Soft-outside and rigid-inside plugging agent, preparation method and water-based drilling fluid

Abstract

The invention provides an external-soft and internal-rigid plugging agent, a preparation method and a water-based drilling fluid, the preparation method of the external-soft and internal-rigid plugging agent comprises the following steps: (1) reacting silicon dioxide with a vinyl-containing silane coupling agent in an ethanol water solution to obtain modified silicon dioxide; (2) mixing the modified silicon dioxide, acrylamide, 2-acrylamido-2-sodium methylpropanesulfonate, sodium p-styrenesulfonate, N, N-methylene bisacrylamide and water, introducing nitrogen to remove oxygen under stirring and heating conditions, and then adding an initiator to react; and (3) filtering, washing and drying the product obtained in the step (2) to obtain the plugging agent with soft outside and rigid inside. The plugging agent can effectively combine the advantages of high strength of a rigid plugging agent and high adaptive capacity of a flexible plugging agent, can adapt to the sizes and shapes of micro-cracks and cracks in a stratum, and effectively plugs pores and cracks in the surface of a well wall.

Description

Technical Field

[0001] The invention relates to the field of oilfield chemistry, and in particular to an outer-soft and inner-rigid plugging agent, a preparation method thereof, and a water-based drilling fluid. Background Art

[0002] Under the macro background of increasing oil and gas exploration and development, oil and gas exploration and development in Changqing area is gradually advancing towards unconventional oil and gas resources, and horizontal wells are becoming the key direction of exploration and development. In recent years, a large number of micro-cracks with high stress and large inclination angles are often encountered in the construction of horizontal wells. Conventional drilling fluid treatment agents are difficult to effectively plug, resulting in serious well wall spalling, short collapse cycle, and complex underground accidents. At present, the effective way to control permeability leakage is mainly to use drilling plugging technology, that is, adding a certain amount of drilling plugging material to the drilling fluid to form a better plugging layer, so as to achieve plugging and leakage prevention while drilling.

[0003] However, the commonly used plugging agents have the following disadvantages: rigid plugging agents are strong and difficult to effectively adapt to the size and shape of formation pores, microcracks and cracks, and the plugging range is narrow, making it difficult to form an effective plugging; flexible plugging agents can adapt to the size and shape of shale surface pores, microcracks and cracks, but due to their own insufficient strength, they are easily pushed to the deep part of the cracks under the action of pressure difference, making it difficult to form an effective plugging around the well wall. Therefore, in view of the disadvantages of existing plugging agents, the advantages of both are combined by combining rigid materials and flexible materials.

[0004] Chinese patent document CN113403042A discloses an organic-inorganic nanocomposite hydrogel plugging agent, which has methacrylic acid, N,N-dimethylacrylamide, and N,N-methylenebisacrylamide as the organic material part and silicon dioxide as the inorganic part. However, the organic part and the inorganic part of the plugging agent are not chemically bonded, and there are problems of poor dispersibility and compatibility in drilling fluid; and the plugging range of the plugging agent is narrow, and it is only applicable to nano-level cracks, and the plugging effect on micron-level cracks is poor, and the application occasions are limited.

[0005] Chinese patent document CN111218261A discloses a soft-outer-hard-inner plugging agent and its preparation method, wherein the plugging agent has acrylamide, acrylic acid, and 2-acrylamido-2-methylpropanesulfonic acid as the flexible part and an organic-inorganic composite material as the rigid part. This plugging agent has a significant plugging effect, but the testing of its compatibility in drilling fluid, such as rheology and filtration loss, is not comprehensive, and it is difficult to meet the needs of plugging and preventing leakage while drilling.

[0006] Therefore, it is of great practical significance to develop a soft-outer-hard-inner plugging agent for water-based drilling fluid. Summary of the invention

[0007] In order to overcome the defects of the prior art that rigid plugging agents are difficult to effectively adapt to the shape of formation pores and cracks and that flexible plugging agents have low strength, the present invention provides an outer-soft and inner-rigid plugging agent and a preparation method and a water-based drilling fluid. The plugging agent can effectively combine the advantages of high strength of rigid plugging agents and strong adaptability of flexible plugging agents, can adapt to the size and shape of micro-cracks and cracks in the formation, and effectively plug the pores and cracks on the surface of the well wall.

