2.2 Characterization of the Composite Emulsion
2.2.1 Infrared Spectroscopy

Figure 4 shows the infrared spectra of E54, modified nano-SiO2 (6% KH550), and the composite emulsion. For E54, the absorption peak at 916 cm–1 is the absorption peak of the epoxy group, and the absorption peaks at 2926 cm–1 and 2855 cm–1 come from the stretching vibrations of —CH2 — and —CH, respectively. The composite emulsion shows obvious SiO2 characteristic peaks at 470 cm–1, 800 cm–1, and 1096 cm–1, indicating that the nano-SiO2 successfully adsorbs as a surfactant on the surface of the resin droplets, thus forming a stable emulsion system. In addition, compared with E54, the absorption peaks of the epoxy group and —OH group in the composite emulsion have weakened, which may be due to the active —OH and —NH 2 on the surface of nano-SiO2 reacting with some epoxy groups to open up, resulting in a decrease in the number of epoxy groups, which helps to make the entanglement between nano-SiO2 and resin droplets more stable, thereby further improving the stability of the emulsion.

2.2.2 Particle Size Distribution
Figure 5 shows the particle size distribution of the composite emulsion. It can be observed that the particle size distribution of the emulsion droplets is relatively narrow, with a median particle size of 33.6μm. Generally speaking, the high viscosity of the epoxy resin makes it difficult to be emulsified into fine droplets. However, the particle size of the composite emulsion droplets can be maintained at a smaller size, which is beneficial for the epoxy resin to form better dispersion in the cement slurry.

2.3 Impact of Composite Emulsion on the Performance of Oil Well Cement

2.3.1 Mechanical Strength of Set Cement

Figure 6 presents a comparison of the 48-hour compressive and flexural strengths of cement pastes with varying lotion dosages. Compared to the blank cement paste, the mechanical strength of the composite lotion cement paste has been significantly enhanced. As the dosage of composite lotion increases, the 48-hour compressive strength of cement pastes with 15%, 30%, 45%, and 60% lotion dosages increases by 11.5%, 18.6%, 24.6%, and 30.8%, respectively, compared to the blank cement paste. Similarly, the 48-hour flexural strength increases by 10.9%, 21.8%, 28.1%, and 39.1%, respectively. The incorporation of lotion effectively enhances both the compressive and flexural strengths of the cement paste, with mechanical strength increasing in tandem with the lotion content. The epoxy resin in the cement slurry forms a three-dimensional network structure with superior mechanical properties through crosslinking with a curing agent, thereby providing strength. Meanwhile, the cured resin fills the voids within the cement matrix, rendering the structure denser and facilitating the further enhancement of its mechanical properties.

2.3.2  Uniaxial Compression Stress-Strain Test

Figure 7(a) shows the uniaxial compressive stress-strain curves of cement stones with different emulsion content, while Figure 7(b) displays the peak strain and elastic modulus of the different samples. The failure process of all five samples can be divided into four stages: the compaction stage, the elastic stage, the stable crack propagation stage, and the accelerated crack propagation stage [14].

It can be observed that the strain of the plain cement stone at peak stress is 0.55%, and the curve drops sharply immediately after reaching the peak, indicating high brittleness. In contrast, for the cement stone samples with 15%, 30%, 45%, and 60% emulsion content, the peak strain increased by 23.6%, 40.0%, 54.5%, and 69.1%, respectively, compared to the plain sample. Simultaneously, the elastic modulus decreased by 12.1%, 26.4%, 42.9%, and 51.6%, respectively.

Furthermore, the variation trend of peak stress in the emulsion-modified cement stones is consistent with the change in compressive strength. The experimental results demonstrate that the incorporation of the composite emulsion enhances both the peak stress and peak strain of the cement stone to varying degrees. As the emulsion dosage increases, the stress-strain curves of the samples with 45% and 60% emulsion content become similar to those of elastic-plastic materials. Notably, these samples retain a certain residual strength after failure, signifying a significant improvement in the elastic deformation capability of the cement stone.

2.3.3 Impact Resistance of Cement Stone

Figure 8 illustrates the initial crack impact energy and fracture impact energy of various cement stones. It is evident that as the dosage of composite emulsion increases, both the initial crack impact energy and fracture impact energy of the cement stones significantly improve. Specifically, the fracture impact energy of cement stones with 15% and 60% composite emulsion dosages increases by 25.1% and 175.1%, respectively, compared to blank cement stones. When compared to cement, cured epoxy resin demonstrates a greater capacity to absorb impact energy. Moreover, the epoxy resin disperses effectively within the cement stone, leading to a more uniform stress distribution upon impact. The resin absorbs energy through deformation, thereby enhancing the impact resistance of the cement stone. Additionally, as the emulsion dosage increases, the cement stone absorbs more impact energy from the initial crack to failure. This is attributed to the even dispersion and curing of more epoxy resin within the cement stone, which strengthens internal bonding by forming an interwoven membrane structure, further improving the impact resistance of the cement stone.

2..4 Phase Composition of Cement Stone

Figure 9 shows the XRD patterns of cement stones with different emulsion content. It can be observed that the phase composition of the emulsion-modified cement stones is consistent with that of the plain cement stone, primarily consisting of Portlandite (CH) and Calcium-Silicate-Hydrate (C-S-H). This indicates that the incorporation of the epoxy resin emulsion does not alter the types of hydration products formed in the cement.

