Modification of polymers to synthesize thermo-salt-resistant stabilizers of drilling fluids

1 Introduction

The most important technological functions of drilling fluids that ensure high-speed drilling of wells are determined by their rheological properties – consistency, mobility, structural, and mechanical parameters. The rheological characteristic of the system is a set of properties that determine its ability to flow to change shape. Broadly speaking, rheology is the science of deformations and flow. Regulation of the rheological properties of drilling fluids, which continuously change during the drilling process, is one of the most important tasks of colloidal chemical science. The rheological parameters of the washing liquids characterize the physico-chemical processes occurring in it [1]. The synthesis of composite highly acidic acrylic polyelectrolytes resistant to salt aggression and temperature when drilling deep wells in complicated conditions is the only way to solve the problem in a promising way [2]. It is known that the creation of thermosalt-resistant composite polymer stabilizers for regulating the rheological properties of drilling fluids can be achieved by using copolymerization methods of strongly acidic monomers or the introduction of sulfogroups and hydrophobic groups into the polymer chain [3]. One of the methods for obtaining such polymers is the copolymerization of acrylonitrile with monomers of the corresponding nature (vinyl sulfonic acid). Unlike weak acids, which include acrylic and methacrylic acids, the effect of the nature of intermolecular interactions on the radical polymerization of strong unsaturated acids has been studied to a much lesser extent. The literature mainly contains data on the copolymerization of acrylamide and vinyl sulfonic acid, as well as styrolsulfonic acid and its salts [4].

In the study by Ghaderi et al. [5], a new terpolymer of acrylamide/styrene/maleic anhydride (PASM-t) was synthesized, exhibiting high salt and temperature resistance along with self-associating properties. This synthesis was achieved through one-step inverse emulsion polymerization. The terpolymer proved to be effective as a rheological modifier and thickener for drilling fluids. Rheological testing showed that the shear viscosity of the aqueous solution increased with rising temperature and salt concentration, maintaining stability even at high temperatures (up to 80°C) and salinity levels (up to 50,000 ppm). The modified drilling fluid based on s-PASM-t demonstrated excellent rheological and corrosion-resistant properties, making it promising for use in extreme deep drilling conditions. Davoodi et al. [6] introduced a synthetic copolymer of acrylamide and styrene (SBASC), which was tested as an additive for water-based drilling fluids. Experimental results confirmed that adding SBASC improved the rheological properties of the drilling fluid both before and after aging, highlighting its effectiveness in challenging drilling conditions.

Continuing from the previous paragraph, it is worth noting that in the study by Li et al. [7], a nano-silica-modified copolymer gel (NS-ANAD) was developed using free-radical copolymerization. The synthesis involved acrylamide, isopropylacrylamide, 2-acrylamide-2-methylpropanesulfonic acid, diallyldimethylammonium chloride, and silica particles modified with double bonds (KH570-SiO₂). The incorporation of nano-silica particles improved the polymer’s resistance to high temperatures and salinity, which is particularly crucial for deep drilling. In another study, Lv et al. [8] synthesized a copolymer gel (PAND) composed of acrylamide, N-isopropylacrylamide, and 3-dimethyl(methacryloyloxyethyl) ammonium propane sulfonate. This gel demonstrated high colloidal stability in drilling fluids, preventing bentonite particle aggregation and promoting the formation of a compact mud cake, which significantly reduced filtration volumes.

The study conducted by Kamali et al. [9] demonstrated the effectiveness of using Fe₃O₄-carboxymethyl cellulose (CMC) nanocomposite in water-based drilling fluids to enhance their rheological and filtration properties. The Fe₃O₄-CMC nanocomposite was added to control fluid loss, and the results showed that it provides increased viscosity and resistance to water intrusion in both deionized and saline water. This is attributed to the unique properties of the nanocomposite, which help improve filtration performance and maintain the stability of the drilling fluid in high-salinity conditions.

