Improved Gelation Performance of an Acidic Low-Polymer Loading Zirconium Cross-Linked CMHPG Fracturing Fluid by Surface Functionalized 1T-Phase Molybdenum Disulfide Nanosheets

The low and ultra-low permeability reservoirs in China, such as the Changqing, Jidong, and Daqing peripheral oil fields, often apply CO2 as a flooding medium to enhance oil recovery. A serial of water-rock interactions will be occurred among the CO2, formation rock, and formation water under the HT/HP conditions. The pH value of the formation will be converted to acidity accordingly. As a side effect, the traditional guar-based fracturing fluids in an alkaline range, such as the borate cross-linked hydroxypropyl guar gum (HPG), cannot result in an effective hydrofracturing operation due to the incompatibility. Consequently, developing an acidic fracturing fluid system with a satisfactory performance is an imperative. Acidic fracturing fluids, such as the zirconium cross-linked carboxymethyl hydroxypropyl guar gum (CMHPG), can protect the formation during the hydrofracturing process from the damage arising from the swelling and migration of the clay particles. However, the shortcomings of the uncontrollable viscosity growth and the irreversible shear-thinning behavior limit the large-scale use of the acidic fracturing fluids. In this work, a novel organic zirconium cross-linker synthesized in the laboratory was applied to control and delay the cross-link reaction under the acidic condition. The ligands coordinated to the zirconium center were the L-lactate and ethylene glycol. The thickener used was the CMHPG at a low loading of 0.3% (approximately 25 pptg). Meanwhile, the surface functionalized metallic phase (1T-phase) molybdenum disulfide (MoS2) nanosheets were employed to improve the rheological performance of the zirconium cross-linked CMHPG fracturing fluid. The modification reagent utilized was the L-cysteine. The morphology, structure, and property of the fabricated functionalized 1T-MoS2 (Cys-1T-MoS2) nanosheets were systematically characterized using the 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) measurements. The results of the characterization tests demonstrated a successful functionalization of the 1T-MoS2 nanosheets with L-cysteine. Then, the effects of this new nanosheet-enhanced zirconium cross-linked CMHPG fracturing fluid systems with different cross-linker and nanosheet loadings on gelation performance were systematically assessed employing the Sydansk bottle testing method combined with a rheometer under the controlled-stress or controlled-rate modes. The results indicated that the nanosheet-enhanced fracturing fluid had a desirable delayed property. Compared with the blank fracturing fluid (without nanosheets), the nanosheet-enhanced fracturing fluid had a much better shear-tolerant and shear-recovery performance.