(1. China Institute of Petrochemical Engineering and Technology, Beijing10010/;

2. School of Petroleum Engineering, Youshi University, China (Beijing), Beijing 102249)

At present, the research on the mechanism of filtrate reducer for oil well cement is mostly speculation, and there is no detailed data support. According to the experimental data of water loss rate, filtrate viscosity and filter cake structure, the mechanism of polyvinyl alcohol filtrate reducer is explained. The results show that the main influencing factor of reducing water loss of polyvinyl alcohol is not its thickening effect on cement slurry, but its reducing the permeability of filter cake, that is, forming a dense polymer film on the surface of filter cake and filter media.

Mechanism of polyvinyl alcohol; Water loss reducer for oil well cement

Mechanism of fluid loss reduction in oil wells

Polyvinyl alcohol cement slurry

Liu Xuepeng 1, 2 Zhang Mingchang 1, Zhang Linhai 1, Ding Shidong 1, Liu Wei 1

(1. China Petrochemical Petroleum Engineering Research Institute, Beijing10010/;

2. School of Petroleum Engineering, China Shiyou University,

Beijing, China 102249)

At present, the understanding of the mechanism of filtrate reducer is mostly speculation, lacking the support of experimental data. In this paper, the action mechanism of polyvinyl alcohol (PVA) was systematically discussed for the first time by measuring filtration loss, filtrate viscosity and electrophoretic mobility of filter cake. The results show that the main factor for PVA to reduce the filtration loss is not the viscosity increase, but 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, and the filtration loss does not increase obviously, and the membrane formed by PVA begins to break, resulting in a sharp increase in filtration loss.

Keywords oil well cement; Fluid loss additive; Polyvinyl alcohol; Operational mechanism

The filtrate reducer for oil well cement is a material that can control the filtration of liquid phase in cement slurry to permeable formation, thus maintaining the proper water cement ratio of cement slurry, which plays an important role in ensuring cementing quality and protecting oil and gas reservoirs [1, 2]. The price of polyvinyl alcohol filtrate reducer for oil well cement is moderate, which has little influence on the retarding time and compressive strength of cement slurry, and has certain film forming and gas channeling prevention effects, and has good application prospects [3, 4].

In this paper, the temperature-resistant 120℃ nonionic polymer filtrate reducer PVA- 120 was synthesized, and the temperature-resistant 160 [3] anionic (AMPS/AM/AA)*** polymer filtrate reducer JHW- 160.

1 experimental part

1. 1 instrument

1) atmospheric thickener, produced by Shenyang Aerospace Industry Research Institute.

2) High temperature and high pressure filtrate reducer, produced by Shenyang Aerospace Industry Research Institute.

3)Zeta potentiometer, JS94H model, produced by Shanghai Chen Zhong Company.

1.2 sample

Industrial product JHW- 160 is a terpolymer polymerized from acrylic acid (AA), acrylamide (AM) and sodium 2- acrylamido -2- methylpropanesulfonate (AMPS). PVA-17-88 was obtained by aldehyde crosslinking.

2 Results and discussion

2. 1 Relationship between water loss and dosage and filtrate viscosity

Table 1 is the measured data of the influence of filtrate reducer dosage on water loss of cement slurry and filtrate viscosity.

Table 1 Effect of filtrate reducer dosage on water loss of cement slurry and filtrate viscosity

Note: "-"indicates that no data measurement has been made.

It can be seen from the table 1 that the water loss of JHW- 160 decreases with the increase of the content. However, when the dosage of PVA- 120 is less than 0.4% BWOC, the water loss is great. With the further increase of dose, the water loss decreased sharply from 95 1 ml 30 min- 1 to 34 ml 30 min- 1, which is the so-called "threshold effect". When the dosage of filtrate reducer is further increased, the water loss will no longer decrease obviously. It can be seen that PVA- 120 and JHW- 160 have different water loss laws and different water reduction mechanisms. The data results are consistent with the literature reports [3].

It can be seen from the table 1 that the viscosity of the filtrate of PVA- 120 does not change obviously with the dosage, while the viscosity of the filtrate of JHW- 160 increases gradually with the dosage, showing a correlation. When the dosage of PVA- 120 was increased from 0.3% BWOC to 0.4% BWOC, the water loss decreased sharply from 951ml for 30 min-1to 34 ml for 30 min-1,while the viscosity of filtrate hardly changed, indicating that the viscosity of filtrate was not PVA-65438. On the contrary, for ionic polymer filtrate reducer JHW- 160, the influence of polymer viscosity or concentration can not be ignored. With the increase of dosage, the viscosity of filtrate of JHW- 160 cement slurry gradually increased and the water loss gradually decreased.

2.2 Relationship between water loss and adsorption capacity

When the dosage of PVA- 120 is 0.4%BWOC, the water loss at 75℃ is 34 ml for 30 min-1; However, when the dosage is 0.3%BWOC, the water loss cannot be controlled. Therefore, these two dosage points were selected to study the difference of surface adsorption capacity of cement particles, and the results are shown in Table 2.

