A kind of coated acid for online acidification of offshore sandstone reservoir and its preparation method and application

Abstract

The present invention relates to the field of oil and gas field development, and discloses an online acidizing coated acid for offshore sandstone reservoirs, a preparation method thereof, and an application thereof. The coated acid is composed of a base acid, an acid isolation membrane, ammonium bifluoride, and an external coating membrane; the base acid is one or more of solid hydrochloric acid, solid nitric acid powder, aminosulfonic acid, and citric acid; the acid isolation membrane is composed of urethane acrylate, 1,6-hexanediol diacrylate, and methacryloyloxyethyltrimethylammonium chloride; and the external coating membrane is composed of acrylamide and 2-acrylamido-2-methylpropanesulfonic acid. The present invention achieves dual-factor control of acidity and temperature by using an outer acidity-responsive membrane and an inner temperature-responsive membrane, thereby greatly improving the release accuracy; it can be mixed and injected online without the need for additional fluoride ions, is non-corrosive to injection equipment, has a good acidizing effect, and has a low economic cost; it is easy to transport and can be stored for a long time at room temperature; the raw materials are readily available, the process is simple, and it is conducive to promotion.

Description

Technical Field

[0001] The present invention relates to the field of oil and gas field development, and in particular to an on-line acidizing coated acid for offshore sandstone reservoirs, a preparation method thereof, and an application thereof. Background Art

[0002] Acidizing operations in offshore oilfields mostly utilize a stationary tubing string process, making tubing corrosion prevention crucial. Furthermore, offshore acidizing operations are challenging and require long-term effectiveness, requiring an acid with deep penetration capabilities. Existing deep acidizing systems, such as emulsified, mixed, micellar, and cross-linked acids, are mostly liquid acids. While they mitigate hydrogen ion release to some extent, they inevitably corrode tubing string equipment. Therefore, a treatment acid that is harmless to metal equipment and offers long-term effectiveness is urgently needed.

[0003] Early solid acid solutions primarily solidified conventional acid solutions or used solid forms of acid, such as solid nitric acid powder, solid hydrochloric acid, aminosulfonic acid, and citric acid. With the continuous advancement of technology, domestic and international researchers have adopted microencapsulation to encapsulate solid acids, isolating them from the reservoir and preventing direct contact. When the outer encapsulation material is damaged by formation conditions, the solid acid inside is exposed, dissolving in the formation fluid and releasing hydrogen ions.

[0004] There are a variety of solid acids and preparation methods in the prior art, but most of them are for acid fracturing of carbonate reservoirs, and there is little disclosure on online acidizing of offshore sandstone reservoirs.

[0005] Patent publication number CN106479477A discloses an encapsulated solid acid comprising a capsule core and a capsule wall, wherein the capsule core is made of nitric acid powder. The encapsulated solid acid is used as an additive in fracturing acidizing fluids to effectively slow the acid release rate. The microencapsulated solid acid uses nitric acid powder, which is severely corroded in high-temperature formations and requires the addition of special additives. Furthermore, the method only considers the thickness of the membrane without considering other properties, making it impossible to achieve precise release.

[0006] Patent publication number CN107880869A discloses a microcapsule-encapsulated acid for oil fields and a preparation method thereof, which comprises a base acid, a solidified filler material, an encapsulating material, a surfactant, and a solvent. The preparation method includes material preparation, mixing, heating, fluidization, and encapsulation. The microcapsule-encapsulated acid is encapsulated in a fluidized bed at an operating temperature of 80 to 90°C, which easily causes the acid to be inactivated under high temperature conditions, resulting in insufficient effective content of the microcapsule acid. The coating material is a high molecular polymer, which is not easily degraded and flowed back.

[0007] Patent publication number CN109593518A provides a solid acid declogging system for oilfield acid declogging. The system consists of 10% to 20% solid acid, 3% to 5% ammonium chloride, 1% to 2% acidizing additive, and the remainder water. This solid acid declogging system requires on-site solution preparation and cannot meet the needs of online acidification.

