An enzyme-responsive core-shell structured composite temporary plugging material, its preparation method and application

Abstract

The present invention provides an enzyme-responsive core-shell structured composite temporary plugging material, a preparation method thereof and an application thereof. The enzyme-responsive core-shell structured composite temporary plugging material comprises a modified ceramsite core and a chitosan and sodium alginate outer shell wrapped around the core. The preparation method comprises the steps of: mixing ceramsite and an aqueous solution of 3-aminopropyltriethoxysilane, carrying out a stirring reaction and drying to obtain modified ceramsite; sequentially adding chitosan, modified ceramsite and sodium alginate into a mixed solvent of dichloromethane and water, carrying out a reaction, and then filtering, washing and drying to obtain. The composite temporary plugging material of the present invention plugs the wellbore during drilling, prevents the drilling fluid from entering the formation, protects the reservoir, and uses biological enzymes to remove the plug after drilling, restores the formation permeability, reduces the damage of the reservoir, and has the advantages of high strength during drilling, good plugging property, easy plug removal and high permeability after the action of biological enzymes.

Description

Technical Field

[0001] The present invention belongs to the technical field of gas hydrate drilling and production, and particularly relates to an enzyme-responsive core-shell structure composite temporary plugging material, a preparation method thereof and an application thereof. Background Art

[0002] Oil and gas resources are one of the most important energy sources in the world, and are of great significance to the economy, energy security and national security. Oil and gas resources are an important part of national energy security, which can provide an independent energy supply and reduce the risk of relying on imported energy. The foreign dependence on oil and gas in China remains high, and it is an inevitable choice to vigorously enhance domestic oil and gas exploration and development efforts to ensure national energy security. There are approximately 80 billion tons of oil equivalent in our sea areas, and its reasonable development and utilization is of great strategic significance for solving China's energy shortage, addressing climate change, maintaining national energy security and social sustainable development. As a new type of gas energy, natural gas hydrate has obvious advantages in terms of gas content, reserves per unit area, etc., and can provide strong support for China's energy security. In addition, since the greenhouse gas emissions generated after its combustion are much lower than those of traditional fossil fuels, the development and utilization of natural gas hydrate can also play an important role in alleviating climate change problems.

[0003] During the drilling process of natural gas hydrates, since the drilling process is non-adiabatic and the formations where hydrates are located are mostly porous media, heat and mass transfer occur between the drilling fluid and the hydrate formation, that is, the drilling fluid penetrates into the formation around the wellbore and the natural gas hydrate decomposes, resulting in mechanical instability of the wellbore and seriously affecting the normal drilling operation. Therefore, during the drilling process, the wellbore and the formation around the wellbore are often blocked to minimize the interference of the drilling fluid on the reservoir. However, traditional conventional plugging agents are difficult to remove after plugging the formation, which affects subsequent construction operations. At present, many different temporary plugging materials and plug removal technologies have also been developed. Mainly through the scientific and reasonable selection of pre-screening temporary plugging and post-acid plug removal, bio-enzyme plug removal technologies, etc. for different plugging and reservoir conditions, to reduce the damage degree of oil and gas reservoirs, thereby increasing the production of oil and gas wells and improving economic benefits. However, the current plugging technology has low plugging strength during plugging, and at the same time, the plug removal process is complex. If the plug removal measures are not properly handled, the permeability of the formation will be reduced. Chinese patent document CN114250067A discloses a temporary plugging agent for natural gas hydrate drilling fluid and its preparation method. The temporary plugging agent has a core-shell structure, including an inner core ceramic particle with porous channels and a polymer outer shell grafted to the outer core through a silane coupling agent; the porous channels include bio-enzyme polymer micelles; the composition of the polymer outer shell includes cellulose derivatives. However, the preparation process of the temporary plugging agent prepared by this invention is relatively complex, with high cost and great difficulty; the prepared temporary plugging agent is to introduce bio-enzyme polymer micelles into the porous channels of the ceramic particles, and the degradation time is controlled by controlling the amount of grafted polymer in the outer shell. However, in actual construction, it is impossible to cope with early or delayed degradation, which is likely to affect the normal drilling process.

