Preparation of a Low-Temperature Expandable Modified Graphite Plugging Agent and Its Application in Water-Based Drilling Fluids

Abstract

This invention belongs to the field of oilfield chemistry, and discloses a low-temperature expandable modified graphite, its preparation method, and its application in the field of plugging water-based drilling fluids. The preparation method of the low-temperature expandable modified graphite includes: (1) subjecting natural flake graphite and a pre-oxidizing agent to a pre-oxidation reaction to obtain pre-oxidized graphite; (2) contacting the pre-oxidized graphite with a main intercalating agent and a main oxidizing agent to conduct an oxidation intercalation reaction, and adding an auxiliary oxidizing agent after reacting for a period of time to obtain low-temperature expandable graphite; (3) using a modifier to modify the low-temperature expandable graphite, and then polymerizing the modified low-temperature expandable graphite with a surfactant using an initiator to obtain the low-temperature expandable modified graphite.

Description

Technical Field

[0001] The present invention relates to the field of oilfield chemistry, and particularly relates to a preparation method of low-temperature expandable modified graphite and its application in water-based drilling fluids. Background Art

[0002] Water-based drilling fluids are still one of the drilling fluid systems most widely used in oil and gas engineering on-site due to their advantages such as environmental friendliness and low cost. However, the problem of lost circulation in water-based drilling fluids is one of the greatest risks faced by countries during drilling operations and is also one of the most complex situations in drilling engineering. Lost circulation not only causes a large amount of drilling fluid loss, restricts the drilling speed, and reduces production efficiency, but also causes a series of major economic losses such as wellbore collapse and stuck pipe. In recent years, with the in-depth exploration of global unconventional oil and gas resources, the number of complex structure well layers has increased. According to statistics, in the Middle East and North America, the cases of severe lost circulation during drilling account for about 30% and more than 40% of the total number of wellbores respectively. Currently, how to solve the problem of lost circulation has become the biggest problem faced by global drilling researchers. Traditional bridging plugging materials have high requirements for the lost circulation channels and are easily further lost deep into the fractures, lacking wide adaptability. With the development and progress of plugging technologies, new plugging materials have become a hot research topic today. However, most new plugging materials focus on polymer-based plugging materials, which have disadvantages such as high cost, environmental pollution, and poor high-temperature resistance. There are few reports on plugging materials based on expanded graphite.

[0003] Graphite, the precursor of expanded graphite, is one of the allotropes of carbon and the softest of carbon minerals. It has a layered structure formed by the hexagonal network planes of carbon. Since each carbon atom is connected to a P electron in the form of a large bond, the atomic structure in the same plane is very stable, while the carbon layers in the vertical direction are combined by van der Waals forces with a bond energy of 16.7KJ / mol, and the bonding force is weak. Therefore, under certain conditions, intercalated substances can easily enter to form graphite interlayer compounds, which provides a prerequisite for the preparation of expandable graphite. As a new type of plugging material, expandable graphite can be added to the high-temperature drilling fluid formula to allow it to enter the high-temperature formation with the drilling fluid. When a certain temperature is reached, the molecules or ions inserted into the expandable graphite interlayer can be instantly vaporized to produce gas, which can expand the expandable graphite with a layered structure, greatly expanding the interlayer spacing of the expandable graphite, and then forming a porous and high-specific surface area worm-like structure - expanded graphite. It can expand 100-200 times more than the original at a certain temperature to achieve the plugging effect of plugging the pore throat; not only that, the addition of expandable graphite particles can also change the apparent viscosity and other parameters of the drilling fluid, improve the plugging performance of the drilling fluid, and optimize the lubricity of the drilling fluid, improve the chip carrying capacity of the drilling fluid, and extend the life of the drill bit and drill pipe. However, most expandable graphites usually have a minimum expansion temperature of up to 400°C, and the formation temperature in the current high-temperature drilling process is mostly between 180°C and 240°C. Ordinary expandable graphites cannot be well adapted to the drilling fluid system. In addition, expandable graphite has the properties of oleophilicity and hydrophobicity, and its interface adhesion with polymers is poor, which is not conducive to dispersion in a water-based drilling fluid system with water as the dispersed phase. If it has poor dispersion in the drilling fluid, it is easy to produce a popcorn effect.

[0004] At present, the research on reducing the initial expansion temperature of expandable graphite is very mature, but the poor dispersibility of expandable graphite limits its application in the field of drilling fluid. The graphite after oxidation intercalation not only has the property of expansion, but also contains a large number of ketones, hydroxyls and carboxyl groups on its surface after oxidation intercalation. Among them, the carbon-oxygen functional groups in expandable graphite mainly exist in the form of single bonds. If it is not modified, the hydrogen bonds between the surface hydroxyls will act and aggregate, which will make the interface compatibility of expandable graphite and drilling fluid base slurry worse.

[0005] Therefore, providing a plugging material with good hydrophilicity, good dispersibility in water-based drilling fluid and high temperature resistance is of great significance for plugging projects in drilling projects. Summary of the invention

[0006] The object of the present invention is to overcome the problems of high cost and unsatisfactory plugging effect existing in the existing plugging materials for water-based drilling fluids, and to provide a low-temperature expandable modified graphite, its preparation method and application, as well as a water-based drilling fluid.

[0007] To achieve the above object, in the first aspect of the present invention, a preparation method of low-temperature expandable graphite is provided, and the method includes:

[0008] (1) Natural flake graphite and a pre-oxidizing agent are contacted for a pre-oxidation reaction to obtain pre-oxidized graphite;

[0009] (2) The pre-oxidized graphite, a main intercalating agent and a main oxidizing agent are contacted for an oxidation intercalation reaction, and after a period of time, an auxiliary oxidizing agent is added to obtain low-temperature expandable graphite;

[0010] In the second aspect of the present invention, a low-temperature expandable modified graphite prepared by the method described in the first aspect is provided.