[0008] The patent with publication number CN109913195A provides a packaged acid that increases the effective action distance of acid fracturing. It uses acidic carrier fluid to enter the formation and uses pH to control the release. + When the temperature drops to a certain level, the structure of the packaging material is destroyed, solid acid is released, and ionization produces H + This encapsulated acid release relies on the acid-rock reaction to reduce the H+ concentration to control the encapsulated acid release. However, the acid-rock reaction has many uncontrollable factors, and the change in H+ concentration cannot be accurately predicted. This makes it impossible to achieve precise acid control in offshore sandstone reservoirs.

[0009] Patent publication number CN112646563A provides a solid sustained-release acid and a preparation method thereof. The sustained-release acid is prepared by mixing p-toluenesulfonic acid, hydroxyethylidene diphosphonic acid, a cross-linking conditioner, a temperature stabilizer, and a dehydrating agent, followed by polycondensation under the action of a catalyst. The sustained-release acid has a sustained-release time of 3-7 days and takes 5-8 days to completely dissolve in water at 25°C. It has good dispersibility and is suitable for temperatures of 80-150°C. However, the preparation process is overly complex and costly, the polymer molecular weight is too high, and degradation is difficult. Moreover, the sustained-release acid only has a sustained release effect and cannot achieve release under specific conditions.

[0010] Patent publication number CN112795375A provides a controlled-release solid acid material, a preparation method, and an application thereof. The controlled-release solid acid material comprises a degradable shell, and a solid acid and a corrosion inhibitor encapsulated in the degradable shell. The controlled solid acid is not released at room temperature, but is released at 40°C-120°C for 1 minute to 120 minutes. However, the preparation process is complex, requiring the acid solution and corrosion inhibitor to be solidified, crushed, and then wrapped with a degradable material. The preparation temperature is 155-275°C, making it difficult to ensure the wrapping rate and effective content. Summary of the Invention

[0011] To overcome the defects of the prior art, the present invention provides an on-line acidizing coated acid for offshore sandstone reservoirs, a preparation method thereof, and an application thereof. The technical solution is as follows:

[0012] The invention discloses an on-line acidizing coating acid for offshore sandstone reservoirs. The coating acid comprises the following components and their mass percentages: base acid accounts for 75%-80%, acid isolation membrane accounts for 3%-5%, ammonium bifluoride accounts for 7%-10%, and external coating membrane accounts for 5%-15%.

[0013] The basic acid is one or more of solid hydrochloric acid, solid nitric acid powder, aminosulfonic acid and citric acid.

[0014] The acid isolation film is composed of the following components and their mass percentages: 40%-50% of urethane acrylate, 40%-45% of 1,6-hexanediol diacrylate, and 5%-20% of methacryloyloxyethyltrimethylammonium chloride; the external wrapping film is composed of the following components and their mass percentages: 55%-65% of acrylamide, and 35%-45% of 2-acrylamido-2-methylpropanesulfonic acid.

[0015] According to the above-mentioned method for preparing coated acid for online acidification of offshore sandstone reservoirs, it comprises the following steps:

[0016] S1: dissolving urethane acrylate, 1,6-hexanediol diacrylate, and methacryloyloxyethyl trimethylammonium chloride in anhydrous ethanol; adding dimethyl azobisisobutyrate as an initiator; performing a polymerization reaction, and obtaining an acid isolation membrane A solid after precipitation;

[0017] S2: dissolving the acid isolation film A solid and the base acid in anhydrous ethanol to obtain a mixed solution; and preparing the coating acid by spray drying;

[0018] S3: dissolving acrylamide and 2-acrylamido-2-methylpropanesulfonic acid in anhydrous ethanol, adding dimethyl azobisisobutyrate as an initiator; performing a polymerization reaction, and obtaining an outer wrapping film B after precipitation;

[0019] S4: The coating acid prepared in S2 is mixed with ammonium bifluoride to obtain a solid mixture C, and the outer coating film B prepared in S3 is dissolved in anhydrous ethanol to obtain a solution D; the solution D is sprayed onto the surface of the solid mixture C and dried to obtain the coating acid.