[0008] The present invention is achieved through the following technical solutions:

[0009] A method for preparing a soft-outer-hard-inner plugging agent comprises the following steps:

[0010] (1) reacting silicon dioxide with a vinyl-containing silane coupling agent in an ethanol aqueous solution to obtain modified silicon dioxide;

[0011] (2) mixing modified silica, acrylamide, sodium 2-acrylamido-2-methylpropane sulfonate, sodium p-styrene sulfonate, N,N-methylenebisacrylamide and water, introducing nitrogen to remove oxygen under stirring and heating conditions, and then adding an initiator to react;

[0012] (3) filtering, washing, and drying the product obtained in step (2) to obtain a plugging agent that is soft on the outside and hard on the inside.

[0013] Preferably, in step (1), the mass ratio of silicon dioxide to silane coupling agent is 1:(1.5-3).

[0014] Preferably, in step (1), the silane coupling agent is one or a combination of two or more of methylvinyldimethoxysilane, vinyltrimethoxysilane, vinyltriethoxysilane, vinyltri(2-methoxyethoxy)silane and γ-methacryloxypropyltrimethoxysilane.

[0015] Preferably, in step (1), the silane coupling agent is a combination of vinyltrimethoxysilane, vinyltriethoxysilane and vinyltri(2-methoxyethoxy)silane in a mass ratio of 1:(1-2):(1-3).

[0016] Preferably, in step (1), the reaction temperature of silicon dioxide and silane coupling agent is 60-80° C., and the reaction time is 2-3 h.

[0017] Preferably, in step (2), the weight ratio of the modified silica, acrylamide, 2-acrylamido-2-methylpropanesulfonic acid, sodium p-styrenesulfonate, N,N-methylenebisacrylamide and the initiator is (1-3):12:3:1:0.2:0.2.

[0018] Preferably, in step (2), the reaction temperature is 60-80°C and the reaction time is 4-6h.

[0019] Preferably, in step (2), the initiator is one or a combination of two or more of ammonium persulfate, sodium bisulfite, hydrogen peroxide, and azobisisobutylamidine hydrochloride.

[0020] The outer-soft and inner-hard plugging agent is obtained by the preparation method.

[0021] A water-based drilling fluid comprises drilling fluid and the above-mentioned soft-outer-hard-inner plugging agent.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] The reaction mechanism of the present invention is as follows: the silane alkoxy group (-Si(OR)3) in the silane coupling agent can be hydrated to a hydroxyl group in an aqueous solution, and the hydroxyl group can undergo a dehydration reaction with some hydroxyl groups (-OH) on the surface of silica, which can improve the dispersibility of silica on the one hand, and on the other hand, connect the copolymerizable group -CH=CH2 to the surface of silica, so that the rigid part (silicon dioxide) and the reactive monomer can be combined; acrylamide is easily soluble in water, and the amide group in the molecular structure has a strong adsorption and hydration ability, which plays a role in protecting the stability of the drilling fluid system, and the carbon The carbon-carbon single bond energy formed by the polymerization of carbon double bonds is high and is not easy to break at high temperatures; 2-acrylamide-2-methylpropanesulfonate sodium contains unsaturated carbon-carbon double bonds, which is easy to copolymerize with other monomers, and the sulfonic acid group in the molecular structure makes the synthetic product have strong salt resistance; the benzene ring structure in the molecular structure of sodium p-styrene sulfonate makes the synthetic product have strong thermal stability; the cross-linking agent N,N-methylenebisacrylamide is easily soluble in water, and both ends of the molecule also contain vinyl groups, which can undergo polymerization reaction with the monomer, so that the plugging agent obtains a complex three-dimensional network structure with higher strength. The plugging agent of the present invention has a mechanism of action: the outer soft inner hard plugging agent plugs through physical action, and uses the plugging agent particles to plug the pores and cracks on the surface of the shale, which can prevent the drilling fluid from invading the formation cracks, and then slow down the pressure transmission between the cracks and pores. The outside of the plugging agent is a flexible gel, which can adapt to the size and shape of the pores and cracks in the formation; the internal rigid material is silicon dioxide, which increases the strength of the plugging agent. The outer-soft and inner-rigid plugging agent of the present invention has strong compatibility with water-based drilling fluid and little effect on the rheological properties of the drilling fluid. Under the action of pressure difference, it can not only effectively reduce the filtration loss of the drilling fluid, but also fill the pores and cracks around the well wall, giving full play to the advantages of strong adaptability of flexible materials and high strength of rigid materials, forming an effective pressure-bearing plugging layer on the surface of the well wall, thereby preventing pressure transmission between cracks and pores and intrusion of drilling fluid. DETAILED DESCRIPTION

