Abstract

Brittle cement sheaths inevitably produce microcracks during oil and gas production, which can lead to seal failure. Based on the Pickering emulsion method, a new organic-inorganic composite emulsion was prepared by using KH550 modified nano-SiO 2 emulsified epoxy resin to improve the mechanical properties of oil well cement. Infrared spectroscopy, thermogravimetric analysis and contact angle test were used to determine the optimal modification conditions of nano-SiO2 , and a stable resin emulsion with uniform particle size was prepared. The properties of cement stone with different emulsion dosages were studied, and the results showed that the incorporation of resin emulsion could improve the mechanical strength and deformation ability of cement stone. Compared with the blank cement stone, the 7-day compressive strength, flexural strength and elastic modulus of 60%(mass fraction, same below) emulsion cement stone were increased by 22.8%, 39.1%, and the elastic modulus was reduced by 51.6%, the breaking impact energy increased by 175.1%, and the mechanical properties of cement stone were significantly improved. The organic-inorganic composite emulsion improves the dispersion properties of epoxy resin in cement slurry, which is conducive to further exerting the elastic deformation properties of epoxy resin, thereby reducing the brittleness of cement stone. At the same time, nano-SiO2 improved the interface between resin and cement matrix through volcanic ash reaction, optimizes the pore structure of cement stone, thereby significantly improving the mechanical properties of cement stone, and ultimately improving the cementing quality.

In the process of oil and gas well development, the cementing cement sheath plays a role in supporting the wellbore,protecting the casing, and preventing interlayer flow. Therefore, the sealing integrity of the cementing ring is the key to ensuring the quality of cementing. However, the solidified cement stone belongs to brittle materials, which have defects such as high brittleness and poor deformation ability. This leads to the formation of micro annular gaps and micro cracks in the cement ring under complex pressure differential conditions, resulting in sealing integrity failure and seriously affecting the safe exploitation of oil and gas resources [2-3]. Therefore, improving the brittleness of oil well cement is the key to improving the quality of oil and gas well cementing. At present, adding materials such as rubber powder, fibers, or liquid latex to cement slurry is a commonly used method to improve the brittleness of cement paste [4]. Among them, the poor compatibility between rubber powder and cement slurry can cause serious deterioration of the strength of cement paste [5]. The addition and dispersion of fiber materials in cement slurry are limited by the mixing process [6]. The large amount of surfactants in liquid latex can lead to the generation of numerous bubbles in cement slurry that cannot be effectively eliminated, thereby affecting the quality of well cementing [7]. Epoxy resin, as a thermosetting resin, has high mechanical strength and deformation capacity after curing. As an additive for oil well cement, it can significantly improve the mechanical properties of cement stone [8]. However, epoxy resin is oily and cannot be effectively dispersed in cement slurry, which limits its application. Therefore, enhancing the dispersion and binding ability of epoxy resin in cement slurry is the key to realizing its role as a material for improving the mechanical properties of oil well cement. To improve the dispersion of epoxy resin in cement slurry, preparing epoxy resin emulsion by adding appropriate emulsifiers is a simple and efficient method [9]. Industrial epoxy resin emulsions use a large amount of emulsifiers to maintain the stability of the emulsion, but the extensive use of traditional emulsifiers has negative impacts on the environment and free emulsifiers can affect the final performance of the material [10]. Different from traditional emulsions, the emulsifiers in Pickering emulsion systems are solid nanoparticles, which irreversibly adsorb at the oil-water interface to form a barrier, thereby restricting the aggregation of droplets [11]. The emergence of Pickering emulsions reduces the use of traditional emulsifiers and enables the introduction of nanoparticles into the curing system, thus forming composite materials with excellent performance. Among numerous nanomaterials, nano-SiO2 has received extensive attention due to its excellent emulsifying performance [12].In addition, in addition to its excellent emulsifying ability, nano-SiO2 has also become the optimal material for improving the performance of cement-based materials due to its high surface area and chemical reactivity [13]. However, the high hydrophilicity of nano-SiO2 itself makes it unable to effectively stabilize the lotion. Therefore, nano-SiO2 usually needs to be hydrophobic modified to varying degrees before being used as an emulsifier. In this paper, based on Pickering lotion method, a new nano SiO2/epoxy organic-inorganic composite lotion was prepared to improve the compatibility of epoxy resin and cement slurry, and finally to enhance the mechanical properties of cement paste. Firstly, nano SiO 2 was grafted by KH550, and a stable composite lotion was obtained by using the modified nano SiO 2 emulsified epoxy resin. The influence of the lotion on the mechanical properties of oil well cement was studied, and the strengthening mechanism of the composite lotion on the mechanical properties of oil well cement was discussed.

