Experimental Evaluation of Authigenic Acid Suitable for Acidification of Deep Oil and Gas Reservoirs at High Temperatures

Abstract

During the acid pressure conversion process in high-temperature, deep oil and gas reservoirs, a number of challenges are encountered that hinder the effectiveness of acid fracturing. These obstacles include significant corrosion of acidized pipe strings, rapid reaction rates of acid with rock, limited reach of acid liquids, and shallow penetration depth of active acids. Additionally, the transportation of highly corrosive acids presents safety risks, necessitating surface conditions that are free of acidity. However, underground conditions require strongly acidic liquids to meet enhanced ecological and environmental protection requirements. To address these limitations, experimental investigations have been conducted to examine the reaction rates of low-corrosive and low-acid rocks in alkaline systems involving halides and carbonyl compounds. Through meticulous assessments of reaction rates and dissociation effects in acid rocks, parameters have been successfully optimized to incorporate erythropoiesis and other compounding agents into acid-pressing designs. The experimental findings indicate that the concentration of released H⁺ ions after 60 min exceeded that of the conventional acid solution processed for 15 min. Enhanced dissolution was observed when erythropoietin content was increased to 20%. Furthermore, combining 10% acetic acid with 20% caustic acid resulted in a significant increase of 6.08% in the dissolution rate from 10 to 120 min, while exhibiting lower dissolution values compared with other types of acids. The development of naturally occurring acids with reduced rates of dissolution and acid–rock reaction holds significant potential for enhancing the efficacy of high-temperature, deep oil and gas reservoirs through acid fracturing stimulation.

1. Introduction

With the continuous development of oil and gas exploration, the number of high-temperature deep wells (≥150 °C) has been increasing. The technical challenges in acidizing such wells mainly include the following [1,2]: Firstly, the high reservoir temperature leads to a rapid acid–rock reaction speed and limited penetration distance of the acid liquid. Second, high temperatures increase the rate of dissolution, causing severe damage to underground pipes. Various nonstandard acid systems have been studied, such as authigenic acid, thickening, emulsifying, and in situ cross-linked acids. Among them, autochthonous acids have a unique advantage in high-temperature deep wells. They do not exhibit acidity or weak acidity at ground level but gradually becomes acidic at the bottom hole temperature. This slows the acid–rock reaction rate, extends the penetration distance of the acid liquid, and reduces the corrosive effect on underground pipes. However, the current applicable temperature range for authigenic acid is relatively low (low temperature and medium temperature), and at high temperatures (120~150 °C), the acid production rate is fast, which cannot effectively delay the acid–rock reaction rate [3,4]. Moreover, the final released acid concentration is low, limiting its role in reservoir transformation [5,6,7]. Therefore, it is necessary to develop nucleic acid solutions suitable for high-temperature conditions.

Templeton et al. proposed the concept of authigenic acids in 1975 [8]. The development of authigenic acid technology has shifted from a single authigenic hydrochloric acid and authigenic organic acid system to a composite authigenic acid system. At present, there are mainly six types of authigenic acids, namely ammonium halide systems, inorganic halides, halogenated hydrocarbons and metal halides, chlorocarboxylates, organic lipid hydrolysis, and polybasic acid ionization [9].

In the presence of initiators (aldehydes, acids, etc.), slowly released aldehydes react with ammonium salts to generate active acids, such as formaldehyde reacting with ammonium formate to produce formic acid, formaldehyde reacting with ammonium acetate to produce acetic acid, and formaldehyde and ammonium halide to produce halogenated acids. Once ammonium chloride and formaldehyde are mixed, hydrochloric acid is gradually produced, and high-temperature conditions can accelerate the reaction. Adding substances such as hexamethylenetetramine or binary soluble carboxylate to the reaction system can inhibit the progress of the reaction, slow down the rate of acid generation, and thus achieve acid production in high-temperature formations. Acetic acid and acetate can also be added to the reaction system to inhibit the rate of acid generation. In addition, there is a system composed of formaldehyde and organic carboxylates, which react under geological conditions to form slow-moving acid.