[0024] In order to further understand the present invention, the present invention is described below in conjunction with embodiments. These descriptions are only to further explain the features and advantages of the present invention and are not intended to limit the claims of the present invention.

[0025] The present invention discloses a method for preparing a soft-outer-hard-inner plugging agent for water-based drilling fluid, comprising the following steps:

[0026] (1) Preparation of modified silica: silica is fully dispersed in a water / anhydrous ethanol dispersion system, and the pH value is adjusted to 3-5 with dilute HCl, and then a silane coupling agent containing a functional group -CH=CH2 is added and mixed, and then heated and stirred for a period of time. After the reaction is completed, the modified silica with surface reactivity is obtained by filtering, washing and drying. The modification mechanism of silica is as follows:

[0027]

[0028] Wherein, R is a group with -CH=CH2, and OX is a methoxy group, an ethoxy group, or a 2-methoxyethoxy group.

[0029] (2) mixing the modified silica prepared in step (1), acrylamide, sodium 2-acrylamido-2-methylpropane sulfonate, sodium p-styrene sulfonate, N,N-methylenebisacrylamide (MBA) and water, introducing nitrogen to remove oxygen under stirring and heating conditions, and then adding an initiator to react;

[0030] Taking R as -CH=CH2 group as an example, the reaction mechanism of the copolymer of modified silica, acrylamide, 2-acrylamido-2-methylpropanesulfonic acid and sodium p-styrenesulfonate is as follows:

[0031]

[0032] The cross-linking effect of N,N-methylenebisacrylamide is shown below:

[0033]

[0034] (3) After the reaction is completed, the product obtained in step (2) is filtered, washed, dried, and crushed to obtain a soft-outer-hard-inner plugging agent.

[0035] In step (1), the water / anhydrous ethanol dispersion system is preferably a water / anhydrous ethanol dispersion system with a volume ratio of 1:3, and the water is preferably deionized water. The particle size range of the silicon dioxide is 30nm-30μm, and the silicon dioxide is dispersed by ultrasonic dispersion, the dispersion time is 30min, and the stirring is performed on a magnetic stirrer for 1min every 5min. The concentration of the dilute HCl is 0.1mol / L.

[0036] In step (1), the mass ratio of silicon dioxide to silane coupling agent is 1:(1.5-3). The silane coupling agent is one or a combination of two or more of methylvinyldimethoxysilane, vinyltrimethoxysilane, vinyltriethoxysilane, vinyltri(2-methoxyethoxy)silane, and γ-methacryloxypropyltrimethoxysilane.

[0037] In the present invention, in order to better impart reactive activity to the silica surface, the above-mentioned silane coupling agents are preferably used in combination, and specifically preferably a combination of vinyltrimethoxysilane, vinyltriethoxysilane and vinyltri(2-methoxyethoxy)silane in a mass ratio of 1:(1-2):(1-3).

[0038] In step (1), the reaction temperature of silicon dioxide and silane coupling agent is 60-80° C., the reaction time is 2-3 hours, and the stirring rate is 200-400 r / min.

[0039] In step (1), the washing is performed by washing with anhydrous ethanol for 3-5 times, and the drying is performed by drying in a forced air drying oven at 50° C. for 24-36 hours.