[0020] The ratio of the total mass of the S1 urethane acrylate, 1,6-hexanediol diacrylate and methacryloyloxyethyltrimethylammonium chloride to the mass of anhydrous ethanol is (1:6)-(1:8); the amount of the initiator dimethyl azobisisobutyrate added is 1%-2% of the total mass of anhydrous ethanol, urethane acrylate, 1,6-hexanediol diacrylate and methacryloyloxyethyltrimethylammonium chloride; the polymerization reaction temperature is 40°C-60°C, and the reaction time is 7-8 hours.

[0021] The ratio of the total mass of the S2 acid isolation membrane A and the base acid to the mass of anhydrous ethanol is (1:2)-(1:3).

[0022] The ratio of the total mass of the S3 acrylamide and 2-acrylamido-2-methylpropanesulfonic acid to the mass of anhydrous ethanol is (1:6)-(1:8); the amount of the initiator dimethyl azobisisobutyrate added is 1%-2% of the total mass of anhydrous ethanol, acrylamide and 2-acrylamido-2-methylpropanesulfonic acid; the polymerization reaction temperature is 40°C-60°C, and the reaction time is 3-4 hours.

[0023] The mass ratio of the S4 wrapping film B to anhydrous ethanol is (1:1)-(1:2).

[0024] In both the S1 and S3 polymerization reactions, acetone is used to completely precipitate the polymerized material; the amount of acetone added is 3%-5% by mass of the reaction solution.

[0025] The invention discloses an application of coated acid for online acidizing of offshore sandstone reservoirs in a sandstone deblocking system, wherein the amount of coated acid used accounts for 15%-25% of the total mass of the sandstone deblocking system.

[0026] Compared with the prior art, the present invention has the following beneficial technical effects:

[0027] 1. The coated acid of the present invention can be mixed and injected online without corrosiveness to the injection equipment, thus ensuring the long-term effectiveness of the acidizing construction, reducing the number of acidizing constructions, improving the acidizing effect, reducing the economic cost, and playing an important role in the production increase and transformation of oil and gas wells.

[0028] 2. Compared to conventional solid acids or single-layer solid acids, the present invention utilizes a double-layer coating, resulting in improved strength. While most conventional solid acids were controlled by either temperature or acidity, or by one factor as a primary and the other as a secondary, the present invention utilizes an outer acidity-responsive membrane and an inner temperature-responsive membrane to achieve dual control of both acidity and temperature, significantly improving release accuracy.

[0029] 3. The present invention encapsulates the fluoride-containing salt and the treatment acid, and no additional fluoride ions need to be added during the treatment of the sandstone reservoir, thereby reducing the amount of additives used and simplifying the procedure.

[0030] 4. The solid acid of the present invention is easy to transport and has a long storage time at room temperature, which can reach 30 days.

[0031] 5. The synthetic raw materials of the present invention are widely available, the production process is simple, it is easy to industrialize, and the application prospect is broad. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is a schematic diagram of the structure of the on-line acidification package acid of the present invention;

[0033] Figure 2 The release rate of the coated acid under different temperature conditions of the online acidification coated acid of the present invention;

[0034] Figure 3 It is the hydrogen ion concentration consumption rate under different usage concentrations of the on-line acidification coating acid of the present invention. DETAILED DESCRIPTION

[0035] The present invention is described in detail below with reference to the embodiments and accompanying drawings. Example 1

[0036] An online acidizing coating acid for offshore sandstone reservoirs is composed of the following components by weight: 80% base acid, 3% acid separator, 8% ammonium bifluoride, and 9% outer coating. The base acid is a mixture of solid hydrochloric acid and citric acid in a 2:1 mass ratio. The acid separator is composed of 50% urethane acrylate, 40% 1,6-hexanediol diacrylate, and 10% methacryloyloxyethyltrimethylammonium chloride. The outer coating is composed of 60% acrylamide and 40% 2-acrylamido-2-methylpropanesulfonic acid.