[0004] Therefore, there is an urgent need to develop a temporary plugging material for natural gas hydrate drilling fluid with high strength during drilling time, good plugging performance, easy plug removal after drilling, and high permeability. Summary of the Invention

[0005] Aiming at the deficiencies of the prior art, especially for the problem of wellbore instability faced by complex structure wells such as horizontal wells in current hydrate exploitation, the present invention provides an enzyme-responsive core-shell structure composite temporary plugging material, its preparation method and application. The composite temporary plugging material of the present invention plugs the wellbore during drilling to prevent the drilling fluid from entering the formation, protects the reservoir, and uses bio-enzyme to remove the plug after drilling to restore the formation permeability and reduce the damage of the reservoir. It has the advantages of high strength during drilling time, good plugging performance, easy plug removal after the action of bio-enzyme, and high permeability.

[0006] The technical solution of the present invention is as follows:

[0007] An enzyme-responsive core-shell structure composite temporary plugging material, the enzyme-responsive core-shell structure composite temporary plugging material includes a modified ceramic particle inner core and a chitosan and sodium alginate outer shell wrapped around the inner core.

[0008] Preferably according to the present invention, the modified ceramsite is a ceramsite surface-modified with 3-aminopropyltriethoxysilane.

[0009] According to the present invention, the preparation method of the above enzyme-responsive core-shell structure composite temporary plugging material comprises the following steps:

[0010] (1) Preparation of modified ceramsite

[0011] Mix ceramsite and an aqueous solution of 3-aminopropyltriethoxysilane, and obtain modified ceramsite through stirring reaction and drying;

[0012] (2) Preparation of composite temporary plugging material

[0013] Add chitosan, modified ceramsite and sodium alginate into a mixed solvent of dichloromethane and water in sequence, carry out reaction, evaporate the organic solvent, and then obtain the enzyme-responsive core-shell structure composite temporary plugging material through filtration, washing and drying.

[0014] Preferably according to the present invention, in step (1), the particle size of the ceramsite is 10-100 μm; the ceramsite is spherical; there is no specific limitation on the type of ceramsite, and preferably it is petroleum proppant ceramsite sand.

[0015] Preferably according to the present invention, in step (1), the mass fraction of the aqueous solution of 3-aminopropyltriethoxysilane is 0.5-15%, and more preferably 0.8-3%.

[0016] Preferably according to the present invention, in step (1), the mass ratio of the ceramsite to 3-aminopropyltriethoxysilane is 10-25:1.

[0017] Preferably according to the present invention, in step (1), the temperature of the stirring reaction is room temperature, the time of the stirring reaction is 30-60 min, the stirring rate is 150-300 r / min, and the room temperature has the meaning well known in the art, referring to 25±5°C; the drying is carried out at 60-70°C for 10-15 h.

[0018] Preferably according to the present invention, in step (2), the weight-average molecular weight of the chitosan is 100,000-200,000, and the degree of deacetylation is ≥99.5%.

[0019] Preferably according to the present invention, in step (2), the weight-average molecular weight of the sodium alginate is 300,000-400,000.

[0020] Preferably according to the present invention, in step (2), the mass ratio of the modified ceramsite, chitosan and sodium alginate is 2-8:2:1.

[0021] Preferably according to the present invention, in step (2), the mass ratio of the modified ceramsite to the mixed solvent is 1:3 to 5; the volume ratio of dichloromethane to water in the mixed solvent is 1:1.

[0022] Preferably according to the present invention, in step (2), after adding each component to the mixed solvent, stirring treatment is required, the stirring time is 20 to 30 min, and the stirring rate is 100 to 300 r / min.

[0023] Preferably according to the present invention, in step (2), the temperature of the reaction is 40 to 50 °C, the time of the reaction is 1 to 3 h, and the stirring rate during the reaction process is 100 to 300 r / min.

[0024] Preferably according to the present invention, in step (2), the washing is carried out 3 to 5 times with deionized water; the drying is carried out at 80 to 90 °C for 10 to 20 h.

