Development And Evaluation Of Acidic Low Polymer Concentration Gel Systems For Hydraulic Fracturing

Enhanced oil recovery techniques related to carbon utilization and storage have broad applications in the tight reservoirs of low or ultralow permeabilities in China,such as the Changqing,Jidong,and Daqing peripheral oilfields.CO₂-water-rock reactions occur in the formation zone under high-temperature and high-pressure conditions,altering the p H of the formation environment;this p H can be as low as 3.5.In this circumstance,the performance of the most utilized borate cross-linked hydroxypropyl guar（HPG）fracturing fluid system,which is gelled in an alkaline range,is restricted due to the incompatibility of the p H reaction conditions.Zirconium cross-linked carboxymethyl hydroxypropyl guar（CMHPG）,the other common guar-based fracturing fluid system that has been applied in the acidic range,can be used to address this problem.The acidic fracturing fluid not only adapts to a formation treated with CO₂ but also prevents formation damage arising from the swelling and migration of clay particles.Moreover,the acidic fluid can promote polymer hydrolysis,which may assist in gel breaking during the flowback process in hydraulic fracturing.A better proppant-pack conductivity is accordingly induced.However,the drawbacks of uncontrolled cross-linking time,high polymer loading,fast viscosity growth and shear-sensitive behavior limit the large-scale use of acidic guar-based fracturing fluids.In this paper,an organic zirconium cross-linker with desirable delayed cross-linking performance was successfully synthesized using zirconium oxychloride,L-lactic acid and ethylene glycol as the raw materials.The effects of various factors of synthesis reaction on the cross-linker properties were systematically analyzed.The results show that the optimized organic zirconium cross-linker can obtain a satisfactory delayed gelation time of 3 to 8 min under the acidic condition at a p H of 2 and 9 to 17 min at a p H of 3,respectively.The cross-linker effectively solves the problem that the gelation time of the acidic guar-based fracturing fluid system is too short or even cannot be delayed at all.A low polymer concentration CMHPG-Zr acidic cross-linked gel fracturing fluid system was developed.The effects of different gelation parameters on the gelation performance of the gel system were systematically studied using the Sydansk bottle testing method combined with a controlled-stress rheometer.The results show that the viscoelastic properties and the gelation time of the gel system can be adjusted and controlled within a CMHPG polymer concentration range of 0.25 to 0.35%and an organic zirconium cross-linker concentration range of 0.03 to 0.07%.The applicable temperature and p H ranges of the gel system are 45 to 125℃and 2 to 4,respectively.It effectively overcomes the disadvantage of the large amount of polymer used in the acidic cross-linked gel fracturing fluid system.Surface-functionalized molybdenum disulfide（Mo S₂）nanosheets（2D nano-black-card fracturing synergist）for enhancing the performance of the traditional guar-based fracturing fluid systems were successfully developed using molybdenum-containing oxide,sulfur-containing organic compound and amino-containing reducing agent as the raw materials,（1-hexadecyl）trimethylammonium bromide and L-cysteine as the intercalating reagent and modifying reagent.The morphology and structure of the modified nanosheets were systematically characterized.The results show that the fabricated modified nanosheets are hydrophilic and have amino-and carboxyl-rich surfaces at a degree of functionalization of about 48%.A nanosheet-enhanced CMHPG-Zr acidic cross-linked gel fracturing fluid system was designed.The effects of the modified nanosheets on the gelation and fracking performance of the nanosheet-enhanced gel system were systematically evaluated.The results show that the viscoelasticity property and temperature,acid,and filtration resistance performance of the CMHPG-Zr fracturing fluid system will be greatly increased by adding the modified nanosheets due to the physical cross-linking structures formed through hydrogen bonding.The inherent defects of poor shear resistance and shear recovery properties of the CMHPG-Zr fracturing fluid system will be effectively improved.The laboratory hydraulic fracturing experiment and 3D digital core technique were employed to conduct a feasibility study to evaluate whether a complex hydraulic fracture network could be formed by the nanosheet-enhanced CMHPG-Zr fracturing fluid system.The results show that after adding the modified nanosheets to the fracturing fluid system,the complexity of the fracture is enhanced,and the fracture density is increased about 40%under the experimental conditions of this paper.The results in this paper can provide a reference for petroleum engineers in the synthesis and application of the organic zirconium cross-linker,as well as the development and enhancement of the traditional guar derivative-based fracturing fluid systems.