CaCO3 / polymer-based plugging agent, preparation method thereof and water-based drilling fluid

Abstract

The invention provides a CaCO3 / polymer-based plugging agent, a preparation method thereof and a water-based drilling fluid, the preparation method of the CaCO3 / polymer-based plugging agent comprises the following steps: S1, synthesizing CaCO3 in the presence of a modifier to obtain modified CaCO3; s2, taking the modified CaCO3 and a functional monomer as raw materials, adding a cross-linking agent, an initiator and a dispersing agent, and synthesizing a CaCO3 / polymer-based plugging agent by adopting a reversed-phase suspension polymerization method; wherein the functional monomers are styrene, acrylamide and N-vinyl pyrrolidone. The water-based drilling fluid based on the CaCO3 / polymer-based plugging agent is low in cost, has a good plugging effect, has temperature resistance and pressure bearing performance, and can solve the problem of well wall instability of a high-temperature well section.

Description

Technical Field

[0001] This invention belongs to the field of drilling fluid technology, and relates to micro-nano plugging agents in oilfield chemical agents and materials, specifically to a CaCO3 / polymer-based plugging agent and its preparation method, as well as a water-based drilling fluid. Background Technology

[0002] As drilling operations extend to greater depths, the scale of high-temperature deep well drilling continues to expand; wellbore instability in microfractured mudstone and shale formations is a crucial issue that urgently needs to be addressed in the current work on wellbore stabilization of high-temperature deep wells.

[0003] In high-temperature well sections, wellbore instability caused by water-based drilling fluids is usually not due to a single factor, but rather a phenomenon resulting from multiple factors including chemical and mechanical aspects. On the one hand, free water seeps into fracture channels, causing hydration of the wellbore rock and leading to instability. On the other hand, when drilling into brittle shale, stress damage and drill string impact contribute to wellbore instability. Especially when drilling into high-temperature sections, the hydration reaction between the fracture surface rock and the water-based drilling fluid filtrate is intensified under high temperatures, increasing the risk of wellbore instability. Therefore, addressing wellbore instability in high-temperature fractured formations is of great significance.

[0004] Currently, the research and development of water-based drilling fluid plugging agents is very extensive in the drilling fluid field. SiO2 / polymer-based plugging agents are plugging agents with good plugging performance, but the high cost of SiO2 keeps the cost of water-based drilling fluids high. Summary of the Invention

[0005] The purpose of this invention is to provide a CaCO3 / polymer-based plugging agent, its preparation method, and a water-based drilling fluid, which can not only solve the wellbore instability problem in high-temperature fractured formations, but also greatly reduce the cost of water-based drilling fluids.

[0006] This invention is achieved through the following technical solution:

[0007] A method for preparing a CaCO3 / polymer-based plugging agent includes the following steps:

[0008] S1, under the condition of the presence of a modifier, CaCO3 is synthesized to obtain modified CaCO3; wherein, the modifier is selected from one of oleic acid, linoleic acid, ethylene trioxane, γ-aminopropyltriethoxysilane and γ-methacryloyloxypropyltrimethoxysilane.

[0009] S2, using modified CaCO3 and functional monomers as raw materials, and adding crosslinking agent, initiator and dispersant, a reverse suspension polymerization method is used to synthesize CaCO3 / polymer-based plugging agent; wherein, the functional monomers are styrene, acrylamide and N-vinylpyrrolidone.

[0010] Preferably, S1 specifically includes: dissolving Ca(OH)2 in water to obtain a Ca(OH)2 solution; dissolving the modifier in ethanol to obtain a modifier solution; adding the modifier solution to the Ca(OH)2 solution and stirring to obtain a mixed solution; introducing CO2 into the mixed solution to carry out the reaction until the pH of the solution reaches neutral, and the reaction ends to obtain modified CaCO3.

[0011] Furthermore, the mass ratio of Ca(OH)2 to the modifier is 1:(0.05-1.12).

[0012] Preferably, S2 specifically includes: mixing liquid paraffin and a dispersant under heating and a protective atmosphere to obtain liquid paraffin containing a dispersant; mixing modified CaCO3, water, functional monomers, crosslinking agents and initiators, adding the resulting mixed solution to the liquid paraffin containing the dispersant, and carrying out a reverse suspension polymerization reaction to obtain the CaCO3 / polymer-based blocking agent.

[0013] Preferably, the mass ratio of modified CaCO3, styrene, acrylamide and N-vinylpyrrolidone is 1:(25-58):(30-55):(12-30).

[0014] Preferably, in S2, the temperature for reverse suspension polymerization is 55-70℃ and the time is 4.5-6.5h.

[0015] This invention provides a CaCO3 / polymer-based plugging agent prepared by the method described above.

[0016] The present invention provides a water-based drilling fluid comprising the CaCO3 / polymer-based plugging agent as described above.

[0017] Preferably, the water-based drilling fluid comprises the following components by weight: 100 parts water, 5-11 parts bentonite, 0.4-0.8 parts sodium carbonate, 4-9 parts viscosity reducer, 5-9 parts filtration reducer, 10-15 parts inhibitor, 4-8 parts alkalinity adjuster, 28-42 parts weighting agent, and 2-5 parts CaCO3 / polymer-based plugging agent.