Halogenated hydrocarbons mainly include CCl₄, chloroform, tetrachloroethane, etc. They hydrolyze to produce halogen acids at formation temperatures of 121~171 °C. Metal halides are mainly AlCl₃ and MgCl₂ [10]. Halogenated hydrocarbons can hydrolyze to produce HCl at high temperatures (121~171 °C). These halogenated hydrocarbons can hydrolyze to produce strong acids and have strong dissolution ability, but these halogenated hydrocarbon substances are insoluble in water. Pure chlorinated hydrocarbons are injected into the well on site or dissolved in organic solvents before being injected into the well. However, this poses potential safety issues, and the extensive use of organic solvents will greatly increase costs. Wang et al. proposed adding emulsifiers to the solution to form an oil-in-water emulsion between the organic compound and water, injecting the emulsion into the formation to achieve acidification of the formation [11].

Organic esters react with water at high temperatures or under the action of catalysts to produce organic acids, such as methyl formate, methyl acetate, ethyl acetate, and glycerol triacetate. The esters of lower alcohols have a good effect on slowing down the acid–rock reaction rate, and the acid consumption time can be in the range of 40~400 min. In addition, due to the hydrolysis of organic esters, the products also include low-level alcohols, which can not only play a certain role in drainage and waterproof locking but also reduce the concentration of water, resulting in a decrease in the acid–rock reaction rate.

In general, the hydrolysis rate of methyl formate is slower, while the reverse reaction produces esters more easily. Formic acid itself can serve as a catalyst for the hydrolysis of methyl formate; so, in practical applications, an appropriate amount of formic acid can be added to accelerate the hydrolysis reaction. Methyl formate can be used in formations ranging from 54 to 82 °C. The hydrolysis equilibrium constant of methyl acetate is very small, and the one-way hydrolysis conversion rate is also low. The use of catalysts in industry can significantly improve its one-way hydrolysis conversion rate. Methyl acetate can be used in formations ranging from 88 to 138 °C.

Nasr El Din et al. studied the use of self-generated organic acids to remove calcium carbonate scale in gas well filter cakes, using methyl acetate as the parent acid [12]. The hydrolysis of 10% methyl acetate can produce 2~3% acetic acid at 80 °C and 5~6% acetic acid at 160 °C. The methanol produced during the hydrolysis of methyl acetate can significantly reduce the surface/interface tension of the acid, which is conducive to the reflux of the acid [13]. But even at temperatures as high as 160 °C, methyl acetate can only effectively release 6% acetic acid, and the weak acidity of acetic acid is an objective problem.

Abrams et al. [14] studied the applicable temperature ranges of three low-molecular-weight ester systems commonly used in acidification, namely methyl formate, ethyl formate, and methyl acetate. They pointed out that their suitable temperature ranges were 54~82 °C, 82~102 °C, and 88~130 °C, respectively, suitable for low-temperature oil and gas reservoirs. Al Otaibi et al. found that CO₂ generated in reservoirs has a weak inhibitory effect on methyl formate, and methyl formate should be selected as the main acid-generating agent for self-generated organic acids [15]. However, the application conditions of self-generated formic acid are very strict, and the main acid-generating agent is severely affected by temperature and catalyst. Then, Al Otaibi et al. proposed to switch to methyl lactate, which not only overcomes the disadvantage of poor thermal stability of methyl formate but also improves the dissociation ability of organic acids, but it may result in the secondary precipitation of calcium lactate [15].

Polybasic acids are divided into two categories: inorganic acids and organic acids. Polybasic inorganic acids usually refer to acids that can ionize three or more hydrated hydrogen ions H₃O⁺ in aqueous solution, such as phosphoric acid H₃PO₄, which is a moderately strong ternary acid [16]. Phosphoric acid reacts with calcium carbonate in the formation to produce dihydrogen calcium phosphate, which can form a buffer solution with phosphoric acid to maintain the pH value within a certain range. Smith et al. pointed out that under geological conditions, phosphoric acid becomes a self-slowing acid, and even if in long-term contact with the formation, the pH of the acid solution is rarely greater than three [17]. Polybasic organic acids mainly refer to carboxylic acids containing three or more carboxyl groups (-COOH), such as hydroxyethylidene diphosphonic acid (HEDP), citric acid, pyromellitic acid C₆H₂ (COOH)₄, etc. Polybasic acids used for reservoir transformation are generally developed for high-temperature and low-permeability sandstone reservoirs [18]. Compared with conventional acid fluids, polybasic composite acids have good retarding performance while ensuring strong dissolution ability, and they do not easily form oil–water emulsions, preventing secondary sedimentation and acid residue formation.