[0025] According to the present invention, the above-mentioned enzyme-responsive core-shell structure composite temporary plugging material is applied in the process of drilling for offshore natural gas hydrate exploitation.

[0026] Preferably according to the application of the present invention, during drilling, the enzyme-responsive core-shell structure composite temporary plugging material is used to plug the wellbore to prevent the drilling fluid from entering the formation and protect the reservoir; after drilling, a bio-enzyme is used to degrade the composite temporary plugging material to achieve plug removal.

[0027] Preferably, the bio-enzyme is a low-temperature composite enzyme, cellulase or amylase; more preferably, it is a low-temperature composite enzyme.

[0028] Preferably, after drilling, an aqueous solution of a bio-enzyme with a mass fraction of 2 to 10% is injected into the formation to degrade the composite temporary plugging material.

[0029] According to the present invention, during drilling, the dosage and injection method of the enzyme-responsive core-shell structure composite temporary plugging material can be according to the existing method.

[0030] The technical features and beneficial effects of the present invention are as follows:

[0031] 1. Aiming at the problems of wellbore instability and reservoir damage faced by complex structure wells such as horizontal wells in current hydrate exploitation, the present invention develops an enzyme-responsive core-shell structure composite temporary plugging material, which has the characteristics of high strength during drilling, good plugging performance, easy plug removal after the action of bio-enzyme, and high permeability. The composite temporary plugging material of the present invention plugs the wellbore during drilling, blocks the channel between the wellbore and the hydrate, prevents the drilling fluid from entering the formation, enhances the wellbore stability during the drilling process, protects the reservoir, and after drilling, uses bio-enzyme to remove the plug, opens the formation channel, restores the formation permeability, and reduces the damage to the reservoir.

[0032] 2. The enzyme-responsive core-shell structured composite temporary plugging material of the present invention uses modified ceramsite as the core. The alkoxy groups in the silane coupling agent hydrolyze to form Si-OH, which reacts with the -OH on the surface of the ceramsite, thereby connecting the silane coupling agent to the surface of the ceramsite to modify the ceramsite. The modification of the silane coupling agent in the present invention can effectively reduce the entry of water into the pores of the ceramsite and at the same time serve as a bridge connecting the ceramsite and the wall material. The modification of the silane coupling agent in the present invention provides amino groups for the ceramsite. The hydroxyl groups in chitosan form hydrogen bonds with the amino groups on the surface of the modified ceramsite, enabling chitosan to wrap the ceramsite. If the ceramsite is not modified, the ceramsite and chitosan cannot be stably connected, and the wrapping effect of chitosan is not ideal, affecting the plugging performance of the composite material. The specific silane coupling agent and polysaccharide compound in the present invention ensure that each part of the composite temporary plugging material can be effectively connected. Through a large number of experiments, the present invention has selected specific solvents, dichloromethane and water. The weakly acidic environment of dichloromethane makes the amino groups of chitosan positively charged, forming capsules with a positively charged surface. Sodium alginate forms a polyelectrolyte complex through ionic complexation to form a multi-layer microcapsule wall. However, if the types of solvents are changed (such as ethanol and / or water), the performance of the obtained temporary plugging material is poor. The wrapping of the wall material on the modified ceramsite can plug the pores of the ceramsite, prevent the fluid from passing through, and make the drilling fluid unable to enter the formation, protecting the oil and gas reservoir. Using chitosan alone as the wall material can improve the plugging rate to a certain extent. The co-wrapping of chitosan and sodium alginate can enhance the plugging performance of the composite material. The present invention is based on the layer-by-layer self-assembly method and the coupling agent modification method. The surface of the ceramsite is modified by the coupling agent to connect the ceramsite and chitosan, and chitosan and sodium alginate are self-assembled through electrostatic interaction, thereby forming an enzyme-responsive core-shell structured composite material. The types of wall materials, the feeding sequence, and the solvents in the present invention are specific. For example, replacing the types of wall materials will reduce the plugging and degradable properties of the temporary plugging material of the present invention. At the same time, the ratios of chitosan, sodium alginate, and modified ceramsite need to be controlled within the scope of the present invention, otherwise the plugging performance of the composite material will be reduced. If sodium alginate is added first, the wrapping degree of sodium alginate on the modified ceramsite is poor, resulting in a poor plugging effect of the finally formed composite temporary plugging material. As a whole, the preparation method of the present invention can achieve the excellent effects of the present invention only when each step and each condition act together.