[0018] Furthermore, the viscosity reducer is one of tannin alkali solution, iron-chromium lignin sulfonate, tannic acid, and tannin.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] The present invention describes a method for preparing CaCO3 by synthesizing modified CaCO3 in the presence of a modifier. In-situ modification of the modified CaCO3 imparts more hydroxyl groups, enabling it to be grafted with polymers to form a CaCO3 / polymer-based plugging agent. The rigidity of CaCO3 provides the CaCO3 / polymer-based plugging agent with a certain pressure-bearing capacity. The polymer formed by the polymerization of styrene, acrylamide, and N-vinylpyrrolidone exhibits excellent water absorption and swelling properties, which significantly increase with rising temperature. The presence of this polymer allows the CaCO3 / polymer-based plugging agent to transform into a highly elastic state above its glass transition temperature. Therefore, when the CaCO3 / polymer-based plugging agent enters a formation above its glass transition temperature, it can transform into a highly elastic state and penetrate fractures much smaller than its particle size. Furthermore, due to its water absorption and swelling properties, it can absorb water and swell upon entering the formation, deformably plugging the fractures, reducing their collapse pressure, and thus maintaining wellbore stability. Furthermore, the presence of benzene rings in styrene can enhance the pressure resistance of the plugging agent, and the addition of a certain amount of styrene can improve steric hindrance and increase the flexibility of the polymer chain, giving it a certain degree of deformation capability; introducing N-vinylpyrrolidone into the polymer backbone can improve the backbone stability. The CaCO3 / polymer-based plugging agent prepared by the method of this invention has good compatibility with water-based drilling fluids.

[0021] This invention relates to a water-based drilling fluid based on CaCO3 / polymer-based plugging agent, which is low in cost and has a good plugging effect. It also has temperature resistance and pressure resistance, and can solve the problem of wellbore instability in high-temperature well sections.

[0022] Furthermore, the water-based drilling fluid of the present invention contains a viscosity reducer. This is because the present invention contains N-vinylpyrrolidone, whose polymer polyvinylpyrrolidone generates a gel with a certain degree of crosslinking under the action of a crosslinking agent, thereby increasing the viscosity of the polymer. Therefore, the addition of a viscosity reducer prevents the drilling fluid rheological properties from changing too much. Detailed Implementation

[0023] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0024] It should be noted that the process equipment or apparatus not specifically mentioned in the following embodiments are all conventional equipment or apparatus in the art.

[0025] It should be noted that the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or apparatus that includes a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to these processes, methods, products, or apparatuses. Furthermore, unless otherwise stated, the numbering of each method step is merely a convenient tool for identifying each method step, and not intended to limit the order of the method steps or define the scope of the invention. Changes or adjustments to their relative relationships, without substantially altering the technical content, should also be considered within the scope of the invention.

[0026] The method for preparing the CaCO3 / polymer-based plugging agent provided by the present invention includes:

[0027] S1, the raw materials for synthesizing CaCO3 are modified with a modifier during the CaCO3 synthesis process to obtain modified CaCO3; the modifier is selected from one of oleic acid, linoleic acid, ethylene trioxane, γ-aminopropyltriethoxysilane and γ-methacryloyloxypropyltrimethoxysilane.

[0028] S2, Modified CaCO3 is contacted with three functional monomers, and CaCO3 / polymer-based plugging agent is synthesized by reverse suspension polymerization under the action of crosslinking agent, initiator and dispersant; wherein, the functional monomers are styrene, acrylamide and N-vinylpyrrolidone.

[0029] In this invention, step S1 specifically includes: dissolving Ca(OH)₂ in water to obtain a Ca(OH)₂ solution; dissolving a modifier in ethanol to obtain a modifier solution; adding the modifier solution to the Ca(OH)₂ solution and stirring to obtain a mixed solution; introducing CO₂ into the mixed solution by heating (NH₄)₂CO₃ to carry out the reaction until the pH of the solution reaches neutral, at which point the reaction ends, yielding modified CaCO₃. The mass ratio of Ca(OH)₂ to the modifier is 1:(0.05-1.12). This invention performs in-situ modification during the synthesis of CaCO₃. Because of the presence of Ca(OH)₂, the -OH content is relatively high, and -OH adsorbs onto CaCO₃, facilitating the reaction between the modifier and CaCO₃, thus achieving successful modification. This in-situ modification method avoids the modification failure problem caused by insufficient -OH content when directly modifying CaCO₃.

[0030] In step S1, in order to improve the modification of CaCO3 and the purity of the product, preferably, the mass ratio of Ca(OH)2:water:modifier:ethanol:(NH4)2CO3 is 1:(35-65):(0.05-1.12):(35-65):(1.2-4.9), and more preferably 1:(40-60):(0.065-1.05):(40-60):(1.5-4.2).

[0031] According to the present invention, the modifier is further preferably γ-aminopropyltriethoxysilane.

[0032] In this invention, S2 specifically includes: mixing liquid paraffin and a dispersant under heating and a protective atmosphere to obtain liquid paraffin containing a dispersant; mixing modified CaCO3, water, functional monomers, crosslinking agents and initiators, adding the resulting mixed solution to the liquid paraffin containing the dispersant, and carrying out a reverse suspension polymerization reaction to obtain the CaCO3 / polymer-based blocking agent.

[0033] In step (2), the functional monomer is further preferably styrene, acrylamide, or N-vinylpyrrolidone. To ensure that the rigidity and water absorption swelling rate of the CaCO3 / polymer-based plugging agent both reach good levels, preferably, the mass ratio of modified CaCO3:liquid paraffin:dispersant:water:styrene:acrylamide:N-vinylpyrrolidone:crosslinking agent:initiator is 1:(300-510):(30-50):(0.45-1.5):(25-58):(30-55):(12-30):(0.85-2.73):(18-47), and more preferably 1:(340-420):(32-48):(0.72-1.34):(32-43):(42-50):(15-18):(1.2-1.95):(22-37).

[0034] In this invention, the dispersant is selected from one of Span-60, Span-80, and OP-10, more preferably Span-80. The crosslinking agent is selected from one of divinylbenzene, N,N-methylenebisacrylamide, methacrylic acid, and boric acid, more preferably N,N-methylenebisacrylamide. The initiator is selected from one of benzoyl peroxide, azobisisobutyronitrile, azobisisoheptanenitrile, cumene peroxide, and ammonium persulfate, more preferably ammonium persulfate.