The advancement of high-temperature authigenic acids systems that are capable of self-diffusion is a promising trend in acid technology. An ideal system must possess three key features. Firstly, the system should exhibit minimal or zero acidity at low temperatures to reduce the risk of corrosion to underground pipes. Conversely, at high temperatures, the system must provide concentrated acid to intensify the acidification effect. Secondly, self-diffusing acid systems should maintain long-lasting and stable acid concentrations, ensuring effective acid fracturing and reduced acid–rock reactions at elevated temperatures. Lastly, an optimal self-diffusing acid system should comprise a single product while generating minimal industrial byproducts, promoting reliability and compliance with safety and environmental regulations. It is essential to justify each of these features systematically to establish their significant contribution to the field of self-diffusing acids. The provision of relevant examples and background information can facilitate the audience’s comprehension of the link between academic standards and practical applications.

The acidification of deep oil and gas reservoirs at high temperatures has been a persistent research focus in the field of enhanced oil recovery. This study aims to experimentally evaluate the efficacy of erythropoiesis under such extreme temperature conditions by optimizing hydrochloric acid system formulations through indoor experiments. By incorporating halide salts, carbonyl compounds, and esters, autologous acid systems capable of stable and continuous acid production at elevated temperatures have been successfully developed. The authigenic acid system exhibits exceptional performance in terms of efficient and sustained acid production. Experiments were conducted to assess the efficiency of release, corrosion ability at different concentrations, rates of acid–rock reaction, effects of various acids on native acid properties, and storage conditions. The experimental setup involved simulating reservoir temperature and injecting a bicarbonate solution into an oil and gas core sample. Additionally, compatibility experiments were performed to select suitable corrosion inhibitors and iron ion stabilizers, resulting in a well-suited system for percolation conversion in deep wells operating at high temperatures. The results demonstrate the effectiveness of erythropoiesis in enhancing oil and gas recovery using core samples under elevated temperatures.

2. Materials and Methods

2.1. Authigenic Acid Formula

Halogens and carbonyl compounds can react to yield hydrochloric acid at high yields, with both reactions achieving nearly 20% efficiency. The carbonyl group’s carbon-oxygen double bonds are readily attacked by nucleophiles, and the carbon atoms directly attached to the carbonyl group are highly reactive. Hydrolysis of halide salts yields weak acidity due to their strong electrolytic nature, and the presence of carbonyl compounds further enhances acidity. Halogens and carbonyl compounds are both economical and feasible industrial raw materials. Accordingly, this report aims to investigate the system for hydrochloric acid production from the reaction of halides and carbonyl compounds (Figure 1).

2.2. Determination Method of Acid Solution Dissolution Rate

The calcium carbonate particles undergo crushing and sieving, with selection of particles ranging from 10 mesh to 20 mesh, which are subsequently dried in an oven at 60 °C for 24 h for standby. A certain amount of calcium carbonate is weighed and transferred into a beaker containing 20 mL of acid solution and is subsequently diluted with a large volume of water after a period of reaction time. The unreacted calcium carbonate is immediately filtered under reduced pressure and washed multiple times with water. The filtered calcium carbonate is dried in an oven for 12 h, and its mass is recorded. The dissolution rate of the acid solution to calcium carbonate can be calculated using the following formula:

$$\mathrm{S}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{d}\mathrm{i}\mathrm{s}\mathrm{s}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}=\frac{m_o-m_t}{m}\times 100\%$$

(1)

• m—Quality of calcium carbonate before dissolution, g.
• m_o—Total mass of calcium carbonate and filter paper before dissolution, g.
• m_t—Total mass of calcium carbonate after dissolution and filter paper after drying, g.

3. Results and Evaluation of Autogenous Acid Dissolution Effect

3.1. Comparison of Dissolution Effect between Authigenic Acids and Hydrochloric Acid

A comparison of the dissolution rates of calcium carbonate using 15% hydrochloric acid (volume fraction) and 12% authigenic acid at different time intervals shows that the dissolution rate of 15% hydrochloric acid remains relatively steady, with only a small increase of 0.36% from 10 min to 60 min at 150 °C and atmospheric pressure. On the other hand, the dissolution rate of 12% authigenic acid displays a noticeable increase of 2.79% during the same time frame, indicating that authigenic acid exhibits certain retardation compared with hydrochloric acid (Figure 2).