[0033] 3. The temporary plugging material prepared by the present invention uses polysaccharide materials to coat the surface of the ceramsite and plug the pores of the porous ceramsite. Its method can effectively plug the ceramsite and endow it with good temporary plugging performance. When in use, it is first plugged with the temporary plugging material and then degraded with biological enzymes, which can control the plugging and unplugging effects of the temporary plugging material. The present invention uses biological enzymes to degrade the wall material of the composite material. The enzyme solution penetrates through the polyelectrolyte layer on the surface of the composite material, thereby degrading chitosan and sodium alginate. The outer layer of the composite material is damaged, and the pores of the ceramsite are opened, enabling the reservoir permeability to be restored. The ceramsite serves as a proppant to stabilize the wellbore. The specific types of biological enzymes in the present invention are conducive to the degradation of the composite material of the present invention to effectively achieve unplugging.

[0034] 4. The raw materials used in the preparation of the bio-enzyme responsive composite temporary plugging material of the present invention are non-toxic and harmless, have a wide range of sources, the preparation method is simple, the reaction conditions are mild, and it has the prospect of large-scale industrial production, which is of great significance for the exploitation of natural gas hydrates. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 SEM images of the enzyme-responsive core-shell structured composite temporary plugging material prepared in Example 1 at different magnifications.

[0036] Figure 2 Permeability test graphs (a) and plugging rate graphs (b) of the composite temporary plugging materials obtained in Example 1, Comparative Example 1 and Comparative Example 2.

[0037] Figure 3 Permeability test graphs (a) and permeability recovery graphs (b) of the enzyme-responsive core-shell structured composite temporary plugging material prepared in Example 1.

[0038] Figure 4 Filter loss performance test graphs of the enzyme-responsive core-shell structured composite temporary plugging materials prepared in Examples 1-3. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0039] The present invention will be further described below in conjunction with specific embodiments, but not limited thereto.

[0040] Meanwhile, the experimental methods described in the following embodiments are all conventional methods unless otherwise specified; the reagents, materials and equipment are all commercially available unless otherwise specified.

[0041] The weight-average molecular weight of the chitosan used in the examples is 150,000, and the degree of deacetylation is ≥99.5%; the weight-average molecular weight of the sodium alginate used is 360,000.

[0042] Example 1

[0043] A preparation method of an enzyme-responsive core-shell structured composite temporary plugging material, comprising the following steps:

[0044] (1) Preparation of modified ceramsite

[0045] 0.95 g of 3-aminopropyltriethoxysilane was mixed with 100 g of deionized water to obtain a silane coupling agent solution. 20 g of ceramsite (ceramsite particle size is 20-40 μm) was fully dispersed in the silane coupling agent solution, stirred and reacted at room temperature for 50 min, the stirring rate was 250 r / min, and then dried at 60 °C for 10 h to obtain modified ceramsite.

[0046] (2) Preparation of composite temporary plugging material

[0047] Dissolve 5 g of chitosan in 100 g of a mixed solvent (the mixed solvent is obtained by mixing dichloromethane and water in a volume ratio of 1:1), and stir at a stirring rate of 250 r / min for 30 min at room temperature; then add 20 g of modified ceramsite, stir at a stirring rate of 200 r / min for 30 min at room temperature, and finally add 2.5 g of sodium alginate. After stirring at a stirring rate of 200 r / min for 30 min at room temperature, heat up to 45 °C and react at 45 °C for 2 h to evaporate the organic solvent. The stirring rate during the reaction is 200 r / min; after the reaction is completed, filter, wash the obtained solid 3 times with deionized water, and then dry at 85 °C for 15 h to obtain an enzyme-responsive shell-core structure composite temporary plugging material.

