Preparation and performance evaluation of viscoelastic zwitterionic surfactant for cleaning fracturing fluids

In response to the limitations of traditional guar gum fracturing fluids, characterized by inadequate proppant carrying capacity and substantial reservoir damage, we have synthesized a zwitterionic surfactant EAPHS using ethyl acid (EA), N,N-dimethyl-1,3-propanediamine (DMAPA), and 3-chloro-2-hydroxypropyl sulfonic acid sodium salt. Through the optimization of the NaSal counterion salt ratio, we developed a viscoelastic surfactant (VES) clean fracturing fluid system, with the optimal formulation identified as 4.0 % EAPHS + 3.0 % NaSal. The research results indicate that the critical micelle concentration (CMC) of the EAPHS surfactant is 8.3 × 10⁻⁴ mol/L, with the surface tension ${\gamma }_{\mathrm{CMC}}$ is 38.5 mN/m. The observation results from cryogenic transmission electron microscopy (Cryo-TEM) indicate the formation of worm-like micelles in the VES clean fracturing fluid system, facilitating viscosity enhancement and sand-carrying. The viscosity of the VES clean fracturing fluid system remains stable at 34 mPa·s after undergoing continuous shearing at 120 °C and 170 s⁻¹ for 60 min. Furthermore, this system exhibits favorable rheological properties and sand-carrying capabilities. The drag reduction efficiency of the VES clean fracturing fluid system exceeds 60 %, demonstrating excellent micelle-breaking performance. Moreover, the damage rate to the permeability of natural cores caused by the breaking fluid is only 7.34 %. This indicates the low-damage characteristics of the VES clean fracturing fluid system, effectively reducing secondary damage to reservoirs after fracturing. It provides valuable reference and guidance for the further application of VES clean fracturing fluid systems in oilfield operations.