Abstract
The inorganic scaling in wells is a common problem faced by mining companies. At present, the use of protective coatings for tubing as a measure to prevent or reduce the formation of inorganic scale deposits on pipe walls has not been fully studied. To use protective coatings as a measure to counteract the deposition of inorganic salts, it is necessary to develop a method that allows assessing the ability of coatings, as well as polymer and metal materials, to prevent the formation of inorganic scale deposits on the inner surface of pipes.
The article proposes a method for assessing the ability of protective coatings to resist the inorganic scaling on the inner surface of tubing. The proposed assessment method allows to make an informed decision on the advisability of using internal protective coatings of tubing to prevent (or reduce) the formation of inorganic scale deposits. The authors consider design features of a test bench for assessing the resistance of coatings to inorganic scale deposits, which allows to simulate the conditions for the formation of scale deposits that are as close as possible to the real conditions of oil production facilities. The article presents the results of bench tests of nine coating samples, two polymer samples and one sample made of St 40G2 steel. To assess the effectiveness of using tubing with an internal anti-corrosion coating as a measure to combat scale deposits, additional research is required to assess the possibility of complex use of coatings in conjunction with other methods of preventing processes of inorganic scaling. Thus, the authors developed the Bench for assessing the resistance of protective coatings of tubing to inorganic scale deposits. A dynamic testing technique is proposed to evaluate the resistance of protective coatings to inorganic scale deposits. Based on the presented results, conclusions were drawn about the possibility of using protective coatings on tubing as a measure to prevent the formation of inorganic scale deposits on the inner surface of the tubing.
1 Introduction
The inorganic scaling on the walls of tubing is an urgent problem, which is associated with a deterioration in the operational characteristics of wells and an increase in the cost of their maintenance. The problem is relevant for almost all oil production facilities, the well products of which reach a high degree of water cut and mineralization.
One of the methods for preventing the deposition of inorganic salts on the working surface of oil and gas equipment is the use of protective coatings for tubing pipes. Persiyantsev [1] mentions the positive experience of using tubing with coating of the internal surface with glass, varnishes and enamels, and describes the positive experience of using equipment at the Samotlor field, with coatings based on epoxy resins, polyamide compounds, and fluoroplastic.
Zang et al. [2] in their research develop scalable and reliable organogel coatings for sustainable protection of equipment surfaces from scale deposits, which can be used for practical applications in oil production.
Yu et al. [3] discuss the use of polydopamine coating (PDA) to reduce the formation of calcium carbonate on the working surface of equipment. A series of laboratory tests are described that compare the adsorption of calcium carbonate on a polydopamine coating (PDA) and on carbon steel, noting that less calcium carbonate is formed on the polydopamine coating.
Xixi et al. [4] use a new composite coating based on organosilicon epoxy resin (EP) (EP/iDCNTs/Zn/PVDF) to protect against scaling and corrosion. There was a reduction in the formation of calcium carbonate when using this coating by 81%.
Li et al. [5] describe in their research the ability of a superhydrophobic coating to prevent scaling because the CaCO3 crystals on the superhydrophobic coating were predominantly needle-shaped compared to the rhombohedral CaCO3 crystals on the surface of a steel substrate.
Zhang et al. [6] claim that the use of superhydrophilic coatings can prevent the formation of calcium salts on the surface of these coatings.
Currently, the question of the possibility of using protective coatings on tubing as a reagent-free method to prevent or reduce the intensity of the formation of inorganic salts on the walls of tubing pipes remains open. In this regard, it is relevant to develop a laboratory method that allows us to evaluate the possibility of using protective coatings on tubing, as well as polymer lining materials, to prevent or reduce the formation of scale deposits on the inner surface of the tubing.
2 Development of a method for assessing the resistance of protective coatings of tubing pipes to inorganic scaling
The authors formed the following requirement for a method for assessing the resistance of protective coatings to inorganic scale deposits: the tests must be dynamic, allowing to simulate the flow of a salt-forming medium relative to the test surface.
Initially, the option of creating a laboratory circulation test bench [7] was considered, the principle of operation being similar to the method of assessing protective coatings for resistance to the loss of asphalt, resin, and paraffin (hereinafter referred to as ARP) deposits [8]. But this option had to be abandoned due to the impossibility of using the principle of a circulation bench when studying scaling processes. Since the scaling processes have a different nature from paraffin, scaling cannot be localized in a strictly defined area of the circulation bench circuit under study, as it was done when studying the processes of precipitation of paraffin. In addition, one of the factors in the formation of inorganic salts is the mixing of incompatible waters from different layers, and this process cannot be simulated using a circulation bench, since after the scaling, the flow medium will become depleted of the fallen salt and further circulation of the liquid in the circulation bench will not lead to further growth of inorganic salts on the surface of pipes.