[0035] In S2 of the present invention, the conditions of the polymerization process include: a temperature of 55-70°C, more preferably 60°C, and a time of 4.5-6.5h, more preferably 5h.

[0036] The CaCO3 / polymer-based plugging agent prepared by the above-described preparation method of the present invention has an average particle size range of 90-680 μm, more preferably 95-180 μm, and can be used as a plugging agent in water-based drilling fluids.

[0037] Therefore, the present invention provides a water-based drilling fluid containing the CaCO3 / polymer-based plugging agent as described above.

[0038] Specifically, by weight, the water-based drilling fluid comprises the following components: 100 parts water, 5-11 parts bentonite, 0.4-0.8 parts sodium carbonate, 4-9 parts viscosity reducer, 5-9 parts filtration loss reducer, 10-15 parts inhibitor, 4-8 parts alkalinity adjuster, 28-42 parts weighting agent, and 2-5 parts CaCO3 / polymer-based plugging agent.

[0039] According to the present invention, the water-based drilling fluid, in addition to satisfying the above-mentioned composition, preferably comprises the following components by mass: 100 parts water, 6-9 parts bentonite, 0.55-0.75 parts sodium carbonate, 5.8-7.9 parts viscosity reducer, 6.2-8.3 parts filtration loss reducer, 12-14 parts inhibitor, 5.4-7.2 parts alkalinity adjuster, 32-39 parts weighting agent, and 2.5-4.5 parts of the CaCO3 / polymer-based plugging agent.

[0040] According to the present invention, the bentonite is selected from one of calcium-based bentonite, sodium-based bentonite, calcium-sodium-based bentonite, and calcium-magnesium-based bentonite, and is more preferably calcium-based bentonite.

[0041] According to the present invention, the viscosity reducer is selected from one of tannin alkali solution, iron-chromium lignin sulfonate, tannic acid, and tannin gum, and is more preferably tannic acid.

[0042] According to the present invention, the filtration loss reducing agent is selected from one of sodium nitrohumate, potassium humate, chromium humic acid, and hydroxypropyl starch, and is more preferably chromium humic acid.

[0043] According to the present invention, the inhibitor is selected from one of polymethyl glucoside, potassium chloride, potassium silicate, potassium formate, and polyamine, and is more preferably potassium formate. Increasing the potassium ion content of the water-based drilling fluid can improve its inhibitory properties (the ability to inhibit clay hydration). Potassium ions have low hydration energy, which can promote dehydration between crystal layers, causing the crystal layers to be compressed and forming a compact structure, thereby inhibiting clay hydration.

[0044] According to the present invention, the alkalinity regulator is selected from sodium hydroxide and calcium oxide, and is more preferably calcium oxide.

[0045] According to the present invention, the weighting agent is selected from one of API barite, manganese tetroxide, magnetite powder, manganese tetroxide, and ultrafine calcium carbonate, and is more preferably API barite.

[0046] Preparation Example 1

[0047] (1) Ca(OH)₂ and deionized water were added to a four-necked flask equipped with a condenser, dropping funnel, and nitrogen inlet at a mass ratio of 1:50 and stirred vigorously for 20 min. γ-aminopropyltriethoxysilane and ethanol were mixed at a mass ratio of Ca(OH)₂:γ-aminopropyltriethoxysilane:ethanol of 1:0.08:50, and then added to the Ca(OH)₂ solution using a separatory funnel, with vigorous stirring. (NH₄)₂CO₃ was heated in a sealed flask and connected to the reaction flask at a mass ratio of Ca(OH)₂:(NH₄)₂CO₃ of 1:3. CO₂ bubbles were generated through the reaction solution until the pH of the solution reached 7, at which point the reaction was complete, yielding modified CaCO₃.

[0048] (2) The modified CaCO3 was centrifuged. Specifically, the modified CaCO3 was mixed with an ethanol solution and centrifuged at 10,000 r / min for 35 min. The above operation was repeated 3 times to ensure that the impurities were completely removed. After the above operation was completed, the modified CaCO3 was dried in a vacuum drying oven at 50℃ for 24 h to obtain the modified CaCO3.

[0049] (3) First, add liquid paraffin and Span-80 to a three-necked flask according to the mass ratio of modified CaCO3: liquid paraffin: Span-80 of 1:380:0.9. Place the three-necked flask in a water bath and heat it to 40°C. At the same time, purge with nitrogen and stir at 350 r / min for 30 min. Then, add modified CaCO3, deionized water, styrene, acrylamide, N-vinylpyrrolidone, N,N-methylenebisacrylamide, and ammonium persulfate to a beaker according to the mass ratio of 1:38:38:45:17:1.48:30 and stir at 500 r / min for 30 min. After stirring, adjust the temperature of the water bath to 60°C. At the same time, add the above solution dropwise to the three-necked flask using a separatory funnel and react in a water bath at 500 r / min for 5 h until the reaction is complete to obtain the CaCO3 / polymer-based sealing agent.

[0050] (4) The CaCO3 / polymer-based plugging agent obtained by polymerization is mixed with an ethanol solution and washed thoroughly. After repeating the above operation 3 times, the product is dried in a vacuum drying oven at 70°C for 48 hours to obtain the CaCO3 / polymer-based plugging agent (denoted as A1).

