The surface hydrophilicity of thermoplastic rubber (TPR) is poor, and the effect of using it directly in oil well cement is not good. TPR was modified by different silane coupling agents, and the hydrophilicity of the modified TPR was studied by Fourier-transform infrared (FT-IR) spectroscopy and dispersion stability photography. The application effect of modified TPR in oil well cement slurry was also evaluated. The fracture surface morphology of TPR cement stone was observed by macrophotography and scanning electron microscopy (SEM). The results demonstrated that the hydrophilicity of TPR particles was improved after modification with silane coupling agent 3-methacryloxypropyltrimethoxysilane (KH570), and its application effect in cement slurry was excellent. Compared with the pure cement paste, the compressive strength of the cement paste with addition of TPR modified by KH570 was reduced, but the flexural strength and impact strength of the cement paste were effectively enhanced. Moreover, the modified TPR greatly improved the deformation capacity and decreased the elastic modulus of the cement paste. The modified TPR particles formed a plastic polymer network structure in the cement stone and penetrated the cement hydration products, filling in the cement paste to form a flexible structural center. Thus, it improved the mechanical properties and reduced the brittleness of cement paste.
Cementing involves injection of cement slurry into the annulus between casing and formation, thus forming cement sheath after solidification. The cement sheath seals the formation effectively and supports and protects the casing at the same time [
Improving the mechanical properties of cement-based composites is helpful to ensure the integrity of the cement sheath. Basically, the three main varieties of flexible materials used to improve the mechanical properties of oil well cement include fiber, latex, and rubber. Among them, rubber has the best effect on the elasticity of cement paste, which is conducive to designing a flexible cement slurry system. Li and Guo [
To the best of our knowledge, the most widely applied modifier in cement-based materials is waste tire rubber, while little work has been reported on the oil well cement with other rubber used as reinforcing materials. Thermoplastic rubber is a type of elastomer material. It has the elasticity of rubber at room temperature and can be softened at high temperature. The structure of thermoplastic rubber consists of different resin segments and rubber segments linked by chemical bonds. The resin segments rely on the interchain force to form physical crosslinking points. The rubber segments are highly elastic chain segments and contribute to the elasticity [
Due to the poor hydrophilicity and low density of thermoplastic rubber particles, they are not stable when they are added directly to cement slurry. It is difficult for these particles to disperse evenly, and they easily float on the surface of the cement slurry, which affects the performance of the cement slurry. Therefore, if thermoplastic rubber is to be applied to oil well cement-based composites, the problem of hydrophobicity should first be solved. Surface modification of thermoplastic rubber particles can improve the hydrophilicity and application effect of thermoplastic rubber particles. The modification methods for rubber particles include water washing, acid and alkali corrosion, plasma pretreatment, and various coupling agent treatments [
To develop new flexible materials to improve the flexibility and enhance the mechanical properties of cementing stone, thermoplastic rubber is applied in the mechanical properties modification of oil well cement. In view of the defect of the poor hydrophilicity of TPR particles, TPR is modified by silane coupling agents and the effect of modified TPR on the mechanical properties of oil well cement-based composites is studied.
The cementing materials obtained from Gezhouba Special Cement Co., Ltd., China, were conventional class G oil well cement. A filtration reducer, a retarder, and a dispersant were purchased from Jingzhou Jiahua Technology Co., Ltd., China. A filtration reducer is mainly used to decrease the water loss of cement slurry. The effect of the retarder is to adjust the thickening time of the cement slurry, and the dispersant can improve the fluidity of the cement slurry. An enhancer and a defoamer were produced in the laboratory. The enhancer can improve the strength of cement paste, and the defoamer is used to prevent foaming when preparing cement slurry.
Thermoplastic rubber (TPR) was obtained from Hunan Yueyang Baling Petrochemical Co., Ltd., China. TPR is used as a flexible material. The silane coupling agents (3-aminopropyl)triethoxysilane (KH550), 3-glycidoxypropyltrimethoxysilane (KH560), and 3-methacryloxypropyltrimethoxysilane (KH570) were obtained from Jiangxi Chenguang New Materials Co., Ltd., China. The silane coupling agents KH550, KH560, and KH570 were used to modify the surface properties of TPR and to improve the hydrophilicity of TPR and its application effect in cement slurry.