[0037] The above-mentioned method for preparing the coated acid for online acidizing of offshore sandstone reservoirs is as follows:

[0038] S1: Dissolve 5 g of urethane acrylate, 4 g of 1,6-hexanediol diacrylate, and 1 g of methacryloyloxyethyltrimethylammonium chloride in 80 g of anhydrous ethanol, add 0.9 g of dimethyl azobisisobutyrate, and carry out polymerization reaction at 55°C for 7-8 h. Add 2.7 g of acetone for precipitation to obtain an acid isolation membrane A solid.

[0039] S2: dissolving the acid isolation membrane A solid and 270 g of the base acid in 560 g of anhydrous ethanol to obtain a mixed solution, and preparing a coating acid by spray drying.

[0040] S3: Dissolve 18 g of acrylamide and 12 g of 2-acrylamido-2-methylpropanesulfonic acid in 240 g of anhydrous ethanol, add 2.7 g of dimethyl azobisisobutyrate, and carry out polymerization reaction at 40° C. for 3-4 h. Add 8.1 g of acetone for precipitation to obtain the outer wrapping film B.

[0041] S4: The coating acid prepared in S2 is mixed with 28 g of ammonium bifluoride to obtain a solid mixture C, and the outer coating film B obtained in S3 is dissolved in 270 g of anhydrous ethanol to obtain a solution D. The solution D is sprayed onto the surface of the solid mixture C and dried to obtain the coating acid.

[0042] The effective content and release rate of the prepared encapsulated acid were evaluated, and the results are shown in Table 1. Figure 2 ; The structure of the prepared encapsulated acid is shown in Figure 1 . Example 2

[0043] A coating acid for online acidizing of offshore sandstone reservoirs is composed of the following components by weight: 75% base acid, 3% acid separator, 7% ammonium bifluoride, and 15% outer coating. The base acid is solid nitric acid powder. The acid separator is composed of 50% urethane acrylate, 45% 1,6-hexanediol diacrylate, and 5% methacryloyloxyethyltrimethylammonium chloride. The outer coating is composed of 55% acrylamide and 45% 2-acrylamido-2-methylpropanesulfonic acid.

[0044] The above-mentioned method for preparing the coated acid for online acidizing of offshore sandstone reservoirs is as follows:

[0045] S1: Dissolve 5 g of urethane acrylate, 4.5 g of 1,6-hexanediol diacrylate, and 0.5 g of methacryloyloxyethyltrimethylammonium chloride in 60 g of anhydrous ethanol, add 1.4 g of dimethyl azobisisobutyrate, and carry out polymerization reaction at 40°C for 8 h. Add 3.5 g of acetone for precipitation to obtain an acid isolation membrane A solid.

[0046] S2: dissolving the acid isolation membrane A solid and 253 g of the base acid in 789 g of anhydrous ethanol to obtain a mixed solution, and preparing a coating acid by spray drying.

[0047] S3: 28.5 g of acrylamide and 23.5 g of 2-acrylamido-2-methylpropanesulfonic acid were dissolved in 312 g of anhydrous ethanol, 7.3 g of dimethyl azobisisobutyrate was added, and polymerization reaction was carried out at 60° C. for 3 h. 18.2 g of acetone was added for precipitation to obtain the outer wrapping film B.

[0048] S4: The coating acid prepared in S2 is mixed with 23 g of ammonium bifluoride to obtain a solid mixture C, and the outer coating film B obtained in S3 is dissolved in 728 g of anhydrous ethanol to obtain a solution D. The solution D is sprayed onto the surface of the solid mixture C and dried to obtain the coating acid.

[0049] The effective content and release rate of the prepared encapsulated acid were evaluated, and the results are shown in Table 1. Figure 2 ; The structure of the prepared encapsulated acid is shown in Figure 1 . Example 3

[0050] An online acidizing coating acid for offshore sandstone reservoirs is composed of the following components by weight: 80% base acid, 5% acid separator, 10% ammonium bifluoride, and 5% outer coating. The base acid is aminosulfonic acid. The acid separator comprises 40% urethane acrylate, 40% 1,6-hexanediol diacrylate, and 20% methacryloyloxyethyltrimethylammonium chloride. The outer coating comprises 65% acrylamide and 35% 2-acrylamido-2-methylpropanesulfonic acid.

