Temporary plugging agent for fracturing, preparation method and application

Abstract

The invention discloses a temporary plugging agent for fracturing as well as a preparation method and application thereof. Belongs to the technical field of oilfield chemistry. According to the temporary plugging agent for fracturing, the preparation method and the application, an acrylic acid blend and plant fibers are adopted to construct a plant fiber enhanced double-network structure, an acid response synergistic degradation mechanism is given to the plant fiber enhanced double-network structure, and the contradiction among high-temperature stability, plugging strength and plugging removal efficiency of an existing temporary plugging agent is overcome; the temporary plugging agent provided by the invention can keep stable plugging in a 200 DEG C high-temperature environment, has the pressure bearing capacity exceeding 50 MPa, and can actively trigger plug removal according to construction requirements. And technical support is provided for further development of the temporary plugging agent.

Description

Technical Field

[0001] The present invention relates to the technical field of oilfield chemistry, and more particularly to a temporary plugging agent for fracturing, a preparation method and application thereof. Background Art

[0002] As oil and gas exploration and development expand into deep and ultra-deep formations, temporary plugging and fracturing technology has become a key means of improving reservoir reconstruction. This technology uses temporary plugging agents to seal existing fractures or high-permeability channels, forcing the fracturing fluid to divert to areas that have not been fully reconstructed, thereby increasing reservoir utilization. However, existing temporary plugging agents have obvious technical bottlenecks in high-temperature environments: Insufficient high temperature resistance: Conventional polymer temporary plugging agents will undergo thermal degradation or softening when the temperature exceeds 160°C. For example, the compressive strength of polyglycolic acid-based materials will drop by more than 30% at 180°C, resulting in plugging failure.

[0003] There is a contradiction between plugging strength and deplugging efficiency: although high-strength metal alloy temporary plugging agents (such as magnesium-aluminum alloy) can withstand a pressure of 30MPa, they require strong acid to deplug and have a residue rate of 10-15%, which seriously damages the reservoir; while water-soluble polymers that are easy to deplug (such as starch / acrylic acid copolymers) have insufficient pressure bearing capacity (<15MPa) and are difficult to seal wide cracks.

[0004] Insufficient controllability of unblocking: The degradation behavior of existing degradable temporary plugging agents is controlled by a single factor, temperature, and it is impossible to accurately adjust the unblocking time according to construction requirements, which affects the production increase effect.

[0005] To address the above issues, existing technological development focuses on material modification: the knotted temporary plugging agent developed by China National Petroleum Corporation adopts a skin-core structure design, and its temperature resistance is increased to 180°C, but its degradation rate is still dominated by temperature and cannot be actively regulated; although the fully degradable temporary plugging agent of Southwest Petroleum Engineering Company achieves a temporary plugging efficiency of over 80%, it lacks stability in an ultra-high temperature environment of 200°C; and although the intelligent and controllable temporary plugging agent newly developed by China National Petroleum Corporation achieves controllable gel breaking time, its plugging strength is still difficult to meet the requirements of ultra-deep wells.

[0006] Therefore, there is an urgent need to develop a new temporary plugging agent that has ultra-high temperature stability, high pressure bearing strength and precise and controllable plugging removal properties to meet the needs of efficient development of deep unconventional oil and gas resources. This is a technical problem that technical personnel in this field urgently need to solve. Summary of the Invention

[0007] In view of this, the present invention provides a temporary plugging agent for fracturing, a preparation method and an application, which improves the high-temperature stability of the temporary plugging agent, overcomes the contradiction between plugging strength and unblocking efficiency, and provides a temporary plugging agent that can maintain stable plugging in a high-temperature environment of 200°C, has a pressure bearing capacity exceeding 50MPa, and can actively trigger unblocking according to construction requirements.

[0008] Secondly, a preparation method of the temporary plugging agent is provided, which solves the compatibility problem of high-temperature deformation and controllable degradation through a double-network gel construction and acid-responsive material composite process.

[0009] Third, the application of the temporary plugging agent in oil and gas well fracturing is provided to achieve precise temporary plugging of cracks and low-residue unplugging.