[0011] In the third aspect of the present invention, the expansion effect of the low-temperature expandable modified graphite described in the second aspect and its application as a plugging agent in a water-based drilling fluid are provided.

[0012] In the fourth aspect of the present invention, a water-based drilling fluid is provided, and the water-based drilling fluid contains the low-temperature expandable modified graphite described in the second aspect.

[0013] Through the above technical solutions, the present invention can achieve the following beneficial effects:

[0014] (1) The low-temperature expandable modified graphite prepared by the method provided by the present invention has a particle size range of 80-110 μm, good hydrophilicity, good dispersibility in a water-based drilling fluid, and a small filtration loss, and can effectively plug cracks with corresponding pore sizes.

[0015] (2) The water-based drilling fluid provided by the present invention contains the above-mentioned, and the water-based drilling fluid has good rheological properties, low filtration loss, good plugging effect, and strong adaptability, and is a very potential plugging material. Specific Embodiments

[0016] For the endpoints and any values disclosed in this article, they are not limited to the precise range thresholds. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed in this article.

[0017] The following is a detailed description of the specific embodiments of the present invention. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0018] In the first aspect of the present invention, a preparation method of low-temperature expandable graphite is provided, and the method includes:

[0019] (1) Contacting natural flake graphite with a pre-oxidant to carry out a pre-oxidation reaction to obtain pre-oxidized graphite;

[0020] (2) Contacting the pre-oxidized graphite, a main intercalating agent and a main oxidant to carry out an oxidation intercalation reaction, and adding an auxiliary intercalating agent after a period of time to obtain low-temperature expandable graphite;

[0021] (3) Modifying the low-temperature expandable graphite with a modifier, and then polymerizing the modified low-temperature expandable graphite with a surfactant using an initiator to obtain low-temperature expandable modified graphite.

[0022] According to the present invention, in step (1), the natural flake graphite is contacted with a pre-oxidant to carry out a pre-oxidation reaction. The pre-oxidant pre-oxidizes the natural flake graphite through oxidation to remove impurities in the natural flake graphite, and then a main oxidant and a main intercalating agent are added. The main oxidant oxidizes the edges of the lamellar graphite, expands the graphite layer spacing, and at the same time enables intercalating agent ions and molecules to enter the graphite carbon layer to obtain graphite intercalation oxide. After completing the above steps, the low-temperature expandable graphite is modified with a silane coupling agent and polymerized with dimethylacrylamidodimethylpropanesulfonic acid to obtain low-temperature expandable modified graphite.

[0023] According to the present invention, the natural flake graphite preferably uses natural flake graphite. Preferably, the natural flake graphite has a particle size of 300 μm and a purity greater than 99%.

[0024] According to the present invention, the pre-oxidant is selected as a hydrogen peroxide solution.

[0025] According to the present invention, preferably, the main intercalating agent is selected from at least one of perchloric acid, periodic acid, nitric acid, and phosphoric acid, and is further preferably perchloric acid and nitric acid. Among them, the weight ratio of nitric acid to perchloric acid is 1:(58 - 64), and is further preferably 1:60.

[0026] According to the present invention, preferably, the main oxidant is selected from at least one of dinitrogen pentoxide, potassium permanganate, and potassium perchlorate, and is further preferably potassium permanganate.

[0027] Preferably, the auxiliary intercalating agent is selected from at least one of formic acid, glacial acetic acid, and hypochlorous acid, and is further preferably glacial acetic acid.

[0028] According to the present invention, in step (1), in order to obtain a better pre-oxidation effect on natural flake graphite, preferably, the weight ratio of the natural flake graphite to the pre-oxidant is 1:(1 - 4), and more preferably 1:(2 - 3).

[0029] According to the present invention, in step (1), the pre-oxidation reaction is carried out at a temperature of 25°C. In the reaction system composed of the natural flake graphite and the pre-oxidant, the pre-oxidation reaction of graphite occurs at the temperature required for the pre-oxidation reaction. During this process, the pre-oxidant removes impurities in the natural flake graphite to the greatest extent. Therefore, it should be placed at the temperature required for the pre-oxidation reaction for a sufficient long time until the reaction is complete.

[0030] According to the present invention, in step (1), the operation process of the pre-oxidation reaction includes: at room temperature, mixing natural flake graphite and hydrogen peroxide solution according to a weight ratio of 1:2, stirring at a speed of 500r / min for 2h, and then placing it in a constant temperature oven at 25°C for static placement for 6 - 8h while ensuring sealing.

[0031] In the present invention, the pre-oxidation reaction treatment is centrifugal water washing. Specifically, mixing the product containing low-temperature expandable graphite with deionized water, shaking well to ensure that the low-temperature expandable graphite is relatively evenly dispersed in water, adjusting the low-speed centrifuge to gear 2, centrifuging at a speed of 5000r / min for 15min, repeating the above operation 4 - 5 times, and drying at 60°C for (6 - 8h) after centrifugation is completed.

[0032] According to the present invention, in step (2), the pre-oxidized graphite, the main intercalating agent and the main oxidizing agent are contacted to carry out an oxidation intercalation reaction, and after a period of time, an auxiliary intercalating agent is added to obtain low-temperature expandable graphite;

[0033] According to the present invention, in step (2), preferably, according to the present invention, preferably, the main intercalating agent is selected from at least one of perchloric acid, periodic acid, nitric acid, and phosphoric acid, and more preferably perchloric acid and nitric acid.

[0034] Preferably, the main oxidizing agent is selected from at least one of dinitrogen pentoxide, potassium permanganate, and potassium perchlorate, and more preferably potassium permanganate.

[0035] In the present invention, the nitric acid refers to a nitric acid solution with a concentration of not less than 68wt%.

