Preparation method of self-degradable plugging agent and reservoir protection drilling fluid

Abstract

The invention relates to a preparation method of a self-degradable plugging agent and a reservoir protection drilling fluid, and belongs to the technical field of petroleum and natural gas extraction. The preparation method of the self-degradable blocking agent comprises the following steps: enabling 16.5 to 31.3 parts of polyethylene glycol, 50 to 68 parts of lactide monomer and 7.5 to 20.7 parts of carbonate to react under the condition of 0.056 to 0.084 parts of catalyst; the reaction temperature ranges from 135 DEG C to 150 DEG C. The self-degradable plugging agent prepared by the invention can realize compact plugging in the drilling process, reduces the invasion risk of liquid-phase and solid-phase particles of drilling fluid, and achieves the purpose of protecting a reservoir. The oil-gas reservoir oil-gas reservoir oil-gas reservoir oil-gas reservoir oil-gas reservoir oil-gas reservoir oil-gas reservoir oil-gas reservoir oil-gas reservoir oil-gas reservoir oil-gas reservoir oil-gas reservoir oil-gas reservoir oil-gas reservoir oil-gas reservoir oil-gas reservoir oil-gas reservoir oil-gas reservoir oil-gas reservoir oil-gas reservoir oil-gas reservoir oil-gas reservoir oil-gas reservoir oil-gas reservoir oil-gas reservoir oil-gas reservoir oil-gas reservoir oil-gas reservoir

Description

Technical Field

[0001] The invention relates to a preparation method of a self-degradable plugging agent and a reservoir protection drilling fluid, belonging to the technical field of oil and natural gas exploitation. Background Art

[0002] In order to ensure the security of domestic oil and gas supply and resolve the contradiction between supply and demand, in recent years, my country's oil and gas resource exploration and development has been continuously developing towards complex, low-permeability, fractured, unconventional oil and gas reservoirs, which has put forward higher requirements for efficient, rapid and economical oil and gas exploitation. Strengthening the protection of oil and gas reservoirs and reducing damage to oil and gas reservoirs are key means to ensure the efficient exploitation of oil and gas resources.

[0003] Drilling is the first step to open the oil and gas reservoir. Therefore, how to effectively prevent or reduce reservoir damage during oil and gas drilling, thereby improving the efficiency of single-well oil and gas production, is a key technology for achieving efficient oil and gas development. During the drilling process, the oil and gas reservoir is first exposed to the drilling fluid after it is opened. Solid particles of different types and sizes in the drilling fluid can easily invade the natural pores near the well wall, causing pollution and damage to the oil and gas reservoir. The existing unblocking measures are complex and have limited effects. They are prone to form new chemical and mechanical blockages, reduce the permeability of the formation, and cause secondary pollution to the reservoir. In addition, the unblocking working fluid is difficult to contact the solid-phase blocking particles deep in the reservoir pores and cannot completely unblock them, which seriously reduces the permeability of the oil and gas reservoir, reduces the production of oil and gas wells, and increases the cost of oil and gas production. How to minimize the invasion of solid particles in the drilling fluid and efficiently remove the invading solid particles is the key to protecting oil and gas reservoirs.

[0004] In recent years, domestic and foreign scholars have developed many different reservoir protection methods and plugging removal technologies, mainly through scientific and reasonable selection of early shielding temporary plugging and later acid plugging removal, bio-enzyme plugging removal technology, etc. for different blockages and reservoir conditions, thereby reducing the degree of damage to oil and gas reservoirs, thereby increasing the production of oil and gas wells and improving economic benefits. However, the current reservoir protection technology still needs to use conventional plugging removal technologies such as chemical methods, physical methods, microbial methods and composite plugging removal methods when dealing with formation blockages. These plugging removal processes are complicated. If the plugging removal measures are not handled properly, new chemical blockages, mechanical blockages, etc. will occur, reducing the permeability of the formation.