Tung et al. [9] and Zhou et al. [10] used flow benches with an open loop to study the processes of scaling in pipes, that is, the salt-forming medium was passed through the test pipe samples once. The disadvantage of this technical solution is the high consumption of solutions, the supply of which to the flow circuit is necessary to create conditions for scaling and simulate various flow rate regimes.
To evaluate protective coatings and materials for resistance to scaling, Bethke et al. [11] showed the use of the rotating cylindrical electrode (RCE) method. The peculiarity of this method is that it is based on the electrochemical mechanism of salt precipitation (cathode or anodic deposition), which does not simulate the processes occurring in wells. Also, in [11], the “flow loop” method was used to study the formation of barite and halite on the coating. It has been shown that the protective coating does not reduce the mass of the formed barite but changes its texture and makes cleaning pipes from the formed barite less labor-intensive. The disadvantage of the flow loop method is that not all samples of protective coatings have the technical possibility of applying a “flow loop” to the inner surface due to its small diameter.
Considering the advantages and disadvantages of existing equipment, as well as the necessary requirements for developing a method for assessing the resistance of coatings to scale deposits, Samara Research and Production Center LLC developed and assembled a test bench, which was called the “Bench for assessing the resistance of coatings to inorganic scale deposits.” Berkov et al. in their previous research [12] showed that the tested protective coatings reduce the mass of salt deposits (gypsum deposits with a halite component) compared to the rough sample but are not able to completely prevent the formation of a layer of inorganic salts on the surface. In this work, the list of coatings and materials tested for resistance to scaling processes on the surface was expanded. Thus, this work is a continuation of the work [12]. In this work, in addition to liquid epoxy coatings, a silicate-enamel coating was tested, as well as polymer materials: polyethylene and polyamide. The choice of polymer materials as objects of study is not accidental: Kashavtsev and Mishchenko [13] describe the positive experience of using pipes with a polyethylene insert in the Orenburg region. They [13] stated that gypsum crystals are not able to grow on a polyethylene surface even if there are scratches.
3 Materials and methods
Tests of coatings and materials were performed at the “Bench for assessing the resistance of coatings to inorganic scale deposits”, developed by the team of Samara RPC LLC. The operating principle of the bench is based on testing cylindrical samples with a protective coating on the outside, immersed in a reactor with a salt-forming medium (Fig. 1).
Bench for assessing the resistance of coatings to inorganic scale deposits (Own source)
1. Working chamber (reactor)
2. Containers for supplying water samples to the reactor
3. Peristaltic pumps for feeding samples into the reactor
4. Electric drive providing rotation of samples at a given speed
5. Lifting mechanism
6. System for heating and maintaining temperature in the reactor
7. Control unit for setting the required test conditions (Rate of water supply from containers to the reactor, temperature of the medium in the reactor, rotation speed of the samples)
Citation: International Review of Applied Sciences and Engineering 16, 1; 10.1556/1848.2024.00819
The purpose of the bench tests was to identify the coating (material) whose surface is most resistant to scaling under flow conditions. The criterion for assessing the resistance of coatings (materials) to scaling processes is the mass of inorganic salts formed on the outer surfaces of a cylindrical sample, as well as the thickness of the formed layer of inorganic salts.
Similar studies were performed in [12], and in this work the list of tested materials and coatings was expanded.
Rotating cylindrical samples were immersed in a reactor filled with mineralized water. Then the rotation of the cylindrical samples was started at a given identical speed rpm, and in parallel with this, solutions of sodium sulfate and calcium chloride were continuously dosed into the reactor. Throughout the test, a constant temperature was maintained in the reactor. This method allows to test coatings and materials for resistance to scale deposits gypsum-halite type.
Using an electron scanning microscope “TESCAN VEGA3” using the adapter “Oxford Instruments X-Act” the qualitative composition of the salt deposits formed on the samples was determined (energy dispersive analysis). Figure 2 shows the microstructure of the scale layer formed on the surface of a rotating cylindrical sample, and Figs 3 and 4 show the corresponding energy-dispersive spectra.
Microstructure of the salt deposit layer (Own source)
Citation: International Review of Applied Sciences and Engineering 16, 1; 10.1556/1848.2024.00819
Energy dispersive spectrum (Own source)
Citation: International Review of Applied Sciences and Engineering 16, 1; 10.1556/1848.2024.00819
Energy dispersive spectrum (Own source)
Citation: International Review of Applied Sciences and Engineering 16, 1; 10.1556/1848.2024.00819
Thus, the inorganic salt deposits formed as a result of tests on the test samples are gypsum with halite impurities.
The test objects were specially made cylindrical samples with applied protective coatings, a polyethylene sample, a polyamide sample, and a steel sample of grade St 40G2 (Fig. 5).
Cylindrical samples (Own source)
Citation: International Review of Applied Sciences and Engineering 16, 1; 10.1556/1848.2024.00819
Table 1 presents the compositions of internal protective coatings of the tubing.