[0040] In step (2), the weight ratio of the modified silica, acrylamide, sodium 2-acrylamido-2-methylpropane sulfonate, sodium p-styrene sulfonate, N,N-methylenebisacrylamide and the initiator is (1-3):12:3:1:0.2:0.2.

[0041] In step (2), the initiator is one or a combination of two or more of ammonium persulfate, sodium bisulfite, hydrogen peroxide, and azobisisobutylamidine hydrochloride, and preferably a combination of ammonium persulfate / sodium bisulfite in a mass ratio of 1:1.

[0042] In step (2), the nitrogen deoxygenation conditions are: temperature of 60-80°C, time of 25-40 min, and stirring rate of 200-400 r / min.

[0043] In step (2), the reaction conditions are: temperature of 60-80°C, time of 4-6h, stirring rate of 200-400r / min. After the reaction is completed, heating is stopped, stirring is continued, and stirring is stopped after cooling to room temperature.

[0044] In step (3), the washing is performed by washing with anhydrous ethanol for 3-5 times, and the drying is performed by drying in a forced air drying oven at 50° C. for 24-36 hours.

[0045] In step (3), the pulverizing time is required to be at least 10 minutes, and the powder must be evenly dispersed without lumps.

[0046] The present invention will be described in detail below by way of examples, but the present invention is not limited thereby. In the following preparation examples, embodiments and comparative examples, unless otherwise specified, all materials used are commercially available, and all methods used are conventional methods in the art.

[0047] Deionized water was homemade in the laboratory, and anhydrous ethanol, acrylamide, sodium 2-acrylamido-2-methylpropanesulfonate, sodium p-styrenesulfonate, N,N-methylenebisacrylamide, methylvinyldimethoxysilane, vinyltrimethoxysilane, vinyltriethoxysilane, vinyltri(2-methoxyethoxy)silane, γ-methacryloxypropyltrimethoxysilane, ammonium persulfate, sodium bisulfite, hydrogen peroxide, and azobisisobutylamidine hydrochloride were purchased from Sinopharm Chemical Reagent Co., Ltd.

[0048] Comparative Example 1:

[0049] Preparation of modified silica: 4 g of silica was fully dispersed in 400 mL of water / anhydrous ethanol dispersion system (volume ratio of 1:3), and the pH value was adjusted to 4 with dilute HCl. Then 2 g of vinyltrimethoxysilane, 3 g of vinyltriethoxysilane and 4 g of vinyltri(2-methoxyethoxy)silane were added and mixed. The mixture was heated and stirred at 70°C and a stirring rate of 300 r / min for 3 h. After the reaction was completed, the mixture was filtered, washed and dried to obtain modified silica F-1 with surface reactivity.

[0050] Comparative Example 2:

[0051] 12 g of acrylamide, 3 g of sodium 2-acrylamido-2-methylpropanesulfonate, 1 g of sodium p-styrenesulfonate, 0.2 g of N,N-methylenebisacrylamide and 100 mL of water were mixed, nitrogen was introduced under stirring and heating conditions for deoxygenation (temperature was 70° C., stirring rate was 300 r / min), and then a mixed aqueous solution containing 0.2 g of ammonium persulfate-sodium bisulfite (1:1) was added dropwise to react for 4 h;

[0052] After the reaction is completed, the obtained product is filtered, washed, dried and crushed to obtain plugging agent F-2.