[0036] According to the present invention, in step (2), in order to obtain a better oxidation intercalation effect, preferably, the weight ratio of the pre-oxidized graphite: the main intercalating agent: the main oxidizing agent: the auxiliary intercalating agent is 1:(2 - 10):(4 - 8):(0.8 - 1.2), and more preferably 1:(5.5 - 6.5):(4 - 6):(0.9:1.1).

[0037] According to the present invention, in step (2), the conditions for the oxidation intercalation reaction include: the temperature is 30 - 60 °C, further preferably 40 °C, and the time is 30 min.

[0038] According to the present invention, in step (2), the operation process of the oxidation intercalation reaction includes: adding the pre-oxidized graphite into a mixed solution of a main intercalating agent and a main oxidizing agent, wherein the weight ratio of the main intercalating agent nitric acid to perchloric acid is 1:60, stirring at a rotation speed of 500 r / min until the solid particles are uniformly dispersed in the solution, and then placing the mixed solution in a constant temperature water bath for reaction. After the reaction proceeds for a period of time, an auxiliary intercalating agent glacial acetic acid is added, and the constant temperature water bath is continued at 40 °C for 30 min until the reaction ends.

[0039] According to a preferred embodiment of the present invention, in step (2), preferably, while stirring the mixed solution of the intercalating agent and the oxidizing agent with a glass rod, the oxidized graphite is slowly added, avoiding adhesion to the cup wall, which may lead to a decrease in the yield. Preferably, each time a reagent is added, it needs to be fully stirred for at least 30 min, and the addition time of the auxiliary intercalating agent is controlled within 5 min to avoid the influence of temperature change on the intermediate product and the experimental results.

[0040] According to the present invention, after the oxidation intercalation reaction in step (2), a second product system of low-temperature expandable graphite is obtained. After the reaction ends, the product is centrifuged to ensure complete washing and removal of the strong acid solution (including perchloric acid, nitric acid, potassium permanganate, glacial acetic acid remaining after incomplete reaction during the oxidation intercalation reaction, and pre-oxidized graphite that has not been completely reacted), and low-temperature expandable graphite is obtained.

[0041] In the present invention, after the oxidation intercalation reaction, the product is centrifuged. Specifically, the product containing low-temperature expandable graphite is mixed with deionized water, shaken well to ensure that the low-temperature expandable graphite is relatively uniformly dispersed in water, the low-speed centrifuge is adjusted to gear 2, and centrifuged at a rotation speed of 5000 r / min for 15 min. The above operation is repeated until the pH of the supernatant is 7. After centrifugation, it is dried at 60 °C for (6 - 8 h). Among them, the lower layer solid is the low-temperature expandable graphite solid powder, and the upper layer supernatant is the impurity.

[0042] According to the present invention, in step (3), the low-temperature expandable graphite is first modified by a modifier in an acidic environment, and then under the action of an initiator, a free radical polymerization is carried out between the surfactant and the modified low-temperature expandable graphite to obtain low-temperature expandable modified graphite.

[0043] According to the present invention, in step (3), preferably, the modifier is selected from at least one of silane coupling agents KH-550, KH-560, and KH-570, and further preferably the silane coupling agent KH-550.

[0044] Preferably, the initiator is selected from at least one of ammonium persulfate, potassium persulfate, and sodium persulfate, and more preferably ammonium persulfate.

[0045] Preferably, the surfactant is selected from diallylamidodimethylpropylsulfonic acid, sodium styrenesulfonate, and N,N-dimethylacrylamide, and more preferably diallylamidodimethylpropylsulfonic acid.

[0046] According to the present invention, in step (3), in order to obtain low-temperature expandable modified graphite with the best performance, preferably, the weight ratio of the low-temperature expandable graphite: silane coupling agent KH-550: diallylamidodimethylpropylsulfonic acid: ammonium persulfate is 1: (0.1-0.3): (4-10): (30-70), and more preferably 1: 0.2: (5-7): (40-60).

[0047] According to the present invention, in step (3), the operation process of the modification process includes: adding the low-temperature expandable graphite into an ethanol solution, adjusting the pH to about 4 with hydrochloric acid at a rotation speed of 250 r / min, adding a corresponding weight ratio of silane coupling agent, and carrying out a water bath reaction at 70 °C for 7 h to obtain expandable graphite modified with silane coupling agent. Subsequently, the modified expandable graphite and ethanol solution are mixed according to the weight ratio, diallylamidodimethylpropylsulfonic acid and deionized water are mixed, the pH value is adjusted to 7 with an aqueous sodium hydroxide solution, the two solutions are mixed, and ammonium persulfate is added at 70 °C under a nitrogen atmosphere and reacted for 8 h.

[0048] In the present invention, the sodium hydroxide solution refers to a self-prepared aqueous sodium hydroxide solution with a concentration of 0.1 mol / L.

[0049] According to a preferred embodiment of the present invention, in step (3), a four-necked flask is used for the polymerization reaction. One opening is for passing nitrogen, one opening is for inserting a thermometer to ensure that the temperature does not change significantly, one opening is for slowly adding the ammonium persulfate initiator, and the remaining opening has a slight slit during the nitrogen passing process to facilitate the overflow of oxygen in the bottle. After stopping the nitrogen passing, it is tightened to ensure that the bottle is completely in a nitrogen atmosphere to avoid the reaction of the persulfate radical in the ammonium persulfate initiator with oxygen and loss of activity.

[0050] The second aspect of the present invention provides low-temperature expandable modified graphite prepared by the method described in the first aspect above, wherein the particle size range of the low-temperature expandable modified graphite is 80-110 μm, and it meets the plugging requirements for fractures in the formation with a size of 100-300 μm.

[0051] According to the present invention, the low-temperature expandable modified graphite has strong hydrophilicity and can be well dispersed in the water-based drilling fluid. The positively charged sulfonic groups in the low-temperature expandable modified graphite can form ionic bonds with the bentonite in the water-based drilling fluid, and the amide groups and sulfonic groups form hydrogen bonds with the main component montmorillonite in the bentonite, forming an adsorption film on the surface of the bentonite in the water-based drilling fluid to prevent free water from increasing the filtration loss by passing through.

