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Water swelling rubber downhole sealing application

the self tightness (or swelling) of water swelling rubber has been used in the application of seals in downhole operation environment. For example, the expandable gasket, which can seal the non-circular hole by swelling, is used to seal the open hole partition. The swelling gasket can replace the traditional bonding and drilling, and avoid the potential decline of permeability along the well edge caused by bonding. Water swelling rubber is also combined with expandable naked eye liner to separate the water belt. The rubber part has the sealing function after the swelling of the lining, and the swelling of the rubber further enhances the sealing ability

in sealing applications, two characteristics of water swelling rubber are noteworthy: water swelling rubber takes time to swell, and the mechanical properties of water swelling rubber will weaken with the increase of swelling rate. It is this decline in mechanical properties that imposes the greatest restrictions on the use of this type of sealing tools. In general, binder can be added to reduce this downward trend, but the final use effect is determined by the final swelling mechanism

the basic swelling mechanism is briefly discussed below

swelling in oil

the degree of equilibrium swelling of rubber in a solvent depends on the difference in cohesive energy density between rubber and solvent. Cohesive energy is the total energy required for a molecule (which can be solid or liquid) to move from its surrounding molecules. It is approximately equal to the heat of vaporization or sublimation. The square root of cohesive energy density is called "solubility parameter". According to the dissolution law, that is, "similar solubility", so if the solubility parameter of rubber is similar to that of solvent, it can be expected to obtain a greater degree of rubber swelling. For example, ethylene propylene diene monomer (EPDM) and oil are the same, so they are often used for swelling in oil

swelling in water (salt water)

the mechanism of water swelling in water or salt water can be illustrated by taking the well-known super absorbent polymer and sodium polyacrylate as examples. Sodium polyacrylate is a polymer (polymer) electrolyte and swells due to osmotic pressure. Fixed anionic charges on suspended polymer chains or solvents need to be kept electrically neutral through the balance of cation balance ions. Therefore, equilibrium ions do not diffuse outward from the polymer matrix, although they are movable. It is further inferred that the chemical potential energy of the equilibrium ions in the polymer matrix is higher than that of the polymer not immersed in water. In order to balance the chemical potential energy of ions (or maintain the thermodynamic balance), water enters the polymer matrix, resulting in the swelling of the polymer matrix. The swelling degree of expanded polymer matrix comes from the balance between the elasticity of polymer complex and the osmotic pressure of equilibrium ions. Water swelling rubber can be obtained by copolymerization or compounding of polyelectrolyte polymer and rubber at the same time

this study explains how to use low molecular weight organic salt and rubber mixing to produce water swelling rubber


in this study, nitrile rubber (NBR) was selected as the basic material because of its wide temperature range and good mechanical properties in downhole sealing applications. Different swelling agents, such as sodium polyacrylate, crosslinked poly sodium salt (isobutylene co maleic acid), sodium acrylate, sodium acetate and sodium formate, are mixed into the compound to evaluate the swelling performance. The swelling experiment is carried out in fresh water and salt water, or there may be no pressure, and the temperature is different. The influence of salt concentration on the swelling rate and the influence of filler and vulcanization conditions on the swelling rate were studied at the same time. This paper only provides the results obtained from organic acid salts. The salt concentration provided below is based on the specific gravity of water

results and discussion

first, the low molecular weight organic salt was used to screen the compounds mixed with sodium acetate, NBR and EPDM respectively. No reinforcing filler was added during the mixing process. Samples are obtained by punching 2mm thick vulcanized film. Soak the sample in fresh water and a solution containing 2% KCl and place it at room temperature. The results are shown in Figure 1

Figure 1. Weight changes of NBR and EPDM at room temperature

after 13 days of immersion, the weight of NBR samples increased by more than 400% in fresh water and more than 200% in 2% KCl solution. The increase of EPDM sample weight is less than 50% in fresh water and 2% KCl solution respectively. Because the swelling rate of EPDM is so low, it can be ignored in the study. Compared with sodium acetate, its lack of polarity is the main reason. However, the situation of nitrile rubber is opposite, which is due to its semi polar nitrile group (- C ≡ n) composition. In order to remain electrically neutral, the relatively high swelling rate indicates that an environment has been identified that requires the diffusion of water (or brine) into the rubber

Figure 2 compares the test results of adding acetate to acrylate and the changes of molecular shape. The test was carried out in KCl solution with a concentration of 2% at 82 ℃. After eight days of immersion, NBR samples obtained weight gain of 180%, 245% and 175% in acetate solutions of 20%, 30% and 35%. Higher acetate concentration improves the swelling performance in terms of mass increase and rate, but the increasing length of carboxylic acid carbon chain seems to be disadvantageous. The swelling degree at 35% sodium acrylate concentration is similar to that at 20% sodium acetate concentration. These experimental results, which are therefore not suitable for plastic and rubber materials, show that acetate is a more effective swelling agent (and/or commonly used long-chain organic salt) than acrylate