The primary aim of the study is to develop a novel approach for synthesizing thermo-salt-resistant stabilizers for drilling fluids by modifying polymers. The focus is on introducing sulfonic acid and hydrophobic groups into the polymer chain to enhance the thermal and salt resistance of drilling fluids, which are critical for maintaining stability and performance under extreme conditions. The synthesis involves copolymerization of acrylonitrile and vinyl sulfonic acid, followed by hydrolysis and further modification of polyacrylonitrile using various chemical agents, optimizing their rheological properties for effective use in challenging drilling environments.

The scientific novelty of the study lies in the development of a two-stage copolymerization process that allows for the creation of composite multifunctional polymer stabilizers with improved hydrophilicity and amorphous structure. The research introduces an innovative method of combining acrylonitrile with vinyl sulfonic acid, achieving a thermo-salt-resistant composition that incorporates amide, imide, carboxylate, and sulfonic groups. Additionally, the use of dispersion polymerization and hydrolysis under specific pH conditions results in the synthesis of new polyelectrolytes with enhanced thermal and salt resistance, providing a significant advancement in the field of drilling fluid technology. This approach offers a versatile solution for the stabilization of drilling fluids in deep and ultra-deep drilling operations.

2 Materials and method

2.2 Experimental process

2.2.1 The process of obtaining a thermo-salt-resistant stabilizer reagent

The process of obtaining a thermo-salt-resistant reagent stabilizer of drilling fluids based on acrylonitrile and vinyl sulfonic acid SANVSK-1 consists of two stages (Figure 1):

Figure 1

Lab setup: 1 – magnetic stirrer; 2 – water bath; 3 – thermometer; 4 – reflux condenser; 5 – addition funnel; 6 – three-necked round-bottom flask.

Stage 1: The first stage of the joint polymerization of monomers is carried out as follows: polymerization of acrylonitrile and vinyl sulfonic acid (vinyl sulfate) (SANVSK) in an aqueous medium in the presence of a redox system of PP and sulfuric acid sodium, by weight of monomers with different ratios of acrylonitrile and vinyl sulfate at 70–90:30–10, at pH 2–12. Next, the reaction volume was purged with an inert nitrogen gas and sealed, and the joint polymerization was carried out in a vacuum [10,11].

The polymerization process proceeds at (stirring the reaction mixture) a temperature of 328–343 K for 2.0–2.5 h. Copolymerization at low pH values = 2.0 and below leads to the formation of denser and harder masses, poorly soluble and poorly hydrolyzed. This is due to the crosslinking of polymer chains due to the imidization of nitrile groups. During the copolymerization reaction in a slightly acidic (pH 3–6) and alkaline (pH 8–12) medium, partial hydrolysis of acrylonitrile into acrylamide apparently occurs simultaneously, while the nitrogen content decreases from 23 to 18%.

During the copolymerization of monomers, solvents can affect not only the speed of the process, but also the composition and microstructure of the copolymers formed, depending on the nature of the terminal and pre-terminal links [12].

The copolymer obtained by us from the data in Figures 1–4, it can be seen that the yield of copolymers, depending on temperature, initiator concentration, synthesis duration, and pH of the medium, was determined by fixing certain parameters in a set of test tubes (10 pieces) installed in a thermostat. To determine the optimal reaction time and temperature, the elemental composition was determined, the viscosity of the selected copolymer samples was measured in a Ubbelode viscometer at 298 K, as well as the acid number depending on the copolymerization time (Table 2), the effective viscosity of the selected samples of copolymer solutions was measured on a rotary viscometer “Polymer RPE 1M1” at a shear rate of γ = 32c⁻¹ on the T ₁–B ₁ sensing element system (cylinder-cylinder), as well as by IR spectroscopy (Figure 6).

Figure 2

Dependence of the yield of the copolymer of acrylonitrile and vinylsulfonic acid on time in the presence of PP and sodium bisulfite (SBS). T = 308 K; [PP + SBS] = 1.0%.