[0052] According to the present invention, the low-temperature expandable modified graphite can increase the viscosity and shear force of the water-based drilling fluid, improve the ability of the drilling fluid to carry cuttings in the well, and has good compatibility with the drilling fluid and strong adaptability.

[0053] The third aspect of the present invention provides the application of the low-temperature expandable modified graphite described in the second aspect as a lost circulation material in the water-based drilling fluid.

[0054] The fourth aspect of the present invention provides a water-based drilling fluid containing the low-temperature expandable modified graphite described in the second aspect.

[0055] According to the present invention, the water-based drilling fluid comprises the following components: 100 parts by weight of water, 6-9 parts by weight of bentonite, 0.2-0.5 parts by weight of sodium carbonate, 7-10 parts by weight of high-temperature resistant filtration reducer, 5-8 parts by weight of high-temperature resistant surfactant, 7-10 parts by weight of high-temperature protection agent, adding 6-9 parts by weight of the low-temperature expandable modified graphite, and 100 parts by weight of weighting material.

[0056] Preferably, the high-temperature resistant filtration reducer is selected from at least one of sulfomethylated phenolic resin, sulfonated lignite, hydrolyzed polyacrylonitrile, and sulfonated tannin, and further preferably sulfonated lignite.

[0057] Preferably, the high-temperature resistant surfactant is selected from at least one of Span 80, Tween 80, and Tween 40, and further preferably Tween 80.

[0058] Preferably, the high-temperature protection agent is selected from one of sodium polyacrylate, potassium polyacrylate, and ammonium polyacrylate, and further preferably potassium polyacrylate.

[0059] Preferably, the weighting material is a mixture of API barite and ultrafine barite, and the weight ratio of API barite to ultrafine barite is 1:(1-10), and further preferably 1:(1.5:9)

[0060] According to the present invention, preferably, the weighting agent is selected from at least one of API barite, ultrafine barium sulfate, ultramicro barite, magnetite powder, ilmenite powder, and manganese tetraoxide, and further preferably a compound of API barite and ultramicro barite, with a weight ratio of 1:(1-10), and further preferably 1:(1.5:9).

[0061] The water-based drilling fluid provided by the present invention has excellent filtration loss reduction ability, good rheological properties, strong adaptability, and can greatly improve the wellbore stability ability.

[0062] The present invention will be described in detail below through examples. In the following preparation examples, examples, comparative examples, and test examples,

[0063] Preparation Example 1

[0064] (1-1) Put natural flake graphite (300 μm, purity 99%) and hydrogen peroxide solution (concentration 30% wt) into a beaker according to a weight ratio of 1:2, wrap the bottle mouth with plastic wrap, stir for 2 h, and leave it to stand in an incubator at 25 °C for 6-8 h. The obtained product is mixed with deionized water, shaken well to ensure that the low-temperature expandable graphite is relatively evenly dispersed in water. Adjust the low-speed centrifuge to gear 2, centrifuge at a speed of 5000 r / min for 15 min, repeat the above operation 4-5 times, and after centrifugation, dry it at 60 °C for (6-8 h).

[0065] (1-2) React by adding pre-oxidized graphite: perchloric acid: nitric acid: potassium permanganate: glacial acetic acid in a weight ratio of 1:5.9:0.1:5:1 into the beaker in sequence. First, add the pre-oxidized graphite into the mixed solution of potassium permanganate and perchloric acid, seal the bottle mouth with plastic wrap, stir for 30 min, then react in a water bath at 40 °C for 30 min, and then add glacial acetic acid to continue the reaction for 30 min until the reaction ends.

[0066] (1-3) Mix the product containing low-temperature expandable graphite with deionized water, shake well to ensure that the low-temperature expandable graphite is relatively evenly dispersed in water. Adjust the low-speed centrifuge to gear 2, centrifuge at a speed of 5000 r / min for 15 min, repeat the above operation until the pH of the supernatant is 7, and after centrifugation, dry it at 60 °C for (6-8 h).

[0067] (1-4) Add the low-temperature expandable graphite into 300 ml of ethanol solution, adjust the pH to about 4 with hydrochloric acid at a speed of 250 r / min, add the corresponding weight ratio of silane coupling agent KH-550, and react in a water bath at 70 °C for 7 h to obtain silane coupling agent-modified expandable graphite. Subsequently, mix the modified expandable graphite with 10 g of ethanol solution according to the weight ratio respectively, mix diacrylamidodimethylpropylsulfonic acid with 80 ml of deionized water, adjust the pH value to 7 with 0.1 mol / L sodium hydroxide aqueous solution, mix the two solutions, and react at 70 °C for 8 h in a nitrogen environment. Among them, the weight ratio of low-temperature expandable graphite: silane coupling agent kh-550: diacrylamidodimethylpropylsulfonic acid: ammonium persulfate is 1:0.2:6:50.

[0068] (1 - 5) Mix the low - temperature expandable modified graphite with deionized water, shake well to ensure its relatively uniform dispersion in water. Adjust the low - speed centrifuge to gear 2, centrifuge at a speed of 5000 r / min for 15 min, and repeat the above operation until the pH of the supernatant is 7. After centrifugation, conduct drying treatment at 60 °C for (20 - 24 h) to obtain low - temperature expandable modified graphite (denoted as S1).

