Double-nano low-density thixotropic cement slurry system and application thereof

Abstract

The invention discloses a double-nano low-density thixotropic cement paste system, relates to the technical field of oil and gas well drilling engineering, and solves the problem that the existing cement paste system is difficult to adapt to stratums with low pore pressure and serious leakage. The cement paste system comprises the following components in parts by weight: 40-60 parts of G-grade oil well cement, 14-25 parts of a solid lightening agent, 20-45 parts of a solid admixture, 3-6 parts of a solid thixotropic agent, 0.5-2 parts of a nano stabilizer, 1-3 parts of a nano early strength agent, 1-3 parts of a fluid loss agent, 0.5-1 part of a dispersing agent and 70-100 parts of water. The double-nano low-density thixotropic cement paste system has the characteristics of lower density, high thixotropy and short setting waiting time, can enable a cement paste body to effectively reside in a shaft when a lower-density cement plug is achieved, is high in thixotropy and quickly forms a gelling structure, is controllable in compressive strength, is beneficial to plug drilling, shortens the setting waiting time and saves the production cost, and is suitable for popularization and application. Therefore, the production purposes of efficiently plugging and improving quality and efficiency are achieved.

Description

Technical Field

[0001] This invention relates to the field of oil and gas well drilling engineering technology, and in particular to a dual-nano low-density thixotropic cement slurry system and its applications. Background Technology

[0002] Formation loss is a common and complex problem during drilling, especially in fractured, cavernous, and low-pressure formations. Severe drilling fluid loss leads to reduced drilling efficiency, increased costs, and may even cause wellbore instability and other safety accidents. Traditional plugging techniques involve bridging the leakage with drilling fluid using plugging materials or gel materials. Chinese patent "A Leakage Prevention Drilling Fluid Material and Its Application Method" (Publication No. CN113773816A) discloses a leakage prevention drilling fluid material mainly composed of fiber materials, core-shell particles and plant shell particles, oil-soluble resin, porous materials, calcium carbonate, and quartz sand. Chinese patent "A Gel Plugging Agent and Its Preparation Method" (Publication No. CN105567190A) discloses a gel-expanding plugging agent containing functional groups that enhance interaction with the formation, using aqueous solution polymerization and chemical cross-linking to improve the retention capacity of the plugging material in the lost formation. These methods are more suitable for leakage velocities of 30m³ / h. 3 Formations with a leakage velocity of less than 30 m / h, for formations with a leakage velocity exceeding 30 m / h 3 The effectiveness of plugging grouting is not significant in formations experiencing severe or complete leakage (loss of return). For example, in the Rumaila Block in Iraq, during the second drilling phase, the leaking strata encountered contained porous dolomite or limestone with well-developed pores and fractures, and the presence of karst caves or karst-like structures. Conventional plugging grouting methods are ineffective, and cement grouting is commonly used for plugging, but this is hampered by the pore pressure of the leaking strata being 1.06-1.11 g / cm³. 3 Conventional density cement slurries suffer from problems such as high density, poor thixotropy, difficulty in retention, and the high strength of the cement stone making it easy to drill new wellbores, thus failing to effectively seal leakage channels. Research has found that thixotropic cement slurries have the characteristics of rapidly developing shear-diluted and static cementitious structures, making them more suitable for situations with severe wellbore leakage. Furthermore, lower cement slurry densities (1.20-1.30 g / cm³) are also beneficial. 3 This method can balance the pressure inside and outside the lost circulation zone (after successful plugging), allowing more cement plug to remain in the wellbore, which is beneficial for drilling to test the plug and understand the actual situation of the plugging. Publication number CN113429163A discloses a low-density plugging cement slurry, but the 24-hour compressive strength of the cement stone exceeds 10MPa. The excessive strength of the cement stone can easily cause the drill plug to deviate and form a new wellbore. Adding cement admixtures (such as fly ash and slag) can reduce the early strength, but the setting time is greatly extended, affecting the plugging efficiency.