Figure 3

Dependence of the copolymer yield of acrylonitrile and vinylsulfonic acid on temperature in the presence of PP and SBS. t = 4 h; [PP + SBS] = 1.0%.

Figure 4

Dependence of the copolymer yield of acrylonitrile and vinylsulfonic acid on the pH concentration of the medium. T = 308 K; [PP + SBS] = 1.0%.

The absence of acrylonitrile and vinyl sulfonic acid (0.2–0.1%) in the reaction mixture indicates the end of the copolymerization process.

3 Results and discussion

The results of determining the dependence of the copolymer yield of acrylonitrile and vinylsulfonic acid on the reaction conditions are shown in Figures 2–5.

Figure 5

Dependence of the copolymer yield of acrylonitrile and vinylsulfonic acid on the concentration. T = 308 K; t = 4 h; [PP + SBS] = 1.0%.

Thus, the selection of the ratio of reactants and copolymerization conditions ensures a high conversion of monomers, which increases the yield of the final product.

On the IR spectrum of the studied sample of the copolymer of acrylonitrile and vinylsulfonic acid (Figure 6), an intense absorption band of 3,100–2,850 cm⁻¹ is observed, which corresponds to the valence vibrations of the S–H and C–H groups of NH involved in the hydrogen bonding of aliphatic compounds. An absorption band with low wavelength intensities of the C–O–C compounds present in esters, acetogroup 1,350–1,030 cm⁻¹, is also observed. IR spectra of wavelengths with low intensity 1,969–1,813 cm⁻¹ characterize valence vibrations of C═O of non-conjugated bond types in amides, and the absorption bands of 1,700–1,530 cm⁻¹ correspond to valence vibrations of C═C.

Figure 6

IR absorption spectrum of a sample of the copolymer of acrylonitrile and vinylsulfonic acid.

The results of the conducted research – elemental analysis (in %, C – 30.82, O – 41.47, Na – 20.78, Si – 0.33, S – 6.60) acid number, mineralogical composition, and IR spectroscopic studies confirm that the resulting copolymer of acrylonitrile and vinylsulfonic acid is of the following composition:

Stage 2. For the synthesis of the stabilizer reagent of drilling fluids, the process of modification of the synthesized copolymer of acrylonitrile and vinylsulfonic acid was carried out by incomplete saponification using a 4–6% aqueous solution of sodium hydroxide and further modification in the presence of fatty acids of gossypol resin. Gossypol resin contains unsaturated fatty acids with a predominant fraction of C₁₁–C₁₇, i.e., (R-COON). The process of copolymer hydrolysis is carried out at a temperature of 95–98°C for 2.0–2.5 h. The process of hydrolysis and modification of the copolymer in the presence of fatty acids of gossypol resin (LCGS) proceeds with an increase in the acid number and reduced viscosity of the system (Table 1); this is also indicated by the IR spectra of the product of hydrolysis of the copolymer (Figure 7) initially, internal cyclization of the nitrile groups of the polymer occurs, accompanied by the appearance of conjugated double vinyl and allyl (relative to nitrogen) connections [13,14].

Figure 7

IR absorption spectrum of a sample of an unpolically hydrolyzed copolymer (90 min) acrylonitrile and vinylsulfonic acid.

Then, in the period from 60 to 90 min (Figure 7), saponification of free nitrile groups leads to the appearance of a certain number of amide and carboxylate groups in the macromolecule chain, which have absorption bands at 950–1,007, 1,311, 1,404, and 1,416 cm⁻¹ indicates the formation of a band, the intensity of which increases with increasing saponification time. This band can be attributed to the valence vibrations C═C and the group –COON.