According to the structural characteristics of the silane coupling agent, KH550 and KH560 were prepared with water as the solvent and KH570 with ethanol as the solvent. The specific processing methods were as follows:
1–2 mg TPR modified by a silane coupling agent was mixed with 200 mg pure potassium bromide and then ground evenly. The sample was put into a mold and pressed into a thin sheet. Spectral analysis of TPR particles before and after modification was performed using a Fourier-transform infrared spectrometer (EQUINOX 55, Bruker, Germany).
Unmodified TPR particles and TPR particles modified by a silane coupling agent were put into distilled water. The dispersion of TPR particles in water was observed after a period of time, and the hydrophilicity of the particles was judged from the macroscopic observation.
The preparation of cement slurry is based on the corresponding regulations of Chinese standard GB/T 10238-2005. According to the proportions of the experimental formulas, cement slurry samples were prepared using a constant speed agitator (TG-3060A, Shenyang Taige Petroleum Instrument & Equipment Co., Ltd.). The formulas of the cement slurry are presented in Table
Slurry samples with different TPR contents.
Sample number | Cement (wt.%) | Water (wt.%) | Filtrate reducer (wt.%) | Enhancer (wt.%) | Dispersant (wt.%) | Retarder (wt.%) | Defoamer (wt.%) | TPR (wt.%) |
---|---|---|---|---|---|---|---|---|
T0 | 100 | 44 | 2 | 2 | 0.6 | 0.4 | 0.5 | 0 |
T1 | 100 | 44 | 2 | 2 | 0.6 | 0.4 | 0.5 | 1 |
T2 | 100 | 44 | 2 | 2 | 0.6 | 0.4 | 0.5 | 2 |
T3 | 100 | 44 | 2 | 2 | 0.6 | 0.4 | 0.5 | 3 |
T4 | 100 | 44 | 2 | 2 | 0.6 | 0.4 | 0.5 | 4 |
The performance test of cement slurry was carried out in accordance with the corresponding provisions of the Chinese standard GB/T 19139-2012.
The cement stone sample was prepared according to the preparation process of a cement stone sample. After curing for 28 d, the stress-strain behavior of the cement stone was tested according to Chinese standard GB/T 50266-2013 using a universal testing machine (HY-20080, Shanghai Hengyi Precision Instrument Co., Ltd., China) under a constant speed load of 2 kN/min.
After curing for 7 d, the cement stone was destroyed under an external load, and specimens with smooth and unpolluted surfaces were selected. The microstructure of cement stone was observed by scanning electron microscopy (SEM) (SU8010, Hitachi, Japan).
TPR was modified by different silane coupling agents according to the experimental method of TPR treatment in the laboratory. The spectra and changes of the surface groups of modified TPR were analyzed. The results of the experiment are shown in Figure
FT-IR spectra of TPR before and after modification (0: unmodified TPR; 1: TPR modified by KH550; 2: TPR modified by KH560; 3: TPR modified by KH570).
As shown in Figure
As shown in Figure
Dispersion stability of unmodified and modified TPR in water (0: unmodified TPR; 1: TPR modified by KH550; 2: TPR modified by KH560; 3: TPR modified by KH570).
The main purpose of applying TPR in oil well cement is to develop a flexible cement slurry system and improve the mechanical properties of cement paste. Therefore, the evaluation of the application effect is mainly focused on the mechanical properties of cement paste with TPR addition. The compressive strength is the maximum external force that a cement sample can bear when it is destroyed [
Figure
Compressive strength of TPR cement slurry modified by different silane coupling agents.
Flexural strength of TPR cement slurry modified by different silane coupling agents.
Impact strength of TPR cement slurry modified by different silane coupling agents.
In addition, the compressive strength, flexural strength, and impact strength of TPR cement paste modified with KH570 are higher than those modified with KH550 and KH560. The molecular formula of the silane coupling agent KH570 is CH2=C (CH3) COO (CH2)3Si (OCH3)3. The organic functional groups of the KH570 molecular formula can form chemical bonds with the surface of rubber particles, and the hydrophilic groups formed on the surface of TPR particles help them disperse in cement slurry [
Subsequent experiments were carried out with TPR modified by silane coupling agent KH570.
The conventional properties of cement slurry mainly include the rheology, water loss, free liquid, and thickening time. The rheology determines the pumping ability and construction safety of cement slurry during cementing operation [
Rheology and free liquid of modified TPR cement slurry.
