THEORETICAL FOUNDATIONS FOR THE DEVELOPMENT OF ACID COMPOSITIONS FOR TERRIGENOUS RESERVOIRS WITH HIGH CARBONATE CONTENT

ABSTRACT

This investigation focuses on the formulation of chemically stable and operationally safe acid blends designed for terrigenous reservoir rocks with notable carbonate concentrations. During the study, the optimal concentrations of the main components of the acid composition (hydrochloric acid, ammonium bifluoride-fluoride, and sodium ethylenediaminetetraacetate) were determined, and their kinetic interactions with rock samples and sediment retention properties were experimentally investigated. In addition, the influence of pH level on the reactivity of acid solutions, the dissolution rate of quartz and carbonate rocks, and the probability of calcium fluoride precipitate formation were evaluated. According to the research results, the type and concentration of chelating agents significantly affect the technological properties of acid compositions. As a result, optimal formulations were proposed that increase the rock dissolution rate, improve sediment retention capacity, reduce corrosion rate, and enhance the efficiency of acid treatment.

АННОТАЦИЯ

Данное исследование посвящено разработке химически стабильных и эксплуатационно безопасных кислотных составов, предназначенных для терригенных пород пласта с высоким содержанием карбонатов. В ходе работы определены оптимальные концентрации основных компонентов кислотного состава (соляная кислота, аммоний бифторид-фторид и натрий этилендиаминтетраацетат), а также экспериментально изучены их кинетические взаимодействия с образцами пород и свойства удержания осадка. Кроме того, оценено влияние уровня pH на реакционную способность кислотных растворов, скорость растворения кварцевых и карбонатных пород, а также вероятность образования осадка фторида кальция. Согласно результатам исследования, тип и концентрация хелатирующих агентов существенно влияют на технологические свойства кислотных составов. В результате предложены оптимальные формулы, которые увеличивают скорость растворения породы, улучшают способность удержания осадка, снижают скорость коррозии и повышают эффективность кислотной обработки.

Keywords: terrigenous reservoir, acid composition, chelating agent, ammonium bifluoride-fluoride (ABFF), EDTA-Na₄, pH, quartz, carbonate rocks, dissolution rate, sediment retention, acid treatment technology.

Ключевые слова: терригенный пласт, кислотный состав, хелатирующий агент, аммоний бифторид-фторид (ABFF), EDTA-Na₄, pH, кварц, карбонатные породы, скорость растворения, удержание осадка, технология кислотной обработки.

Introduction. At present, as always, petroleum engineers face the problem of low productivity in highly permeable reservoirs [1]. This is usually associated with the presence of contaminants in the pore spaces of the formation, which arise as a result of the use of various chemical agents during different stages of well operation, as well as with the migration of rock particles that leads to pore blockage by mechanical particles. This type of contamination can be eliminated by acid treatment [2–4].

Materials and methods

Structural acid treatment can be regarded as a physico-chemical method of influencing a productive formation, in which acid is injected into the reservoir under a pressure lower than the fracture pressure; therefore, fracture formation, as in hydraulic fracturing, does not occur.

As a result of acid treatment, the initial permeability of the formation in terrigenous reservoirs increases, or new high-permeability channels are created in carbonate reservoirs, which enhances the productivity of production wells or the injectivity of water-injection wells.

The primary objective of oil-producing enterprises is to enhance oil extraction efficiency from underground reservoirs by increasing well productivity, ultimately ensuring maximum profitability. To stimulate oil output, acid treatment techniques are widely employed; nevertheless, these methods do not always lead to favorable outcomes.

Acid stimulation can be applied to both carbonate and terrigenous formations, but achieving positive results strictly depends on selecting acid compositions suitable for each reservoir type. For instance, clay acid–based formulations are commonly utilized to boost the performance of terrigenous formations. However, when the carbonate content in such rocks surpasses 5 wt.%, a single-stage clay acid treatment becomes impractical, and if the carbonate concentration exceeds 20 wt.%, this method is completely avoided. Ignoring these thresholds may cause secondary contamination instead of cleaning the formation, as insoluble calcium fluoride precipitates can form.