[0005] At present, the drilling fluid protection materials for oil and gas reservoirs during drilling are mainly acidifiable materials, oil-soluble materials, biodegradable materials, biodegradable polymers, and temperature-sensitive polymer materials. They all require external auxiliary conditions to unblock and do not have good self-degradation properties. Their own temperature resistance, plugging performance, and degradation performance still need further research. For this reason, in view of the difficult-to-reconcile contradiction between plugging and reservoir protection during the current oil and gas reservoir drilling process, it is very urgent to research and develop self-unblocking plugging agents, which can not only meet the requirements of efficient plugging during the drilling process, but also achieve efficient self-degradation after the drilling is completed. It is the key to the current oil and gas reservoir protection technology. Summary of the invention

[0006] The purpose of the present invention is to provide a method for preparing a self-degradable plugging agent, so as to solve the problem in the prior art that the plugging agent needs external auxiliary conditions to be unblocked.

[0007] The second object of the present invention is to provide a reservoir protection drilling fluid to solve the problem in the prior art that plugging and reservoir protection are difficult to reconcile during the oil and gas reservoir drilling process.

[0008] In order to achieve the above objectives, the technical solution of the preparation method of the self-degradable plugging agent of the present invention is:

[0009] A method for preparing a self-degradable plugging agent comprises the following steps: reacting 16.5 to 31.3 parts of polyethylene glycol, 50 to 68 parts of lactide monomers and 7.5 to 20.7 parts of carbonate under the condition of 0.056 to 0.084 parts of catalyst; the reaction temperature is 135 to 150° C. The parts refer to the parts by weight.

[0010] The invention introduces ethylene glycol functional groups into the molecular structure of polylactide to improve the hydrophilicity and dispersibility of the polylactide material. The prepared self-degradable plugging agent is easily dispersed in the drilling fluid and has good compatibility with the drilling fluid, thereby achieving tight plugging during the drilling process, reducing the risk of intrusion of liquid and solid phase particles of the drilling fluid, and protecting the reservoir.

[0011] The present invention introduces ethylene glycol functional groups, carbonates and molecular structure optimization, and the obtained self-degradable plugging agent is a particle in which a polymer wraps carbonates, which improves the degradation efficiency of the self-degradable plugging agent. The self-degradable plugging agent can automatically undergo physical and chemical degradation under the conditions of formation temperature and pressure within a certain time range to generate small molecular organic acids, inorganic salts, etc. It is an intelligent material that can be fully automatically degraded, ensuring that the filter cake is automatically removed efficiently, achieving extremely low damage to the oil and gas reservoir, maximizing the restoration of the permeability of the oil and gas reservoir, and significantly improving the reservoir protection effect. The self-degradable plugging agent of the present invention can be applied to self-degradation at 50-120°C, which improves the applicable temperature range of the self-degradable plugging agent, so that the self-degradable plugging agent can be applied to more oil and gas reservoir conditions. The self-degradable plugging agent of the present invention has good compressive resistance, high strength, low cost, and is easy to mechanically crush, wherein carbonate can react with the organic acid produced by the decomposition of the polymer to degrade.

[0012] More preferably, the composition comprises 20 to 30 parts of polyethylene glycol, 60 to 65 parts of lactide monomer, 8 to 15 parts of carbonate and 0.075 to 0.08 parts of catalyst.

[0013] Preferably, the molecular weight of the polyethylene glycol is 2000-3000 g / mol.

[0014] For better blocking, preferably, the reaction time is 5 to 6 hours.

[0015] In order to make the polyethylene glycol, lactide monomer and carbonate react better, preferably, the catalyst is a combined catalyst system of compound A and compound B; wherein compound A is selected from one of stannous octoate and stannous isooctanoate, and compound B is selected from one of stannous dichloride and boron trichloride.

[0016] In order to improve the reaction efficiency, preferably, the mass ratio of compound A to compound B is (70-75): (25-30).

[0017] Preferably, the carbonate is a structural unit containing formula (I), wherein the R1 group is one of H atoms, sodium, potassium, calcium, lithium, and magnesium atoms, and the R2 group is one of sodium, potassium, calcium, lithium, magnesium, zinc, and iron atoms;

[0018]

[0019] Further preferably, the carbonate is one or more of potassium carbonate, lithium carbonate, zinc carbonate, sodium bicarbonate, calcium bicarbonate, magnesium carbonate, and ferrous carbonate.

[0020] Preferably, the preparation method further comprises drying and crushing after the reaction. The drying temperature is 70 to 80° C. and the drying time is 12 to 24 hours.