The dissolution rate of calcium carbonate in 15% hydrochloric acid (volume fraction) for 60 min was determined, and the effective hydrogen ion concentration released from the acid solution was calculated. When the authigenic acids dosage was about 20%, the hydrogen ion concentration released in 60 min of acid reaction was more than that released in 60 min of 15% HCl (

Table 1. Comparison of dissolution rates of autogenous acid and hydrochloric acid with different dosages (150 °C and atmospheric pressure).

Acid Type	Time/min	Dissolution Rate/%	Release Concentration of H+/mol/L	pH
10% Authigenic acid	60	27.01	1.17	5.0
15% Authigenic acid	60	29.42	1.35	4.5
20% Authigenic acid	60	48.86	2.18	4.5
25% Authigenic acid	60	64.58	2.73	4.5
30% Authigenic acid	60	77.63	3.37	4.5
15% HCl	60	44.32	1.93	6.0

).

3.2. Effect of Authigenic Acids Dosage on Dissolution Rate

3.2.1. Comparison of 60 min Dissolution Rate of Authigenic Acid Reaction at Different Concentrations

To prepare aqueous solutions of authigenic acid with mass-to-volume ratios of 15%, 20%, 25% and 30%, we utilized 100 mL volumetric flasks at a temperature of 150 °C and atmospheric pressure. We determined the rate of dissolution of calcium carbonate in an acid solution for 60 min to observe the effect of authigenic acid. Our findings showed that the dissolution rate increased significantly upon adding authigenic acid. However, beyond a 20% addition of authigenic acid, the rate of increase in dissolution slowed (as depicted in Figure 3). In general, increasing the addition of authigenic acid is conducive to improving the dissolution rate of calcium carbonate in the acid solution.

3.2.2. Comparison of Dissolution Rate of Authigenic Acid Reaction at Different Concentration and Time

Dissolution rates of aqueous solutions of authigenic acid on calcium carbonate at concentrations of 10%, 15%, 20%, 25%, and 30% were determined at various time intervals in a 100 °C water bath. Indoor testing yielded data indicating that the dissolution rate of calcium carbonate increased with reaction time for all concentrations of autogenic acid.

Comparing the increase in dissolution rate for different concentrations of authigenic acid between 10 and 90 min (as shown in Figure 4), it is observed that the increase in the dissolution rate of 12% authigenic acid increases with concentration. Additionally, the increase in the dissolution rates of 27% and 30% authigenic acid are 2.55% and 2.62%, respectively. The significant increase in the dissolution rate occurred before 27% authigenic acid was added; thereafter, the increase in the dissolution rate slowed down.

The dissolution rate results for 10% autogenic acid were taken as the reference point, and changes in dissolution rate over time were analyzed for a series of concentrations including 15%, 20%, 25%, and 30% autogenic acid.

The experimental results suggest that compared with the dissolution rate of 10% authigenic acid on calcium carbonate, an increase in the concentration of authigenic acid does not yield a proportionate increase in the dissolution rate over time. When the authigenic acid concentration remains below 25%, the dissolution rate curve remains below the baseline at the 40 min mark, and the increase range of dissolution rate is lower than that of the concentration on a year-on-year basis. Beyond 40 min, all dissolution rate curves surpass the baseline, with a higher increase in dissolution rate relative to the increase in concentration on a year-on-year basis. At an authigenic acid concentration of 30%, the dissolution rate curve surpasses the 10% authigenic acid baseline with the highest rate of increase over time.

As the concentration of authigenic acid increases, the rate of dissolution demonstrates a slower growth before 40 min and accelerates thereafter. The trend of dissolution rate over time indicates a gradually increasing slope when the authigenic acid concentration exceeds 25%. This implies that increasing the concentration of authigenic acid beyond 25% beneficially impacts the dissolution of calcium carbonate after 40 min.

A thorough analysis of these findings leads to the conclusion that when the authigenic acid concentration exceeds 25%, the rate of dissolution increase between 10 and 90 min decreases, and the optimal dosage of authigenic acid is 20% (refer to

Table 2. Increase in autogenous acid dissolution rate with different dosage (100 °C and atmospheric pressure).

Authigenic Acid Concentration	10%	15%	20%	25%	30%
Increase value of dissolution rate corresponding to 10 to 90 min	0.33%	1.01%	1.67%	1.76%	2.22%

).