[0051] The above-mentioned method for preparing the coated acid for online acidizing of offshore sandstone reservoirs is as follows:

[0052] S1: Use 119 g of anhydrous ethanol to dissolve 6.8 g of amino acrylate, 6.8 g of 1,6-hexanediol diacrylate, and 3.4 g of methacryloyloxyethyl trimethylammonium chloride, add 2 g of dimethyl azobisisobutyrate, and carry out polymerization reaction at 60°C for 7 h. Add 5.4 g of acetone for precipitation to obtain an acid isolation membrane A solid.

[0053] S2: dissolving the acid isolation membrane A solid and 270 g of the base acid in 718 g of anhydrous ethanol to obtain a mixed solution, and preparing a coating acid by spray drying.

[0054] S3: Dissolve 11 g of acrylamide and 6 g of 2-acrylamido-2-methylpropanesulfonic acid in 119 g of anhydrous ethanol, add 2 g of dimethyl azobisisobutyrate, and carry out polymerization reaction at 50° C. for 3.5 h. Add 5.4 g of acetone for precipitation to obtain the external wrapping film B.

[0055] S4: The coating acid prepared in S2 is mixed with 34 g of ammonium bifluoride to obtain a solid mixture C, and the outer coating film B obtained in S3 is dissolved in 204 g of anhydrous ethanol to obtain a solution D. The solution D is sprayed onto the surface of the solid mixture C and dried to obtain the coating acid.

[0056] The effective content and release rate of the prepared encapsulated acid were evaluated, and the results are shown in Table 1. Figure 2 ; The structure of the prepared encapsulated acid is shown in Figure 1 .

[0057]

[0058] As can be seen from Table 1, the average effective contents of encapsulated acids 1, 2, and 3 are 79.79%, 76.58%, and 74.62%, respectively, which are basically consistent with the contents designed when preparing the encapsulated acids.

[0059] like Figure 1 The figure shows a schematic diagram of the structure of the coated acid. The outer coating provides support and protection and responds to changes in acidity. The irregular solid black solid represents ammonium bifluoride. The small circle with black dots is the coated acid. The regular black dots inside represent the base acid. The acid isolation membrane prevents leakage of the base acid.

[0060] like Figure 2As shown, the release rate of the encapsulated acid, whether solid hydrochloric acid, citric acid, solid nitric acid or aminosulfonic acid, is relatively stable before 80°C; in the range of 80°C-90°C, the release rate of the encapsulated acid whose base acid is aminosulfonic acid increases significantly, and the other two base acids also increase slightly; after 90°C, the release rate of solid hydrochloric acid, citric acid and solid nitric acid increases significantly; aminosulfonic acid will be hydrolyzed in large quantities after exceeding 90°C, so the release rate of the encapsulated acid whose base acid is aminosulfonic acid remains almost unchanged after 90°C. Example 4

[0061] A conventional soil acid declogging system is configured, whose ingredients are: 12% HCl + 2% HF, 2% corrosion inhibitor, 1% iron ion stabilizer, 1% demulsifier, 1% clay stabilizer, 1% drainage aid, and the rest is water.

[0062] The main substances selected for each component in the system are as follows:

[0063] Basic acid: 12% HCl + 2% HF;

[0064] Corrosion inhibitor: pentadecyl imidazoline;

[0065] Iron ion stabilizer: EDTA;

[0066] Demulsifier: sulfonate;

[0067] Clay stabilizer: polyquaternium;

[0068] Depletion aid: polyoxyethylene alkyl alcohol amide.

[0069] The prepared acid solution was subjected to corrosion test, steel sheet corrosion test and retarding performance evaluation, and the results are shown in Table 2, Table 3 and Figure 3 . Example 5

[0070] The coated acid is configured into a sandstone declogging system, the composition of which is: 15% by mass of coated acid, 2% by mass of corrosion inhibitor, 1% by mass of iron ion stabilizer, 1% by mass of demulsifier, 1% by mass of clay stabilizer, 1% by mass of drainage aid, and the remainder by mass of water.

