Recent developments in horizontal drilling and fracking technology have had a crucial role in increased shale gas and oil production in the U.S and the world. Natural resources kept in low permeability reservoirs, namely shale formations, can be recovered by a well-designed hydraulic fracturing process. Optimization of production and efficiency in hydraulic fracturing significantly depend on properties and efficiency of fracturing fluids. This work presents a pH-responsive supramolecular assembly solution system synthesized via complexation of maleic acid with an amino-amide in an aqueous media. Our previous studies on this system have verified its significant potential to be used as displacement fluids in EOR due to its highly adjustable and reversible viscosity behavior. Herein, we investigated its potential as fracturing fluids considering its mobility control, and proppant transfer and settling capacities.

The main protocol is: the viscosity of injected supramolecular solution is set to an optimum (moderate to high) value enabling both enhanced proppant transport and easy injection of the fluid, and then reduced with an external pH stimulus near fissures for proppant sedimentation and easy flowback. Flow-sweep experiments of the fracturing fluid using the supramolecular solution and proppants (silica sand) revealed that for only a 2wt.% concentration, viscosity could be varied nearly 1600 times by changing pH from ~4 to ~8, reversibly. Moreover, sedimentation studies showed that the settling rate of the silica proppants descended five orders of magnitude by increasing pH from 4 to 8 in a reversible fashion. On the other hand, our lab-scale preliminary experiments showed that these supramolecular assembly solutions both have reversible pH-responsive properties and high salinity and temperature tolerance. While degradation and breakage are observed in polymer viscosity modifiers used in hydraulic fracturing under high temperatures and sudden extreme shear stresses, supramolecular assembly solutions have ability to disassemble and re-assemble. Use of supramolecular solutions in fracturing fluids can also prevents reduction of permeability of reservoirs arising from polymer blockage during flowback period. Therefore, supramolecular solutions can be thought as self-healing polymer solutions.

Briefly, supramolecular assembly solution system described here has a potential to be a novel type fracturing fluid due to its pH-tunable and reversible viscosity and proppant transfer capacity, high salt tolerance, self-healing property, and sustainability and environmental friendliness.

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