4. Effect of Other Acids on the Acidizing Properties of Authigenic Acids

Authigenic acid is highly susceptible to hydrolysis upon dissolution in water, resulting in an increase in acidity in the aqueous solution over time. To prevent this, it is recommended to add other acids to the authigenic acid to maintain a high concentration of hydrogen ions in the solution, which can inhibit its hydrolysis and disrupt its hydrolysis balance. This leads to a decreased release of hydrogen ions and improved retardation performance of the acid solution. Hydrochloric acid is commonly used in carbonation and acidification systems. Additionally, both organic acids like formic and acetic acid, as well as inorganic acids such as phosphoric acid, are commonly employed for this purpose.

$$\mathrm{C}\mathrm{l}\mathrm{C}{\mathrm{H}}_2\mathrm{C}\mathrm{O}{\mathrm{O}}^{-}+{\mathrm{H}}_2\mathrm{O}\triangleq \mathrm{O}\mathrm{H}\mathrm{C}{\mathrm{H}}_2\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}+{\mathrm{C}\mathrm{l}}^{-}$$

(2)

4.1. Physical and Chemical Properties of Common Acids

Formic acid is a colorless transparent liquid and is soluble in water. The dissociation constant of formic acid Ka = 1.75 × 10⁻⁴. Formic acid is more acidic and corrosive to steel than acetic acid. Formic acid reacts with calcium carbonate or magnesium carbonate to form calcium formate or magnesium formate, which are soluble in water. Formic acid has the same characteristics of speed and dissolution inhibition as acetic acid and can be used in high-temperature deep well acidizing operations.

Acetic acid, a colorless, transparent liquid, is very soluble in water. Acetic acid is a weak electrolyte with a dissociation constant Ka = 1.8 × 10⁻⁶ at 25 °C. Due to the low solubility of calcium acetate, the concentration of acetic acid in its acidizing solution is usually 10–12%. The corrosion rate of acetic acid to metal is much lower than that of hydrochloric acid, with uniform corrosion and no serious pitting corrosion. It does not corrode aluminum alloy materials and can be used in acid perforation operations with long contact times with acid. The acid–rock reaction rate of acetic acid is lower than that of hydrochloric acid, so the active penetration distance is longer, which can be used as retarded acid. In addition, acetic acid has the ability to complex Fe³⁺, which can prevent the formation of iron hydroxide precipitation.

Phosphoric acid is a medium-strength acid with a dissociation constant Ka = 1.75 × 10⁻³ at 25 °C. The acid–rock reaction is as follows:

CaCO₃ + 2H₃PO₄ → Ca(H₂PO₄)₂ + CO₂↑ + H₂O

(3)

Because the strength of polyacid is determined by the first-order ionization constant, the reaction rate of phosphoric acid is much slower than that of hydrochloric acid. H₃PO₄ and reaction product Ca(H₂PO₄)₂ also form a buffer solution. The pH value of the acid solution is kept at a low value (pH ≤ 3) within a certain period of time, making it a slow acid and inhibiting the secondary precipitation.

4.2. Effect of Common Acid on Autogenous Acid Acidification

In the current study, we investigated the dissolution rate of calcium carbonate in a mixture of acetic acid, formic acid, phosphoric acid, and autogenous acid at a temperature of 100 °C. Our research demonstrates that the dissolution rate of autogenous mixed acids with 3% formic acid and 10% acetic acid increases over time and is considerably higher than that of autogenous mixed acids with 3% phosphoric acid. Notably, when reacting with calcium carbonate, the phosphoric acid and autogenous acid mixture resulted in a significant number of white powders in the residual acid and caking within calcium carbonate particles. Therefore, our findings suggest that the use of phosphoric acid in combination with autogenous acid is not recommended. In conclusion, our results highlight the necessity of selecting the appropriate acid for efficient dissolution and emphasize the significant differences in dissolution rates among various acids and their mixed solutions with calcium carbonate. This study has the potential to aid researchers in selecting the optimal acid for further study and could lead to advancements in the field of calcium carbonate dissolution.

The compound effect of formic acid, acetic acid, and authigenic acid is better than that of phosphoric acid, but after the mixed acid of formic acid and authigenic acid reacts with calcium carbonate, a large number of white turbidities appear in the residual acid, producing a large number of white powder particles. After the mixed acid of acetic acid and authigenic acid reacts, the residual acid is relatively clear, with a small number of white powder particles.