Composition of internal protective coating of tubing (Own source)
| Coating | Composition of the protective coating |
| Coating 1 | Epoxy phenolic polymer based protective system |
| Coating 2 | Epoxy phenolic polymer based protective system |
| Coating 3 | Two-layer epoxy phenolic coating |
| Coating 4 | Two-layer epoxy phenolic coating |
| Coating 5 | A coating consisting of a layer of epoxy-phenolic or phenolic primer and a layer based on epoxy materials |
| Coating 6 | A coating consisting of a layer of epoxy-phenolic primer and a layer based on epoxy materials, modified by non-thermal microwave treatment |
| Coating 7 | Silicate enamel coating |
| Coating 8 | Suspension of pigments and fillers in a polyurethane binder |
| Coating 9 | Suspension of pigments and fillers in a polyurethane binder |
4 Results of bench tests
Bench tests at the “Bench for assessing the resistance of coatings to inorganic scale deposits” were performed with the rotation of cylindrical samples at speeds of 100, 500, 1,000, 2,000, 3,000 rpm, which is equivalent to the linear velocities of the liquid in the pipe 0, 1, 0.5, 1, 2, 3 m s−1 and at a temperature of 30 ⁰C. The duration of each experimental cycle is 6 h.
The figures show samples made of polyamide, polyethylene and a sample with a silicate-enamel coating, tested at various speed conditions.
Table 2 presents the results of the bench tests.
Bench test results (Own source)
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The obtained results of bench tests are presented in the form of graphical dependences of the scale deposit mass on the speed regime (Fig. 7).
100 rpm equivalent to linear flow velocity 0.1 m s−1 (Own source)
Citation: International Review of Applied Sciences and Engineering 16, 1; 10.1556/1848.2024.00819
The influence of flow speed on the process of scaling on various brands of protective coatings and on polymer materials (Own source)
Citation: International Review of Applied Sciences and Engineering 16, 1; 10.1556/1848.2024.00819
The authors studied the processes of forming a salt deposit layer (gypsum-halite type) on various brands of internal protective coatings of tubing. They defined a relationship between the mass of the scaling layer and the rotation speed of the sample (equivalent to the flow speed in the pipe).
5 Discussion
As a result of bench tests, it was revealed that none of the tested brands of protective coatings is capable of completely preventing the formation of gypsum scale deposits with halite impurities on its surface, but a decrease in the mass increase of the scale deposit layer is observed compared to the steel sample that models the rough tubing. The authors found that the use of protective coatings does not prevent salt formation without the additional preventing methods. None of the tested coatings can prevent the formation of scale deposits by 100%. It was revealed that the use of protective coatings leads to a reduction in the mass of 147 formed scale deposits – up to 60.14% (at a simulated flow speed of 3 m s−1).
The thesis that gypsum crystals are unable to grow on a polyethylene surface even in the presence of scratches, outlined in [13], has not been confirmed. Gypsum-halite deposits were consistently formed on both polyethylene and polyamide (see Fig. 6, Table 1).
We assumed that gypsum deposits can form solid structures without forming a strong adhesive bond with the surface on which they form. Figure 8 shows the layers of scale deposits formed on the polymer shafts of the “Bench for assessing the resistance of coatings to inorganic scale deposits”.
Scale deposits removed from the shafts of the “Bench for assessing the resistance of coatings to inorganic scale deposits” (Own source)
Citation: International Review of Applied Sciences and Engineering 16, 1; 10.1556/1848.2024.00819
The formed scale deposits were easily removed from the polymer shafts without damaging them, therefore the adhesive connection between the shafts and the formed scale deposits is minimal or absent. We can conclude that scale deposits can form even on surfaces with minimal adhesive bonding, and the deposits will be fixed due to the non-ideal geometric shape of the object on which they are formed. A similar situation will occur in a pipeline: a layer of scale deposits can be formed in the pipeline, but will not be torn off by the flow, since it will be a solid structure and fixed in the pipe because the pipeline does not have an ideal geometric shape (pipe joints, bends, inter-nipple space, etc.).
Thus, protective coatings of tubing can reduce the increase in the mass of gypsum deposits with halite impurities on the surface of pipes but are not able to completely prevent scaling. In this regard, it is necessary to assess the possibility of complex use of coatings together with other methods of preventing the processes of inorganic salt deposition.
6 Conclusion
The use of protective coatings to prevent or reduce inorganic scale deposits in tubing is a promising but not fully studied preventive method. To evaluate the ability of internal protective coatings of tubing pipes to prevent or reduce the formation of inorganic scaling in pipes, a dynamic bench test method was developed. This method models the process of formation of inorganic scale deposits on samples of protective coatings under dynamic conditions (simulation of flow in a pipe relative to the protective coating). As a result of bench tests, the authors revealed that none of the tested coatings and polymer materials can prevent the formation of gypsum halite deposits on its surface. There was a decrease in the mass growth of salt deposits on protective coatings compared to the steel surface.
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