[0021] The technical solution of the reservoir protection drilling fluid of the present invention is:

[0022] A reservoir protection drilling fluid contains a plugging agent prepared by the preparation method of the self-degradable plugging agent.

[0023] The self-degradable plugging agent contained in the reservoir protection drilling fluid of the present invention can be processed and crushed into particles of different particle sizes, and the particle size distribution is relatively wide, which is conducive to reasonably optimizing the particle size of the self-degradable plugging agent according to the pore structure and size characteristics of the oil and gas reservoir, adding it to the drilling fluid to form a dense filter cake, achieving a strong plugging effect, and can play a good anti-permeation, anti-intrusion, anti-leakage and plugging effect, reduce and prevent the leakage of drilling fluid in the oil and gas reservoir, etc., thereby ensuring normal drilling and reducing damage to the oil and gas reservoir.

[0024] The self-degradable plugging agent contained in the reservoir protection drilling fluid of the present invention has a relatively good plugging effect and a completely automatic degradation characteristic, can achieve a dense plugging effect during the drilling process, and after the drilling is completed, the dense filter cake can be automatically and efficiently degraded and unblocked, and no other treatment measures such as acidification, biological enzymes, and gel-breaking liquids are required subsequently, which saves operating procedures and costs, fundamentally improves traditional oil and gas reservoir protection concepts and methods, achieves the goal of extremely low pollution and damage to oil and gas reservoirs, and achieves the goals of safe drilling of oil and gas reservoirs, maximum protection of oil and gas reservoirs, and simplification of construction operations, and has great application prospects.

[0025] In order to achieve a better plugging effect, preferably, the mass percentage of the plugging agent in the reservoir protection drilling fluid is 0.85-6.2%.

[0026] Preferably, the reservoir protection drilling fluid is mainly composed of water and the following components in percentage by mass: 0.2-0.5% viscosity enhancer, 1.0-2.0% fluid loss reducer, 2.6-3.3% KCl and 0.85-6.2% plugging agent. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a self-degradation performance diagram of the self-degradable plugging agent of Example 1 of the present invention;

[0028] Figure 2 This is a self-degradation performance diagram of the self-degradable plugging agent of Example 2 of the present invention;

[0029] Figure 3 This is a self-degradation performance diagram of the self-degradable plugging agent of Example 3 of the present invention;

[0030] Figure 4 This is a graph showing the self-degradation performance of the reservoir protection drilling fluid of Example 4 of the present invention;

[0031] Figure 5It is a diagram of the reservoir protection performance of Examples 4-6 of the present invention and the comparative example drilling fluid. DETAILED DESCRIPTION

[0032] The technical solution of the present invention is further described below in conjunction with specific implementation methods.

[0033] 1. The specific embodiment of the preparation method of the self-degradable plugging agent of the present invention is as follows:

[0034] Example 1

[0035] The self-degradable plugging agent of this embodiment is composed of the following components in parts by mass: 31.3 parts of polyethylene glycol with a molecular weight of 2000 g / mol, 0.0836 parts of stannous isooctanoate and boron trichloride catalyst in a mass ratio of 72:28, 50 parts of lactide monomer, 2.5 parts of potassium carbonate, 2.5 parts of sodium bicarbonate and 2.5 parts of calcium bicarbonate.

[0036] The preparation method of the self-degradable plugging agent in this embodiment is as follows: 31.3 parts by mass of polyethylene glycol with a molecular weight of 2000 g / mol is weighed and placed in a reactor, heated to 128° C., and vacuumed for 15 minutes. The reaction solution is naturally cooled, and after the temperature of the reaction solution drops to 25° C., 0.0836 parts by mass of a catalyst prepared by mixing stannous isooctanoate and boron trichloride in a mass ratio of 72:28 is added, and then 50 parts by mass of lactide monomer and 7.5 parts by mass of carbonate are added, and vacuumed for 25 minutes. The vacuum degree of the reaction solution is maintained at 70 Pa, and the temperature is raised to 135° C. and stirred for reaction for 5 hours. The reaction product is dried in a drying oven at 70° C. for 24 hours and then mechanically crushed to prepare a self-degradable plugging agent with a particle size of 100 mesh.