Due to the fact that the absorption of the –COONa groups takes place at several frequencies in the region of 1,550 and 1,651–1,724 cm⁻¹ and these frequencies are quite closely located, two wide bands are formed due to the overlap of the bands, characteristic of the carbonyl and amide groups. The appearance of a wide absorption band in the 3,100–3,600 cm⁻¹ region with peaks of 3,352 cm⁻¹ corresponds to the carboxylate group formed as a result of hydrolysis of amide groups. During this period, the absorption bands’ 2,850 cm⁻¹ characteristic of S–H valence oscillations decrease (Figures 6 and 7), and their mixing towards the absorption band 3,100–3,600 cm⁻¹ is observed. However, absorption bands of deformation vibrations in the region of 1,490–1,530 cm⁻¹ characterizing the imide cycles cannot be detected in the spectra, since they are closed by absorption bands of valence vibrations of the carboxylate and amide groups. This is indicated by the works of other authors [15,16].

Thus, the modification process leads to the formation, along with amide and carboxylic, also ether groups (COOR) due to the reorganization of fragments and the rupture of hydrogen bonds of carboxylic groups (Figure 7) and the appearance of an absorption band 1,180–1,250 cm⁻¹, characteristic of ether bonds in samples of the band at 1,650–1,550 cm⁻¹ can be attributed to ions –COO⁻, while the carboxyl groups interact with R-COON.

Thus, in the process of hydrolysis and modification of the copolymer of acrylonitrile and vinylsulfonic acid sodium hydroxide in the presence of fatty acids, the acid number increases sharply, and the reduced viscosity first gradually increases as hydrolysis proceeds, then decreases slightly (Table 2). This behavior is explained by the fact that during this period, the degree of dissociation of carboxylate groups increases as a result, dissociated negatively charged ions of carboxylate groups in the aqueous medium repel and contribute to the formation of expanded conformations of macromolecules.

The hydrolysis process takes place within 2–2.5 h, and by the end of the reaction, the resulting product is a thick viscous paste of creamy yellow color, well soluble in water due to densely arranged functional groups (amide, imide, carboxylate, ether, and sulfogroups).

The synthesis of the thermo-salt-resistant reagent stabilizer of drilling fluids SANVSK-2 based on polyacrylonitrile is carried out by incomplete saponification using a mixture: when adding a solution of sodium hydroxide, the hydrolysis process with the formation of salts occurs, when adding a solution of sulfuric acid, the formation of initial acids occurs, and water is released. The process of saponification of polyacrylonitrile, in this case, proceeds at pH 4–6 in a slightly alkaline solution, due to the formed sodium sulfate, at a temperature of 95–98°C for 2.0–2.5 h. The process of polyacrylonitrile hydrolysis using a mixture of an aqueous solution of sodium hydroxide and a solution of sulfuric acid was evaluated by a change in the elemental composition, the amount of nitrogen (Table 2), acid number, degree of saponification, viscosity change, and the ratio of functional groups. In the initial stage, it is characterized by a significant rate of hydrolysis (Table 2). i.e., the content of residual nitrogen decreases sharply from 23.35 to 12.20%, and the degree of hydrolysis of the polymer is reached to 47%. In the period from 1.0 to 1.5 h, the acid number increases sharply, and with further saponification from 1.5 to 4.0 h, it changes slightly, which is due to a decrease in the saponification rate during this period, as was previously found by the release of ammonia and a change in the amount of nitrogen in saponification products.

The results of the conducted studies show that the hydrolysis of polyacrylonitrile at pH 1.0–3.0 is due to the crosslinking of polymer chains due to the imidization of acrylamide units in an acidic medium and passes into an insoluble state in water in the form of resin. In this case, the elemental composition of the polymer is as follows: Figure 8a shows the microstructure of hydrolyzed polyacrylonitrile using a mixture of 4–6% aqueous solution of sodium hydroxide and 4% solution of sulfuric acid at pH 3 consists of parts of both crystalline and amorphous structures, as evidenced by the structure of cumulus-like vague crystals and inclusions of fine granular sodium compounds. During the reaction at pH 4–6 (Figure 8b) (sharply differs from pH 3), the total hydrophilicity of the reaction mass increases, which contributes to the formation of expanded conformations of macromolecules, the crystalline part passes into an amorphous state.