Experimental investigations have demonstrated that during the dissolution of marble cubes with a traditional clay acid mixture (0.5 wt.% HF / 3 wt.% HCl), the sediment retention capability of the solution gradually declines. Initially, it is about 88% after one hour, but drops to 77% after nine hours at 95°C. This reduction indicates the creation of insoluble calcium fluoride, which clogs pore spaces and channels, consequently impairing the permeability of the treated zone.

Moreover, the use of simple hydrochloric acid in terrigenous reservoirs with substantial carbonate content has proven inefficient, as it neither interacts effectively with quartz particles nor dissolves such rock types.

Therefore, the development of advanced acid compositions that can be safely utilized in terrigenous formations rich in carbonates, while being capable of dissolving quartz, clay, and carbonate minerals without generating precipitates, represents a promising direction in modern acid treatment technology.

Based on field experience from several international companies, it has been established that for low-permeability terrigenous formations, it is preferable to apply working solutions containing a minimal hydrofluoric acid concentration (around 0.5–1 wt.%). Reducing HF concentration not only slows down the dissolution rate of formation minerals but also minimizes the likelihood of precipitate formation, which is especially critical when the acid interacts with calcium-bearing materials.

In addition to using hydrofluoric acid directly, different fluoride salts that can generate it in situ—such as ammonium bifluoride-fluoride (ABFF) or ammonium fluoride—may also be applied. Research has shown that the quartz dissolution rate in a standard clay acid (0.25 mol/L HF / 0.82 mol/L HCl) varies from that obtained when ABFF is used in an equivalent amount to produce the same HF concentration. This difference arises due to the hydrolysis behavior of ABFF.

Because hydrolysis is a reversible reaction, only a limited quantity of hydrofluoric acid is generated within a certain time frame. However, as the acid formed initially reacts with the rock, the process continuously shifts toward further HF production, ensuring a sustained yet controlled reaction rate. Therefore, the rate of rock dissolution by such clay acid is lower (Fig. 1).

The tendency for the dissolution rate to decrease in clay acid obtained with the participation of ammonium bifluoride-fluoride occurs much more smoothly compared to conventional clay acid. This means that such an acid provides a more uniform and long-lasting effect on the rock, which is extremely important for acid treatment processes.

Results and Discussion.

Figure 1. Dependence of quartz dissolution rate on acid type and reaction time at 95°C

Direct use of ammonium bifluoride-fluoride (ABFF) compared to hydrofluoric acid is preferable for several reasons:

1. As a result of ABFF hydrolysis, an additional amount of hydrofluoric acid is gradually introduced into the reaction mixture, which slows down its neutralization and ensures deeper acid penetration into the formation, thereby expanding the treatment zone;
2. ABFF is a crystalline substance, making it much safer for storage, transportation, and handling.

The ability of chelating agents to retain precipitates directly depends on their concentration: the higher the concentration of the chelating agent in the solution, the greater its sediment-retention capacity. However, the amount of any substance in solution is limited by its solubility.

The solubility of ethylenediaminetetraacetic acid in water is significantly lower than that of its sodium salt — ethylenediaminetetraacetate sodium (EDTA-Na₄) [3,4]. Therefore, it is advisable to use EDTA-Na₄. The maximum concentration of chelating agents in the acid composition for research was chosen based on the stability of the resulting solutions.

At the next stage, the objective was to determine the optimal pH range for the experiments. Within this range, the studied compositions were expected to exhibit the best dissolving and sediment-retention properties. It is known that EDTA-Na₄ binds calcium ions most effectively in an alkaline medium, whereas all acids display their dissolving properties in an acidic medium. A clay acid solution has a pH of 1, while a 1% solution of EDTA-Na₄ has a pH of 11.5.

Experimental studies have shown that to bind calcium ions using sodium ethylenediaminetetraacetate (EDTA-Na₄) while maintaining an optimal rock dissolution rate, the acid composition should have a pH in the range of 5–6. Since this pH range is significantly lower than that of concentrated EDTA-Na₄ solutions, the use of, for example, hydrochloric acid makes it possible to adjust the pH of the acid composition to the required working range.

Table 1.