[0037] Example 2

[0038] The self-degradable plugging agent of this embodiment is composed of the following components in parts by mass: 26 parts of polyethylene glycol with a molecular weight of 2000 g / mol, 0.075 parts of stannous octoate and boron trichloride catalyst with a mass ratio of 75:25, 63 parts of lactide monomer, 11 parts of lithium carbonate and 6.5 parts of magnesium carbonate.

[0039] The preparation method of the self-degradable plugging agent in this embodiment is as follows: 26 parts by mass of polyethylene glycol with a molecular weight of 2000 g / mol are weighed and placed in a reactor, heated to 128° C., and vacuumed for 15 minutes. The reaction solution is naturally cooled, and after the temperature of the reaction solution drops to 25° C., 0.075 parts by mass of a catalyst prepared by mixing stannous octoate and boron trichloride in a mass ratio of 75:25 is added, and then 63 parts by mass of lactide monomer and 17.5 parts by mass of carbonate are added, and vacuumed for 25 minutes. The vacuum degree of the reaction solution is maintained at 75 Pa, and the temperature is raised to 140° C. and stirred for reaction for 5 hours. The reaction product is dried in a drying oven at 70° C. for 24 hours and then mechanically crushed to prepare a self-degradable plugging agent with a particle size of 100 mesh.

[0040] Example 3

[0041] The self-degradable plugging agent of this embodiment is composed of the following components in parts by mass: 16.5 parts of polyethylene glycol with a molecular weight of 3000 g / mol, 0.056 parts of stannous isooctanoate and stannous dichloride catalyst in a mass ratio of 75:30, 68 parts of lactide monomer, 15 parts of zinc carbonate and 5.7 parts of ferrous carbonate.

[0042] The preparation method of the self-degradable plugging agent in this embodiment is as follows: 16.5 parts by mass of polyethylene glycol with a molecular weight of 3000 g / mol is weighed and placed in a reactor, heated to 130° C., and vacuumed for 15 minutes. The reaction solution is naturally cooled, and after the temperature of the reaction solution drops to 25° C., 0.056 parts by mass of a catalyst prepared by mixing stannous isooctanoate and stannous dichloride in a mass ratio of 70:30 is added, and then 68 parts by mass of lactide monomer and 20.7 parts by mass of carbonate are added, and vacuumed for 25 minutes. The vacuum degree of the reaction solution is maintained at 75 Pa, and the temperature is raised to 150° C. and stirred for reaction for 6 hours. The reaction product is dried in a drying oven at 75° C. for 24 hours and then mechanically crushed to prepare a self-degradable plugging agent with a particle size of 100 mesh.

[0043] 2. Embodiments of the reservoir protection drilling fluid of the present invention

[0044] In the following examples, the viscosity enhancer MC VIS and the fluid loss reducer Dual Flo were provided by Sinopec North China Petroleum Engineering Co., Ltd.

[0045] Example 4

[0046] The reservoir protection drilling fluid of this embodiment is composed of the following components in percentage by mass: 0.3% of viscosity enhancer MCVIS, 1.5% of fluid loss reducer Dual Flo, 2.6% of KCl and 0.85% of the self-degradable plugging agent of Embodiment 1, and the balance is water.

[0047] The self-blocking reservoir protection drilling fluid is prepared according to the standard GB / T 16783.1-2014 "Drilling fluid field test part 1 water-based drilling fluid" and is marked as YS1.

[0048] Example 5

[0049] The reservoir protection drilling fluid of this embodiment is composed of the following components in percentage by mass: 0.3% of viscosity enhancer MCVIS, 1.5% of fluid loss reducer Dual Flo, 3% of KCl and 3.1% of the self-degradable plugging agent of Embodiment 2, and the balance is water.

[0050] The preparation method of the reservoir protection drilling fluid in this embodiment is the same as that in Example 4 and is marked as YS2.

[0051] Example 6

[0052] The reservoir protection drilling fluid of this embodiment is composed of the following components in percentage by mass: 0.3% of viscosity enhancer MCVIS, 1.5% of fluid loss reducer Dual Flo, 3.3% of KCl and 6.2% of the self-degradable plugging agent of Embodiment 3, and the balance is water.

[0053] The preparation method of the reservoir protection drilling fluid in this embodiment is the same as that in Example 4 and is marked as YS3.