1. Experimental Section

1.1 Instruments and Reagents

WQF 520 Fourier-transform infrared (FTIR) spectrometer (KBr pellet method, Beijing Rayleigh Analytical Instrument Co., Ltd.). DSC823 TGA/SDTA85/e simultaneous thermogravimetric-differential thermal analyzer (Mettler-Toledo Instruments (Shanghai) Co., Ltd.). SDC-350 contact angle goniometer (Dongguan Shengding Precision Instrument Co., Ltd.). Mastersizer 2000 laser diffraction particle size analyzer (Malvern Panalytical, UK). X’Pert PRO MPD X-ray diffractometer (XRD, PANalytical, Netherlands). Thermo Scientific Apreo 2C field-emission scanning electron microscope (FE-SEM, Thermo Fisher Scientific Co., Ltd.). RTR-150 triaxial rock mechanics testing system (Geotechnical Consulting & Testing Systems (GCTS), USA). SHT4106 microcomputer-controlled electro-hydraulic servo universal testing machine (MTS Systems Corporation, China). DIT122Z3 drop-weight impact fatigue testing machine (Shenzhen Wantest Testing Equipment Co., Ltd.). AutoPore V fully automatic mercury intrusion porosimetry (MIP) analyzer (Micromeritics Instrument (Shanghai) Co., Ltd.).

Reagents: Nano-silica (nano-SiO₂), γ-aminopropyltriethoxysilane (KH550), anhydrous ethanol (Chengdu Kelong Chemical Reagent Co., Ltd.). epoxy resin E54 (Nantong Xingchen Synthetic Materials Co., Ltd.). 669 active diluent (Jinan Xinkai New Materials Co., Ltd.). polyetheramine D230 curing agent (Shanghai Jizhi Biochemical Technology Co., Ltd.). Jiahua G-grade oil well cement (Jiahua Special Cement Co., Ltd.).fluid loss additive (HT-2), ketone-aldehyde condensation dispersant (SXY), and organic phosphonate defoamer (XP) (industrial-grade, Chengdu Chuanfeng Chemicals Co., Ltd.).tap water.

1.2 Preparation of Composite Emulsion

Firstly, KH550 was used to perform graft modification on nano-SiO2: 5.0 g of nano-SiO2 was mixed with 100 mL of ethanol-water solution (ethanol : water = 2 : 1, V : V) and stirred evenly to obtain a nano-SiO2 suspension. According to the mass of SiO2, 3%, 6%, and 9% of KH-550 were respectively added to the nano-SiO2 suspension, heated to 70 ℃ and stirred for 4 h. After centrifugation, washing, and drying, different degrees of modification of modified nano-SiO2 were obtained. E54 and 669 diluents were diluted in a mass ratio of 4 : 1 and mixed evenly. 20.0 g of the diluted epoxy resin was slowly added to 19.6 g of the modified nano-SiO2 suspension (mass fraction of 2.0%, the same below) and stirred at 8000 rpm for 30 min to obtain the composite emulsion (solid content: 50%).

1.3 Characterization and Testing

1.3.1 Characterization of Modified Nano-SiO2 and Composite Emulsions

The chemical structures of different nano-SiO2 and composite emulsions were studied using an infrared spectrometer; the thermal stability of different nano-SiO2 was tested using a thermal analyzer; the nano-SiO2 powders were pressed into thin sheets using a press machine and placed on a glass slide, and the static contact angles of different nano-SiO2 with water were measured using a fully automatic contact angle measuring instrument; the microscopic morphology of the emulsions was observed using an inverted fluorescence digital microscope without a magnifying glass; the particle size distribution of the composite emulsions was analyzed using a laser particle size analyzer.

1.3.2 Mechanical Properties Testing of Oil Well Cement

According to the standard "Test Methods for Oil Well Cement" GB/T 19139-2012, cement slurry was prepared. The cement slurry formula is shown in Table 1. The cement slurry was respectively poured into a 50.8 mm × 50.8 mm × 50.8 mm cube mold and a 160.0 mm × 40.0 mm × 40.0 mm rectangular mold, placed in a 90 ℃ water bath for 48 hours, then taken out, and the compressive strength and flexural strength of the cement stone were tested using a microcomputer-controlled electro-hydraulic servo universal testing machine. Circular cylindrical cement stone specimens with a diameter of 25.8 mm and a height of 50.8 mm and circular cylindrical specimens with a diameter of 150.0 mm and a height of 60.0 mm were respectively prepared and cured in a 90 ℃ water bath for 48 hours. The elastic modulus of the cement stone at 48 hours was tested using the RTR1500 rock mechanics testing system, and the impact resistance of the cement stone was tested using a drop hammer impact fatigue testing machine. In this case, the drop hammer weight was 4.5 kg and the drop height was 500.0 mm. When the specimen showed initial cracks, the number of drop hammer impacts recorded as N1, when the specimen was destroyed, the number of drop hammer impacts recorded as sN2, and the impact energy was calculated simultaneously.