[0071] The main substances selected for each component in the system are as follows:

[0072] Base acid: 15% encapsulated acid;

[0073] Corrosion inhibitor: pentadecyl imidazoline;

[0074] Iron ion stabilizer: EDTA;

[0075] Demulsifier: sulfonate;

[0076] Clay stabilizer: polyquaternium;

[0077] Depletion aid: polyoxyethylene alkyl alcohol amide.

[0078] The prepared acid solution was subjected to corrosion test, steel sheet corrosion test and retarding performance evaluation, and the results are shown in Table 2, Table 3 and Figure 3 . Example 6

[0079] The coated acid is configured into a sandstone declogging system, the composition of which is: 20% by mass of coated acid, 2% by mass of corrosion inhibitor, 1% by mass of iron ion stabilizer, 1% by mass of demulsifier, 1% by mass of clay stabilizer, 1% by mass of drainage aid, and the remainder by mass of water.

[0080] The main substances selected for each component in the system are as follows:

[0081] Base acid: 20% encapsulated acid;

[0082] Corrosion inhibitor: pentadecyl imidazoline;

[0083] Iron ion stabilizer: EDTA;

[0084] Demulsifier: sulfonate;

[0085] Clay stabilizer: polyquaternium;

[0086] Depletion aid: polyoxyethylene alkyl alcohol amide.

[0087] The prepared acid solution was subjected to corrosion test, steel sheet corrosion test and retarding performance evaluation, and the results are shown in Table 2, Table 3 and Figure 3 . Example 7

[0088] The coated acid is configured into a sandstone declogging system, the composition of which is: 25% by mass of coated acid, 2% by mass of corrosion inhibitor, 1% by mass of iron ion stabilizer, 1% by mass of demulsifier, 1% by mass of clay stabilizer, 1% by mass of drainage aid, and the remainder by mass of water.

[0089] The main substances selected for each component in the system are as follows:

[0090] Base acid: 25% encapsulated acid;

[0091] Corrosion inhibitor: pentadecyl imidazoline;

[0092] Iron ion stabilizer: EDTA;

[0093] Demulsifier: sulfonate;

[0094] Clay stabilizer: polyquaternium;

[0095] Depletion aid: polyoxyethylene alkyl alcohol amide.

[0096] The prepared acid solution was subjected to corrosion test, steel sheet corrosion test and retarding performance evaluation, and the results are shown in Table 2, Table 3 and Figure 3 .

[0097] As can be seen from Table 2, the dissolution rate of 15% coating acid is 14.24%, the dissolution rate of 20% coating acid is 16.76%, the dissolution rate of 25% coating acid is 17.23%, and the dissolution rate of soil acid is 18.67%; the dissolution ability of coating acid and soil acid is basically equivalent; when the mass fraction of coating acid is continuously increased, the dissolution rate continues to increase, but the dissolution rate increases less when the mass fraction of coating acid increases from 20% to 25%.

[0098] As shown in Table 3, under the condition of the same corrosion inhibitor dosage, the corrosion rate of solid acid with a mass fraction of 25% reaches 3.1205 g / m2·h, which is much lower than the 6.0213 g / m2·h of mud acid on steel sheets. In addition, the corrosion rates of the three mass fractions of encapsulated acid all meet the national standards.

[0099] like Figure 3 As shown in the figure, at the same temperature, the hydrogen ion concentration consumption rate of 12% HCl+2% HF is significantly faster than that of the coated acid; the time for the hydrogen ion concentration of 25% coated acid to decrease to 0.1 mol / L is about 180 minutes, the time for the hydrogen ion concentration of 20% coated acid to decrease to 0.1 mol / L is about 140 minutes, the time for the hydrogen ion concentration of 15% coated acid to decrease to 0.1 mol / L is about 110 minutes, and the time for the hydrogen ion concentration of 12% HCl+2% HF to decrease to 0.1 mol / L is about 10 minutes. This shows that the coated acid has good retarding properties.