[0051] Preparation Example 2

[0052] (1) Ca(OH)₂ and deionized water were added to a four-necked flask equipped with a condenser, dropping funnel, and nitrogen inlet at a mass ratio of 1:40 and stirred vigorously for 20 min. γ-aminopropyltriethoxysilane and ethanol were mixed at a mass ratio of Ca(OH)₂:γ-aminopropyltriethoxysilane:ethanol of 1:0.065:40, and then added to the Ca(OH)₂ solution using a separatory funnel, with vigorous stirring. (NH₄)₂CO₃ was heated in a sealed flask and connected to the reaction flask at a mass ratio of 1:1.5, generating CO₂ bubbles through the reaction solution. The reaction was continued until the pH of the solution reached 7, at which point the reaction was complete, yielding modified CaCO₃.

[0053] (2) The modified CaCO3 was centrifuged. Specifically, the modified CaCO3 was mixed with an ethanol solution and centrifuged at 10,000 r / min for 35 min. The above operation was repeated 3 times to ensure that the impurities were completely removed. After the above operation was completed, the modified CaCO3 was dried in a vacuum drying oven at 50℃ for 24 h to obtain the modified CaCO3.

[0054] (3) First, add liquid paraffin and Span-80 to a three-necked flask according to the mass ratio of modified CaCO3: liquid paraffin: Span-80 of 1:340:0.72. Place the three-necked flask in a water bath and heat it to 40°C while simultaneously purging with nitrogen. Stir at 350 r / min for 30 min. Then, add modified CaCO3, deionized water, styrene, acrylamide, N-vinylpyrrolidone, N,N-methylenebisacrylamide, and ammonium persulfate to a beaker according to the mass ratio of 1:32:32:42:15:1.2:22 and stir at 500 r / min for 30 min. After stirring, adjust the temperature of the water bath to 60°C. At the same time, add the above solution dropwise to the three-necked flask using a separatory funnel. React in a water bath at 500 r / min for 5 h until the reaction is complete to obtain the CaCO3 / polymer-based sealing agent.

[0055] (4) The CaCO3 / polymer-based plugging agent obtained by polymerization is mixed with an ethanol solution and washed thoroughly. After repeating the above operation 3 times, the product is dried in a vacuum drying oven at 70°C for 48 hours to obtain the CaCO3 / polymer-based plugging agent (denoted as A2).

[0056] Preparation Example 3

[0057] (1) Ca(OH)₂ and deionized water were added to a four-necked flask equipped with a condenser, dropping funnel, and nitrogen inlet at a mass ratio of 1:60 and stirred vigorously for 20 min. γ-aminopropyltriethoxysilane and ethanol were mixed at a mass ratio of Ca(OH)₂:γ-aminopropyltriethoxysilane:ethanol of 1:1.05:60, and then added to the Ca(OH)₂ solution using a separatory funnel, with vigorous stirring. (NH₄)₂CO₃ was heated in a sealed flask and connected to the reaction flask at a mass ratio of 1:4.2, generating CO₂ bubbles through the reaction solution. The reaction was continued until the pH of the solution reached 7, at which point the reaction was complete, yielding modified CaCO₃.

[0058] (2) The modified CaCO3 was centrifuged. Specifically, the modified CaCO3 was mixed with an ethanol solution and centrifuged at 10,000 r / min for 35 min. The above operation was repeated 3 times to ensure that the impurities were completely removed. After the above operation was completed, the modified CaCO3 was dried in a vacuum drying oven at 50℃ for 24 h to obtain the modified CaCO3.

[0059] (3) First, add liquid paraffin and Span-80 to a three-necked flask according to the mass ratio of modified CaCO3: liquid paraffin: Span-80 of 1:420:1.34. Place the three-necked flask in a water bath and heat it to 40°C while purging with nitrogen. Stir at 350 r / min for 30 min. Then, add modified CaCO3, deionized water, styrene, acrylamide, N-vinylpyrrolidone, N,N-methylenebisacrylamide, and ammonium persulfate to a beaker according to the mass ratio of 1:48:43:50:18:1.95:37 and stir at 500 r / min for 30 min. After stirring, adjust the temperature of the water bath to 60°C. At the same time, add the above solution dropwise to the three-necked flask using a separatory funnel. React in a water bath at 500 r / min for 5 h until the reaction is complete to obtain the CaCO3 / polymer-based sealing agent.

[0060] (4) The CaCO3 / polymer-based plugging agent obtained by polymerization is mixed with an ethanol solution and washed thoroughly. After repeating the above operation 3 times, the product is dried in a vacuum drying oven at 70°C for 48 hours to obtain the CaCO3 / polymer-based plugging agent (denoted as A3).

[0061] Example 1

[0062] The water-based drilling fluid of this embodiment comprises: 100 parts by weight of tap water, 8 parts by weight of calcium-based bentonite, 0.65 parts by weight of sodium carbonate, 6.4 parts by weight of tannic acid, 7.8 parts by weight of chromium humic acid, 13 parts by weight of potassium formate, 6.5 parts by weight of calcium oxide, 35 parts by weight of API barite, and 3.5 parts by weight of CaCO3 / polymer-based plugging agent A1.

[0063] The water-based drilling fluid preparation method of this embodiment is as follows: tap water, calcium-based bentonite, and sodium carbonate are stirred at a stirring rate of 6000 r / min for 30 min and allowed to stand for 24 h for hydration to obtain a base slurry. Tannic acid is added to the base slurry and stirred for 10 min, then chromium humic acid is added and stirred for 10 min, then potassium formate is added and stirred for 10 min, then calcium oxide is added and stirred for 10 min, then API barite is added and stirred for 30 min, and finally the CaCO3 / polymer-based plugging agent prepared in this invention is added and stirred for 10 min. The stirring rate after each addition is 6000 r / min to obtain the water-based drilling fluid (denoted as B1).

[0064] Example 2-3

[0065] The process is essentially the same as in Example 1, except that CaCO3 / polymer-based plugging agents A2 and A3 are used instead of CaCO3 / polymer-based plugging agent A1 in Example 1. The resulting water-based drilling fluids are designated as B2 and B3, respectively.