[0053] Embodiment 1:

[0054] (1) Preparation of modified silica: 4 g of silica was fully dispersed in 400 mL of water / anhydrous ethanol dispersion system (volume ratio of 1:3), and the pH value was adjusted to 4 with dilute HCl, and then 2 g of vinyl trimethoxysilane, 3 g of vinyl triethoxysilane and 4 g of vinyl tri(2-methoxyethoxy)silane were added and mixed, and heated and stirred at a temperature of 70° C. and a stirring rate of 300 r / min for 3 h. After the reaction was completed, the modified silica with surface reactivity was obtained by filtering, washing and drying;

[0055] (2) 1 g of modified silica prepared in step (1), 12 g of acrylamide, 3 g of sodium 2-acrylamido-2-methylpropanesulfonate, 1 g of sodium p-styrenesulfonate, 0.2 g of N,N-methylenebisacrylamide and 100 mL of water were mixed, nitrogen was introduced under stirring and heating conditions for deoxygenation (temperature was 70° C., stirring rate was 300 r / min), and then 0.2 g of a mixed aqueous solution of ammonium persulfate-sodium bisulfite (1:1) was added dropwise to react for 5 h;

[0056] (3) After the reaction is completed, the product obtained in step (2) is filtered, washed, dried, and crushed to obtain a soft-outer-hard-inner plugging agent FD-1.

[0057] Embodiment 2:

[0058] The same process as in Example 1 was adopted, except that the mass of modified silica added in step (2) was 2 g.

[0059] The rest is the same as in Example 1, and a plugging agent FD-2 with a soft exterior and a hard interior is obtained.

[0060] Embodiment 3:

[0061] The same process as in Example 1 was adopted, except that the mass of modified silica added in step (1) was 3 g.

[0062] The rest is the same as in Example 1, and a plugging agent FD-3 with a soft exterior and a hard interior is obtained.

[0063] Embodiment 4:

[0064] The same process as in Example 1 was adopted, except that the silane coupling agent added in step (1) was γ-methacryloxypropyltrimethoxysilane with a mass of 6 g.

[0065] The rest is the same as in Example 1, and a plugging agent FD-4 with a soft exterior and a hard interior is obtained.

[0066] Embodiment 5:

[0067] The same process as in Example 1 was adopted, except that the mass of silicon dioxide added in step (1) was 3 g.

[0068] The rest is the same as in Example 1, and a plugging agent FD-5 with a soft exterior and a hard interior is obtained.

[0069] Embodiment 6:

[0070] The same process as in Example 1 was adopted, except that the mass of silicon dioxide added in step (1) was 6 g.

[0071] The rest is the same as in Example 1, and a plugging agent FD-6 with a soft exterior and a hard interior is obtained.

[0072] Embodiment 7:

[0073] The same process as in Example 1 was adopted, except that the reaction temperature of silicon dioxide and silane coupling agent in step (1) was 60°C.

[0074] The rest is the same as in Example 1, and a plugging agent FD-7 with a soft exterior and a hard interior is obtained.

[0075] Embodiment 8:

[0076] The same process as in Example 1 was adopted, except that the reaction temperature of silicon dioxide and silane coupling agent in step (1) was 80°C.

[0077] The rest is the same as in Example 1, and the outer-soft and inner-hard plugging agent FD-8 is obtained.

[0078] Embodiment 9:

[0079] The same process as in Example 1 was adopted, except that the reaction time of silicon dioxide and the silane coupling agent in step (1) was 2 h.

[0080] The rest is the same as in Example 1, and the outer-soft and inner-hard plugging agent FD-9 is obtained.

[0081] Embodiment 10:

[0082] The same process as in Example 1 was adopted, except that the reaction temperature for synthesizing the plugging agent in step (2) was 60°C.

[0083] The rest is the same as in Example 1, and the outer-soft and inner-hard plugging agent FD-10 is obtained.

[0084] Embodiment 11:

[0085] The same process as in Example 1 was adopted, except that the reaction temperature for synthesizing the plugging agent in step (2) was 80°C.

[0086] The rest is the same as in Example 1, and a plugging agent FD-11 with soft exterior and hard interior is obtained.

[0087] Embodiment 12:

[0088] The same process as in Example 1 was adopted, except that the reaction time for synthesizing the plugging agent in step (2) was 4 h.

[0089] The rest is the same as in Example 1, and a plugging agent FD-12 with a soft exterior and a hard interior is obtained.

[0090] Embodiment 13:

[0091] The same process as in Example 1 was adopted, except that the reaction time for synthesizing the plugging agent in step (2) was 6 h.