[0048] Example 2

[0049] A preparation method of an enzyme-responsive shell-core structure composite temporary plugging material includes the following steps:

[0050] (1) Preparation of modified ceramsite

[0051] Mix 0.95 g of 3-aminopropyltriethoxysilane with 100 g of deionized water to obtain a silane coupling agent solution. Disperse 20 g of ceramsite (ceramsite particle size is 20 - 40 μm) thoroughly in the silane coupling agent solution, stir and react at room temperature for 50 min, with a stirring rate of 250 r / min, and then dry at 60 °C for 10 h to obtain modified ceramsite.

[0052] (2) Preparation of composite temporary plugging material

[0053] Dissolve 10 g of chitosan in 100 g of a mixed solvent (the mixed solvent is obtained by mixing dichloromethane and water in a volume ratio of 1:1), and stir at a stirring rate of 250 r / min for 30 min at room temperature; then add 20 g of modified ceramsite, stir at a stirring rate of 200 r / min for 30 min at room temperature, and finally add 5 g of sodium alginate. After stirring at a stirring rate of 200 r / min for 30 min at room temperature, heat up to 45 °C and react at 45 °C for 2 h to evaporate the organic solvent. The stirring rate during the reaction is 200 r / min; after the reaction is completed, filter, wash the obtained solid 3 times with deionized water, and then dry at 85 °C for 15 h to obtain an enzyme-responsive shell-core structure composite temporary plugging material.

[0054] Example 3

[0055] A preparation method of an enzyme-responsive shell-core structure composite temporary plugging material includes the following steps:

[0056] (1) Preparation of modified ceramsite

[0057] Mix 0.95 g of 3-aminopropyltriethoxysilane with 100 g of deionized water to obtain a silane coupling agent solution. Disperse 20 g of ceramsite (ceramsite particle size is 20 - 40 μm) thoroughly in the silane coupling agent solution, stir and react at room temperature for 50 min with a stirring rate of 250 r / min, and then dry at 60 °C for 10 h to obtain modified ceramsite.

[0058] (2) Preparation of composite temporary plugging material

[0059] Dissolve 20 g of chitosan in 100 g of a mixed solvent (the mixed solvent is obtained by mixing dichloromethane and water in a volume ratio of 1:1), stir at a stirring rate of 250 r / min at room temperature for 30 min; then add 20 g of modified ceramsite, stir at a stirring rate of 200 r / min at room temperature for 30 min, and finally add 10 g of sodium alginate, stir at a stirring rate of 200 r / min at room temperature for 30 min and then raise the temperature to 45 °C, react at 45 °C for 2 h to evaporate the organic solvent, and the stirring rate during the reaction is 200 r / min; after the reaction is completed, filter, wash the obtained solid 3 times with deionized water, and then dry at 85 °C for 15 h to obtain an enzyme-responsive core-shell structured composite temporary plugging material.

[0060] Comparative Example 1

[0061] A preparation method of a temporary plugging material includes the following steps:

[0062] Mix 0.95 g of 3-aminopropyltriethoxysilane with 100 g of deionized water to obtain a silane coupling agent solution. Disperse 20 g of ceramsite (ceramsite particle size is 20 - 40 μm) thoroughly in the silane coupling agent solution, stir and react at room temperature for 50 min with a stirring rate of 250 r / min, and then dry at 60 °C for 10 h to obtain modified ceramsite, which is the temporary plugging material.

[0063] Comparative Example 2

[0064] A preparation method of a composite temporary plugging material includes the following steps:

[0065] (1) Preparation of modified ceramsite

[0066] Mix 0.95 g of 3-aminopropyltriethoxysilane with 100 g of deionized water to obtain a silane coupling agent solution. Disperse 20 g of ceramsite (ceramsite particle size is 20 - 40 μm) thoroughly in the silane coupling agent solution, stir and react at room temperature for 50 min with a stirring rate of 250 r / min, and then dry at 60 °C for 10 h to obtain modified ceramsite.