[0066] Example 4

[0067] The water-based drilling fluid of this embodiment comprises: 100 parts by weight of tap water, 6 parts by weight of calcium-based bentonite, 0.55 parts by weight of sodium carbonate, 5.8 parts by weight of tannic acid, 6.2 parts by weight of chromium humic acid, 12 parts by weight of potassium formate, 5.4 parts by weight of calcium oxide, 32 parts by weight of API barite, and 12.5 parts by weight of CaCO3 / polymer-based plugging agent A1.

[0068] The water-based drilling fluid preparation method of this embodiment is as follows: tap water, calcium-based bentonite, and sodium carbonate are stirred at a stirring rate of 6000 r / min for 30 min and allowed to stand for 24 h for hydration to obtain a base slurry. Tannic acid is added to the base slurry and stirred for 10 min, then chromium humic acid is added and stirred for 10 min, then potassium formate is added and stirred for 10 min, then calcium oxide is added and stirred for 10 min, then API barite is added and stirred for 30 min, and finally the CaCO3 / polymer-based plugging agent prepared in this invention is added and stirred for 10 min. The stirring rate after each addition is 6000 r / min to obtain the water-based drilling fluid (denoted as B4).

[0069] Examples 5-6

[0070] The process is essentially the same as in Example 4, except that CaCO3 / polymer-based plugging agents A2 and A3 are used instead of CaCO3 / polymer-based plugging agent A1 in Example 4. The resulting water-based drilling fluids are designated as B5 and B6, respectively.

[0071] Example 7

[0072] The water-based drilling fluid of this embodiment comprises: 100 parts by weight of tap water, 9 parts by weight of calcium-based bentonite, 0.75 parts by weight of sodium carbonate, 7.9 parts by weight of tannic acid, 8.3 parts by weight of chromium humic acid, 14 parts by weight of potassium formate, 7.2 parts by weight of calcium oxide, 39 parts by weight of API barite, and 4.5 parts by weight of CaCO3 / polymer-based plugging agent A1.

[0073] The water-based drilling fluid preparation method of this embodiment is as follows: tap water, calcium-based bentonite, and sodium carbonate are stirred at a stirring rate of 6000 r / min for 30 min and allowed to stand for 24 h for hydration to obtain a base slurry. Tannic acid is added to the base slurry and stirred for 10 min, then chromium humic acid is added and stirred for 10 min, then potassium formate is added and stirred for 10 min, then calcium oxide is added and stirred for 10 min, then API barite is added and stirred for 30 min, and finally the CaCO3 / polymer-based plugging agent prepared in this invention is added and stirred for 10 min. The stirring rate after each addition is 6000 r / min to obtain the water-based drilling fluid (denoted as B7).

[0074] Examples 8-9

[0075] The process is essentially the same as in Example 7, except that CaCO3 / polymer-based plugging agents A2 and A3 are used instead of CaCO3 / polymer-based plugging agent A1 in Example 7. The resulting water-based drilling fluids are designated as B8 and B9, respectively.

[0076] Comparative Example 1

[0077] The only difference from Example 1 is that CaCO3 / polymer-based plugging agent A1 is not added, and the resulting water-based drilling fluid is denoted as DB1.

[0078] Comparative Example 2

[0079] The only difference from Example 1 is that the CaCO3 / polymer-based plugging agent A1 is replaced with the modified CaCO3 prepared in Preparation Example (1-1). The resulting water-based drilling fluid is designated DB2.

[0080] Comparative Example 3

[0081] The only difference from Example 1 is that the CaCO3 / polymer-based plugging agent A1 is replaced with natural bitumen powder, a commonly used plugging agent in drilling fluids, and the resulting water-based drilling fluid is designated as DB3.

[0082] Comparative Example 4

[0083] The only difference from Example 1 is that the amount of CaCO3 / polymer-based plugging agent A1 used is 15 parts by mass, and the resulting water-based drilling fluid is designated as DB4.

[0084] Comparative Example 5

[0085] The only difference from Example 1 is that the CaCO3 / polymer-based plugging agent A1 is replaced with plugging agent A4 prepared by the following direct modification method:

[0086] (1) Industrial CaCO3 and deionized water were placed in a four-necked flask equipped with a condenser, a dropping funnel, and a nitrogen inlet at a mass ratio of 1:50 and stirred vigorously for 20 minutes. γ-aminopropyltriethoxysilane and ethanol were mixed at a mass ratio of CaCO3:γ-aminopropyltriethoxysilane:ethanol of 1.35:0.08:50, and then added to the CaCO3 solution using a separatory funnel and stirred to obtain modified CaCO3.

[0087] (2) The modified CaCO3 was centrifuged. Specifically, the modified CaCO3 was mixed with an ethanol solution and centrifuged at 10,000 r / min for 35 min. The above operation was repeated 3 times to ensure that the impurities were completely removed. After the above operation was completed, the modified CaCO3 was dried in a vacuum drying oven at 50℃ for 24 h to obtain the modified CaCO3.

[0088] (3) First, add liquid paraffin and Span-80 to a three-necked flask according to the mass ratio of modified CaCO3: liquid paraffin: Span-80 of 1:380:0.9. Place the three-necked flask in a water bath and heat it to 40°C. At the same time, purge with nitrogen and stir at 350 r / min for 30 min. Then, add modified CaCO3, deionized water, styrene, acrylamide, N-vinylpyrrolidone, N,N-methylenebisacrylamide, and ammonium persulfate to a beaker according to the mass ratio of 1:38:38:45:17:1.48:30 and stir at 500 r / min for 30 min. After stirring, adjust the temperature of the water bath to 60°C. At the same time, add the above solution dropwise to the three-necked flask using a separatory funnel and react in a water bath at 500 r / min for 5 h until the reaction is complete to obtain the CaCO3 / polymer-based sealing agent.