[0010] In order to solve the above technical problems, this application adopts the following technical solutions: A temporary plugging agent for fracturing comprises the following components in percentage by weight: 25-40% acrylic polymer, 8-15% plant fiber, 20-30% acid-responsive inorganic filler, 3-8% slow-release breaker, 2-5% thermal stabilizer, 0.5-1.5% cross-linking agent and 0.1-0.3% initiator, with the balance being water to make up 100%.

[0011] As a preferred technical solution, the acrylic polymer is composed of acrylamide and acrylic acid in a mass ratio of 4:6-5:5; under the action of a crosslinking agent, the acrylic polymer forms a three-dimensional crosslinked skeleton, giving the material high elasticity and deformation recovery ability.

[0012] The plant fibers are sisal fibers and / or bamboo fibers that have been treated with alkali; The acid-responsive inorganic filling material is a composite of magnesium hydroxide and zinc oxide; the acid-responsive inorganic filling material is acid-soluble: the magnesium hydroxide / zinc oxide composite particles dissolve rapidly when exposed to acid (pH ≤ 4.5) (reaction formula: Mg(OH)2+ 2H + → Mg 2+ +2H2O), forming microchannels inside the material to achieve acid-responsive unblocking; The slow-release breaker is a silver-1,4-dicyanobenzene complex; the slow-release breaker releases silver ions within a certain period of time, catalyzing the oxidative degradation of the polymer blend to achieve time-controlled breaking; The thermal stabilizer is hydrophobic fumed silica, which improves thermal stability; The cross-linking agent is N,N'-methylenebisacrylamide, and the acrylic polymer forms a chemical cross-linking network through the cross-linking agent to strengthen the network cross-linking; The initiator is ammonium persulfate.

[0013] Through the above-mentioned formulation of the present application, a temporary plugging agent with a plant fiber-reinforced double-network structure can be successfully constructed, and has an acid-responsive synergistic unblocking mechanism, as follows: Plant fiber reinforced double network structure: Primary network: A chemically cross-linked network formed by acrylic polymers through cross-linking agents, providing basic elasticity; Secondary network: After alkali treatment, the hydroxyl and carboxyl groups on the surface of the plant fiber are connected to the polymer molecular chain through hydrogen bonds and chemical bonds to form physical cross-linking points. At high temperatures (>150°C), the plant fiber gelatinizes and hardens, effectively inhibiting excessive hydration of the gel and maintaining structural strength.

[0014] Acid response synergistic plugging removal mechanism: Acid dissolution of filling materials: Magnesium hydroxide / zinc oxide composite particles dissolve rapidly in acid, forming microchannels inside the material; Acid hydrolysis of polymer networks: Hydrogen ions catalyze the hydrolysis and chain scission of amide groups, and the degradation rate is increased compared to neutral conditions; Slow-release gel breaker trigger: The slow-release gel breaker releases silver ions within a certain period of time, catalyzing the oxidative degradation of the polymer, and realizing "time-chemistry" dual-control unblocking.

[0015] As a preferred technical solution, the plant fiber has a fiber length of 0.5-2 mm and an aspect ratio of ≥20:1.

[0016] As a preferred technical solution, the particle size distribution of the magnesium hydroxide and zinc oxide composite is 1-10 μm; The molar mass ratio of silver ions to 1,4-dicyanobenzene in the sustained-release gel breaker is 1:1.5-2.5.