Figure 8

Elemental composition and microstructure of hydrolyzed polyacrylonitrile using a mixture of a mixture of: 4–6% aqueous solution of sodium hydroxide and 4% solution of sulfuric acid at pH 3 (a) and at pH 4–6 (b).

The process of hydrolysis of polyacrylonitrile proceeds as in the process of hydrolysis of the copolymer of polyacrylonitrile and vinylsulfonic acid, i.e., hydration, cyclization of the nitrile groups of the polymer and destruction of the nefteride cycle with transition to an amide, then a carboxylate group, accompanied by the appearance of conjugated double vinyl and allyl (relative to nitrogen) bonds, which is consistent with the literature data [17,18,19]. Further saponification of free nitrile groups by the release of ammonia leads to swelling of polymer particles due to the transition of the polymer from a hydrophobic to a partially hydrophilic state.

In the IR spectra of hydrolysis of saponification polyacrylonitrile using a mixture of: 4–6% aqueous solution of sodium hydroxide and 4% solution of sulfuric acid (Figure 9), absorption bands are observed, which can be attributed to valence fluctuations C═C and the group –COON (950–1,007, 1,311, 1,404, and 1,416 cm⁻¹). Absorption bands also appear at 845 cm⁻¹, a vague band at 1,080 cm⁻¹ (C–C), and a maximum of its mixing towards low frequencies and a decrease in the intensity of the band 1,245 cm⁻¹ (CH) are observed. The appearance of the shoulder at 1,416 cm⁻¹ indicates the formation of a band, the intensity of which increases with increasing saponification time. This band can be attributed to the valence vibrations of the C═C and –COON groups; in addition, absorption of the COON groups takes place at several frequencies in the 1,500–1,620 cm⁻¹ region, and these frequencies are quite close, due to the overlap of the bands, one wide band is formed in the 1,500–1,700 cm⁻¹ region. An increase in the intensity of these bands in the range of 1,680 and 1,570 cm⁻¹, characteristic of the carbonyl and amide groups [20,21]. Since the (−COO−) and (−CONH₂) groups are more polar than the nitrile ones, the total hydrophilicity of the reaction mass increases, and it passes from an inhomogeneous dispersed state to a gel-like, and then to a homogeneous state. The saponification process takes place within 2–2.5 h, and by the end of the reaction, the resulting product is a thick viscous paste of creamy yellow color with a content of up to 10% active substance, well soluble in water [22,23].

Figure 9

IR absorption spectrum of a sample of non-hydrolyzed polyacrylonitrile using at pH 6 in a mixture of a mixture of 4–6% aqueous solution of sodium hydroxide and 4% solution of sulfuric acid pH 4–6.

The synthesis of the thermo-salt-resistant reagent of the SANVSK-3 drilling fluid stabilizer based on polyacrylonitrile (10 g) is carried out by incomplete hydrolysis using a 5% aqueous solution of sodium hydroxide (100 ml) and further sulfomethylation (10 ml, 25% formalin and 10 g of sodium sulfate) at 95–98°C within 2.5–3.0 h.

In the initial period, (30–60 min) (–OH) groups cause chemical transformations of nitrile groups into naphtheridine groups, then into amide and carboxylate groups [10,19]. With further saponification (Table 2), in the range of 1.0–2.5 h, the nitrogen content decreases slightly from 10.0 to 8.85%, i.e., the degree of saponification increases by an average of 5%, while the saponification process decreases, which is apparently due to the predominance of the effect of electrostatic repulsion of the saponifying agent (−OH) by negatively charged carboxylate groups over the effect of wedging pressure created by the interaction of the latter with each other. In this case, the sulfomethylol groups bind to the amide groups. Table 2 shows that in the period from 1.0 to 1.5 h, depending on time, the acid number and the reduced one increase, and with further saponification from 1.5 to 4.0 h, it changes slightly, which is associated with a decrease in the saponification rate during this period.