[0003] Therefore, for formations with low pore pressure and severe leakage, there is an urgent need for a cement slurry system that combines lower density, high thixotropy, and short setting time. Summary of the Invention

[0004] This invention addresses the problem that existing cement slurry systems are unsuitable for formations with low pore pressure and severe leakage, by providing a dual-nano low-density thixotropic cement slurry system. This system combines lower density, high thixotropy, and short setting time, enabling the cement slurry to achieve a lower density (cement slurry density 1.20-1.30 g / cm³). 3 The cement plug effectively resides in the wellbore, rapidly forms a gel structure due to its high thixotropy, has controllable compressive strength which facilitates drilling, and shortens the setting time, thus saving production costs. This achieves the production goals of efficient leak sealing and improved quality and efficiency. This invention also provides an application for a dual-nano low-density thixotropic cement slurry system.

[0005] The present invention solves its problem through the following technical solution: the dual-nano low-density thixotropic cement slurry system, the composition of which, by weight, includes:

[0006] 40-60 parts of Grade G oil well cement, 14-25 parts of solid weight-reducing agent, 20-45 parts of solid admixture, 3-6 parts of solid thixotropic agent, 0.5-2 parts of nano stabilizer, 1-3 parts of nano early strength agent, 1-3 parts of water loss reducing agent, 0.5-1 part of dispersant, and 70-100 parts of water.

[0007] Furthermore, the solid weight-reducing agent comprises hollow glass microspheres and vitrified perlite; the weight ratio of the hollow glass microspheres to the vitrified perlite is 20-40:60-80.

[0008] The hollow glass microspheres have a density of 0.39-0.42 g / cm³. 3 Specific surface area 0.2-2m² 3 / g;

[0009] The vitrified perlite has a density of 0.80-1.20 g / cm³. 3 Specific surface area 1-3m² 3 / g.

[0010] Furthermore, the solid admixtures include fly ash, slag powder, and microsilica; the weight ratio of fly ash: slag powder: microsilica is 40-60: 15-25: 20-50.

[0011] The fly ash is secondary fly ash with a specific surface area greater than 300 m². 3 / kg, loss on ignition less than 8%;

[0012] The slag powder is of grade S95, with a specific surface area of ​​400-500 m².3 / kg;

[0013] The density of the microsilicon is 1.20 g / cm³. 3 The SiO2 content is greater than 85%, and the particle size is 0.1-0.3μm.

[0014] Furthermore, the solid thixotropic agent comprises attapulgite and sulfate, wherein the weight ratio of attapulgite to sulfate is 40-50:50-60.

[0015] The attapulgite clay has a colloidal value of 55-60 ml / 15 g and a specific surface area of ​​400-500 m². 3 / g;

[0016] The sulfate is one or more of aluminum sulfate, sodium sulfate, and calcium sulfate.

[0017] Furthermore, the nano-stabilizer is hydrophilic nano-silica with a particle size of 10-100 nm; the water loss reducing agent is an AMPS-type water loss reducing agent;

[0018] The dispersant is a comb-type polycarboxylic acid dispersant.

[0019] Furthermore, the nano-early strength agent is a nano-sized hydrated calcium silicate type early strength agent with a particle size of 10-100 nm;

[0020] Its composition by weight ratio is: soluble calcium salt: soluble silicate: dispersant: coupling agent in a weight ratio of 30-45: 25-45: 20-30: 5-10.

[0021] Furthermore, the soluble calcium salt is at least one of calcium nitrate tetrahydrate and calcium formate;

[0022] The soluble silicate is at least one of sodium metasilicate pentahydrate and sodium metasilicate nonahydrate.

[0023] Furthermore, the coupling agent is one or more of polyvinyl alcohol, polyvinylpyrrolidone, polyethylene glycol, or sodium dodecylbenzenesulfonate;

[0024] The dispersant is a comb-type polycarboxylic acid dispersant.

[0025] Furthermore, the preparation method of the nano-sized hydrated calcium silicate type early strength agent includes the following steps:

[0026] The dispersant and coupling agent are prepared into a first solution;

[0027] A second solution is prepared from soluble calcium salts, and a third solution is prepared from soluble silicates.

[0028] Under continuous stirring and nitrogen atmosphere, the second solution and the third solution are simultaneously added dropwise to the first solution. The dropping system is controlled under a set temperature condition and a set time is added. Then, under the same temperature condition, the dropping system is stirred for a set time to obtain a hybrid suspension.

[0029] After centrifugation, filtration, and drying of the hybrid suspension, the resulting particles are ground into solid nano-sized hydrated calcium silicate.