[0054] 3. Comparative Proportion

[0055] Comparative Example 1

[0056] The drilling fluid of this comparative example is basically the same as that of Example 4, except that calcium carbonate plugging material is used instead of the self-degradable plugging agent of Example 1. The particle size of the acid-soluble calcium carbonate material is in the range of 2.0 to 200 μm.

[0057] The preparation method of the drilling fluid in this comparative example is the same as that in Example 4, and is marked as DS1.

[0058] Comparative Example 2

[0059] The plugging agent of this comparative example does not contain carbonate, and is composed of the following components in percentage by mass: 16.5 parts of polyethylene glycol with a molecular weight of 3000 g / mol, 0.056 parts of stannous isooctanoate and stannous dichloride catalyst with a mass ratio of 75:30, and 68 parts of lactide monomer.

[0060] The preparation method of the self-degradable plugging agent in this embodiment is as follows: 16.5 parts by mass of polyethylene glycol with a molecular weight of 3000 g / mol is weighed and placed in a reactor, heated to 130° C., and vacuumed for 15 minutes. The reaction solution is naturally cooled, and after the temperature of the reaction solution drops to 25° C., 0.056 parts by mass of a catalyst prepared by mixing stannous isooctanoate and stannous dichloride in a mass ratio of 70:30 is added, and then 68 parts by mass of lactide monomer are added, and vacuumed for 25 minutes. The vacuum degree of the reaction solution is maintained at 75 Pa, and the temperature is raised to 150° C. and stirred for reaction for 6 hours. The reaction product is dried in a drying oven at 75° C. for 24 hours and then mechanically crushed to prepare a self-degradable plugging agent with a particle size of 100 mesh.

[0061] The drilling fluid of this comparative example comprises 0.3% of tackifier MC VIS, 1.5% of fluid loss reducer Dual Flo, 2.6% of KCl and 0.85% of the plugging agent of this comparative example, and the balance is water.

[0062] The self-plugging reservoir protection drilling fluid is prepared according to the standard GB / T 16783.1-2014 "Drilling fluid field test part 1 water-based drilling fluid" and is marked as DS2.

[0063] Comparative Example 3

[0064] The plugging agent of this comparative example is composed of the following components in percentage by mass: 16.5 parts of polyethylene glycol with a molecular weight of 3000 g / mol, 0.056 parts of stannous isooctanoate catalyst, 68 parts of lactide monomer, 15 parts of zinc carbonate and 5.7 parts of ferrous carbonate.

[0065] The preparation method of the self-degradable plugging agent in this embodiment is as follows: 16.5 parts by mass of polyethylene glycol with a molecular weight of 3000 g / mol is weighed and placed in a reactor, heated to 130°C, and vacuumed for 15 minutes. The reaction solution is naturally cooled, and after the temperature of the reaction solution drops to 25°C, 0.056 parts of stannous isooctanoate catalyst is added, and then 68 parts by mass of lactide monomer and 20.7 parts by mass of carbonate are added, and vacuumed for 25 minutes. The vacuum degree of the reaction solution is maintained at 75Pa, and the temperature is raised to 150°C and stirred for reaction for 6 hours. The reaction product is dried in a drying oven at 75°C for 24 hours and then mechanically crushed to prepare a self-degradable plugging agent with a particle size of 100 mesh.

[0066] The drilling fluid of this comparative example comprises 0.3% of tackifier MC VIS, 1.5% of fluid loss reducer Dual Flo, 2.6% of KCl and 0.85% of the plugging agent of this comparative example, and the balance is water.

[0067] According to the standard GB / T 16783.1-2014 "Drilling fluid field test part 1 water-based drilling fluid", the self-plugging reservoir protection drilling fluid is prepared and marked as DS3.

[0068] IV. Experimental Examples

[0069] Experimental Example 1 Self-degradation performance of plugging agent in Example 1

[0070] In this experimental example, 6% of the self-degradable plugging agent of Example 1 was added to 100 parts of clean water, and the particle size was 100 meshes. The degradation performance test was carried out at 60°C and 0.1MPa. The results are as follows: Figure 1 shown.