Compared to the plain cement stone, the intensity of the CH diffraction peaks in the emulsion-modified cement stones shows a decreasing trend. Moreover, this decrease becomes more pronounced with higher emulsion content. This phenomenon is likely due to the abundant nano-SiO₂ in the composite emulsion reacting with CH (the pozzolanic reaction), generating additional C-S-H gel, which contributes to the strength of the cement stone. Consequently, this reaction reduces the amount of CH in the hydration products while simultaneously enhancing the mechanical strength of the cement stone.

2.3.5 Porosity of Cement Stone

Typically, the pores in cement stone can be classified into four categories: harmless pores (<20 nm), less harmful pores (20–50 nm), harmful pores (50–200 nm), and multi-harmful pores (>200 nm) [15]. Figure 10 shows the pore size distribution of cement stones with different emulsion content. The average pore diameters of the plain cement stone and the cement stones with 15%, 30%, 45%, and 60% emulsion content are 54.4 nm, 45.3 nm, 42.6 nm, 37.1 nm, and 33.5 nm, respectively. The average pore diameter of the composite emulsion-modified cement stones shows a significant decreasing trend compared to the plain cement stone, and it continues to decrease with increasing emulsion content.

Furthermore, with the addition of the composite emulsion, the volume of harmful and multi-harmful pores in the cement stone decreases markedly. A substantial portion of multi-harmful and harmful pores are transformed into less harmful pores (20–50 nm) or harmless pores (<20 nm), indicating that the incorporation of the emulsion significantly refines the pore structure of the cement stone.The composite emulsion facilitates the good dispersion of epoxy resin within the cement stone matrix and the formation of strong interfacial bonding, thereby improving the pore structure. The reduction in average pore size and the refinement of the pore diameter distribution contribute to further enhancing the mechanical properties and durability of the cement stone.

2.3.6 Microstructure of Cement Stone

Figures 11 (a-d) display the fracture surface morphology of cement stones with 15%, 30%, 45%, and 60% composite emulsion content, respectively. It can be observed that at a lower emulsion content (15%), the cured epoxy resin is uniformly distributed as spherical particles within the cement matrix. This indicates that the epoxy resin achieved excellent dispersion in the cement slurry, and the interface between the resin and the cement hydration products is relatively tight.In the cement stone with 30% emulsion content, besides spherical epoxy resin particles, some film-like structures appear. The increased amount of epoxy resin allowed it to partially cross-link into sheets during the curing process, resulting in the coexistence of both spherical and film-like epoxy resin.

As the emulsion content increases further, the cross-linking of the epoxy resin becomes more intensive. Large, continuous film-like structures appear on the fracture surface of the cement stone, and the structure becomes denser, which corresponds to the observed reduction in cement stone porosity.

The prepared composite emulsion, as an organic-inorganic hybrid, improves the mechanical properties of the cement stone through two main mechanisms:

Excellent Dispersion and Stress Distribution: The good stability of the composite emulsion enables the uniform dispersion of epoxy resin within the cement slurry, overcoming the difficulty of dispersing oily epoxy resin in aqueous cement paste. This uniform dispersion helps distribute internal stress more evenly when the cement stone is under load. The epoxy resin itself can then utilize its mechanical properties to bear and buffer more energy, thereby enhancing the mechanical strength and deformation capacity of the cement stone [16]. Enhanced Interfacial Bonding and Microstructure Refinement: The nano-SiO₂ in the emulsion, bonded to the resin on one end, can react with the cement matrix through the pozzolanic reaction after dispersion into the slurry. This not only enhances the mechanical properties of the cement stone but also effectively strengthens the interfacial bond between the resin phase and the cement matrix. This process optimizes the porosity of the cement stone and further improves stress dissipation during internal crack propagation.In summary, the composite emulsion synergistically combines the advantages of organic and inorganic materials. It achieves excellent dispersion of epoxy resin in the cement slurry and strong interfacial bonding. This significantly enhances the mechanical properties of oil well cement, particularly by markedly improving the deformation capacity of the cement stone under load. This ensures the sealing integrity of the cement sheath under complex stresses, ultimately achieving the goal of improving cementing quality.

3. Conclusion

This study utilized modified nano-SiO2 emulsified epoxy resin to prepare an organic-inorganic composite emulsion. Further research was conducted on the influence of this emulsion on the properties and microstructure of oil well cement, leading to the following conclusions:

(1) Through infrared spectroscopy, thermogravimetric analysis, contact angle, and emulsion morphology analysis, the optimal dosage of KH550 for modifying nano-SiO2 was determined to be 6%. The prepared composite emulsion had a uniform particle size distribution, with a median particle size of 33.6 μm.

(2) The mechanical strength and deformation capacity of the composite emulsion cement stone were significantly improved, and the improvement effect increased with the increase in emulsion dosage. Compared to the blank cement stone, the compressive strength of 15% and 60% composite emulsion cement stones increased by 11.5% and 30.8%, respectively. The 48-hour flexural strength increased by 10.9% and 39.1%, respectively, and the 48-hour elastic modulus decreased by 12.1% and 51.6%, respectively. The 48-hour impact energy increased by 25.1% and 175.1%, respectively. The mechanical strength, deformation capacity, and impact resistance of the emulsion cement stone were significantly enhanced.

(3) The composite emulsion achieved a good dispersion effect of epoxy resin, with the epoxy resin uniformly distributed in the cement stone and curing to form a dense three-dimensional network structure, enhancing the strength and deformation performance of the cement stone. The modified nano-SiO2 in the emulsion reacted with the cement matrix through pozzolanic reaction, effectively improving the interface bonding between the resin phase and the cement matrix, optimizing the pore structure. The composite emulsion achieved a synergistic enhancement of the mechanical strength and deformation performance of the cement stone, ensuring the integrity of the cement ring seal.