[0069] Preparation Example 2

[0070] (2 - 1) Put natural flake graphite (300 μm, purity 99%) and hydrogen peroxide solution (concentration 30% wt) into a beaker in a weight ratio of 1:2, wrap the mouth of the beaker with plastic wrap, stir for 2 h, and let it stand in a constant - temperature oven at 25 °C for 6 - 8 h. Mix the obtained product with deionized water, shake well to ensure the relatively uniform dispersion of low - temperature expandable graphite in water. Adjust the low - speed centrifuge to gear 2, centrifuge at a speed of 5000 r / min for 15 min, and repeat the above operation 4 - 5 times. After centrifugation, conduct drying treatment at 60 °C for (6 - 8 h);

[0071] (2 - 2) React by adding pre - oxidized graphite, perchloric acid, nitric acid, potassium permanganate, and glacial acetic acid into the beaker in a weight ratio of 1:5.4:0.1:4:0.9 successively. First, add the pre - oxidized graphite into the mixed solution of potassium permanganate and perchloric acid, seal the mouth of the beaker with plastic wrap, stir for 30 min, then react in a water bath at 40 °C for 30 min, and then add glacial acetic acid to continue the reaction for 30 min until the reaction ends.

[0072] (2 - 3) Mix the product containing low - temperature expandable graphite with deionized water, shake well to ensure the relatively uniform dispersion of low - temperature expandable graphite in water. Adjust the low - speed centrifuge to gear 2, centrifuge at a speed of 5000 r / min for 15 min, and repeat the above operation until the pH of the supernatant is 7. After centrifugation, conduct drying treatment at 60 °C for (6 - 8 h).

[0073] (2 - 4) Add the low - temperature expandable graphite into 300 ml of ethanol solution, adjust the pH to about 4 with hydrochloric acid at a speed of 250 r / min, add the corresponding weight - ratio silane coupling agent, and react in a water bath at 70 °C for 7 h to obtain silane - coupling - agent - modified expandable graphite. Subsequently, mix the modified expandable graphite with 10 g of ethanol solution respectively by weight ratio. Mix diacrylamidodimethylpropylsulfonic acid with 75 ml of deionized water, adjust the pH value to 7 with 0.1 mol / L sodium hydroxide aqueous solution, mix the two solutions, and add ammonium persulfate in a nitrogen environment and react at 70 °C for 8 h. Among them, the weight ratio of low - temperature expandable graphite: silane coupling agent kh - 550: diacrylamidodimethylpropylsulfonic acid: ammonium persulfate is 1:0.2:5:40.

[0074] (2-5) Mix the low-temperature expandable modified graphite with deionized water, shake well to ensure its relatively uniform dispersion in water, adjust the low-speed centrifuge to gear 2, centrifuge at a speed of 5000 r / min for 15 min, and repeat the above operation until the pH of the supernatant is 7. After centrifugation, dry at 60 °C for (20-24 h) to obtain low-temperature expandable modified graphite (denoted as S2).

[0075] Preparation Example 3

[0076] (3-1) Put natural flake graphite (300 μm, purity 99%) and hydrogen peroxide solution (concentration 30% wt) into a beaker according to a weight ratio of 1:2, wrap the mouth of the bottle with plastic wrap and stir for 2 h, and let it stand in a constant temperature oven at 25 °C for 6-8 h. Mix the obtained product with deionized water, shake well and ensure that the low-temperature expandable graphite is relatively uniformly dispersed in water. Adjust the low-speed centrifuge to gear 2, centrifuge at a speed of 5000 r / min for 15 min, and repeat the above operation 4-5 times. After centrifugation, dry at 60 °C for (6-8 h);

[0077] (3-2) React by adding pre-oxidized graphite: perchloric acid: nitric acid: potassium permanganate: glacial acetic acid in a weight ratio of 1:6.4:0.1:6:1.1 to the beaker in sequence. First, add the pre-oxidized graphite to the mixed solution of potassium permanganate and perchloric acid, seal the mouth of the bottle with plastic wrap, stir for 30 min, then react in a water bath at 40 °C for 30 min, and then add glacial acetic acid and continue to react for 30 min until the reaction ends.

[0078] (3-3) Mix the product containing low-temperature expandable graphite with deionized water, shake well and ensure that the low-temperature expandable graphite is relatively uniformly dispersed in water. Adjust the low-speed centrifuge to gear 2, centrifuge at a speed of 5000 r / min for 15 min, and repeat the above operation until the pH of the supernatant is 7. After centrifugation, dry at 60 °C for (6-8 h).

[0079] (3-4) Add the low-temperature expandable graphite to 300 ml of ethanol solution, adjust the pH to about 4 with hydrochloric acid at a speed of 250 r / min, add the corresponding weight ratio of silane coupling agent, and react in a water bath at 70 °C for 7 h to obtain silane coupling agent-modified expandable graphite. Subsequently, mix the modified expandable graphite with 10 g of ethanol solution according to the weight ratio respectively. Mix diacrylamidodimethylpropylsulfonic acid with 75 ml of deionized water, adjust the pH value to 7 with 0.1 mol / L sodium hydroxide aqueous solution, mix the two solutions and react at 70 °C for 8 h in a nitrogen environment. Among them, the weight ratio of low-temperature expandable graphite: silane coupling agent kh-550: diacrylamidodimethylpropylsulfonic acid: ammonium persulfate is 1:0.2:7:60.

[0080] (3-5) Mix the low-temperature expandable modified graphite with deionized water, shake well to ensure its relatively uniform dispersion in water, adjust the low-speed centrifuge to gear 2, centrifuge at a speed of 5000 r / min for 15 min, repeat the above operation until the pH of the supernatant is 7, and after centrifugation, dry at 60 °C for (20-24 h) to obtain low-temperature expandable modified graphite (denoted as S3).

[0081] Example 1

[0082] Take 100 parts by weight of water, mix it with 8 parts by weight of bentonite and 0.3 parts by weight of sodium carbonate, stir at a stirring rate of 1500 r / min for 30 min, then add 9 parts by weight of sulfonated lignite and stir at a stirring rate of 2000 r / min for 10 min, then add 9 parts by weight of potassium polyacrylate and stir at a stirring rate of 2000 r / min for 10 min, then add 7 parts by weight of Tween 80 and stir at a stirring rate of 2000 r / min for 10 min. Add 70 parts by weight of API barite and 30 parts by weight of ultrafine barite in 5-8 times and stir at a stirring rate of 2000 r / min for 30 min until there is no precipitate at the bottom of the high-speed stirring cup. Finally, add 6 parts by weight of plugging agent S1 and stir at a stirring rate of 2000 r / min for 10 min to obtain a water-based drilling fluid (denoted as F1).

