pH-responsive viscoelastic supramolecular viscosifiers based on dynamic complexation of zwitterionic octadecylamidopropyl betaine and triamine for hydraulic fracturing applications

Abstract

Viscosity modifying agents are one of the most critical components of hydraulic fracturing fluids, ensuring the efficient transport and deposition of proppant into fissures. To improve the productivity index of hydraulic fracturing processes, better viscosifiers with a higher proppant carrying capacity and a lower potential of formation damage are needed. In this work, we report the development of a novel viscoelastic system relying on the complexation of zwitterionic octadecylamidopropyl betaine (OAPB) and diethylenetriamine (DTA) in water. At a concentration of 2 wt%, the zwitterionic complex fluid had a static viscosity of 9 to 200 poise, which could be reversibly adjusted by changing the suspension pH. The degree of pH-responsiveness ranged from 10 to 27 depending on the shear rate. At a given concentration and optimum pH value, the zwitterionic viscosifiers showed a two-orders-of-magnitude reduction in settling velocity of proppant compared to polyacrylamide solution (slickwater). By adjusting the pH between 4 and 8, the networked structure of the gel could be fully assembled and disassembled. The lack of macromolecular residues at the dissembled state can be beneficial for hydraulic fracturing application in avoiding the permeation damage issues encountered in polymer and linear-gel-based fracturing fluids. The reusability and the unnecessary permanent breakers are other important characteristics of these zwitterionic viscosifiers.

Fig. 1. (a) The reaction scheme for the formation of long-chain, large head-group zwitterionic amphiphile (OAPB) using stearic acid, DMPDA, and chloroacetic acid. (b) The supramolecular complexation of OAPB with triamine to yield viscoelastic gel.

Fig. 2. FTIR of long-chain zwitterionic amphiphile and its reactants in the frequency range of (a) 1000–2000 cm⁻¹ and (b) 2000–4000 cm⁻¹.

Fig. 3. Static (i.e., zero-frequency) viscosity of DBC suspension (2 wt%) involving OAPB and DTA building blocks and PAM (2 wt%) as a function of pH at 25 °C. Error bars indicate the relative error of static viscosity for three repetitions.

Fig. 4. The effect of temperature on the viscosity of DBCs (2 wt%) at (a) 25 °C, (b) 45 °C, (c) 65 °C, and (d) 90 °C for pH values of 2, 4, 6, 8, 10, and 12. Error bars indicate the relative error of viscosity for three repetitions.

Fig. 5. Natural logarithm of static viscosity versus inverse temperature for DBC suspension.

Fig. 6. Viscosity of DBCs (2 wt%) with respect to shear rate at different salinity value for (a) the suspension pH of 4, (b) the suspension pH of 6, (c) the suspension pH of 8, and (d) the suspension pH of 10. Error bars indicate the relative error of viscosity for three repetitions.

Fig. 7. The strain amplitude results show storage modulus G′ (solid) and loss modulus G′′ (empty) of DBC at pH 4, 8 and 10 with various strain (%) at 1 Hz.

Fig. 8. The storage modulus G′ (solid) and loss modulus G′′ (empty) of DBC at pH 4, 8 and 10 with various frequency.

Fig. 9. (a) Sand settling in DBC solutions of pH 4, pH 6, pH 8, pH 10 with time at room temperature (25 °C). (b) Sand settling in DBC solutions of pH 4, pH 6, pH 8, pH 10 with time at higher temperature (90 °C).

Fig. 10. (a) Settling velocity of sand in DBC suspension as a function of pH at 25 °C and 90 °C. (b) Comparison of sand settling velocity of DBC suspension and PAM solution (pH 4) at 25 °C and 90 °C. Error bars indicate the standard deviation for three repetition.

Fig. 11. The mean settling velocity of sand proppant as a function of salt concentration for the cases of pH 4 and pH 6 at 90 °C. Error bars indicate the standard deviation for three repetitions.

Fig. 12. Optical micrograph of a sand grain in DBC solution (a) at pH 4 and (b) at pH 8. (c) Zeta potential of sand in water, DBC suspension, and sand (divided by 20 to better show differing scales) in DBC suspension as a function of pH. The long, surface-aligned tubules (yellowish color) that are the supramolecularly assembled dynamic binary complexes localizing near the sand grain. Compared to pH4, the DBC solution at pH 8 contains no long tubules but large aggregates separated from the sand grain. The error bars indicate the standard deviation of triplicate measurements (see ESI for larger images).