The dissolution rate of 20% authigenic acid + 3% formic acid and 20% authigenic acid + 10% acetic acid solution on an N80 steel sheet was determined at 150 °C for 4 h (

Table 3. Corrosion rate of two kinds of acid solutions on a steel sheet (150 °C and atmospheric pressure).

Acid Formula	Corrosion Amount/%	Corrosion Rate/ g·(m2·h)−1 ± 0.01
20% Authigenic acid + 3% phosphoric acid	5.94	141.35
20% Authigenic acid + 10% acetic acid	4.03	93.38

). The results show that the dissolution rate of 20% authigenic acid + 10% acetic acid mixed acid on the N80 steel sheet was lower than that of 25% authigenic acid + 3% formic acid, and the dissolution rate of 20% authigenic acid + 10% acetic acid was about 64% of that of 20% authigenic acid + 3% formic acid.

According to the dissolution rate of calcium carbonate, the residual acid state after reaction, and the corrosion rate of acid solution on the steel sheet, the combination of acetic acid and authigenic acid has a better effect.

4.3. Effect of Acetic Acid on Acidizing Properties of Authigenic Acid

Based on the above experimental results on the effect of the amount of authigenic acid on dissolution rate, it is concluded that the effect of acetic acid combined with 20% authigenic acid is better. The effects of different amounts of acetic acid (acetic acid dosage percentage is volume percentage) on the dissolution rate of authigenic acid were compared and analyzed (Figure 5). When the acetic acid dosage was 8%, 10%, and 15%, the dissolution rate increased from 10 min to 120 min (Δε). They were 5.29%, 5.50%, and 6.08%, respectively, showing an increasing trend. Meanwhile, with an increase in acetic acid dosage, Δε, the increased amplitude showed a trend of first increasing and then decreasing.

Figure 6 displays the time-dependent trend of the dissolution rate for the mixed acid solution containing acetic acid and authigenic acid with varying dosages. The dissolution rate curve for the mixed acid solution lies above that of pure authigenic acid. The addition of acetic acid to authigenic acid led to a significantly higher dissolution rate of calcium carbonate compared with authigenic acid alone. The dissolution rates for the 8%, 10%, and 15% acetic acid dosages in authigenic acid were 5.29%, 5.50%, and 6.08%, respectively, relative to the 20% authigenic acid dosage. The dissolution rate increased significantly when the acetic acid dosage was raised from 8% to 10%, while at 15%, the dissolution rate was high, and there was a sharp increase from 10 min to 120 min. However, beyond the 10% dosage level, the rate of increase in dissolution rate slowed. In conclusion, the optimal acetic acid dosage is 10%.

4.4. Acidification Performance of Mixed Acetic and Authigenic Acids

The dissolution rates of calcium carbonate at a temperature of 90 °C were compared using 20% authigenic acid, a mixed acid of 20% authigenic acid and 10% acetic acid, and 20% (mass fraction) HCl. The dissolution rate of calcium carbonate by 20% HCl remained nearly constant over time, as shown in Figure 7. The dissolution rate increased by only 3.38% from 10 min to 120 min. The dissolution rates of 20.0% authigenic acid and 20% authigenic acid + 10% acetic acid gradually increased over time, with an increase of 4.40% and 6.08%, respectively, from 10 min to 120 min. The dissolution rate of calcium carbonate by 20% authigenic acid + 10% acetic acid was significantly higher than that by 20% authigenic acid alone, suggesting that the mixed acid of acetic acid and authigenic acid exhibits some degree of retardation compared with hydrochloric acid.

The effective concentration of hydrogen ions released from the acidic solution at various time intervals was calculated. Based on the data presented in Figure 8, the initial hydrogen ion concentration released from 20% HCl was relatively high. However, the subsequent changes in hydrogen ion release were minimal, indicating a rapid reaction between hydrochloric acid and calcium carbonate, wherein the acid solution was mostly depleted at the beginning. In comparison with 20% HCl, the concentration of hydrogen ions released from 20% authentic acid and 20% authentic acid + 10% acetic acid was lower, although the rate of increase in hydrogen ion release concentration over time was higher than that of hydrochloric acid. Within the first 120 min of the reaction between 20% authentic acid + 10% acetic acid, the increase in hydrogen ion release concentration was over seven times that of 20% hydrochloric acid, suggesting that the mixed acid of authentic and acetic acid had a certain level of retardation compared with 20% hydrochloric acid.