Figure 2. Effect of swelling agent on swelling rate at 82 ℃

the rubber compound previously used in the experiment did not add any reinforcing filler, such as carbon black. To do a good job in safety and environmental protection for rubber with large swelling ratio, the top leaders of the enterprise need to "transform and upgrade" the concept. The sample becomes very soft and easy to tear. In order to improve its mechanical properties, carbon black was added to the compound formula. The swelling rate of NBR samples with and without carbon black in 20% acetate was compared, and the results are shown in Figure 3. The laboratory was carried out at 82 ℃ in sodium chloride brine with a concentration of 3.5%. The experimental results show that the poor mechanical properties have been improved, but at the cost of reducing the swelling ratio. Since the swelling rubber needs to maintain as much strength as possible, fillers are used in all NBR compounds in the following experiments

Figure 3. Effect of fillers on swelling rate

the effects of vulcanization conditions, such as vulcanization time and vulcanization pressure, on swelling rate and swelling ratio were also studied. Figure 4 shows the swelling properties obtained by vulcanizing the sample at 10 tons of pressure for 10 minutes, at 10 tons of pressure for 30 minutes, and at 30 tons of pressure for 10 minutes at 82 ℃ and in NaCl solution with a concentration of 3.5%. The over vulcanization of the sample (high vulcanization pressure or long vulcanization time) leads to a slower swelling rate and less weight increment

Figure 4. Influence of vulcanization pressure and vulcanization time on swelling rate

since these plastics are best used underground in occasions requiring chemical contact, mechanical damage and repeated cycle load resistance, rubber parts used need to swell under hydrostatic pressure, so we studied the influence of fluid static pressure on swelling ratio and swelling rate. Within the accuracy range of the experiment, the hydrostatic pressure does not affect the swelling speed and swelling ratio of NBR sample in sodium acetate solution. The results are shown in Figure 5. Similarly, the test was carried out in NaCl solution with a concentration of 3.5% at 82 ℃

Figure 5. Effect of pressure on swelling rate

in addition to sodium acetate, sodium formate can also be used as a water swelling agent. Figure 6 shows the results of NBR samples soaked in sodium formate and sodium acetate solutions of the same concentration. It can be seen that even if the initial swelling rate of sodium formate is low, the final swelling rate of the sample immersed in sodium formate is slightly higher than that immersed in sodium acetate

Figure 6. Comparison of sodium formate and sodium acetate

once these organic salts were determined to be effective swelling agents, we began a study using samples 2mm thicker than the previously used rubber samples. Change the shape of the test specimen to the button shaped specimen of American ASTM standard (25.4mm diameter * 12.5mm thickness)

due to the use of a slow initial swelling rate, in the continuous stage of the test, in addition to comparing the effect of surface area, sodium formate was also used as a water swelling agent. Sodium acetate was used in later tests, and the results are shown in Figure 8

one reason for using thicker samples is to better simulate the actual service conditions. Compared with the thinner sample, the swelling rate of the thicker sample is slower than that of the thinner sample due to the longer diffusion time of the larger penetration sample. This result can be obtained by measuring the swelling ratio and swelling rate at two temperatures. Figure 7 shows the test results in NaC1 solution with a concentration of 3.5%. These results show that the swelling of the sample at 93 ℃ is faster than that at 82 ℃

Figure 7. Effect of temperature on swelling rate

since the swelling rate is controlled through the diffusion process, a larger contact area will also lead to faster initial swelling. Therefore, if you want to get a faster initial swelling rate, you must maximize the effective area in the seal design. However, the final equilibrium swelling should be independent of the surface, because the sample is unstable. Figure 8 compares the initial swelling of button samples that increase surface area by drilling. The experiments were carried out at 93 ℃ and in NaCl solution with a concentration of 3.5%. As expected, the initial swelling of button samples with three holes is faster than those with only one hole or no hole. Since the weight of the button sample can still be increased in the later stage of the test, it can be concluded that the final swelling rate has not been reached

figure 8. Effect of water contact area on swelling rate

later, in order to further try to simulate the field conditions, the swelling rate was tested in different brine. Figure 9 compares NBR using sodium formate as swelling agent, in NaCl, CaCl2 and KCl solutions with a concentration of 3.5%, at a temperature of 82 ℃. The swelling rate of CaCl2 solution with a concentration of 5% is slightly higher than that of potassium chloride with a concentration of 3.5%, while that of the latter is higher than that of NaCl solution with a concentration of 3.5%. What is somewhat unexpected in this result is that the diffusion/permeability is usually controlled in a large range by the ionic strength of brine. These results show that the swelling rate decreases with the increase of the molar concentration of the solution, and the problem of domestic 3D printing materials is more prominent. Obviously, CaCl2 plays a leading role in this system, which is in sharp contrast to the properties shown in acrylic based super absorbent polymers. In the latter systems, calcium ions act as inhibitors to inhibit swelling

Figure 9. Effect of the valence of salt on the swelling rate

however, in the same salt solution, higher concentration will lead to weaker swelling, as shown in Figure 10

Figure 10. Effect of salt concentration on swelling rate


in this water swelling study, nitrile

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