Development and Evaluation of an Acidic Shear-Tolerant Nanosheet-Enhanced Cross-Linked Gel System for Hydrofracturing

Acidic fracturing fluids, such as zirconium cross-linked carboxymethyl hydroxypropyl guar gum (CMHPG), can not only adapt to formations treated with CO₂ but also protect such formations from the damage that arises from the swelling and migration of clay particles during the hydrofracturing process. However, the shortcomings of uncontrollable viscosity growth and irreversible shear-thinning behavior limit the large-scale application of acidic fracturing fluids. In this work, a novel organic zirconium cross-linker was synthesized in the laboratory and applied under acidic conditions to control and delay cross-linking reactions. The ligands that were coordinated to the zirconium center were l-lactate and ethylene glycol. A CMHPG thickener was used at a low loading of 0.3% (approximately 25 pptg). Concentrated hydrochloric acid was utilized as a buffer to adjust the fracturing fluid pH to 3.0 for the majority of the tests and approximately simulate the pH attained in formations treated with CO₂ under high-temperature (HT)/high-pressure (HP) conditions. Moreover, surface-functionalized metallic-phase (1T) molybdenum disulfide (MoS₂) nanosheets were employed to improve the rheological performance of the zirconium cross-linked CMHPG fracturing fluid. l-Cysteine was utilized as a modification reagent. The morphologies, structures, and properties of the fabricated functionalized 1T-MoS₂ (Cys-1T-MoS₂) nanosheets were systematically characterized using transmission electron microscopy (TEM), scanning electron microscopy (SEM), Raman spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA). The results of these characterization tests demonstrated the successful functionalization of the 1T-MoS₂ nanosheets with l-cysteine. Then, the effects of this new nanosheet-enhanced zirconium cross-linked CMHPG fracturing fluid system with different cross-linker and nanosheet loadings on gelation performance were systematically assessed through the Sydansk bottle testing method combined with a rheometer under controlled-stress or controlled-rate modes. Microscopic observations and gel-breaking, regained-permeability, and proppant-settling tests were also carried out to further evaluate the fracturing fluid properties of this new system. The results indicated that the nanosheet-enhanced fracturing fluid had a desirable delayed cross-linking property. Compared with the blank fracturing fluid (without nanosheet enhancement), the nanosheet-enhanced fracturing fluid had a much better shear tolerance, shear recovery, and proppant carrying performance. Micrograph observations showed that an even and fine three-dimensional (3D) network structure formed within the nanosheet-enhanced fracturing fluid sample. Additionally, in contrast to the alkaline system, the acidic nanosheet-enhanced zirconium cross-linked CMHPG fracturing fluid system had much less formation damage induced by gel residues and clay swelling and dispersions.