[0071] according to Figure 1 The results show that the self-degradable plugging agent of Example 1 can be effectively self-degraded at 60°C. The degradation rate of the self-degradable plugging agent is very slow in the first 2 days, with a degradation rate of only 16.8%. During this period, the reservoir pores can be plugged, thereby protecting the reservoir. As time goes by, since the degradation reaction gradually penetrates from the surface to the inside, the contact area becomes larger, and more substances participate in the degradation reaction, which synergistically accelerates the degradation rate of the self-degradable plugging agent of Example 1. At 60°C, it is basically completely degraded in 20 days, and the self-degradation rate is close to 100%.

[0072] Therefore, from the perspective of degradation effect, the self-degradable plugging agent of Example 1 of the present invention has a strong degradation ability at 60° C. and can achieve complete self-degradation within a certain time range.

[0073] Experimental Example 2 Self-degradation performance of plugging agent in Example 2

[0074] In this experimental example, 6% of the self-degradable plugging agent of Example 2 was added to 100 parts of clean water, and the particle size was 100 meshes. The degradation performance test was carried out at 90°C and 0.1MPa. The results are as follows: Figure 2 shown.

[0075] according to Figure 2 The results show that the self-degradable plugging agent of Example 2 can be effectively self-degraded at 90°C. The degradation rate of the self-degradable plugging agent is relatively slow in the first two days, and its degradation rate is below 24.6%. During this period, it plays a good plugging role on the reservoir pores. As time goes by, its degradation law is basically the same as that of the self-degradable plugging agent of Example 1, but the degradation rate of the self-degradable plugging agent of Example 2 is accelerated due to the increase in temperature. After 17 days, the degradation rate of the material reaches 100%.

[0076] Therefore, from the perspective of degradation effect, the self-degradable plugging agent of Example 2 of the present invention has a strong self-degradation ability at 90° C. and can be basically completely self-degraded within a certain time range.

[0077] Experimental Example 3 Self-degradation performance of plugging agent in Example 3

[0078] In this experimental example, 6% of the self-degradable plugging agent of Example 3 was added to 100 parts of clean water, and the particle size was 100 meshes. The degradation performance test was carried out at 120°C and 2.0 MPa. The results are as follows: Figure 3 shown.

[0079] according to Figure 3 The results show that the self-degradable plugging agent of Example 3 can be effectively self-degraded at 120°C. The degradation rate of the self-degradable plugging agent is relatively slow within the first 24 hours, and its degradation rate is below 29.2%. During this period, it plays a good plugging role on the reservoir pores. As time goes by, its degradation rate accelerates. After 4 days, the degradation rate of the material reaches more than 90%, and 100% degradation within 7 days. It can be seen that the increase in temperature will greatly accelerate the degradation rate of the self-degradable plugging agent.

[0080] Therefore, from the perspective of degradation effect, the self-degradable plugging agent of Example 3 of the present invention has extremely strong self-degradation ability at 120° C. and can be completely self-degraded in a relatively short time.

[0081] Experimental Example 4 Self-degradation performance of reservoir protection drilling fluid

[0082] This experimental example measures the self-degradation performance of the filter cake formed by the reservoir protection drilling fluid of Example 4.

[0083] This experimental example adopts the standard GB / T 29170-2012 "Laboratory Test of Drilling Fluids in Petroleum and Natural Gas Industry" to test the filter cake formed by the reservoir protection drilling fluid of Example 4 in clean water at 90°C and 0.1MP to observe the self-degradation of the filter cake. The results are as follows: Figure 4 shown.

[0084] according to Figure 4 The results show that the filter cake formed by the reservoir protection drilling fluid of Example 4 can undergo significant self-degradation behavior at 90°C, and the filter cake degradation rate can reach more than 99% after 5 days. This shows that the filter cake formed by the reservoir protection drilling fluid of the present invention still has excellent self-degradation ability, can significantly reduce the damage to the oil and gas reservoir, and achieve the protection effect on the oil and gas reservoir.

[0085] Experimental Example 5 Reservoir Protection Performance

[0086] In this experimental example, the reservoir protection performance of the reservoir protection drilling fluids of Examples 4-6 and the drilling fluids of Comparative Examples 1-3 was tested.