[0083] The components and contents of F1 are shown in Table 1.

[0084] Examples 2-3

[0085] According to the method of Example 1, the difference is that S2 and S3 are respectively used as plugging agents, and other conditions are the same as those in Example 1. Water-based drilling fluids are obtained (denoted as F2 and F3 respectively).

[0086] Example 4

[0087] Take 100 parts by weight of water, mix it with 6 parts by weight of bentonite and 0.2 parts by weight of sodium carbonate, stir at a stirring rate of 1500 r / min for 30 min, then add 7 parts by weight of sulfonated lignite and stir at a stirring rate of 2000 r / min for 10 min, then add 7 parts by weight of potassium polyacrylate and stir at a stirring rate of 2000 r / min for 10 min, then add 5 parts by weight of Tween 80 and stir at a stirring rate of 2000 r / min for 10 min. Add 60 parts by weight of API barite and 40 parts by weight of ultrafine barite in 5-8 times and stir at a stirring rate of 2000 r / min for 30 min until there is no precipitate at the bottom of the high-speed stirring cup. Finally, add 7 parts by weight of plugging agent S2 and stir at a stirring rate of 2000 r / min for 10 min to obtain a water-based drilling fluid (denoted as F4).

[0088] The components and contents of F4 are shown in Table 1.

[0089] Examples 5 - 6

[0090] According to the method of Example 4, the difference is that S2 and S3 are respectively used as the lost circulation materials, and other conditions are the same as those in Example 4. Water-based drilling fluids (denoted as F5 and F6 respectively) are obtained.

[0091] Example 7

[0092] Take 100 parts by weight of water, mix it with 9 parts by weight of bentonite and 0.5 part by weight of sodium carbonate, stir at a stirring rate of 1500 r / min for 30 min, then add 10 parts by weight of sulfonated lignite and stir at a stirring rate of 2000 r / min for 10 min, then add 10 parts by weight of potassium polyacrylate and stir at a stirring rate of 2000 r / min for 10 min, then add 8 parts by weight of Tween 80 and stir at a stirring rate of 2000 r / min for 10 min. Add 80 parts by weight of API barite and 20 parts by weight of ultrafine barite in 5 - 8 times and stir at a stirring rate of 2000 r / min for 30 min until there is no precipitate at the bottom of the high-speed stirring cup. Finally, add 7 parts by weight of the lost circulation material S3 and stir at a stirring rate of 2000 r / min for 10 min to obtain a water-based drilling fluid (denoted as F7).

[0093] The components and contents of F7 are shown in Table 1.

[0094] Examples 8 - 9

[0095] According to the method of Example 7, the difference is that S2 and S3 are respectively used as the lost circulation materials, and other conditions are the same as those in Example 7. Water-based drilling fluids (denoted as F8 and F9 respectively) are obtained.

[0096] Example 10

[0097] Take 100 parts by weight of water, mix it with 8 parts by weight of bentonite and 0.3 part by weight of sodium carbonate, stir at a stirring rate of 1500 r / min for 30 min, then add 9 parts by weight of sulfonated lignite and stir at a stirring rate of 2000 r / min for 10 min, then add 9 parts by weight of potassium polyacrylate and stir at a stirring rate of 2000 r / min for 10 min, then add 7 parts by weight of Tween 80 and stir at a stirring rate of 2000 r / min for 10 min. Add 90 parts by weight of API barite and 10 parts by weight of ultrafine barite in 5 - 8 times and stir at a stirring rate of 2000 r / min for 30 min until there is no precipitate at the bottom of the high-speed stirring cup. Finally, add 7 parts by weight of the lost circulation material S1 and stir at a stirring rate of 2000 r / min for 10 min to obtain a water-based drilling fluid (denoted as F10).

[0098] The components and contents of F10 are shown in Table 1.

[0099] Examples 11 - 12

[0100] According to the method of Example 10, the difference is that S2 and S3 are respectively used as the lost circulation material, and other conditions are the same as those in Example 10. Water-based drilling fluids (denoted as F11 and F12 respectively) are obtained.

[0101] Comparative Example 1

[0102] According to the method of Example 1, the difference is that the lost circulation material S1 is not added, and other conditions are the same as those in Example 1. A water-based drilling fluid (denoted as DF1) is obtained.

[0103] Comparative Example 2

[0104] According to the method of Example 1, the difference is that the low-temperature expandable graphite prepared by step (2 - 2) of Preparation Example 1 is used as the lost circulation material, and other conditions are the same as those in Example 1. A water-based drilling fluid (denoted as DF2) is obtained.

[0105] Comparative Example 3

[0106] According to the method of Example 2, the difference is that the low-temperature expandable graphite prepared by step (2 - 2) of Preparation Example 2 is used as the lost circulation material, and other conditions are the same as those in Example 2. A water-based drilling fluid (denoted as DF3) is obtained.

[0107] Comparative Example 4

[0108] According to the method of Example 3, the difference is that the low-temperature expandable graphite prepared by step (2 - 2) of Preparation Example 3 is used as the lost circulation material, and other conditions are the same as those in Example 3. A water-based drilling fluid (denoted as DF4) is obtained.

[0109] Comparative Example 5

[0110] According to the method of Example 1, the difference is that S1 is replaced by natural flake graphite, and other conditions are the same as those in Example 1. A water-based drilling fluid (denoted as DF5) is obtained.

[0111] Comparative Example 6

[0112] According to the method of Example 1, the difference is that S1 is replaced by micron-sized calcium carbonate, a common laboratory lost circulation material, and other conditions are the same as those in Example 1. A water-based drilling fluid (denoted as DF6) is obtained.