[0017] Another object of the present application is to provide a method for preparing the temporary plugging agent for fracturing, comprising the following steps: (1) Plant fiber pretreatment: The plant fiber is cut into 1-2 mm lengths, immersed in 8-12 wt% NaOH solution, treated at 85-95°C for 1.5-2.5 hours, washed to neutrality, and then dried to prepare pretreated plant fiber; the plant fiber is sisal fiber and / or bamboo fiber that has been treated with alkali; the plant fiber of the present application has the following effects after being pretreated with high temperature and high concentration alkali (under severe conditions): Improve fiber surface reactivity: break down the natural wax / hemicellulose barrier and expose more hydroxyl (-OH) and carboxyl (-COOH) groups; Reinforced fiber-polymer interface bonding: -COOH on the fiber surface forms an amide bond (-CONH-) with the polymer amide group; the exposed -OH forms a hydrogen bond network with the polymer carboxyl group; a stable double network structure is constructed through chemical bonds and hydrogen bonds; Inhibition of high-temperature creep: Long-term treatment with high-temperature and high-concentration alkali triggers the formation of "alkali cellulose" (Cellulose II crystal form) of cellulose, or rearranges cellulose chains after dissolving amorphous impurities, thereby increasing crystallinity. This causes the fiber to gelatinize and harden at high temperatures rather than degrade, thereby inhibiting high-temperature deformation of the polymer network. (2) Preparation of sustained-release gel breaker: silver nitrate and 1,4-dicyanobenzene were mixed in ethanol at a molar ratio and reacted at 45-55°C for 3-6 hours to generate a silver-1,4-dicyanobenzene complex precipitate, which was filtered, dried, and crushed to 180-220 mesh to prepare a sustained-release gel breaker; (3) mixing acrylic polymer, water, and pretreated plant fibers, adding a crosslinking agent and an initiator for polymerization reaction to prepare a gel; (4) After the gel is broken, it is mixed evenly with the acid-responsive inorganic filler, the slow-release breaker, and the thermal stabilizer; (5) The mixed material is granulated by deep cooling extrusion.

[0018] As a preferred technical solution, the conditions of the polymerization reaction in step (3) are as follows: 55-65°C for 3h; As a preferred technical solution, the cryogenic extrusion in step (5) is carried out at -40°C to -60°C, and the screw aspect ratio is 12-18:1.

[0019] Another object of the present application is to provide: application of the temporary plugging agent for fracturing or the temporary plugging agent for fracturing prepared by the above method in oil and gas well fracturing.

[0020] As a preferred technical solution, the temporary plugging agent is added to the fracturing fluid at a concentration of 0.5-3wt% and pumped to the target layer to form temporary plugging; when unblocking, an acidic solution with a pH of 3.5-5.0 is injected or a slow-release breaker is used to automatically trigger unblocking.

[0021] As a preferred technical solution, the acidic solution is formic acid, acetic acid and / or citric acid solution.

[0022] It can be seen from the above technical solution that, compared with the prior art, the present invention has the following beneficial effects: the present application provides a temporary plugging agent for fracturing, which has: Ultra-high temperature stability: The dual network structure and thermal stabilizer work synergistically to enable the temporary plugging agent to maintain structural integrity at 200°C, with a compressive strength of ≥50MPa, breaking through the temperature resistance limit of existing temporary plugging agents; Precise and controllable plugging removal: Controllable plugging removal is achieved through acid injection or timed release of gel breaker. The residue rate after plugging is ≤1%, which is much lower than that of metal alloy temporary plugging agents (>10%). Little damage to reservoir: Fully organic-inorganic composite design, the degradation products are water-soluble small molecules (acrylic acid oligomers, magnesium ions, etc.), and no solid residue is retained in the reservoir.

[0023] Reduced construction costs: No special sealing tools are required for plugging, and pumping is sufficient, saving operating costs. DETAILED DESCRIPTION

[0024] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0025] The magnesium hydroxide / zinc oxide composite of the embodiment of the present invention is prepared by the following method: (1) Magnesium nitrate hexahydrate (Mg(NO3)2·6H2O) and zinc nitrate (Zn(NO3)2) were dissolved in deionized water at a molar ratio of Mg:Zn = 3:1 to prepare a mixed salt solution with a concentration of 1.0 mol / L; (2) Precipitant: sodium hydroxide and ammonia water mixed in a volume ratio of 1:1 (total concentration 2 mol / L); (3) A precipitant was added dropwise to the mixed salt solution until the pH value was 10-11, the solution was stirred at 50°C, and then aged at 70°C for 2 h. The solution was washed by centrifugation until neutrality was achieved, and then dried under vacuum at 80°C. The solution was ground and sieved to obtain a magnesium hydroxide / zinc oxide complex (1-10 μm).

[0026] Example 1

[0027] Acrylamide: 14%; Acrylic acid: 21%; Alkali-treated sisal fiber: 10%; Magnesium hydroxide / zinc oxide complex: 25%; Silver-1,4-dicyanobenzene complex: 5%; Hydrophobic fumed silica: 4%; N,N'-methylenebisacrylamide: 1%; Ammonium persulfate: 0.2%; Deionized water: balance.