1.3.3 Microstructure characterization of oil well cement

Soak the cured cement stone slices in anhydrous ethanol for 48 hours to terminate hydration, dry them, grind them into powder, and analyze the phase composition of the samples using an X-ray diffractometer of the X Pert PRO MPD type; Using Auto Pore V-type fully automatic mercury intrusion porosimeter to analyze the pore size distribution of cement stone; Use Thermo Scientific Apreo 2C field emission scanning electron microscope to observe the microstructure of cement block cross-section.

2 Results and Discussion

2.1 Research on Surface Modification of Nano-SiO2

2.1.1 Infrared Spectroscopy and Thermogravimetric Analysis of Different Nano-SiO2

Figure1(a) shows the infrared spectra of different nano-SiO2. The four types of SiO2 all have distinct characteristic absorption peaks at 471cm–1, 800cm–1, and 1099cm–1, representing the bending and stretching vibrations of Si—O—Si. The absorption peaks at 3426cm–1 and 1635cm–1 are attributed to the asymmetric H—O vibration and the bending vibration of H—O—H, respectively. The modified SiO2 shows weak new absorption peaks at 2855cm–1 and 2926cm–1, which is due to the introduction of—CH2 and—CH3 by KH550. Additionally, the hydroxyl characteristic peak at 3426cm–1 decreases with the increase in the amount of KH550, indicating a reduction in the number of hydroxyl groups on the SiO2 surface. The absorption peaks of—NH2 and—OH overlap, resulting in a slightly wider peak shape at 3300-3400cm–1. As shown in Figure 1(b), the thermal degradation curves of the four types of SiO2 are presented. The weight loss of the unmodified nano-SiO2 mainly occurs within 200℃, and is mainly due to the decomposition of hydroxyl and free water. For the modified nano-SiO2, the weight loss can be divided into two stages: within 200 ℃, the weight loss is mainly caused by water evaporation, and above 200 ℃, the weight loss is related to the decomposition of the organic chain of KH550. The weight loss of the SiO2/KH550 (3%), SiO2/KH550 (6%), and SiO2/KH550 (9%) samples at 200-600 ℃ is 3.5%, 3.6%, and 3.7%, respectively. The weight loss of the samples within this temperature range increases with the increase in the amount of KH550, indicating that different amounts of KH550 have successfully grafted onto the surface of nano-SiO2, thereby forming different thermal degradation trends.  

2.1.2 Different Hydrophilic Properties of Nano-SiO₂

The surface wetting property of nanoparticles is the key to forming stable Pickering emulsions. Figure 2 shows the static contact angles of different nano-SiO2 with water. The contact angle of unmodified SiO2 is 14.6°, and the particles can be almost completely wetted by the aqueous phase. As the amount of KH550 increases, the particle contact angle gradually increases. When the amount of KH550 is 6%, the contact angle between SiO2 and water is 61.8°, indicating that some -OH groups on the surface of SiO2 combine with -OH produced by the hydrolysis of KH550 to form Si-O-Si bonds, reducing the hydrophilicity of SiO2. When the amount of KH550 increases to 9%, the contact angle of the modified nano-SiO2 increases to 118.4°, indicating a transition from hydrophilicity to slightly hydrophobicity.

2.1.3 Analysis of Emulsion Morphology

Figure 3 (a-c) respectively show the microscopic morphology of the emulsions prepared from SiO2/KH550 (3%), SiO2/KH550 (6%), and SiO2/KH550 (9%) samples. Due to the high hydrophilicity of the unmodified nano-SiO2, it cannot be adsorbed on the oil-water interface and thus cannot form a stable emulsion. The emulsion droplets prepared from SiO2/KH550 (3%) are large and uneven, and there is obvious aggregation between the droplets. This is because the nanoparticles still have a high hydrophilicity, and a large amount of SiO2 is dispersed in water rather than on the oil/water interface, causing resin aggregation. With the increase in the amount of KH550, the emulsifying ability of nano-SiO2 is improved, and SiO2/KH550 (6%) and SiO2/KH550 (9%) can effectively stabilize the emulsion, with smaller droplets and relatively uniform particle sizes. In summary, in order to form a stable emulsion, considering that the hydrophobic SiO2 will lead to poor compatibility of the prepared emulsion with the cement slurry, the optimal amount of KH550 is determined to be 6%.