(1. China Institute of Petrochemical Engineering and Technology, Beijing10010/; 2. School of Petroleum Engineering, Youshi University, China (Beijing), Beijing 102249)
Chemically crosslinked polyvinyl alcohol (PVA) changes the permeability of filter cake by forming a uniform and dense crosslinked PVA solid film at the interface between filter cake and filter material, which plays a major role in controlling water loss. In this paper, the mechanism and properties of two kinds of chemical cross-linked polyvinyl alcohol filtrate reducer widely used at present are discussed, and the ways to further modify polyvinyl alcohol and improve its high temperature resistance are put forward from the molecular point of view.
Synthesis of additives for oil well cement: Polyvinyl alcohol filtrate reducer
Study on the method of greatly improving the temperature resistance of polyvinyl alcohol filtrate reducer
Liu Xuepeng 1, 2, Zhang Mingchang 1
(1. China Petrochemical Petroleum Engineering Research Institute, Beijing10010/; 2. School of Petroleum Engineering, China Shiyou University, Beijing 102249.
The main factor to reduce the filtration loss of chemically crosslinked polyvinyl alcohol (PVA) is the reduction of the filter cake permeability: a tough, complete and dense polymer film is formed on the surface of the filter membrane below the filter cake. In this paper, the mechanism and properties of two kinds of chemical crosslinking polyvinyl alcohol (PVA) are discussed. PVA was further modified at molecular level to improve its high temperature resistance, and it can be used as filtrate reducer for oil well cement at about 150℃.
Keywords polyvinyl alcohol; Fluid loss additive; Synthetic; Oil cement; Cement additive
Oil well cement filtrate reducer is a kind of material which can control the filtration of liquid phase in cement slurry to permeable formation, so as to maintain the appropriate water cement ratio of cement slurry. It is the most important additive in oil well cement admixture, and its use is directly related to the success or failure of cementing construction and a series of problems such as oil well life and productivity. Compared with other dosage forms, polyvinyl alcohol filtrate reducer has moderate price, little influence on retarding time and compressive strength, certain film forming and gas channeling prevention, and good application prospect [1].
Generally, unmodified PVA has low water loss efficiency and large dosage, and can only be used in formations below 50℃ [1]. At present, PVA filtrate reducer widely used in cementing construction is mostly chemical cross-linking modified products, and its maximum service temperature has also increased to 70 ~ 120℃ [2 ~ 5]. This chemically modified PVA can form a spatial network structure with certain strength in the application process, which limits the flow of free water, and at the same time, it can form a dense low-permeability membrane with gas channeling prevention function with the interface, further reducing water loss [5].
With the development of oil exploration and development to deep wells and ultra-deep wells, higher bottom hole temperature brings greater challenges to cementing engineering. How to modify PVA from the molecular point of view by chemical means to further improve its application temperature is of great significance to cementing operation. On the basis of discussing the mechanism of water loss reduction of polyvinyl alcohol, the ways to effectively improve the water loss reduction performance of polyvinyl alcohol are discussed.
1 polyvinyl alcohol and its mechanism of reducing water loss
1. 1 polyvinyl alcohol structure
Polyvinyl alcohol (PVA) is a kind of white powder resin, which is hydrolyzed from polyvinyl acetate. Figure 1 is a structural fragment of PVA molecule, which contains a large number of hydroxyl (-OH) structures and a small amount of unhydrolyzed carboxymethyl (-—COCH3). Ordinary PVA can be divided into many types according to different molecular weight and degree of hydrolysis, including 300, 500, 1200, 1700, 2200, 2400, etc. According to the degree of hydrolysis, it can be divided into 99% hydrolysis degree (complete hydrolysis type), 88% hydrolysis degree and 78% hydrolysis degree. There is also a low degree of hydrolysis, but it is not common. The marks of domestic products are the first two molecular weights and the last two degrees of hydrolysis, such as 1788, 1799, etc.
Figure 1 PVA molecular structure fragment
The chemical structure of polyvinyl alcohol is stable. The thermal decomposition temperature of 10% is higher than 200℃, and its chemical structure is very stable in high temperature alkaline solution. It has strong calcium and magnesium ion resistance, belongs to nonionic polymer, has little influence on the setting time of cement slurry, and has moderate price, so it is suitable as a raw material or component for developing high-temperature cement filtrate reducer [1].
