Abstract

In response to the problem of smooth and easily agglomerated surface of basalt fiber (BF), this paper uses coating method to modify the surface of BF. A hydrophilic cationic coating agent is prepared by using poly (methyl methacrylate), coupling agent KH-570, and hexadecyltrimethylammonium chloride, and coated on the surface of BF to obtain modified basalt fiber (MBF).The structure of MBF was characterized by infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (EDS), optical contact angle measurement instrument, and Zeta potential analyzer.The FT-IR, SEM, and EDS analysis results indicate that the surface of MBF is rough and contains hydrophilic carboxyl, hydroxyl, and quaternary ammonium groups. The contact angle test results indicate that the hydrophilicity of MBF is significantly improved after modification.The Zeta potential test results indicate that the absolute value of the Zeta potential of MBF is significantly higher than that of BF. The dispersion performance of fibers was evaluated using image analysis method, and the results showed that the distribution coefficient of MBF increased by 73% compared to BF, indicating better dispersion.By studying the effect of MBF on the mechanical properties of oil well cement, the results showed that when the cement stone was cured for 2 days at a curing temperature of 80 ℃ and a fiber content of 2% (mass fraction), the compressive strength and flexural strength of MBF cement stone increased by 40.1% and 37.1% respectively compared to blank cement stone; Compared with BF cement stone, MBF cement stone has increased its compressive strength and flexural strength by 26.6% and 21.4%, respectively.Through the single wire pull-out test, it was found that the bonding performance between MBF and cement increased by 35.5% compared to BF. Based on SEM analysis of MBF cement paste, BF surface coating modification increased its dispersibility in cement paste, while increasing the surface roughness of fibers, enhancing the bonding performance between fibers and cement, and improving the mechanical properties of cement paste.

The cement sheath of oil and gas well cementing mainly plays a role in isolating formation fluids, supporting and protecting casing. And cement sheath is a material with high brittleness, low tensile strength, and poor impact toughness.During the processes of pressure testing, perforation, production, and production increase operations in oil and gas wells, the cement sheath may crack and break under various complex impact forces, resulting in seal failure, underground oil, gas, and water flow, causing pressure in the annulus of the oil and gas well, posing safety production risks, and causing serious economic losses. Therefore, improving the mechanical properties of cementing cement, especially the impact toughness of cement, is essential for ensuring the safe and efficient production of oil well gas.

Adding fiber materials to the cement matrix is one of the common measures to improve the toughness of cement paste.HUANG et al. found that adding carbon fiber to cement matrix can effectively improve the mechanical properties of cement paste without changing the hydration composition of cement. By changing the surface roughness of carbon fibers, the interfacial adhesion between carbon fibers and cement matrix was improved, resulting in a 27.25% and 70.56% increase in the flexural strength and tensile strength of cement paste compared to unmodified materials.However, the high cost of carbon fiber limits its further development in the engineering field.Huang Jiasheng et al. studied the influence of polyvinyl alcohol fiber (PVA) reinforced concrete on its mechanical properties at high temperatures. Research has shown that the addition of PVA enhances the high temperature resistance of concrete to a certain extent.Steel fiber reinforced concrete is widely used in construction and highways due to its excellent tensile, bending, shear, crack resistance, impact resistance, fatigue resistance, and high toughness properties. However, during the cementing process, due to its complex and harsh environment, steel fibers are prone to corrosion, leading to a decrease in the strength of cement paste.Whisker material is a type of artificially controlled growth fiber material that has the effect of reducing the brittleness of cement paste and increasing its toughness. Whisker materials mainly include CaCO₃ whiskers, ZnO whiskers, and nano-SiO₂ whiskers, which have the advantages of high tensile strength and few surface defects. However, due to the complex preparation process and high cost of whisker materials, their further development in the field of oil well cement is limited.