[0067] (2) Preparation of composite temporary plugging material

[0068] Dissolve 5 g of chitosan in 100 g of a mixed solvent (the mixed solvent is obtained by mixing dichloromethane and water in a volume ratio of 1:1), stir at a stirring rate of 250 r / min at room temperature for 30 min, then add 20 g of modified ceramsite, stir at a stirring rate of 200 r / min at room temperature for 30 min, and then heat to 45 °C. React at 45 °C for 2 h to evaporate the organic solvent, and the stirring rate during the reaction is 200 r / min; after the reaction is completed, filter, wash the obtained solid 3 times with deionized water, and then dry at 85 °C for 15 h to obtain the composite temporary plugging material.

[0069] Comparative Example 3

[0070] A preparation method of a composite temporary plugging material, comprising the following steps:

[0071] (1) Preparation of modified ceramsite

[0072] Mix 0.95 g of 3-aminopropyltriethoxysilane with 100 g of deionized water to obtain a silane coupling agent solution. Disperse 20 g of ceramsite (the ceramsite particle size is 20 - 40 μm) fully in the silane coupling agent solution, stir and react at room temperature for 50 min, the stirring rate is 250 r / min, and then dry at 60 °C for 10 h to obtain the modified ceramsite.

[0073] (2) Preparation of composite temporary plugging material

[0074] Dissolve 5 g of sodium alginate in 100 g of a mixed solvent (the mixed solvent is obtained by mixing dichloromethane and water in a volume ratio of 1:1), stir at a stirring rate of 250 r / min at room temperature for 30 min, then add 20 g of modified ceramsite, stir at a stirring rate of 200 r / min at room temperature for 30 min, and then heat to 45 °C. React at 45 °C for 2 h to evaporate the organic solvent, and the stirring rate during the reaction is 200 r / min; after the reaction is completed, filter, wash the obtained solid 3 times with deionized water, and then dry at 85 °C for 15 h to obtain the composite temporary plugging material.

[0075] The composite temporary plugging material prepared in this comparative example has too high hardness and is not suitable for use as a temporary plugging material.

[0076] Test Example 1

[0077] Evaluate the performance of the temporary plugging materials prepared in Example 1 and Comparative Examples 1 - 2 through the following experimental methods:

[0078] 1. Scanning electron microscopy analysis

[0079] Perform scanning electron microscopy testing on the enzyme-responsive core-shell structure composite temporary plugging material prepared in Example 1, and the results are as Figure 1As shown, the composite temporary plugging material is approximately spherical microscopically, with a relatively smooth surface. The wall material wraps the ceramsite to block the pore channels, and the wall materials are all polysaccharides, which can be unblocked by biodegradation with bioenzymes. Figure 1 The scale bar of the left middle figure is 40μm, and the scale bar of the right figure is 50μm.

[0080] 2. Plugging Test

[0081] Test the initial permeability of the core, and test the permeability of the core after plugging with the enzyme-responsive core-shell structured composite temporary plugging material prepared in Example 1, the modified ceramsite in Comparative Example 1, and the composite temporary plugging material in Comparative Example 2. Calculate the plugging rate according to the core plugging rate formula to evaluate the plugging effect of the temporary plugging material.

[0082]

[0083] E is the plugging rate, %; K 0 is the core permeability before plugging, md; K w is the core permeability after plugging, md.

[0084] The permeability of the core is calculated according to the following formula:

[0085]

[0086] In the formula, k is the core permeability, md; Q is the volume of fluid passing through the core sample per unit time, cm 3 / s; μ is the fluid viscosity, mPa·s; L is the core length, m; ΔP is the pressure difference between the front and rear ends of the core, MPa; A is the cross-sectional area of the core, cm 2 ;

[0087] The specific test method is as follows: First, place the washed and dried core (5 cm long and 2.5 cm in diameter) in the core holder, seal and fix it, turn on the vacuum pump, evacuate, then apply a confining pressure of 6 MPa, adjust the flow rate to 1 mL / min, displace water into the core, set the pressure upper limit to 3 MPa, and the back pressure to 2 MPa to test the core permeability before plugging. Then take out the core, plug 5 g of the temporary plugging material at the front end of the core, turn on the vacuum pump again to evacuate, apply a confining pressure of 6 MPa, adjust the water flow rate to 1 mL / min and continue to displace, record the core permeability after plugging, and calculate the plugging rate.

