Self-Assembled Viscoelastic Surfactant Micelles with pH-Responsive Behavior: A New Fracturing-Displacement Integrated Working Fluid for Unconventional Reservoirs

Abstract

The integrated fracturing and oil recovery strategy is a new paradigm for achieving sustainable and cost-effective development of unconventional reservoirs. However, a single type of working fluid cannot simultaneously meet the different needs of fracturing and oil displacement processes. Here, we develop a pH-responsive fracturing-displacement integrated working fluid based on the self-assembled micelles of N,N-dimethyl oleoamine propylamine (DOAPA) and succinic acid (SA). By adjusting the pH of the working fluid, the DOAPA and SA molecules can be switched repeatedly between highly viscoelastic wormlike micelles and aqueous low-viscosity spherical micelles. The zero-shear viscosity of the working fluid enriched the wormlike micelles can reach more than 93,100 mPa·s, showing excellent viscoelasticity and sand-carrying properties. The working fluid is easy to gel-break when it encounters oil, generating a low-viscosity liquid without residue. In addition, the system has strong interfacial activity, which can greatly reduce the oil-water interfacial tension to form emulsions and can achieve reversible demulsification and re-emulsification by adjusting pH. Through the designed and fabricated microfluidic chip, it can be visualized that under the synergistic effect of viscoelasticity and interfacial activity DOAPA/SA can effectively expand the swept volume of tight fractured formations, promote pore wetting reversal and crude oil emulsification, and improve the displacement efficiency. The DOAPA/SA meets the design requirements of the fracturing-displacement integrated working fluids and provides a novel method and idea for constructing the integrated working fluids suitable for fracturing and displacement in unconventional reservoirs.

Figure 1

Molecular structural formulas of DOAPA and SA.

Figure 2

(A) Steady shear rheology and (B) zero-shear viscosity of DOAPA/SA at different SA concentrations. (C) Steady shear rheology with different concentrations of DOAPA/SA. (D) Frequency sweep rheology and Cole–Cole diagram of DOAPA/SA.

Figure 3

(A) Steady shear rheology and (B) zero-shear viscosity of the DOAPA/SA with different pH. (C) Variation of zero shear viscosity with the number of cycles by changing pH. (D) Frequency sweep rheology of DOAPA/SA with different pH.

Figure 4

(A) Cryo-TEM micrograph of DOAPA/SA solution at pH 6.15 and (B) at pH 7.56.

Figure 5

(A) Steady shear viscosity plots of DOAPA/SA with different temperatures. (B) Viscosity variation curves of DOAPA/SA with shear time. (C) Variation of proppant settling velocity with temperature. (D) Breaking time and viscosity of fracturing fluid vary with kerosene content.

Figure 6

(A) Optical microscopic images of gel-broken fluids. (B) Interfacial tension of DOAPA/SA at different pH values. (C) Interfacial tension of DOAPA/SA at different concentrations. (D) O/W emulsion appears at different pH values.

Figure 7

(A) Fabrication templates and microscopic characterization of microfluidic chips. (B) Real-time recorded images of microfluidic chip pore throat wetting transformation and oil droplet stripping process. (C) Real-time recorded images of the exfoliation process from a macroscopic viewpoint. (D) Cumulative sweep efficiency and oil displacement efficiency as a function of injection volume. (E) Variation curves of DOAPA/SA imbibition capacity with time at different temperatures.