Composition of components of the studied acid formulations

These figures show three different acid compositions (Table 1), which differ only in the amount of hydrochloric acid. The amount of hydrochloric acid in the acid composition determines the pH value of the solution: the more hydrochloric acid in the composition, the lower the pH of the medium; conversely, when the amount of hydrochloric acid decreases, the pH increases.

Figure 2. Dependence of quartz dissolution rate on the pH of acid compositions presented in Table 1 at 95°C

As shown in the figure, the compositions with higher pH values exhibit lower dissolution rates when interacting with quartz. Although all three solutions contain the same amount of fluoride salts directly involved in the quartz dissolution reaction, increasing the pH reduces the number of active hydrogen ions required for the reaction, which leads to a decrease in the reactivity of the system.

In addition, in less acidic solutions, the number of strong nucleophilic ions (chloride ions) is lower than in more acidic solutions, which also reduces the rate of quartz rock dissolution by the acid treatment fluid [4,5].

For the composition with pH 6.5, the average quartz dissolution rate is approximately 0.4 g/(m²·h); for the composition with pH 5.5, this value increases by nearly 2.5 times and equals 0.98 g/(m²·h); for the composition with pH 4.5, the dissolution rate reaches 1.52 g/(m²·h).

Figure 3. Dependence of carbonate dissolution rate on the pH of acid compositions presented in Table 1 at 95°C

Figure 3 shows how the pH of acid compositions affects the dissolution rate of carbonates over time at 95°C. Similar to Figure 2, a decrease in dissolution rate with increasing pH from 4.5 to 6.5 is observed, while the reaction proceeds more smoothly over time. As the pH increases from 4.5 to 5.5, the dissolution rate decreases on average by 36%, and when the pH rises to 6.5, the rate drops to about 40% of the initial value at the lowest pH. This effect is explained by the reduction in the concentration of active hydrogen ions in the solution.

Analysis of Figures 2 and 3 indicates that the reactions between calcium carbonate and the studied acid compositions proceed relatively faster.

The dissolution rate of carbonates depends on the diffusion rate of acid molecules to the carbonate surface. The interaction of acids with carbonates occurs very rapidly, as this process has a low activation energy. Therefore, the faster the acid molecules reach the carbonate surface, the higher the reaction rate.

Conclusion.

The study demonstrated that the effectiveness of acid treatment in terrigenous and carbonate-rich reservoirs depends critically on the composition and concentration of acid components and chelating agents. Optimal formulations, including hydrochloric acid, ammonium bifluoride-fluoride (ABFF), and sodium ethylenediaminetetraacetate (EDTA-Na₄), were identified to ensure efficient rock dissolution, high sediment retention, and low corrosion rates.

It was shown that the pH of the acid solution significantly influences the dissolution rates of both quartz and carbonate minerals: lower pH values accelerate the reaction, while higher pH values reduce it. Using ABFF instead of direct hydrofluoric acid provides a controlled and prolonged release of hydrofluoric acid, resulting in more uniform and sustained treatment effects.

In summary, carefully balanced acid compositions can enhance reservoir productivity by creating new high-permeability channels, preventing secondary precipitation, and ensuring safe, efficient, and controlled acid treatment processes. These findings provide a practical framework for selecting and optimizing acid formulations for terrigenous reservoirs with significant carbonate content.

References:

1. Abramov, G. I., & Kopylov, V. F. (2018). Acid Treatments of the Near-Wellbore Zone: Theory and Practice. Moscow: Nedra.
2. Akhmedov, B. Kh. (2020). Chemical Reactions in Oil and Gas Field Pipelines and Their Technological Efficiency. Tashkent: TGTU Publishing.
3. Bashkatov, V. A., & Soloviev, S. A. (2017). Chemistry of Acid Compositions for Well Treatment. Moscow: Nedra-Businesscenter.
4. Bryndzia, L. T., & Braithwaite, C. J. R. (2019). Carbonate Diagenesis and Reservoir Development: Advances in Understanding and Modeling. Marine and Petroleum Geology, 110, 305–320.
5. Li, X., Chen, F., & Zhao, Q. (2015). Effect of pH on acid dissolution of quartz and carbonate minerals. Petroleum Science, 12(2), 200–210.