[0028] (1) Sisal fiber pretreatment: Sisal fibers (fiber length 1 mm, aspect ratio ≥ 20:1) were cut into 1-2 mm lengths, immersed in 10 wt% NaOH solution, treated at 90 °C for 2 h, washed to neutrality, and then dried to prepare pretreated sisal fibers; (2) Preparation of sustained-release gel breaker: silver nitrate and 1,4-dicyanobenzene were mixed in ethanol at a molar mass ratio of silver ion to 1,4-dicyanobenzene of 1:2, and reacted at 50°C for 4 hours to generate a silver-1,4-dicyanobenzene complex precipitate, which was filtered, dried, and crushed to 200 mesh to prepare a silver-1,4-dicyanobenzene complex; (3) Acrylic polymer, water, and pretreated sisal fiber were mixed, N,N'-methylenebisacrylamide was added, the temperature was raised to 60°C, and then ammonium persulfate was added. The mixture was reacted at 60°C for 3 h to prepare a gel block, which was mechanically crushed into 5 mm particles. (4) After the gel is crushed, it is mixed with magnesium hydroxide / zinc oxide complex (5 μm), silver-1,4-dicyanobenzene complex, and hydrophobic fumed silica and mixed in a high-speed mixer at 800 rpm for 15 minutes; (5) The mixed material is granulated by a twin-screw extruder (-50℃ deep cold environment) and sieved to obtain 0.8-1.2mm particles. Example 2

[0029] Acrylamide: 12.5%; Acrylic acid: 12.5%; Alkali-treated bamboo fiber: 8%; Magnesium hydroxide / zinc oxide complex: 20%; Silver-1,4-dicyanobenzene complex: 3%; Hydrophobic fumed silica: 2%; N,N'-methylenebisacrylamide: 0.5%; Ammonium persulfate: 0.1%; Deionized water: balance.

[0030] (1) Bamboo fiber pretreatment: bamboo fibers (fiber length 0.5 mm, aspect ratio ≥ 20:1) were cut into 1-2 mm lengths, immersed in 8 wt% NaOH solution, treated at 85 °C for 1.5 hours, washed to neutrality, and then dried to prepare pretreated bamboo fibers; (2) Preparation of sustained-release gel breaker: silver nitrate and 1,4-dicyanobenzene were mixed in ethanol at a molar mass ratio of silver ion to 1,4-dicyanobenzene of 1:1.5, and reacted at 45°C for 3 hours to generate a silver-1,4-dicyanobenzene complex precipitate, which was filtered, dried, and crushed to 180 mesh to prepare a silver-1,4-dicyanobenzene complex; (3) Mixing acrylic polymer, water, and pretreated bamboo fiber, adding N,N'-methylenebisacrylamide, heating to 60°C, and then adding ammonium persulfate, reacting at 60°C for 3 h to prepare gel blocks, which were mechanically crushed into 5 mm particles; (4) The crushed gel was mixed with magnesium hydroxide / zinc oxide complex (2 μm), silver-1,4-dicyanobenzene complex, and hydrophobic fumed silica in a high-speed mixer at 800 rpm for 15 minutes; (5) The mixed material is granulated by a twin-screw extruder (-40℃ deep cold environment) and sieved to obtain 0.8-1.2mm particles. Example 3

[0031] Acrylamide: 20%; Acrylic acid: 20%; Alkali-treated sisal fiber: 15%; Magnesium hydroxide / zinc oxide complex: 25%; Silver-1,4-dicyanobenzene complex: 8%; Hydrophobic fumed silica: 5%; N,N'-methylenebisacrylamide: 1.5%; Ammonium persulfate: 0.3%; Deionized water: balance.