[0088] Such as Figure 2As shown in the figure, when comparing the modified ceramsite (Comparative Example 1) and the composite temporary plugging material wrapped with chitosan (Comparative Example 2), the change range of the permeability before and after plugging the core is relatively large, and the plugging rates are 40.88% and 51.4% respectively. The plugging rate of the composite temporary plugging material prepared only by wrapping with chitosan has a certain increase, but the amplitude is small; while the plugging rate of the composite temporary plugging material wrapped with a mixture of chitosan and sodium alginate in Example 1 is 100%, and the permeability after plugging drops to 0, completely plugging the core. After modification with ceramsite, the composite temporary plugging material with chitosan and sodium alginate as the wall material has good plugging performance. Therefore, the composite material wrapped with a single wall material has a certain plugging ability, while the composite material wrapped with multiple wall materials has a good plugging effect and can be effectively applied to reservoir plugging.

[0089] 3. Permeability Recovery Test

[0090] Test the plugging removal performance of the enzyme-responsive core-shell structure composite temporary plugging material prepared in Example 1. Place the low-temperature composite enzyme, cellulase or amylase at 15°C to degrade the composite temporary plugging material for 24 hours, and test the permeability recovery rate of the composite temporary plugging material for plugging the core after degradation.

[0091] Test method: Prepare an aqueous solution of 0.2 wt% bioenzyme, add the composite temporary plugging material to the enzyme solution, then stir at 15°C for 24 hours, and then dry it. Use the composite temporary plugging material after degradation to test the permeability before and after plugging, and calculate the permeability recovery rate.

[0092]

[0093] In the formula, D is the permeability recovery rate, %; K W is the permeability of the core after degradation, μm 2 ; K 0 is the permeability of the core before plugging, μm 2 .

[0094] As Figure 3 shown, for the composite temporary plugging material degraded by the low-temperature composite enzyme, the change range of the permeability before and after plugging the core is relatively small, and the highest permeability recovery rate is 53%; while the permeability recovery rates of the composite temporary plugging materials degraded by cellulase and amylase are relatively low, which are 29.84% and 29.96% respectively. By comparison, it is obtained that the low-temperature composite enzyme still has good enzyme activity, has strong degradation performance, is suitable for degrading the composite temporary plugging material, and thus improves the plugging removal effect of the composite temporary plugging material.

[0095] 4. Sand Bed Filtration Loss Experiment

[0096] Test the filtration loss performance of the composite temporary plugging materials prepared in Examples 1-3.

[0097] Test method:

[0098] The composite temporary plugging material prepared in Examples 1-3 was added to the bentonite water-based drilling fluid base slurry of 4% (4 g of bentonite was added to 100 mL of water) respectively, and stirred to make it fully and evenly mixed to obtain the composite temporary plugging material drilling fluid of 3% (6 g was added to 200 mL); the invasion depth of the drilling fluid was tested through the sand bed filtration loss experiment. 300 mL of quartz sand with a mesh size of 60-80 was added to the cylindrical and transparent drilling fluid cup, 150 mL of the composite temporary plugging material drilling fluid was poured in, the cup lid was tightened, the gas pressure was adjusted to 0.7 MPa, the air release valve was opened, and the invasion depth of the drilling fluid was tested when the gas source was introduced for 30 min.

[0099] Tested by the sand bed filtration loss experiment, the invasion depth of the composite temporary plugging material with a core-wall ratio of 8:2:1 (the composite temporary plugging material prepared in Example 1) was 4.5 cm, the invasion depths of the composite temporary plugging material with a core-wall ratio of 4:2:1 (the composite temporary plugging material prepared in Example 2) were 3.5 cm respectively, and the invasion depths of the composite temporary plugging material with a core-wall ratio of 2:2:1 (the composite temporary plugging material prepared in Example 3) were 4.5 cm respectively. When the core-wall ratio was 4:2:1, the coating effect of chitosan and sodium alginate on the modified ceramsite was the best, which could enhance the plugging performance of the composite temporary plugging material and reduce the invasion depth of the drilling fluid.