[0089] (4) The CaCO3 / polymer-based plugging agent obtained by polymerization is mixed with an ethanol solution and thoroughly washed. After repeating the above operation 3 times, the product is dried in a vacuum drying oven at 70°C for 48 hours to obtain the directly modified CaCO3 / polymer-based plugging agent A4.

[0090] The component contents of the water-based drilling fluids prepared in Examples 1-9 are shown in Table 1, and the component contents of the water-based drilling fluids prepared in Comparative Examples 1-5 are shown in Table 2.

[0091] Table 1. Components and dosage of water-based drilling fluids in each embodiment.

[0092]

[0093] Table 2. Components and dosage of water-based drilling fluids for each comparison example.

[0094]

[0095]

[0096] Test case

[0097] The expansion ratio and glass transition temperature of the CaCO3 / polymer-based plugging agents prepared in Examples 1-3 were measured. The plugging performance of the water-based drilling fluids prepared in Examples 1-9 and Comparative Examples 1-5 was tested according to GB / T16783.1-2014 "Field Testing of Drilling Fluids for Petroleum and Natural Gas Industry - Part 1: Water-based Drilling Fluids". The water-based drilling fluids prepared in Examples 1-9 and Comparative Examples 1-5 were evaluated.

[0098] In the following test cases:

[0099] The glass transition temperature (Tg) was measured using a differential scanning calorimeter; the differential scanning calorimeter was manufactured by METTLER TOLEDO, model DSC 3, part number 30139229.

[0100] Apparent viscosity (AV, mPa·s), plastic viscosity (PV, mPa·s), and dynamic shear force (YP, Pa) were measured using a six-speed rotational viscometer according to the method in GB / T 16783.1-2014.

[0101] The high-temperature high-pressure filtration loss (HTHP, mL) was measured using a high-temperature high-pressure filtration loss meter according to the method in GB / T16783.1-2014;

[0102] The manufacturer of the six-speed rotational viscometer is Micco Corporation, and the model is ZNN-D6B electronic six-speed rotational viscometer.

[0103] The manufacturer of the high-temperature and high-pressure filter is Meike Company, and the model is GGS42-2A High-Temperature and High-Pressure Filter - PID Electronic Temperature Control.

[0104] The pressure bearing capacity test was completed by the formation microfracture sealing and pressure bearing capacity evaluation device, which was a self-made laboratory device. The experimental equipment consisted of a gas booster station, a hand pump, a piston intermediate container, a core holder, a pressure sensor, a high-pressure manifold, and related sealing components.

[0105] 1. Expansion ratio test

[0106] 1 mL of the CaCO3 / polymer-based plugging agent A1-A3 obtained in the preparation example was placed into a 50 mL pressure-resistant tube and marked as V1. Deionized water was added until the mark was reached to 50 mL. After standing for 24 h at different temperatures, the tube was removed. The test temperatures were 25℃, 95℃, and 160℃. The volume change of the microspheres during this period was observed. The expansion ratio was calculated using formula (1):

[0107]

[0108] Among them, V t V0 represents the volume of CaCO3 / polymer-based plugging agents A1-A3 after absorbing water and swelling, in mL; V0 represents the volume before water absorption, in mL.

[0109] The results of the expansion ratio test are shown in Table 3.

[0110] Table 3. Expansion Ratio Test Results

[0111]

[0112] The test results show that at each test temperature, the expansion ratio A1 > A3 > A2. This may be because the CaCO3 / polymer-based plugging agent A3 has too much crosslinking agent, resulting in an overly tight crosslinking structure that makes it difficult for water molecules to penetrate the plugging agent. For CaCO3 / polymer-based plugging agent A2, the amounts of crosslinking agent and initiator are both relatively small. The smaller amount of crosslinking agent leads to poor crosslinking structure formation within the polymer, resulting in a mud-like product. The smaller amount of initiator prevents the monomers from reacting completely, leading to a significantly lower water absorption expansion rate compared to the other two preparations. Furthermore, the test results show that the water absorption expansion performance increases significantly with increasing temperature, especially for CaCO3 / polymer-based plugging agent A1. During the test, it was found that the CaCO3 / polymer-based plugging agent did not undergo a significant phase change when the water absorption expansion rate was tested at 160℃, indirectly demonstrating its certain temperature resistance.

[0113] 2. Glass transition temperature test

[0114] The test results for glass transition temperature (Tg) are shown in Table 4.

[0115] Table 4. Test results of glass transition temperature

[0116] A1 A2 A3 <![CDATA[T g (℃)]]> 158 134 143

[0117] The test results show that CaCO3 / polymer-based plugging agent A1 has the highest glass transition temperature, which is suitable for the wellbore stability requirements of high-temperature formations in current drilling sites at 160℃. Therefore, it is analyzed that when the CaCO3 / polymer-based plugging agent enters the high-temperature formation, and the temperature exceeds its glass transition temperature, the CaCO3 / polymer-based plugging agent will deform and enter the fractures for effective plugging.

[0118] 3. Blocking performance test

[0119] The base slurry (water + 8% bentonite + 2% sodium carbonate, where the percentage content of each component is based on the mass of water) was placed in the mud cup of a high-temperature and high-pressure filtration loss tester. The high-temperature and high-pressure filtration loss test was conducted for 30 minutes at a temperature of 160℃ and a pressure difference of 3.5MPa. After the test, the slurry was cooled to room temperature, and the base slurry in the mud cup was poured out, leaving a filter cake at the bottom of the mud cup. Distilled water was injected along the inner wall of the mud cup 1-2 times while gently shaking it during the injection process. The distilled water was then poured out to wash away the loose mud on the filter cake. Distilled water was then injected into the mud cup to the mark, and the high-temperature and high-pressure filtration loss test process was repeated at a temperature of 160℃ and a pressure difference of 3.5MPa for 30 minutes to further wash away the loose mud. After the test, the slurry was cooled to room temperature, the distilled water was poured out, the mud cake in the mud cup was removed, and the mud cake permeability of the base slurry was calculated using formula (2) to be 548×10 -2 mD; The above-mentioned base slurry cake is used as a simulated formation with micro-nano fractures;

[0120]

[0121] Where K is the permeability, in mD; q is the average filtration loss per unit time, in cm. 3 / s; l is the mud cake thickness, mm; μ is the drilling fluid viscosity, mPa·s; A is the mud cake area, m² 2 Δp: Pressure difference, MPa.