[0032] (1) Sisal fiber pretreatment: Sisal fibers (fiber length 2 mm, aspect ratio ≥ 20:1) were cut into 1-2 mm lengths, immersed in 12 wt% NaOH solution, treated at 95 °C for 2.5 h, washed to neutrality, and then dried to prepare pretreated sisal fibers; (2) Preparation of sustained-release gel breaker: silver nitrate and 1,4-dicyanobenzene were mixed in ethanol at a molar mass ratio of silver ion to 1,4-dicyanobenzene of 1:2.5, and reacted at 55°C for 6 hours to generate a silver-1,4-dicyanobenzene complex precipitate, which was filtered, dried, and crushed to 220 mesh to prepare a silver-1,4-dicyanobenzene complex; (3) Acrylic polymer, water, and pretreated sisal fiber were mixed, N,N'-methylenebisacrylamide was added, the temperature was raised to 60°C, and then ammonium persulfate was added. The mixture was reacted at 60°C for 3 h to prepare a gel block, which was mechanically crushed into 5 mm particles. (4) The crushed gel was mixed with magnesium hydroxide / zinc oxide complex (10 μm), silver-1,4-dicyanobenzene complex, and hydrophobic fumed silica in a high-speed mixer at 800 rpm for 15 minutes; (5) The mixed material is granulated by a twin-screw extruder (-60℃ deep cold environment) and sieved to obtain 0.8-1.2mm particles.

[0033] Comparative Example 1

[0034] The scheme is basically the same as that of Example 1, except that the sisal fiber is not pretreated, and the other operations are the same as those of Example 1.

[0035] In order to study the effect of the pretreatment operation of the plant fibers of the present application, a comparative example 1 was set up for Example 1. The only difference between the two was that one underwent a pretreatment operation and the other did not. The surface morphology, specific surface area and functional group density of the plant fibers of Example 1 (pretreated fiber) and Comparative Example 1 (untreated fiber) were measured respectively. The experimental results are shown in Table 1.

[0036] Table 1 Surface activation degree of plant fibers in different groups

[0037] Result analysis: As shown in Table 1, the surface of the plant fiber treated with alkali is fully activated, and the specific surface area and the density of -OH and -COOH functional groups are improved, which provides a basis for the subsequent construction of the plant fiber-enhanced double network structure.

[0038] The crystallinity of Example 1 and Comparative Example 1 was further measured, and the experimental results are shown in Table 2.

[0039] Table 2 Crystallinity and cellulose type in different groups

[0040] Result analysis: Crystallinity control (XRD verification), untreated sisal fiber: crystallinity is 63% (cellulose type I).

[0041] After alkali treatment, the crystallinity of Example 1 increased to 82% (cellulose type I was completely converted into high-crystallinity cellulose type II), and high-crystallinity cellulose gelatinized and hardened (not melted) at temperatures above 150°C, inhibiting high-temperature deformation of the polymer network.

[0042] Performance Testing

[0043] (1) High temperature pressure test: The temporary plugging agents for fracturing prepared in Examples 1-3 and Comparative Example 1 were filled into a simulated crack device (crack width 2 mm), and their temperature resistance and pressure bearing properties were measured. The experimental results are shown in Table 3.

[0044] Table 3 Temperature resistance and pressure bearing capacity of different groups of temporary plugging agents

[0045] Results: The temperature resistance and pressure bearing capacity of Examples 1-3 are significantly better than those of Comparative Example 1. The reason is that the crystallinity of the cellulose that has not been treated with alkali is low, the cellulose type is mainly natural fiber type I, and the adsorption mode of the fiber and polymer is mainly physical adsorption. High temperature will cause thermal degradation, resulting in poor temperature resistance and weak pressure bearing capacity.

[0046] (2) Controllable unblocking test: The unblocking time of Examples 1-3 and Comparative Example 1 was measured under Condition 1 and Condition 2, respectively. The specific conditions are as follows: Condition 1: Inject formic acid solution with pH = 4.0 and record the time for complete unblocking; Condition 2: Without adding acid, the unblocking time triggered by the slow-release breaker was tested.

[0047] The experimental results are shown in Table 4.

[0048] Table 4 Unblocking time and residue rate under different conditions in different groups

[0049] Results analysis: Compared with Comparative Example 1, the acid-triggered unblocking time of Comparative Example 1 is significantly prolonged in Examples 1-3. The reason for this is that the -COOH density of the alkali-treated fiber is high. In an acidic environment: Protonation contraction (-COO⁻ → -COOH) triggers network collapse; with acid-responsive filling materials Collaboratively accelerate channel formation.

