Low pH fracturing fluids, such as the transition metal cross-linked guar derivative, can reduce the formation damage caused by clay swelling and fines migration in hydraulic fracturing treatments. However, a major disadvantage of the fluids is the uncontrollable rapid viscosity development, especially cross-linking under a pH below 3.0. In this paper, a novel delayed organic zirconium cross-linker with lactate and ethylene glycol as ligands was designed to overcome the shortcoming. Carboxymethyl hydroxypropyl guar (CMHPG), which results in a low gel residue after breaking, was used as the polymer at low concentration. The impact of gelation parameters on the properties of zirconium-CMHPG gel system was systematically investigated using a Sydansk bottle-testing method combined with a controlled stress rheometer. Desirable gelation time delay with agreeable rheological performance could be obtained by optimizing the polymer and cross-linker concentrations even at a pH as low as 2.0. Uniformly and compactly generated 3D network microstructures could firmly lock the free water within the gel, which further improved the thermostability and acid-resistance of the system. Meanwhile, the structures could also maintain the viscoelasticity of the fluid at a high level to ensure the efficient transportation of the proppants. Proppant settling and gel breaking tests demonstrated that the low pH and low polymer concentration gel system generated fewer insoluble residues with an uncompromised suspension performance. Moreover, the formation damage evaluation revealed that the low pH fracturing fluid system produced a better return permeability than the high pH system.

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.

A novel nonionic fluorocarbon surfactant is synthesized and indicates excellent foaming performance and super surface activity.

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.

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