[0092] The rest is the same as in Example 1, and a plugging agent FD-13 with a soft exterior and a hard interior is obtained.

[0093] The following is a detailed description of the expansion properties of the soft-outer-hard-inner plugging agent prepared in the examples, as well as the test evaluation of its rheological properties, filtration properties, and plugging performance in the drilling fluid.

[0094] 1. Expansibility of plugging agent

[0095] The polymer-containing plugging agent has the property of swelling upon water absorption, but excessive water absorption by the plugging agent reduces its plugging ability, so it is necessary to test the water absorption swelling multiples of the materials in Comparative Example 2 and Examples 1 to 4.

[0096] Method for measuring water absorption expansion multiple: Take 0.1 g of the plugging agent synthesized under the conditions in Comparative Example 2 and Examples 1 to 4 respectively, add it to a beaker containing 100 g of water, stir it appropriately, let it stand for 24 hours, take it out and weigh it, and the water absorption expansion multiple of the plugging agent is obtained. The experimental results are shown in Table 1.

[0097] Table 1 Water absorption expansion ratio of plugging agent under different preparation methods

[0098] Plugging agent Weight after water absorption / g Water absorption expansion ratio / (g / g) F-2 3.66 36.6 FD-1 2.31 23.1 FD-2 1.55 15.5 FD-3 1.09 10.9 FD-4 1.09 10.9

[0099] It can be seen from Table 1 that the water absorption multiple of the gel plugging agent F-2 without rigid material is 36.6 times, and with the addition of modified silica, the water absorption multiple is significantly reduced, and with the increase of modified silica, the water absorption multiple of the plugging agent gradually decreases from 23.1 to 10.9, indicating that the addition of modified silica can alleviate the problem of excessive water absorption of the plugging agent leading to decreased plugging ability.

[0100] 2. Rheology of drilling fluid

[0101] Preparation of drilling fluid:

[0102] (1) 400 mL of distilled water was measured and placed in a stirrer, 0.8 g of sodium carbonate was added and stirred to dissolve, 16 g of bentonite for drilling fluid (purchased from Shandong Huawei Bentonite Co., Ltd.) was added while stirring, and the mixture was stirred at 10,000 rpm for 20 min, and allowed to stand for hydration for more than 48 h to obtain 400 mL of 4 wt% bentonite-based slurry.

[0103] (2) 4 g of plugging agent was added to the bentonite-based slurry, and the mixture was stirred at 10,000 rpm for 30 minutes to obtain drilling fluid.

[0104] Aging of drilling fluid:

[0105] The drilling fluid obtained in the preparation process of the drilling fluid is placed in an aging tank and hot-rolled at 150° C. for 16 hours to obtain the aged drilling fluid.

[0106] Rheology of drilling fluid:

[0107] (1) Inject the drilling fluid sample into the sample cup to the scale line, place the sample cup on the base of the six-speed rotary viscometer (model ZNN-D6), and adjust the base so that the drilling fluid level in the sample cup coincides with the scale line on the measuring tube;

[0108] (2) Use a six-speed rotational viscometer to rotate the measuring cylinder at 600 r / min. After the dial value stabilizes, read and record the dial reading, expressed as R600 (in mPa·s);

[0109] (3) Use a six-speed rotational viscometer to rotate the measuring cylinder at 300 r / min. After the dial value stabilizes, read and record the dial reading, expressed as R300 (in mPa·s);

[0110] (4) Calculation formula of drilling fluid rheological parameters: apparent viscosity (AV) = R600 / 2, plastic viscosity (PV) = R600-R300, dynamic shear force (YP) = AV-PV.

[0111] 3. Drilling fluid filtration

[0112] Normal temperature medium pressure (API) filtration loss FL API test

[0113] (1) Drilling fluid sample is injected into the filter cup, with the liquid level about 1 cm from the sealed end of the cup, filter paper and sealing gasket are placed, and the filter loss meter (model ZNS-2A) is installed;

[0114] (2) Place a dry measuring cylinder under the outlet of the filter cup to receive the filtrate, close the pressure relief valve and adjust the pressure regulator so that the pressure in the cup reaches 690 kPa. Start timing when the first drop of filtrate begins to appear;

[0115] (3) The filtrate volume measured after 30 minutes is the API filtration loss.