Basalt fiber (BF) is an inorganic fiber obtained by melting and drawing natural basalt rock at 1200-1500℃. The production process is environmentally friendly and is known as the "green and environmentally friendly material of the 21st century".BF is increasingly being applied by scholars in cement-based composite materials due to its excellent mechanical properties and natural compatibility with cement materials.JIANG et al. studied the effect of basalt fibers on the mechanical properties of concrete and found that BF can improve the toughness of concrete. ZHENG et al. studied the effects of BF aspect ratio and dosage on the flexural, compressive, and impact resistance of oil well cement.Yu Bin et al. studied the mechanical properties of short cut basalt fiber-reinforced cement-based composites and found that the addition of short cut basalt fibers can effectively improve the toughness of cement-based composites, improving brittle fracture failure under bending and tensile strength to ductile crack propagation failure.However, due to the smooth surface of BF, the bonding with the cement matrix is not very tight, and the fibers are prone to intertwining with each other. When the dosage increases, the dispersibility in the cement slurry is poor, resulting in weak reinforcement effect on the cement stone. Therefore, in order to improve the bonding effect between BF and cement materials and enhance the dispersibility of BF in the cement matrix, it is necessary to modify its fiber interface.The main methods of fiber surface modification include coupling agent modification, acid-base etching modification, plasma modification, and coating modification. Among them, coating modification has a relatively small impact on the fiber body structure, does not damage the mechanical strength of BF, and coating modification has good structural designability. By introducing specific functional groups into the coating material, the bonding force between fibers and composite materials can be improved.

Therefore, this article adopts coating modification to prepare a hydrophilic cationic coating agent using poly (methyl methacrylate), coupling agent KH-570, and hexadecyltrimethylammonium chloride. And it is coated onto the surface of BF, introducing hydrophilic groups such as carboxyl, hydroxyl, and quaternary ammonium groups on the BF surface and increasing the surface roughness of BF to improve the surface properties of BF fibers, increase the dispersibility of BF in cement stone, and ultimately achieve the goal of improving the mechanical properties of BF oil well cement stone.

1. Experimental Section

1.1 Instruments and Reagents

WQF-520 Fourier transform infrared spectrometer; Quanta 450 scanning electron microscope; DSA30S optical contact angle measuring instrument; SurPASS 3 Solid Zeta Surface Potential Analyzer; NYL-300 pressure testing machine; KZJ-500 electric flexural testing machine.

Silane coupling agent (γ-methacryloyloxypropyltrimethoxysilane, KH-570), methyl acrylic acid (MAA), benzoyl peroxide (BPO), hexadecyltrimethylammonium chloride (CTAC), and hydrogen peroxide (H₂O₂, mass fraction of 30%) are all analytical grade and purchased from Chengdu Kelong Chemical Reagent Factory;G-grade oil well cement, purchased from Sichuan Jiahua Cement Co., Ltd; Continuous Basalt Fiber (BF), purchased from Sichuan Aerospace Tuoxin Basalt Fiber Co., Ltd;The fluid loss agent (SD18), dispersant (SD35), and defoamer (SD52) are provided by Chuanqing Drilling Engineering Co., Ltd; Deionized water is self-made in the laboratory.The basic performance parameters of basalt fiber are shown in Table 1.

1.2 Surface Modification of Basalt Fiber

First, add 45 mL of MAA and 90 mL of water to a three necked flask and stir and mix. Then, add a mixture of 5 mL of MAA and 0.25g of initiator BPO dropwise at a constant speed at 80℃ within 0.5 hours, and react for 2 hours.Add a mixture of 2.5 mL of KH-570 and 0.25 g of BPO dropwise at a constant speed within 0.5 hours, and react for 1 hour. Cool the system to 60℃, add 10mL of CTAC aqueous solution dropwise, with a CTAC addition amount of 1.0% of MAA, and incubate for 30 minutes.After the reaction is completed and cooled to room temperature, the obtained lotion is a hydrophilic cationic coating agent.

Soak BF in H₂O₂ solution (mass fraction of 5%) at 40℃ for fiber surface oxidation for 2 hours to enhance the surface hydroxyl activity of BF.After the reaction is complete, wash repeatedly with distilled water and then dry in a 60℃ oven for later use. Finally, soak the dried BF in the above-mentioned cationic coating agent, soak it at 45℃ for 1 hour, then take it out and dry it in a 60℃ oven until constant weight.The sample obtained after drying is modified basalt fiber (MBF). The principle of surface modification of basalt fibers is shown in Figure 1.