The corrosion rates of 20% HCl, 20% authigenic acid, and 20% authigenic acid + 10% acetic acid on an N80 steel sheet in 4 h at a temperature of 150 °C were determined and are summarized in

Table 4. Corrosion rate of several acid solutions on a steel sheet (150 °C and atmospheric pressure).

Acid Formula	Corrosion Amount/%	Corrosion Rate/ g·(m2·h)−1 ± 0.01
20% HCl	41.48	916.34
20% Authigenic acid	6.89	147.38
20% Authigenic acid + 10% acetic acid	5.13	93.29

. Among the three acids, the lowest corrosion rate was observed for 20% authigenic acid + 10% acetic acid on a tubing steel sheet, which was 93.29 g·(m²·h)⁻¹, and it was lower than the corrosion rate of 20% salt acid on the steel sheet. The corrosion rate of the N80 steel sheet in 4 h was about 10% of that observed for 20% hydrochloric acid. Interestingly, the mixed acid solution of authigenic acid and acetic acid caused less corrosion on the steel sheet compared with the authigenic acid alone, implying that a certain amount of acetic acid has a corrosion-inhibiting effect on such a solution while improving its corrosion rate. Thus, 20% authigenic acid + 10% acetic acid may be a promising option for corrosion inhibition.

The residual acid was obtained after the reaction of 20% authigenic acid and 10% acetic acid, 20% authigenic acid, 20% HCl, and calcium carbonate. To this residual acid, a specific amount of ferric chloride was added to attain an iron ion concentration of 11,300 mg/L. The pH of the solution was then adjusted to 4~5 using a 10% sodium hydroxide solution, and the iron deposition was observed.

Experimental observations revealed a clear residual acid in the case of 20% authigenic acid and 10% acetic acid, with some precipitates seen in the beaker. In contrast, numerous precipitates were produced in 20% authigenic acid and 20% HCl residual acid, particularly in the latter. This signifies that 20% authigenic acid and 10% acetic acid possess anti-iron precipitation capabilities relative to authigenic acid and hydrochloric acid.

In conclusion, the mixture of 10% acetic acid and 20% authigenic acid exhibits a high dissolution rate for calcium carbonate, with a significant increase in the dissolution rate within 120 min of the reaction. Additionally, the mixture has corrosion inhibition properties and results in minimal iron precipitation in residual acid.

5. Conclusions

The authigenic acids system, consisting of halogen and carbonyl compounds, exhibits high water solubility and effectively releases H⁺ ions without exhibiting acidity or weak acidity in the reservoir. Autologous acid systems have been developed utilizing halide salts, carbonyl compounds, and esters to ensure stable and continuous acid production even at elevated temperatures. This gradual acid generation at the bottom-hole temperature slows down the rate of acid–rock reaction, extends the coverage area of the acidic liquid, and minimizes corrosion on bottom-hole equipment. Consequently, it can serve as a viable alternative to hydrochloric acid for carbonate reservoirs’ acidification purposes.

Under high-temperature conditions, authigenic acid has the effect of slowly dissolving rocks. Compared with 15% conventional hydrochloric acid, the dissolution rate of authigenic acid on calcium carbonate increases with time. From 10 min to 60 min of reaction time, the dissolution rate of authigenic acid on calcium carbonate increases by 2.79%. However, the change in the dissolution rate of 15% conventional acid is very small, and within the same reaction time, its dissolution rate of calcium carbonate only increases by 0.36%.

The dissolution effect of calcium carbonate is the best when the concentration of authigenic acid is 20%. The experimental results of the dissolution rate of calcium carbonate by different concentrations of authigenic acid from 10 min to 90 min show that the dissolution rate increases significantly before the concentration of authigenic acid reaches 20%, while the increase in dissolution rate slows down after the concentration exceeds 20%. When the concentration of authigenic acid is greater than 20%, the H⁺ concentration released by the acid solution after 60 min of reaction exceeds the H⁺ concentration released by 15% conventional hydrochloric acid during the same reaction time.

The corrosion rate of the mixture of authigenic acid and acetic acid on N80 steel sheets is lower than that of authigenic acid on steel sheets, indicating that the mixture of 20% authigenic acid and 10% acetic acid has lower corrosivity on steel materials. During the process of using authigenic acid in oil and gas wells, a mixture of authigenic acid and acetic acid helps to slow down the corrosion of steel tools such as oil casings, which effectively reduces string corrosion and meets the requirements of deep acidification in high-temperature reservoirs.