[0050] Untreated alkali fibers lack -COOH and mainly rely on the dissolution of filling materials, so the unblocking time should be significantly extended.

[0051] (3) Field test: In a deep shale gas well in southern Sichuan (4,500 m deep, 195°C), 350 kg of the temporary plugging agent from Example 1 was pumped in to seal the already fractured fissures. The operating pressure increased from 45 MPa to 68 MPa, demonstrating effective diversion. After completion, an acidic plugging fluid (pH 4.2) was injected, and flowback resumed six hours later, increasing production by 2.1 times compared to the unfractured state.

[0052] The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referenced to each other.

[0053] The above description of the disclosed embodiments is intended to enable one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be readily apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention is not limited to the embodiments shown herein but is intended to conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A temporary plugging agent for fracturing, characterized in that: The invention comprises the following components in weight percentage: 25-40% of acrylic polymer, 8-15% of plant fiber, 20-30% of acid-responsive inorganic filler, 3-8% of slow-release breaker, 2-5% of heat stabilizer, 0.5-1.5% of crosslinking agent and 0.1-0.3% of initiator, and the balance is water to make up 100%.

2. The temporary plugging agent for fracturing according to claim 1, characterized in that: The acrylic polymer is composed of acrylamide and acrylic acid in a mass ratio of 4:6-5:5; The plant fibers are sisal fibers and / or bamboo fibers that have been treated with alkali; The acid-responsive inorganic filling material is a composite of magnesium hydroxide and zinc oxide; The sustained-release breaker is a silver-1,4-dicyanobenzene complex; The thermal stabilizer is hydrophobic fumed silica; The cross-linking agent is N,N'-methylenebisacrylamide; The initiator is ammonium persulfate.

3. The temporary plugging agent for fracturing according to claim 2, characterized in that: The plant fiber has a fiber length of 0.5-2 mm and an aspect ratio of ≥20:

1.

4. The temporary plugging agent for fracturing according to claim 2, characterized in that: The particle size distribution of the magnesium hydroxide and zinc oxide composite is 1-10 μm; The molar mass ratio of silver ions to 1,4-dicyanobenzene in the sustained-release gel breaker is 1:1.5-2.

5.

5. The method for preparing the temporary plugging agent for fracturing according to any one of claims 2 to 4, characterized in that: The steps include: (1) Plant fiber pretreatment: the plant fiber was cut into 1-2 mm length, immersed in 8-12 wt% NaOH solution, treated at 85-95 ° C for 1.5-2.5 hours, washed to neutrality and then dried to prepare pretreated plant fiber; (2) Preparation of sustained-release gel breaker: silver nitrate and 1,4-dicyanobenzene were mixed in ethanol at a molar ratio, and reacted at 45-55°C for 3-6 hours to generate a silver-1,4-dicyanobenzene complex precipitate, which was filtered, dried, and crushed to 180-220 mesh to prepare a sustained-release gel breaker; (3) mixing an acrylic polymer, water, and pretreated plant fibers, adding a crosslinking agent and an initiator for polymerization reaction to prepare a gel; (4) After the gel is broken, it is mixed evenly with the acid-responsive inorganic filler, the slow-release breaker, and the thermal stabilizer; (5) The mixed material is granulated by deep cooling extrusion.

6. The method for preparing a temporary plugging agent for fracturing according to claim 5, characterized in that: The polymerization reaction conditions in step (3) are as follows: 55-65°C for 3h; The cryogenic extrusion in step (5) is carried out at a temperature of -40°C to -60°C, and the aspect ratio of the screw is 12-18:

1.

7. Use of the temporary plugging agent for fracturing according to any one of claims 1 to 4 or the temporary plugging agent for fracturing prepared by the method according to any one of claims 5 to 6 in fracturing of oil and gas wells.

8. The use according to claim 7, characterized in that The temporary plugging agent is added to the fracturing fluid at a concentration of 0.5-3wt% and pumped to the target layer to form temporary plugging; when unblocking, an acidic solution with a pH of 3.5-5.0 is injected or a slow-release breaker is used to automatically trigger unblocking.

9. The use according to claim 8, characterized in that The acidic solution is formic acid, acetic acid and / or citric acid solution.

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