[0087] The core of this experimental example is a natural sandstone reservoir core from the field oil field of Sinopec North China Petroleum Engineering Co., Ltd. According to the Petroleum and Natural Gas Industry Standard SY / T 6540-2021 of the People's Republic of China "Indoor Evaluation Method for Damage to Reservoirs by Drilling Fluids and Completion Fluids", the core was tested for the degree of damage to the formation by the reservoir protection drilling fluids YS1-YS3 of Examples 4-6 through dynamic pollution experiments, and compared with the drilling fluid DS1-3 system prepared in Comparative Examples 1-3. The results are shown in Tables 1 and Figure 5 shown.

[0088] Table 1 Comparison of reservoir protection performance of drilling fluids

[0089]

[0090] From the data in Table 1 and Figure 5 It can be seen that: for the on-site sandstone reservoir, the reservoir protection drilling fluid formed by the self-degradable plugging agent of the present invention can effectively plug the core, with a plugging rate of up to 98.17%, and the plugging permeability recovery value after 10 days of backwash displacement reaches 98.7% at 10 days. The results show that the calcium carbonate plugging in the comparative example is difficult to unblock, the self-unblocking plugging agent in the comparative example 2 does not add carbonate to reduce the blocking rate, and the self-unblocking plugging agent in the comparative example 3 uses only one catalyst for insufficient reaction, and the single catalyst does not have a catalytic effect on all chemical reactions in the preparation of the self-unblocking plugging agent, resulting in a decrease in the blocking performance of the plugging agent and a too fast degradation rate, so the catalyst in the present invention should be a combined catalyst system; the comparison shows that the self-degradable plugging agent of the present invention and the reservoir protection drilling fluid formed by the self-degradable plugging agent can automatically realize backflow unblocking through automatic degradation reaction after drilling is completed, and release the oil and gas production capacity to the maximum extent, without the need for additional acid washing to remove the filter cake and other operations, thus avoiding secondary damage to the oil and gas reservoir, saving operation procedures and time, and effectively shortening the completion cycle, which is a simple, efficient, economical, and new reservoir protection technology without side effects, and the organic acid formed by degradation helps to remove the blockage of some particles in the formation, which can effectively improve the connectivity between reservoirs, increase the permeability of the oil and gas reservoir, and improve the fluidity of the oil and gas in the reservoir, which is a simple, efficient, economical, and new reservoir protection technology without side effects.

Claims

1. A method for preparing a self-degradable plugging agent, characterized in that: The following steps are involved: 16.5-31.3 parts of polyethylene glycol, 50-68 parts of lactide monomer and 7.5-20.7 parts of carbonate are reacted under the condition of 0.056-0.084 parts of catalyst; the reaction temperature is 135-150°C.

2. The method for preparing the self-degradable plugging agent according to claim 1, characterized in that: The reaction time is 5 to 6 hours.

3. The method for preparing the self-degradable plugging agent according to claim 1, characterized in that: The catalyst is a combined catalyst system of compound A and compound B; wherein compound A is selected from one of stannous octoate and stannous isooctanoate, and compound B is selected from one of stannous dichloride and boron trichloride.

4. The method for preparing the self-degradable plugging agent according to claim 3, characterized in that: The mass ratio of compound A to compound B is (70-75):(25-30).

5. The method for preparing the self-degradable plugging agent according to claim 1, characterized in that: The carbonate is a structural unit having formula (I), wherein the R1 group is one of H atoms, sodium, potassium, calcium, lithium, and magnesium atoms, and the R2 group is one of sodium, potassium, calcium, lithium, magnesium, zinc, and iron atoms; 6. The method for preparing the self-degradable plugging agent according to claim 5, characterized in that: The carbonate is one or more of potassium carbonate, sodium bicarbonate, calcium bicarbonate, lithium carbonate, magnesium carbonate, zinc carbonate, and ferrous carbonate.

7. A reservoir protection drilling fluid, characterized in that: The reservoir protection drilling fluid contains a plugging agent prepared by the method for preparing a self-degradable plugging agent as described in any one of claims 1 to 6.

8. The reservoir protection drilling fluid according to claim 7, characterized in that: The mass percentage of the plugging agent in the reservoir protection drilling fluid is 0.85-6.2%.

9. The reservoir protection drilling fluid according to claim 8, characterized in that: The reservoir protection drilling fluid is mainly composed of water and the following components in percentage by mass: 0.2-0.5% of viscosity enhancer, 1.0-2.0% of fluid loss reducer, 2.6-3.3% of KCl and 0.85-6.2% of plugging agent.

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