[0122] Water-based drilling fluids B1-B9 and DB1-DB5 were used as test slurries and aged at 160℃ for 16 hours. After cooling to room temperature, they were placed in the mud cups of a high-temperature, high-pressure filtration analyzer (the bottom of the mud cup used the aforementioned base slurry cake as the filtration medium). A high-temperature, high-pressure filtration loss test was conducted for 30 minutes at 160℃ and a pressure difference of 3.5 MPa. After the test, the fluid was cooled to room temperature, and the slurry in the mud cup was poured out, leaving a test slurry cake at the bottom. Water was injected 1-2 times along the inner wall of the mud cup, gently agitating it during the injection process. The water was then poured out to wash away any loose mud on the filter cake. Water was then injected back into the mud cup. The slurry cup was filled to the mark, and the above high temperature and high pressure filtration loss test process was repeated under the conditions of 160℃ and 3.5MPa pressure difference. The test time was 30min to further wash away the loose mud. After the test, the slurry cup was cooled to room temperature, the water was poured out, the test slurry cake in the slurry cup was taken out, and the permeability of the cake of B1-B9 and DB1-DB5 was calculated according to the above formula (2) (denoted as K1, mD). On this basis, the plugging rate of B1-B9 and DB1-DB5 before and after the simulated formation was calculated using formula (3) (that is, the plugging rate is used to characterize the plugging effect. The higher the plugging rate, the better the plugging effect of the plugging material). The results are shown in Table 5.

[0123]

[0124] Among them, K r K0 is the plugging rate (%); K1 is the permeability of the base slurry cake (simulated formation) (mD); K2 is the permeability of the test slurry cake (mD).

[0125] Table 5 Evaluation of Blocking Effectiveness

[0126]

[0127]

[0128] The evaluation of the plugging effect shows that the CaCO3 / polymer-based plugging agent prepared in this invention has a very good plugging effect, and the plugging efficiency of CaCO3 / polymer-based plugging agent A1 is as high as 99%, which is consistent with the results of the water absorption swelling rate test and the glass transition temperature test. Compared with the drilling fluid DB1 without plugging agent, it can be seen that the plugging rate of drilling fluid B1 with CaCO3 / polymer-based plugging agent of this invention is significantly improved. Compared with drilling fluid DB2, it can be seen that the plugging rate of drilling fluid B1 is improved, indicating that the combination of modified CaCO3 and polymer can improve the plugging effect of drilling fluid. At the same time, compared with comparative example 3 with added natural bitumen powder, the drilling fluid with plugging agent of this invention has a better plugging effect. As shown in Table 5, the plugging effect of comparative example DB4 is significantly reduced, which is because too much plugging agent was added. The blocking effect of DB5 in Comparative Example 5 was significantly lower than that in Example 1. This is because Comparative Example 5 used industrial calcium carbonate for direct modification. During the experiment, it was found that the properties of the modified calcium carbonate were almost identical to those of the raw calcium carbonate, indicating that industrial calcium carbonate has a very low hydroxyl content, resulting in poor modification effect. Consequently, there was not much chemical bonding between the calcium carbonate and the polymer in the prepared blocking agent, leading to a poor blocking effect. Secondly, the experimental temperature was set at 160℃. At a high temperature of 160℃, the CaCO3 / polymer-based blocking agent still exhibited good blocking performance, indicating good temperature resistance.

[0129] 4. Compatibility test

[0130] Water-based drilling fluid base slurry A was prepared (100 parts by mass of tap water + 8 parts by mass of calcium-based bentonite + 0.65 parts by mass of sodium carbonate + 0.64 parts by mass of tannic acid + 7.8 parts by mass of chromium humic acid + 13 parts by mass of potassium formate + 6.5 parts by mass of calcium oxide + 35 parts by mass of API barite). Then, 3.5% (based on the mass of tap water) of CaCO3 / polymer-based plugging agent A1 prepared in Preparation Example 1 was added to the above base slurry A to obtain water-based drilling fluid B.

[0131] The rheological parameters of water-based drilling fluid A and water-based drilling fluid B at room temperature were tested respectively, and the results are shown in Table 6.

[0132] Table 6. Rheological parameters of water-based drilling fluid A and water-based drilling fluid B at room temperature.

[0133] Drilling fluid AV(mPa·s) PV (mPa·s) YP(Pa) A 45 38 7 B 47 39 8

[0134] As can be seen from Table 6, the rheological parameters of the water-based drilling fluid did not change significantly after the addition of the CaCO3 / polymer-based plugging agent of the present invention. This indicates that the addition of the CaCO3 / polymer-based plugging agent will not have a significant impact on the rheological parameters of the water-based drilling fluid, and that the CaCO3 / polymer-based plugging agent prepared in this invention has good compatibility with the water-based drilling fluid.

[0135] 5. Pressure resistance test

[0136] (1) Drilling fluid preparation: By mass, the drilling fluid comprises: 100 parts tap water, 8 parts calcium-based bentonite, 0.65 parts sodium carbonate, 0.64 parts tannic acid, 7.8 parts chromium humic acid, 13 parts potassium formate, 6.5 parts calcium oxide, 35 parts API barite, and 3.5 parts A1 / A2 / A3. That is, B1, B2, and B3 as described in the above preparation example.