The IR absorption spectrum of the SANVSK-3 sample of hydrolyzed polyacrylonitrile (Figure 10) using a 4–6% aqueous solution of sodium hydroxide in the presence of 25% formalin and sodium sulfuric acid is identical. After 30 min, absorption bands appear at 1,107 cm⁻¹ (C−C), and a maximum of its mixing is observed in the direction of low frequencies and a decrease in the intensity of the band 1,242 cm⁻¹ (CH). Absorption bands in the regions of 1,658 and 1,554 cm⁻¹, characteristic of the carbonyl and amide groups, as well as in the region from 2,924 to 3,217 cm⁻¹, after 30 min of saponification, a wide band with a maximum absorption at 3,325 cm⁻¹ appears, which, by analogy with polyamides. Absorption bands in the 1,107 regions (C–C), which overlaps with the absorption bands 1,041–1,080 cm⁻¹, characteristic of the sulfogroup, and a wide absorption band is formed in the region of 1,041–1,350 cm⁻¹. The content of the sulfogroup (Figure 11) also confirms the elemental composition of the sample of sulfomethylated polyacrylonitrile derivatives.

Figure 10

IR absorption spectrum of a sample of partially hydrolyzed polyacrylonitrile using a 4–6% aqueous solution of sodium hydroxide in the presence of 25% formalin and sodium sulfate.

Figure 11

Elemental composition and microstructure of a sample of non-polyhydrolyzed polyacrylonitrile using a 4–6% aqueous solution of sodium hydroxide in the presence of 25% formalin and sodium sulfate.

Thus, the introduction of sulfomethyl groups into the macromolecule during hydrolysis increases the hydrophilicity of the system, which is expressed in a gradual transition to a gel-like and then visco-fluid state and gives a better stabilizing effect due to the conformational state of the macromolecules of the resulting water-soluble polymer and amorphous structure (Figure 11).

Thus, the results of the study show that the optimal ratio of acrylonitrile and vinyl sulfonic acid is 80:20 at pH 3–6. An increase in the concentration of the vinyl sulfonic acid pressure gauge during the copolymerization of acrylonitrile and vinyl sulfonic acid leads to spatial difficulty due to the effects of repulsion (the presence of strongly acidic sulfogroups) and ultimately a decrease in the reaction rate and further course of the copolymerization reaction.

4 Conclusions

A method for synthesizing a composite reagent for drilling fluids was developed through the copolymerization of acrylonitrile and vinyl sulfonic acid in an aqueous medium, using PP and sodium sulfate as initiators, followed by hydrolysis and modification with sodium hydroxide in the presence of gossypol resin fatty acids. The optimal ratio for dispersion polymerization was determined to be 80:20 of acrylonitrile to vinyl sulfonic acid at pH levels ranging from 3 to 6, with subsequent hydrolysis and modification resulting in the production of thermo-salt-resistant composite multifunctional polymer stabilizers containing groups like amide, imide, carboxylate, and sulfogroups. Additionally, multifunctional polyelectrolytes with sulfonic acid and sulfomethylated groups were synthesized through polyacrylonitrile hydrolysis in slightly alkaline solutions, followed by dehydrolysis with sodium hydroxide, formalin, and sulfuric acid sodium. The synthesis methods enabled the creation of new multifunctional composite polymer stabilizers featuring diverse macromolecular sizes, structures, and compositions, providing enhanced hydrophilicity and amorphous structures, suitable for use in challenging drilling conditions.

Future directions for this research include optimizing the synthesis parameters to further enhance the stability and efficiency of these polymer stabilizers under even more extreme thermal and saline conditions encountered in ultra-deep drilling environments. Additionally, investigating the interactions between these stabilizers and various additives used in drilling fluids will help fine-tune their properties for different geological formations. Exploring the environmental impact and biodegradability of these stabilizers also represents a crucial step toward sustainable applications in drilling technology.