Mechanism of water loss reduction of 1.2 polyvinyl alcohol
The filtrate reducer mainly plays its role in three aspects: first, it increases the viscosity of filtrate and the movement resistance of free water; The second is to adjust the particle size ratio in the mud cake, control the loss of fine particles, make the filter cake denser and reduce the permeability; The third is to change the electrical properties of cement particle surface and increase the wettability of filter cake capillary [1].
The results show that the increase of filtrate viscosity is not the main reason for PVA to reduce its water loss performance. Whether PVA can form a dense temperature-resistant polymer film at the junction of filter cake and filter material is the main reason for reducing the permeability and water loss of filter cake [1, 5]. When using uncrosslinked PVA, although PVA can form hydrogen bonds within and between molecules through hydroxyl groups (-OH) at room temperature, this hydrogen bond is easy to break and its mechanical properties are poor [1], so there is no film formed at the interface between filter cake and filter medium, and its water loss reduction ability is poor. This is also the reason why the water loss reduction efficiency of unmodified PVA is low. How to form a low-permeability film at the joint and make it resistant to high temperature is the key to improve the performance of PVA filtrate reducer. At present, various chemical crosslinking methods are aimed at this main factor.
2 chemical crosslinking modified PVA water loss reducer
In 1980s and 1990s, foreign countries began to modify PVA by chemical means to improve its application temperature from the molecular point of view [6, 7]. In recent years, great progress has been made in this field in China [1 ~ 3], and related products have been widely used. The main ways are divided into two aspects: one is the crosslinking modification of boric acid, titanic acid, chromic acid or corresponding inorganic salts [5,6,8 ~12]; The second is cross-linking modification of glutaraldehyde [1~ 4,7, 13, 14]. The main purpose of these two modification methods is to form a low permeability and temperature-resistant film at the junction.
2. Crosslinking modification with1boric acid, titanic acid, chromic acid or corresponding inorganic salts.
The earliest method for forming and strengthening the low-permeability membrane at the junction of PVA filtrate reducer filter cake and filter media is to mix linear PVA with a certain proportion of gelling agent such as boric acid, titanic acid, chromic acid or corresponding inorganic salts. PVA and boric acid contact in cement slurry to form a complex structure, which is further strengthened under alkaline conditions, as shown in Figure 2. As early as 1990, there was a patent report in the United States [6], and the complexation mechanism was also studied [12]. In recent years, domestic research and application in this field have been very mature [1 1].
Fig. 2 Complexation reaction of polyvinyl alcohol with boric acid
* * * Mixed crosslinked PVA forms a low-permeability gel film through the mutual contact and bonding of molecules and gel molecules on the surface of filter media, which reduces water loss and greatly improves water loss performance. However, this product has certain application limitations. When the temperature is lower than 40℃, it is difficult to form a uniform composite membrane. When the temperature is higher than 95℃, the composite membrane is easy to decompose, so it cannot be used as a filtrate reducer against high temperature [1].
2.2 glutaraldehyde crosslinking modification
In order to solve the problem of instability of polymer film formed by mixed crosslinking, the method of chemical crosslinking with glutaraldehyde to increase the strength of polymer film appeared again (Figure 3). There are patent reports in 1994 [7] abroad, and the crosslinking mechanism has also been studied [13]. In recent years, relevant research has been done in China [1, 3], and related patent applications have been filed [2].
The chemical crosslinking between PVA and glutaraldehyde also controls the loss of water by forming a polymer film on the interface between filter cake and filter media. However, this chemical crosslinking is more stable than the mixed crosslinking of boric acid, and it is easier to make the chemically crosslinked PVA colloidal particles rich in hydroxyl groups gather at the filtration joint to form a continuous whole [1], thus promoting the formation of a uniform solid film. It is pointed out that the chemically crosslinked PVA colloidal particles gathered in the filter cake can also form discontinuous solid films. This makes the use temperature of glutaraldehyde crosslinked PVA reach 65438 0 20℃. When the temperature is further raised above 120℃, the PVA colloidal particles and the solid film formed will gradually dissolve, the low-permeability gel film will gradually disappear, and the water loss will suddenly increase.
Fig. 3 Complexation reaction of polyvinyl alcohol with glutaraldehyde
2.3 ways to improve the temperature resistance of PVA filtrate reducer
The chemical crosslinking method shows that the chemical modification of PVA molecular structure can improve its temperature resistance as a filtrate reducer, and make its maximum service temperature reach 120℃. At present, this is also the highest applicable temperature when PVA filtrate reducer is used alone. As mentioned above, the chemical structure of PVA is stable, and the thermal decomposition temperature of 10% is higher than 200℃. Can it be further improved?