Table 2 Adsorption of PVA-120 on Cement Particles

The experimental results show that the adsorption capacity of PVA- 120 on the surface of cement particles is extremely low, far less than the amount of water reducer. At the same time, when the dosage of PVA- 120 is 0.4%BWOC and 0.3% BWOC, the adsorption capacity of PVA- 1 and PVA-65 1 on the surface of cement particles is not much different, but PVA- 1 and PVA-65/kloc-0. It shows that the adsorption of polyvinyl alcohol on the surface of cement particles is not the main reason for the dehydration of PVA- 120.

2.3 Relationship between water loss and electrical properties of filter cake

The filter cake was redispersed in deionized water to determine the electrophoretic mobility of filter cake particles, and the data shown in Table 3 were obtained.

Table 3 Relationship between water loss and electrophoretic mobility of filter cake particles

Through the study of PVA- 120 and JHW- 160 filter cakes, it can be known that when PVA- 120 can control the water loss, a dense polymer film with certain toughness will be formed at the junction of filter cakes and filter media. At the same time, there is a thin filter cake on the top of the membrane, and an obvious incomplete membrane interlayer can be seen inside the filter cake; However, JHW- 160 only forms a filter cake. The greater the dosage, the smaller the water loss and the thinner the filter cake.

The filter cake obtained from the water loss test of cement slurry containing two polymers was redispersed in deionized water to determine the electrophoretic mobility of filter cake particles. It was found that the electrophoretic mobility of cement filter cake with PVA- 120 did not change much with the increase of dosage, which was basically consistent with the value of pure pulp filter cake, indicating that the electrical properties of filter cake did not change, and its water loss reduction effect had nothing to do with it. This is mainly because PVA- 120 is a nonionic polymer, which is not adsorbed on the surface of cement particles by electrostatic force. For JHW- 160, the electrophoretic mobility of filter cake will change from positive to negative with the increase of dosage, and the absolute value of electrophoretic mobility will increase with the increase of dosage, which shows that the surface electrical properties of cement particles have changed essentially with the addition of JHW- 160, which will inevitably affect the structure and wettability of filter cake, and then play a role in controlling water loss. The research results are consistent with reference [3].

2.4 Relationship between water loss and filter membrane

The results show that forming dense polymer film on filter cake and filter screen is the key to control the filtration loss of polyvinyl alcohol. As long as the polymer concentration required for film formation is reached, there will be no obvious change in water loss. In order to further reduce water loss, it is necessary to understand the structural composition and formation process of this membrane.

From the electron microscope (figure 1), we can clearly see the overall structure of the filter membrane. It can be seen that the filter membrane is composed of many particles with a particle size less than 100μm, and there are adhesion structures between the particles. It is speculated that the formation of filter membrane is a whole composed of PVA molecules and cement particles, in which cement particles pile up and adhere to PVA molecules. At the same time, the particle size distribution of cement particles measured by water-phase laser particle size analyzer (D50 = 17.4 μ m) shows that the particle size of cement particles is mainly below 100μm, which is basically consistent with the particle size observed under electron microscope.

Fig./SEM micrograph of membrane structure of kloc-0/PVA filtrate reducer.

After the formation of filter membrane, the water loss state of cement slurry is obviously improved, and the water loss will decrease instantly, but there will still be a small amount of outflow. This may be because the structure of the filter membrane is composed of cement particles, and the diameter distribution of cement particles shows that there are few particles with the particle size less than 1μm, so there will be some small gaps in the particle accumulation on the filter membrane that cannot be effectively blocked (black holes in the electron microscope), and once they collude, they will show more and more water loss. It is speculated that if this part of the gap is blocked by adding substances with small particle size, the water loss will be reduced.

When the dosage of PVA- 120 is fixed at 0.8%BWOC, the basic slurry formula of 0.5% retarder DZH-2, water cement ratio of 0.44 and Jiahua g cement is adopted, the water loss is 17.2mL at 100℃, as shown in Table 4. Because the particle size of cement is mainly distributed in the range of 1 ~ 100 micron, the medium particle size material ultrafine silicon powder (d50 = 8.3 micron) and the small particle size material nanometer manganese powder (d50 = 0.9 micron) are selected to investigate the improvement effect on water loss. The results are shown in Table 4.

Table 4 is the average value of three tests at 100℃. From the data results, with the decrease of the particle size of the added materials, the water loss gradually decreases. This also proves that improving the pore structure of the filter membrane can improve its water loss control effect.

Table 4 Water loss data after adding small particle size materials

2.5 Mechanism of Fluid Loss Reducer

As the most important additive in oil well cement admixture, the use of filtrate reducer for oil well cement slurry is directly related to the success or failure of cementing construction and a series of problems such as oil well life and productivity. At present, anionic polymer system (AMPS as the main monomer) and nonionic polymer system (including latex system and polyvinyl alcohol system) are widely used. Through the above research, it is further proved that the action mechanisms of these two fluid loss additives are different. Anionic polymer system can control water by changing the electrical properties of filter cake and increasing the viscosity of free liquid. Forming dense polymer film on filter cake and filter screen is the key to water control of polyvinyl alcohol filtrate reducer.

3 Conclusion

1) anionic polymer JHW- 160 controls water by changing the electrical properties of filter cake and increasing the viscosity of free liquid.

2) Forming dense polymer film on filter cake and filter screen is the key to PVA-120 water control.

3) It is the key to improve the temperature resistance and filtration efficiency of PVA membrane and improve the structure of the membrane.

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