polymers used, which was similar to the effects of a temperature increase. The Flopaam 3630S at 1000 ppm has more stability under harsh conditions and enables 22% and 38% oil recovery in carbonate and sandstone core rocks, respectively. Consequently, Flopaam 3630S can be the perfect polymer agent for different chemical flooding procedures in high-salinity oil reservoirs.