Recently, Plank et al. [15] in technical university of munich, Germany, made a detailed and in-depth study on the action mechanism of PVA filtrate reducer, and gave suggestions on improving the performance of PVA filtrate reducer. It can be summarized as three points: one is to improve the adhesion of PVA molecules on the surface of particles at high temperature; Second, increase temperature-resistant plugging particles; Thirdly, PVA raw materials with high molecular weight and high hydrolysis degree are used. This is consistent with the early research conclusion of Chen Juan et al. [1] in China, and its purpose is to promote the formation of uniform solid film and increase its temperature resistance. In view of the above research results, PVA can be further modified and developed to obtain PVA temperature-resistant products with good water loss reduction performance.
2.3. 1 glyoxal and glutaraldehyde crosslinking
Glyoxal and glutaraldehyde were mixed to optimize the synthesis route, and the temperature-resistant film-forming PVA water reducer was obtained. The temperature drop resistance and water loss resistance can be further improved to 65438 025℃ by optimizing the dosage of raw materials and reaction route with the above dialdehyde crosslinking method. When the temperature exceeds this temperature, the formed low-permeability film will gradually dissolve, and the water loss of cement slurry will greatly increase. Fig. 4 shows the filter cake and low-permeability filter membrane formed at 125℃.
Fig. 4 Filter cake and low-permeability filter membrane (125℃)
2.3.2 Modification of inorganic nano-plugging particles
According to Plank et al.' s research, in this paper, nano-silica (30nm) was grafted onto PVA molecules with epichlorohydrin [16], and then crosslinked with glutaraldehyde to obtain another temperature-resistant film-forming PVA water loss reducer. The reaction route is shown in Figure 5. Modified PVA has good water loss reduction ability below 130℃, but its consistency is large, which is not conducive to field practical application. Fig. 6 is a sample diagram of nano-silica modified PVA.
Fig. 5 Graft modification of nano-silica (about 30 nm)
Fig. 6 silica grafted PVA sample
2.3.3 Modification of organic heat-resistant plugging particles
It can be seen from the above research that although the crosslinking of dialdehyde and the introduction of nano-silica modified PVA with adhesion resistance and temperature resistance improve its temperature resistance, the improvement is limited. The reason is that when the temperature rises further, PVA molecules will dissolve quickly and leak with free water. How to reduce its solubility at high temperature and increase its adhesion on the surface of cement particles will be beneficial to further improve its temperature resistance. The research conclusion of Plank et al. was adopted: the strength of polymer membrane was improved by dialdehyde crosslinking, and organic heat-resistant polymer was used as high temperature plugging particles. At the same time, a small amount of chemical functional groups that change the molecular properties of PVA are introduced to reduce its high-temperature solubility, increase its adhesion on the surface of cement particles, and comprehensively improve its temperature resistance.
In this method, a small amount of sodium 2- acrylamide -2- methylpropanesulfonate (AMPS) was introduced to increase the molecular adhesion, a small amount of rigid support structure N- vinylpyrrolidone (NVP) with temperature resistance was introduced, and synthetic temperature-resistant polymer plugging particles were added to obtain a PVA film-forming water loss reducer with good water loss reduction ability at 150℃. The reaction route is shown in Figure 7.
Fig. 7 chemical grafting modification of polyvinyl alcohol and introduction of organic heat-resistant plugging particle products
2.4 Summary
On the basis of discussing the mechanism of water loss reduction of polyvinyl alcohol, the ways to effectively improve the water loss reduction performance of polyvinyl alcohol were discussed, and two kinds of polyvinyl alcohol modified water reducers with good water loss reduction performance and excellent comprehensive performance of cement slurry were synthesized at 125℃ and 150℃ respectively. This provides an effective way to further modify polyvinyl alcohol and improve its high temperature resistance.
3 Conclusion theory
1) chemical crosslinking PVA forms a uniform and compact crosslinked PVA solid film at the interface between filter cake and filter material, which changes the permeability of filter cake and plays a major role in controlling water loss.
2) The solid membrane composed of two aldehydes mixed with chemical crosslinking PVA has high strength and good stability, which can improve the high temperature resistance of PVA filtrate reducer.
3) The high temperature resistance of PVA filtrate reducer can be further improved by using high molecular weight PVA, introducing molecules to increase molecular adhesion and adding plugging particles.
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