[0137] (2) Select a suitable 3D printed core and place it in the core clamping container. Tighten and fix both ends with a screwdriver so that the 3D printed core (crack width is 400μm) is located in the center of the core clamping container. Under the applied confining pressure, the actual crack width is about 4μm.

[0138] (3) Connect the pressure sensing device pipeline in sequence, connect it to the intermediate container, and tighten the screws;

[0139] (4) Set the required confining pressure using a pressure pump and set the temperature to 160℃ on the control panel;

[0140] (5) Add the prepared drilling fluid to the intermediate container, lower the piston, and let the piston slowly descend to the drilling fluid level. Stop when a small amount of drilling fluid overflows from the screw hole. Cover with the sealing bolt, add water to the container until 100mL remains at the container opening, cover the container with the container cover, and connect to the drive top pipeline.

[0141] (6) After ensuring that the data acquisition software and the sensor are connected normally, set the corresponding parameters on the control panel and click Start Recording. After drilling fluid flows out from the outlet, stop pumping liquid into the core. After the test is completed, remove the displacement pressure and confining pressure and save the data.

[0142] The pressure-bearing capacity of the prepared drilling fluid was tested, and the results are shown in Table 7.

[0143] withstand pressure 3MPa 5MPa 7MPa 9MPa 11MPa B1 √ √ √ √ √ B2 √ √ √ × × B3 √ √ √ √ ×

[0144] As shown in Table 7, the CaCO3 / polymer-based plugging agent can withstand a maximum pressure of 11 MPa. As mentioned earlier, during this experiment, the actual width of the fracture was about 4 μm, while the average particle size of the CaCO3 / polymer-based plugging agent was 95-180 μm. Under these conditions, it still achieved a good plugging effect. This is because the CaCO3 / polymer-based plugging agent can transform into a highly elastic state after the external temperature exceeds its glass transition temperature. Under the action of the positive pressure differential of the drilling fluid, it deforms and is squeezed into the fracture. After entering the fracture for a period of time, the CaCO3 / polymer-based plugging agent absorbs water and expands, thus sealing the fracture. Secondly, since the CaCO3 / polymer-based plugging agent of this invention contains inorganic material CaCO3, it has good rigidity and can still maintain rigidity after being compressed, avoiding the loss of rigidity due to water absorption and expansion, which would prevent the CaCO3 / polymer-based plugging agent from being unable to withstand pressure in the formation.

[0145] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A method for preparing a CaC03 / polymer-based plugging agent, characterized in that, The method comprises the following steps: S1, synthesizing CaCO3 in the presence of a modifier to obtain modified CaCO3; wherein the modifier is selected from one of oleic acid, linoleic acid, ethylene trioxysilane, gamma-aminopropyl triethoxysilane and gamma-methacryloyloxypropyl trimethoxysilane; S2, synthesizing the CaCO3 / polymer-based plugging agent by reverse suspension polymerization using the modified CaCO3 and functional monomers as raw materials, adding a crosslinking agent, an initiator and a dispersant; wherein the functional monomers are styrene, acrylamide and N-vinyl pyrrolidone.

2. The method of preparing a CaC03 / polymer-based plugging agent according to claim 1, characterized in that, S1 specifically comprises: dissolving Ca(OH)2 in water to obtain a Ca(OH)2 solution; dissolving the modifier in ethanol to obtain a modifier solution; adding the modifier solution into the Ca(OH)2 solution and stirring to obtain a mixed solution; introducing CO2 into the mixed solution to react until the pH of the solution reaches neutral, and the reaction is completed to obtain the modified CaCO3.

3. The method for preparing the CaCO3 / polymer-based plugging agent according to claim 2, characterized in that, The mass ratio of the Ca(OH)2 to the modifier is 1:(0.05-1.12).

4. The method for preparing the CaCO3 / polymer-based plugging agent according to claim 1, characterized in that, S2 specifically comprises: mixing liquid paraffin and a dispersant under heating and in a protective atmosphere to obtain liquid paraffin containing the dispersant; mixing the modified CaCO3, water, functional monomers, a crosslinking agent and an initiator, and adding the obtained mixed solution into the liquid paraffin containing the dispersant to perform reverse suspension polymerization to obtain the CaCO3 / polymer-based plugging agent.

5. The method for preparing the CaCO3 / polymer-based plugging agent according to claim 1, characterized in that, The mass ratio of the modified CaCO3, styrene, acrylamide and N-vinyl pyrrolidone is 1:(25-58):(30-55):(12-30).

6. The method of claim 1, wherein the CaC03 / polymer-based plugging agent is prepared by the steps of: In S2, the temperature of the reverse suspension polymerization is 55-70℃, and the time is 4.5-6.5h.

7. The CaCO3 / polymer-based plugging agent obtained by the preparation method in any one of claims 1-6.

8. A water-based drilling fluid, characterized by 7. The CaCO3 / polymer-based plugging agent obtained by the preparation method in any one of claims 1-6.

9. The water-based drilling fluid of claim 8, wherein, According to mass fraction, the water-based drilling fluid comprises the following components: water 100 parts, bentonite 5-11 parts, sodium carbonate 0.4-0.8 parts, viscosity reducer 4-9 parts, fluid loss additive 5-9 parts, inhibitor 10-15 parts, alkalinity regulator 4-8 parts, weighting agent 28-42 parts and the CaCO3 / polymer-based plugging agent 2-5 parts.

10. The water-based drilling fluid of claim 9, wherein, The viscosity reducer is one of tannin liquor, iron-chromium lignin sulfonate, tannic acid and tannin extract.

Images