[0116] High temperature and high pressure (HTHP) filtration loss FL HTHP test

[0117] (1) Insert the thermometer into the thermometer jack on the heating jacket, preheat to about 6°C lower than the required experimental temperature (150°C), and adjust the temperature adjustment knob to maintain a constant temperature;

[0118] (2) Close the bottom valve stem, pour the drilling fluid into the filter cup, with the liquid level 1.3 cm from the top, place the filter paper and sealing gasket, and install the high-temperature and high-pressure filter loss meter;

[0119] (3) Connect the adjustable pressure regulator to the upper valve stem and the lower filtrate receiver respectively and lock them;

[0120] (4) With the upper and lower valve stems closed, adjust the upper and lower pressure regulators to 690 kPa respectively, and open the upper valve stem to release the pressure into the filter cup, maintaining the pressure of 690 kPa at 150°C, and keep this temperature constant for 1 hour;

[0121] (5) Increase the top pressure to 4140 kPa, open the bottom valve stem and measure the filtration loss. After 30 minutes, double the filtrate volume is the high temperature and high pressure (HTHP) filtration loss.

[0122] 4. Drilling fluid plugging performance

[0123] Sand bed plugging experiment: Take 350cm 3 The 60-80 mesh quartz sand is loaded into the sand filter tube of the visual medium pressure filter loss meter, filled evenly, and 300 mL of drilling fluid is slowly added. The pressure is increased to 0.69 MPa at room temperature, and the penetration effect of the drilling fluid on the sand bed is tested after 30 minutes.

[0124] The plugging agent used in Comparative Example 3 is commercially available sulfonated asphalt FT-1 (Tianjin Chengli Chemical Co., Ltd.).

[0125] The plugging agent used in Comparative Example 4 is commercially available sulfonated asphalt FT-2 (China Petroleum Engineering Institute).

[0126] The plugging agent used in Comparative Example 5 is commercially available emulsified asphalt FF-3 (Shandong Deshunyuan Petroleum Technology Co., Ltd.).

[0127] The plugging agent used in Comparative Example 6 is commercially available emulsified paraffin PF-2 (China Petroleum Engineering Institute).

[0128] The anti-slump and filtration loss agent used in Comparative Example 7 is commercially available lignite resin SPNH (Shandong Zhengyang New Material Technology Co., Ltd.).

[0129] The added amount of the embodiment and the comparative example is 1 wt %. The performance test results of the embodiment and the comparative example are shown in Table 2.

[0130] Table 2 Performance test results of embodiments and comparative examples

[0131]

[0132]

[0133] As can be seen from Table 2, after aging at 150°C / 16h, the API filtration loss and high temperature and high pressure filtration loss of the 4wt% bentonite slurry with 1wt% of the soft outer and hard inner plugging agent added are significantly lower than those of the 4wt% bentonite slurry and the 4wt% bentonite slurry with 1wt% of modified silica, and the sand bed intrusion depth is significantly lower than that of the 4wt% bentonite slurry, the 4wt% bentonite slurry with 1wt% of modified silica, and the 4wt% bentonite slurry with 1wt% of the flexible plugging agent F-2. This shows that the combination of rigidity and flexibility can greatly enhance the plugging performance of the plugging agent. Among them, the most effective soft outer and hard inner plugging agent is FD-2 (Example 2), with an API filtration loss of 8.0mL, a high temperature and high pressure filtration loss of 33.8mL, and a sand bed intrusion depth of 26.0mm. Moreover, the API filtration loss and high-temperature and high-pressure filtration loss of the plugging agent prepared in the embodiment of the present invention after aging at 150°C / 16h are lower than those of commercially available products, and the sand bed plugging effect is better than that of commercially available anti-collapse plugging agents, indicating that the soft-outer-hard-inner plugging agent prepared by the method of the present invention can be used as a plugging agent in water-based drilling fluids.