[0113] Comparative Example 7

[0114] According to the method of Example 1, the difference is that the addition amount of S1 is adjusted to 15 parts by weight, and other conditions are the same as those in Example 1. A water-based drilling fluid (denoted as DF7) is obtained.

[0115] Table 1

[0116]

[0117] Table 2

[0118]

[0119]

[0120] Note: In Table 2, the graphene pre-products used for DF2 - DF4 are the low-temperature expandable graphite prepared in step (2 - 2) of Preparation Examples 1 - 3 respectively.

[0121] Test Example

[0122] The expansion volume of the nano-graphene products S1 - S3 prepared in Preparation Examples 1 - 3 was tested to evaluate their expansion performance, and the plugging performance of the water-based drilling fluids F1 - F12 prepared in Examples 1 - 12 and the water-based drilling fluids DF1 - DF7 prepared in Comparative Examples 1 - 7 was tested. According to GB / T 16783.1 - 2014 "Petroleum and natural gas industries - Drilling fluids - Field testing - Part 1: Water-based drilling fluids", the high-temperature-resistant drilling fluids prepared in Examples 1 - 12 and Comparative Examples 1 - 7 were evaluated. In the following test examples,

[0123] The apparent viscosity (AV, mPa·s), plastic viscosity (PV, mPa·s), and yield point (YP, Pa) were measured using a six-speed rotational viscometer according to the method specified in GB / T 29170 - 2012;

[0124] The medium-pressure filtration loss (API, mL) was measured using a medium-pressure filtration loss instrument according to the method specified in GB / T 29170 - 2012;

[0125] The high-temperature and high-pressure filtration loss (HTHP, mL) was measured using a high-temperature and high-pressure filtration loss instrument according to the method specified in GB / T 29170 - 2012;

[0126] The manufacturer of the six-speed rotational viscometer is Sidelai Platinum Co., Ltd., model ZNN-D6S

[0127] The manufacturer of the medium-pressure filtration loss instrument is Sidelai Platinum Co., Ltd., model SD4 / SD4A;

[0128] The manufacturer of the high-temperature and high-pressure filtration loss instrument is KenCe Instruments Co., Ltd., model KC-GS173.

[0129] 1. Expansion Performance Test

[0130] The S1 - S3 obtained from the preparation examples were placed in an air environment and a water environment respectively to test their expansion performance, and the expansion volume at temperatures of 180 °C, 200 °C, and 220 °C was tested respectively.

[0131] Table 3

[0132]

[0133]

[0134] Note: Vair and Vwater in Table 3 are the expansion volumes of S1-S3 in water environment and air environment respectively.

[0135] It can be seen from Table 3 that the expansion volume of S1-S3 is very small at 180°C, and the expansion volume gradually increases as the temperature rises. This is because more and more intercalants decompose at high temperatures to produce more gas, which expands the graphite sheets and increases the graphite volume. It can also be seen that the expansion volume of S1-S3 in water is slightly lower than that in the air environment. This is because the pressure of the external water limits the expansion of the graphite, but this will not only not reduce the performance of the drilling fluid, but also avoid the excessive expansion of the expanded graphite, resulting in excessive softness of the expanded graphite and a decrease in its pressure bearing capacity. The low-temperature expandable modified graphite can ensure that it has the ability to expand in water without excessively reducing its pressure bearing capacity.

[0136] 2. Leakage plugging performance test

[0137] A self-made micro-crack plugging simulation device is used to more accurately evaluate the plugging effect. During the plugging evaluation experiment, a self-made simulated crack device is used in combination with a high-temperature and high-pressure filter loss instrument. Specifically, after the micro-crack simulation rigid crack plate is assembled, it is installed at the lower end of the inner cylinder of the GGS42 high-temperature and high-pressure filter loss instrument kettle, and then the lower part of the tank is installed according to the normal steps. The crack width is calibrated with a standard thickness gauge aluminum foil, and then the prepared drilling fluid slurry is poured into the tank. Among them, the crack width of the micro-crack device is set to 150μm, 220μm, and 300μm, respectively, and the water-based drilling fluids F1-F12 prepared in Examples 1-12 and the water-based drilling fluids DF1-DF7 prepared in Comparative Examples 1-7 are tested at a temperature of 200°C and a pressure difference of 3.5MPa to test their filter loss. The filter loss at 100μm, 150μm, and 220μm is recorded as FL1, FL2, and FL3, respectively.

[0138] Table 4 Plugging performance test of water-based drilling fluids F1-F12 prepared in Examples 1-12 and water-based drilling fluids DF1-DF7 prepared in Comparative Examples 1-7

[0139] <![CDATA[FL1 / mL]]> <![CDATA[FL2 / mL]]> <![CDATA[FL3 / mL]]> F1 9 8 6 F2 6 7 7 F3 8 7 7.9 F4 6 7 7 F5 5 5.2 8 F6 7.2 8.7 9 F7 4 6.6 8.3 F8 6 5 10 F9 7 8 15 F10 4.5 5.7 19 F11 5.7 6.7 18 F12 4.9 7 16 DF1 30 31 38 DF2 18 21 28 DF3 16 16 24.8 DF4 14.4 19 20 DF5 27.8 25.6 29 DF6 22 24 29 DF7 15 17.8 16.6

[0140] As can be seen from Table 4, the more the addition amount of low-temperature expandable modified graphite, the more obvious the plugging effect. Moreover, it can be found that the low-temperature expandable modified graphite prepared by us has the best matching effect with cracks of 100μm and 150μm. This is because the natural flake graphite used in the preparation examples is 300μm, and the particle size after modification is about 100μm. After entering the cracks and expanding, it can plug cracks slightly larger than itself. However, if the cracks are too large, the plugging effect will be lost. Considering that the plugging effect of the modified expandable graphite on 300μm cracks is not ideal because the expansion volume of the low-temperature expandable modified graphite will decrease in water, resulting in the inability to plug too large cracks. Moreover, the plugging effect of the low-temperature expandable modified graphite is significantly better than that of calcium carbonate, and is comparable to that of low-temperature expandable graphite. Since natural flake graphite does not have the expansion performance and cannot plug leaks only by virtue of its physical properties, the plugging effect is very poor.