Claims

1. An on-line acidizing package acid for offshore sandstone reservoirs, characterized in that: The coating acid is composed of the following components and their mass percentages: basic acid accounts for 75%-80%, acid isolation film accounts for 3%-5%, ammonium bifluoride accounts for 7%-10%, and external coating film accounts for 5%-15%; The base acid is one or more of solid hydrochloric acid, solid nitric acid powder, aminosulfonic acid and citric acid; The acid isolation film is composed of the following components and their mass percentages: 40%-50% of urethane acrylate, 40%-45% of 1,6-hexanediol diacrylate, and 5%-20% of methacryloyloxyethyltrimethylammonium chloride; the external wrapping film is composed of the following components and their mass percentages: 55%-65% of acrylamide, and 35%-45% of 2-acrylamido-2-methylpropanesulfonic acid.

2. The method for preparing coated acid for online acidification of offshore sandstone reservoirs according to claim 1, characterized in that: The following steps are involved: S1: dissolving urethane acrylate, 1,6-hexanediol diacrylate, and methacryloyloxyethyl trimethylammonium chloride in anhydrous ethanol; adding dimethyl azobisisobutyrate as an initiator; performing a polymerization reaction, and obtaining an acid isolation membrane A solid after precipitation; S2: dissolving the acid isolation film A solid and the base acid in anhydrous ethanol to obtain a mixed solution; and preparing the coating acid by spray drying; S3: dissolving acrylamide and 2-acrylamido-2-methylpropanesulfonic acid in anhydrous ethanol, adding dimethyl azobisisobutyrate as an initiator; performing a polymerization reaction, and obtaining an outer wrapping film B after precipitation; S4: The coating acid prepared in S2 is mixed with ammonium bifluoride to obtain a solid mixture C, and the outer coating film B prepared in S3 is dissolved in anhydrous ethanol to obtain a solution D; the solution D is sprayed onto the surface of the solid mixture C and dried to obtain the coating acid.

3. The method for preparing coated acid for online acidification of offshore sandstone reservoirs according to claim 2, characterized in that: In step S1, the ratio of the total mass of urethane acrylate, 1,6-hexanediol diacrylate, and methacryloyloxyethyltrimethylammonium chloride to the mass of anhydrous ethanol is (1:6)-(1:8); the amount of initiator dimethyl azobisisobutyrate added is 1%-2% of the total mass of anhydrous ethanol, urethane acrylate, 1,6-hexanediol diacrylate, and methacryloyloxyethyltrimethylammonium chloride; the polymerization reaction temperature is 40°C-60°C, and the reaction time is 7-8 hours.

4. The method for preparing coated acid for online acidification of offshore sandstone reservoirs according to claim 2, characterized in that: The ratio of the total mass of the acid isolation film A and the base acid in step S2 to the mass of anhydrous ethanol is (1:2)-(1:3).

5. The method for preparing coated acid for online acidification of offshore sandstone reservoirs according to claim 2, characterized in that: In step S3, the ratio of the total mass of acrylamide and 2-acrylamido-2-methylpropanesulfonic acid to the mass of anhydrous ethanol is (1:6)-(1:8); the amount of initiator dimethyl azobisisobutyrate added is 1%-2% of the total mass of anhydrous ethanol, acrylamide and 2-acrylamido-2-methylpropanesulfonic acid; the polymerization reaction temperature is 40°C-60°C, and the reaction time is 3-4 hours.

6. The method for preparing coated acid for online acidification of offshore sandstone reservoirs according to claim 2, characterized in that: The mass ratio of the wrapping film B to the anhydrous ethanol in step S4 is (1:1)-(1:2).

7. The method for preparing coated acid for online acidification of offshore sandstone reservoirs according to claim 2, characterized in that: In the polymerization reactions in steps S1 and S3, acetone is used to completely precipitate the polymeric material; the amount of acetone added is 3%-5% by mass of the reaction solution.

8. The use of an on-line acidizing packaged acid for offshore sandstone reservoirs in a sandstone plugging removal system according to claim 1, characterized in that: The amount of the coated acid used accounts for 15%-25% of the total mass of the sandstone deplugging system.

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