[0134] The plugging agent of the present invention has the following advantages:

[0135] Advantage 1: The soft-outer-hard-inner plugging agent can effectively combine the advantages of high strength of rigid plugging agents and strong adaptability of flexible plugging agents. It can adapt to the size and shape of microcracks and cracks in the formation and effectively plug the pores and cracks on the well wall surface.

[0136] Advantage 2: The soft-outer-hard-inner plugging agent can be adsorbed on the surface of pores and cracks through hydrogen bonds through the amide group, sulfonic acid group and other groups on the molecular chain to enhance the plugging effect; the soft-outer-hard-inner plugging agent has excellent salt resistance through the sulfonic acid group on the molecular chain; the soft-outer-hard-inner plugging agent has strong thermal stability through the sulfonic acid group and benzene ring structure on the molecular chain.

[0137] Advantage 3: The soft-outer-hard-inner plugging agent has strong compatibility with water-based drilling fluid, has little effect on the rheology of drilling fluid, can effectively reduce the filtration loss of drilling fluid, and can be used while drilling.

[0138] Advantage 4: The preparation process of the soft-outer-hard-inner plugging agent is simple, the materials are easy to obtain, and it is easy to produce.

Claims

1. A method for preparing a soft-outer-hard-inner plugging agent, characterized in that: The following steps are involved: (1) reacting silicon dioxide with a vinyl-containing silane coupling agent in an ethanol aqueous solution to obtain modified silicon dioxide; (2) mixing modified silica, acrylamide, sodium 2-acrylamido-2-methylpropane sulfonate, sodium p-styrene sulfonate, N,N-methylenebisacrylamide and water, introducing nitrogen to remove oxygen under stirring and heating conditions, and then adding an initiator to react; (3) filtering, washing, and drying the product obtained in step (2) to obtain a plugging agent that is soft on the outside and hard on the inside.

2. The method for preparing the soft-outer-hard-inner plugging agent according to claim 1, characterized in that: In step (1), the mass ratio of silicon dioxide to silane coupling agent is 1:(1.5-3).

3. The method for preparing the soft-outer-hard-inner plugging agent according to claim 1, characterized in that: In step (1), the silane coupling agent is one or a combination of two or more of methylvinyldimethoxysilane, vinyltrimethoxysilane, vinyltriethoxysilane, vinyltri(2-methoxyethoxy)silane and γ-methacryloxypropyltrimethoxysilane.

4. The method for preparing the soft-outer-hard-inner plugging agent according to claim 1, characterized in that: In step (1), the silane coupling agent is a combination of vinyl trimethoxy silane, vinyl triethoxy silane and vinyl tri(2-methoxyethoxy) silane in a mass ratio of 1:(1-2):(1-3).

5. The method for preparing the soft-outer-hard-inner plugging agent according to claim 1, characterized in that: In step (1), the reaction temperature of silicon dioxide and silane coupling agent is 60-80° C., and the reaction time is 2-3 h.

6. The method for preparing the soft-outer-hard-inner plugging agent according to claim 1, characterized in that: In step (2), the weight ratio of the modified silica, acrylamide, 2-acrylamido-2-methylpropanesulfonic acid, sodium p-styrenesulfonate, N,N-methylenebisacrylamide and the initiator is (1-3):12:3:1:0.2:0.

2.

7. The method for preparing the soft-outer-hard-inner plugging agent according to claim 1, characterized in that: In step (2), the reaction temperature is 60-80°C and the reaction time is 4-6h.

8. The method for preparing the soft-outer-hard-inner plugging agent according to claim 1, characterized in that: In step (2), the initiator is one or a combination of two or more of ammonium persulfate, sodium bisulfite, hydrogen peroxide, and azobisisobutylamidine hydrochloride.

9. A soft-outer-hard-inner plugging agent obtained by the preparation method according to any one of claims 1 to 8.

10. A water-based drilling fluid, characterized in that: The invention comprises drilling fluid and the outer-soft and inner-hard plugging agent as claimed in claim 9.

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