ADVANCES IN FORMULATING SURFACTANT SOLUTIONS COUPLED TUNABLE VISCOELASTIC CHARACTERISTICS AND OPTIMAL IFT CONSIDERATIONS

Abstract

AbstractDevelopments of surfactant microemulsion systems for residual oil recovery have been carried out for several decades, with only limited success in field pilot scale tests. Further improvements of surfactant formulations, such as in the exploitation of suitable viscoelastic characteristics applying certain types of extended surfactants (i.e., alkylpropoxy sulfates and carboxylates) offer unique opportunities to capture some portion of Original Oil In Place (OOIP) entrapped after waterflooding. In a series of our prior investigations, the unique viscoelastic behaviors associated with certain surfactant-only formulations could be successfully formulated through classical Winsor Type III microemulsion designs, and in parallel adjusting distinctly the overall rheological properties. Prior generation of the developed surfactant-only formulations offered initial evidence capable to prevent any inverse formation blockage and/or lead to undesirable viscous fingering phenomena in subsurface. The current study is intended to further exploit favorable viscoelastic characteristics and designs of the Winsor microemulsion systems. The candidate systems were formulated by combining mostly with primary surfactants, such as Aspiro S 8710 (A) or Aspiro S 8310 (E) (carboxylates (-COO-)) with one of the secondary surfactant candidates, including anionic surfactants, such as dialkyl diphenol oxide disulfonates (Calfax 16L-35), alkyl ethoxylate sulfate (Steol CS-460), internal olefin sulfonate (Petrostep S2), and alkyl benzene sulfonate (SDBS), at various surfactant concentrations (a total of 0.5-2 wt% surfactant) and conducted generally with the salt scans of NaCl (between 11-25 wt%) aimed for achieving the optimal Winsor Type III with the measured IFT values of 10-2-10-3 mN/m (dyne/cm). Results of the single (primary) surfactant systems (at room temperature) showed mostly belonged to either Type I or Type II system with few opaque-only Type III (close to 18.8 wt% NaCl concentration) for 1 wt% solution of Aspiro surfactant systems. In contrast, most binary surfactant systems also resulted in similar behaviors as single surfactant systems concentration with various concentration at 1.5 and 2 wt% of total surfactants also possessing several opaque middle phases optimal Winsor III microemulsions in the binary mixtures were successfully formulated by varying surfactant concentrations in different salt levels and temperature conditions. Once ultra-low IFT characteristics been confirmed, supplementary experiments were done to investigate the effects of induced oil introduced in the recipes on the ultra-low IFT systems to alter their rheological properties (based on the results of the storage and loss moduli) as determined by a Discovery Hybrid Rheometer (DHR – TA instrument). The apparent viscosity of the microemulsion increased with salinity, ranging from 4.55 to 10.88 Pa·s as NaCl concentration rose from 11 to 15 wt% for 0.5 wt% E/D. Both the storage modulus (G’) and loss modulus (G”) showed frequency dependent growth across 0.001–100 rad/s, with G’ values between 1,200 and 4,200 Pa at 100 rad/s. This viscoelastic response reflects the progressive entanglement of micellar structures, leading to the formation of stiffer wormlike networks capable of resisting deformation at short timescales while dissipating energy through viscous relaxation. The systems with good viscoelastic nature (G’ >> G”) were selected for 1-D column studies to quantify the net recovery rate of residual oil. Results of sand pack study revealed the recovery factors of the new viscoelastic (VES) formulations improved by averaging of a factor of 16, and 40 % of better sweep efficiency compared to the oil-free system (SUF) waterflooding recoveries respectively. The improved surfactants-only viscoelastic formulations may offer some technically and economically favorable alternatives and evidence for further pilot test.