[0141] 3. Drilling fluid performance test

[0142] Referring to GB / T16783.1-2014 "Petroleum and natural gas industries - Drilling fluids - Field testing - Part 1: Water-based drilling fluids", the high-temperature resistant drilling fluids prepared in Examples 1 to 12 and Comparative Examples 1 to 7 were evaluated.

[0143] Table 5 Test results of high-temperature resistant drilling fluid performance

[0144]

[0145]

[0146]

[0147] From the test results in Table 5, it can be seen that the high-temperature resistant water-based drilling fluids prepared in Examples 1 to 12 have good rheological properties. After aging at 220°C, the rheological properties change little, indicating that the drilling fluid system has good high-temperature resistance. Moreover, the high-temperature and high-pressure filtration loss can basically be maintained below 30 mL. For Comparative Example DF1 without adding low-temperature expandable modified graphite, it can be clearly seen that the filtration loss is higher than that of the samples with added low-temperature expandable modified graphite. In the performance tests of Examples 1 - 12, it can be seen that the addition amount of low-temperature expandable modified graphite also affects the filtration loss of the drilling fluid. As the dosage increases, the filtration loss decreases. As the addition amount of API barite in the drilling fluid formula increases, the filtration loss also decreases.

Claims

1. A preparation method of a low-temperature expandable modified graphite plugging agent, characterized in that, It includes the following steps: (1) Carry out a pre-oxidation reaction on natural flake graphite and a pre-oxidizing agent to obtain pre-oxidized graphite; (2) Contact the pre-oxidized graphite with a main intercalating agent and a main oxidizing agent to carry out an oxidation intercalation reaction. After reacting for a period of time, add an auxiliary intercalating agent to obtain low-temperature expandable graphite; the main intercalating agent is selected from at least one of perchloric acid, periodic acid, nitric acid, and phosphoric acid; the main oxidizing agent is selected from at least one of dinitrogen pentoxide, potassium permanganate, and potassium perchlorate; the auxiliary intercalating agent is selected from at least one of formic acid, glacial acetic acid, and hypochlorous acid; (3) Modify the low-temperature expandable graphite with a modifier, and then polymerize the modified low-temperature expandable graphite with a surfactant using an initiator to obtain low-temperature expandable modified graphite; the modifier is selected from at least one of silane coupling agents KH-550, KH-560, and KH-570; the surfactant is selected from at least one of diacrylamidodimethylpropylsulfonic acid, sodium styrenesulfonate, and N,N-dimethylacrylamide; the initiator is selected from at least one of ammonium persulfate, potassium persulfate, and sodium persulfate.

2. The preparation method according to claim 1, characterized in that, In step (1), the weight ratio of the natural flake graphite to the pre-oxidizing agent is 1:(2-3).

3. The preparation method according to claim 1, characterized in that, In step (2), the weight ratio of the pre-oxidized graphite to the main intercalating agent to the main oxidizing agent to the auxiliary intercalating agent is 1:(5.5-6.5):(4-6):(0.9:1.1).

4. The preparation method according to any one of claims 1-3, characterized in that, The main intercalating agent is selected as nitric acid and perchloric acid; among them, the weight ratio of the nitric acid to the perchloric acid is 1:(11-13); the main oxidizing agent is selected as potassium permanganate; the auxiliary intercalating agent is selected as glacial acetic acid.

5. The preparation method according to claim 1, wherein, In step (3), the weight ratio of the low-temperature expandable graphite to the modifier to the surfactant to the initiator is 1:0.2:(5-7):(40-60); the modifier is selected as silane coupling agent KH-550; the surfactant is selected as diacrylamidodimethylpropylsulfonic acid; the initiator is selected as ammonium persulfate.

6. The preparation method according to any one of claims 1-3, characterized in that In step (1), the conditions of the pre-oxidation reaction include: the temperature is 25°C; the washing method uses a low-speed centrifuge to wash the low-temperature expandable modified graphite with water; In step (2), the conditions of the reaction oxidation intercalation include: the temperature is 40°C; the time is 1-2 h; In step (3), the modification conditions include: the temperature is 70°C; the time is 7 h; In step (3), the polymerization conditions include: the temperature is 70°C; the time is 8 h.

7. The low-temperature expandable modified graphite prepared by the preparation method according to any one of claims 1-6, characterized in that, The particle size range of the low-temperature expandable modified graphite is 80-110 μm.

8. Application of the low-temperature expandable modified graphite according to claim 7 as a plugging agent in a water-based drilling fluid.

9. A water-based drilling fluid, characterized in that, Containing the low-temperature expandable modified graphite according to claim 7.

10. The water-based drilling fluid according to claim 9, wherein, The water-based drilling fluid comprises the following components: 100 parts by weight of water, 6-9 parts by weight of bentonite, 0.2-0.5 parts by weight of sodium carbonate, 7-10 parts by weight of high-temperature resistant filtration reducer, 5-8 parts by weight of high-temperature resistant surfactant, 7-10 parts by weight of high-temperature protector, 6-9 parts by weight of low-temperature expandable modified graphite, and 100 parts by weight of weighting material; The high-temperature resistant filtration reducer is selected as sulfonated lignite; the high-temperature resistant surfactant is selected as Tween 80; the high-temperature protector is selected as potassium polyacrylate; the weighting agent is selected as API barite and ultrafine barite; the weighting agent is a compound of API barite and ultrafine barite, and the weight ratio is 1:(1.5:9).

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