Sample number | Modified TPR content (wt.%) | Rheology | Free fluid (%) | |
---|---|---|---|---|
|
|
|||
T0 | 0 | 0.78 | 0.37 | 0.2 |
T1 | 1 | 0.76 | 0.45 | 0 |
T2 | 2 | 0.73 | 0.56 | 0 |
T3 | 3 | 0.72 | 0.59 | 0 |
T4 | 4 | 0.68 | 0.67 | 0 |
Water loss of cement slurry with different modified TPR contents.
Thickening time of cement slurry with different modified TPR contents.
Table
As a flexible material, the main function of the modified TPR added to the cement slurry system is to improve the mechanical properties of cement paste. The compressive strength, flexural strength, and impact strength of the modified TPR cement samples were evaluated, and the results are shown in Figures
Compressive strength of cement stone with different TPR contents for different curing times.
Flexural strength of cement stone with different TPR contents for different curing times.
Impact strength of cement stone with different TPR contents for different curing times.
The experimental results show that the compressive strength of cement stone is decreased with the addition of modified TPR at different curing ages, but the flexural strength and impact strength of cement paste are greatly improved. The mechanical properties of the modified TPR cement sample increase rapidly within 7 days of curing. With the prolongation of the curing time, the increase of the compressive strength, flexural strength, and impact strength slows down.
As shown in Figure
As shown in Figure
To achieve the purpose of long-term cement sealing, cement paste also requires great flexibility and deformation capacity. The stress-strain behavior reflects the deformation rule of cement stone under external stress, and the elastic modulus of cement stone can be obtained from the stress-strain test results. The modulus of elasticity is the standard for measuring the difficulty of deformation of cement stone [
Stress-strain curves of pure cement sample and 3% TPR cement stone.
Test results of stress and strain.
Sample number | Modified TPR content (wt.%) | Peak stress (MPa) | Peak strain (%) | Elastic modulus (GPa) |
---|---|---|---|---|
T0 | 0 | 45 | 0.49 | 9.3 |
T3 | 3 | 42.6 | 0.87 | 5.1 |
As can be observed in Figure
The elastic modulus of cement paste can be obtained according to the test results of the stress-strain behavior. The results in Table
The internal morphology of the cement sample with 3% TPR (T3) was observed. The internal macromorphology structure after the destruction of cement paste is shown in Figure
Internal macromorphology of a T3 cement stone specimen.
SEM photos of T3 cement stone specimens: (a) the overall microstructure of TPR cement stone; (b) the partial microstructure of TPR cement stone.
The modification effect of TPR with silane coupling agent KH570 used in the surface treatment is the best. The hydrophilicity of TPR particles is improved, and the dispersion in water is good. Additionally, the compressive strength, flexural strength, and impact strength of modified TPR cement stone are 13.2%, 19.2%, and 10.2% higher than that of the unmodified cement stone, respectively. After adding TPR, the rheology of the cement slurry system meets the construction requirements. The settlement stability and water loss are better than those of pure cement slurry, and there is no adverse effect on the thickening time. Modified TPR will slightly reduce the compressive strength of cement paste but effectively improve the flexural strength and impact strength. When the content of modified TPR is 3%, the compressive strength of TPR cement paste decreases by 5.33% after 28 days of curing, while the flexural strength and impact strength increase by 9.8% and 10.4%, respectively. In addition, the content of modified TPR applied in oil well cement slurry needs to be controlled. Modified TPR can effectively improve the deformation capacity of cement paste and reduce the elastic modulus. Compared with the pure cement sample, the maximum strain of 3% modified TPR cement paste increases by 77.55% and the elastic modulus decreases by 45.16%. When the modified TPR cement slurry is solidified at 90°C, the connection structure formed by the softening of TPR particles dispersed in the cement slurry penetrates the hydration products of oil well cement. The structure and properties of the cement matrix are improved, and the brittleness of the cement stone is reduced.
The data used to support the findings of this study are included within the article.
The authors declare that they have no conflicts of interest.
The authors received the financial support from the National Science and Technology Major Project (Nos. 2016ZX05060-015, 2016ZX05025-004-003, and 2017ZX05032004-004) grant funded by the Chinese Government and Open Fund (PLN201715) of State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Southwest Petroleum University).