Supramolecular dynamic binary complexes with pH and salt-responsive properties for use in unconventional reservoirs

Abstract

Hydraulic fracturing of unconventional reservoirs has seen a boom in the last century, as a means to fulfill the growing energy demand in the world. The fracturing fluid used in the process plays a substantial role in determining the results. Hence, several research and development efforts have been geared towards developing more sustainable, efficient, and improved fracturing fluids. Herein, we present a dynamic binary complex (DBC) solution, with potential to be useful in the hydraulic fracturing domain. It has a supramolecular structure formed by the self-assembly of low molecular weight viscosifiers (LMWVs) oleic acid and diethylenetriamine into an elongated entangled network under alkaline conditions. With less than 2 wt% constituents dispersed in aqueous solution, a viscous gel that exhibits high viscosities even under shear was formed. Key features include responsiveness to pH and salinity, and a zero-shear viscosity that could be tuned by a factor of ~280 by changing the pH. Furthermore, its viscous properties were more pronounced in the presence of salt. Sand settling tests revealed its potential to hold up sand particles for extended periods of time. In conclusion, this DBC solution system has potential to be utilized as a smart salt-responsive, pH-switchable hydraulic fracturing fluid.

Fig 1. Effect of salinity on the viscosity of 2 wt% DBC at pH 11.

Upon the addition of salt, the viscosity of the system increased in the salinity range of 0 wt% and 3 wt%. The error bars indicate standard error.

Fig 2

(a) Viscosity variation with pH of a 2 wt% DBC solution + 3 wt% NaCl and (b) Viscosity variation with pH of a 2 wt% DBC solution + 1 wt% NaCl. In both figures, the error bars indicate the standard error.

Fig 3. Comparison of zero shear viscosity for 2wt% and 1wt% polyacrylamide (PAM) solution and 2wt% and 1wt% DBC solution.

All the solutions contain 3 wt% NaCl. pH-tunable nature is observed for the DBC solutions. The error bars indicate standard error.

Fig 4

Storage (G’) and loss (G”) moduli of 2wt% DBC+3wt% salt at (a) pH 9, (b) pH 10, (c) pH 11, (d) pH 12. The error bars indicate standard error.

Fig 5

Sand settling test for (a) 1 wt% DBC + 1 wt% salt and (b) 1 wt% DBC + 3 wt% salt at room temperature (23˚C).

Fig 6. Images showing the stability of proppant suspension in saline DBC solution (2 wt% DBC + 3 wt% salt) at 90˚C during various time intervals.

Fig 7

Optical microscopy images of supramolecular DBC solutions at (a) pH 9 and (b) pH 11. Scale bar indicates 50 μm.

Fig 8. Illustration of the use of stimuli-responsive dynamic binary complexes as viscosifying agents in shale gas recovery.

The reversibly adjustable viscosity is a key property that can allow a precise control over proppant deposition and fluid flowback.