[0100] It can be concluded from this that the enzyme-responsive core-shell structure composite temporary plugging material of the present invention has the characteristics of high drilling time strength, good plugging performance, easy plug removal after the action of biological enzymes, and high permeability. It is suitable for the exploitation of marine natural gas hydrates. At the same time, the preparation method is simple, meeting the environmental protection requirements of exploitation, and has good application prospects in the exploitation of natural gas hydrates.

Claims

1. An enzyme-responsive core-shell structured composite temporary plugging material, characterized in that, the enzyme-responsive core-shell structured composite temporary plugging material comprises a modified ceramsite core and a chitosan and sodium alginate outer shell wrapped around the core; the modified ceramsite is ceramsite surface-modified with 3-aminopropyltriethoxysilane; The preparation method of the enzyme-responsive core-shell structured composite temporary plugging material comprises the following steps: (1) Preparation of modified ceramsite Mix ceramsite and an aqueous solution of 3-aminopropyltriethoxysilane, and obtain modified ceramsite through stirring reaction and drying; the mass ratio of ceramsite to 3-aminopropyltriethoxysilane is 10~25:1; (2) Preparation of composite temporary plugging material Add chitosan, modified ceramsite and sodium alginate to a mixed solvent of dichloromethane and water in sequence, carry out a reaction, and then obtain the enzyme-responsive core-shell structured composite temporary plugging material through filtration, washing and drying; the mass ratio of the modified ceramsite, chitosan and sodium alginate is 2~8:2:1; the volume ratio of dichloromethane to water in the mixed solvent is 1:

1.

2. The enzyme-responsive core-shell structured composite temporary plugging material according to claim 1, characterized in that, in step (1), the particle size of the ceramsite is 10~100μm; the mass fraction of the aqueous solution of 3-aminopropyltriethoxysilane is 0.5~15%.

3. The enzyme-responsive core-shell structured composite temporary plugging material according to claim 1, characterized in that, in step (1), the mass fraction of the aqueous solution of 3-aminopropyltriethoxysilane is 0.8~3%.

4. The enzyme-responsive core-shell structured composite temporary plugging material according to claim 1, characterized in that, in step (1), the temperature of the stirring reaction is room temperature, the stirring reaction time is 30~60min, and the stirring rate is 150~300r / min; the drying is carried out at 60~70°C for 10~15h.

5. The enzyme-responsive core-shell structured composite temporary plugging material according to claim 1, characterized in that, in step (2), the weight average molecular weight of the chitosan is 100000~200000, and the degree of deacetylation ≥99.5%; the weight average molecular weight of the sodium alginate is 300000~400000.

6. The enzyme-responsive core-shell structured composite temporary plugging material according to claim 1, characterized in that, in step (2), the mass ratio of the modified ceramsite to the mixed solvent is 1:3~5; In step (2), after adding each component to the mixed solvent, stirring treatment needs to be carried out, the stirring time is 20~30min, and the stirring rate is 100~300r / min.

7. The enzyme-responsive core-shell structured composite temporary plugging material according to claim 1, characterized in that, in step (2), the temperature of the reaction is 40~50°C, the reaction time is 1~3h, and the stirring rate during the reaction process is 100~300r / min; The washing is carried out by washing 3~5 times with deionized water; the drying is carried out at 80~90°C for 10~20h.

8. Application of the enzyme-responsive core-shell structured composite temporary plugging material according to claim 1 in the process of drilling for offshore natural gas hydrate exploitation.

9. The application according to claim 8, wherein, during drilling, an enzyme-responsive core-shell structured composite temporary plugging material is used to plug the wellbore, preventing the drilling fluid from entering the formation and protecting the reservoir; after drilling is completed, a bio-enzyme is used to degrade the composite temporary plugging material to achieve plug removal.

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