[0030] Further, after preparing the dispersant into a solution with a mass concentration of 10%-15%, the coupling agent is added, and after stirring evenly, the pH value is adjusted to between 11.0 and 13.5 to obtain the first solution;

[0031] The second solution is a soluble calcium salt solution with a mass concentration of 10%-20%, and the third solution is a soluble silicate solution with a mass concentration of 10%-20%, wherein the molar ratio of calcium ions to silicate ions is 1:1-1.75:1.

[0032] The stirring speed is 700 rpm - 7000 rpm, the temperature of the dropping system is set at 20℃ - 40℃, the dropping time is set at 3h - 5h, and the second solution and the third solution are dropped simultaneously; during the dropping process, the pH value of the dropping system is controlled between 11.0 and 13.5, and the stirring time of the dropping system is set at 1 day;

[0033] The drying process involves drying in a vacuum drying oven at a temperature of 50℃-70℃ for 1 day, followed by continued drying at room temperature in a desiccator for 2 hours.

[0034] This invention also provides an application of a dual-nano low-density thixotropic cement slurry system in plugging operations for severe and malignant leakage during drilling.

[0035] Compared with the above-mentioned background technology, the present invention has the following beneficial effects:

[0036] This invention discloses a dual-nano low-density thixotropic cement slurry system. Compared with existing technologies, this cement slurry system uses hollow glass microspheres and vitrified perlite as weight-reducing agents, forming a 1.20-1.30 g / cm³ cement slurry. 3The lower-density cement slurry system, due to the lower cost of vitrified perlite, not only reduces costs but also facilitates the formation of a longer cement plug within the wellbore. External admixtures are used to reduce the strength of the cement stone, while nano-hydrated silica can shorten the setting time while maintaining lower cement stone strength, thus saving plugging time and preventing plug deviation. Nano-sized hydrated calcium silicate has excellent early strength properties, effectively shortening the setting time of low-density cement slurry while reducing the early strength of the cement stone. Nano-silica can effectively improve the stability of cement slurry and has a certain thixotropic effect. The composite thixotropic agent can form a network structure through chemical bonds, giving the cement slurry good pseudoplasticity and shear dilution properties. When at rest, the chemical bonds reconnect and interact with the charge of cement particles to form a network structure, rapidly developing a certain gel strength, thereby achieving the production goals of efficient plugging, improved quality, and increased efficiency.

[0037] In the preparation process of the nano-early strength agent of this invention, the molar ratio of calcium ions to silicate ions is controlled between 1:1 and 1.75:1. Under this ratio, the particle size of hydrated calcium silicate is appropriate and the size distribution is uniform. Selecting the optimal reaction temperature and reaction time are factors affecting the preparation of nano-hydrated calcium silicate. At low reaction temperatures, the reaction rate is slow, resulting in smaller calcium silicate hydrate particles and slower crystal growth, but more uniform calcium silicate hydrate particles can be obtained. At high reaction temperatures, the reaction rate is faster, resulting in larger calcium silicate hydrate particles and faster crystal growth, but this may lead to the formation of uneven calcium silicate hydrate particles. 20℃-40℃ is the optimal reaction temperature. If the reaction time is too short, the calcium silicate hydrate particles will not form completely, and crystal growth will be insufficient. If the reaction time is too long, the calcium silicate hydrate particles will be too large or have poor crystal morphology, affecting the performance of the calcium silicate hydrate particles. Therefore, controlling the reaction time to 3-5 hours, followed by stirring for 1 day, yields the best results. In addition, maintaining an alkaline environment will also help stabilize the calcium silicate hydrate particles, and changes in pH value will affect the calcium content. 2+ and SiO3 2- The complexing ability of SiO3 is affected, thus influencing the formation and growth of hydrated calcium silicate particles. Adjusting the alkaline pH value can promote the formation and growth of SiO3. 2- The hydrolysis and precipitation reactions cause it to form nano-hydrated calcium silicate particles. Detailed Implementation

[0038] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to embodiments.

[0039] The embodiments of this application are described below. Those skilled in the art should recognize that these specific embodiments are merely illustrative of the implementation techniques chosen to achieve the objectives of this application and are not intended to limit the technical solutions. Based on the teachings of this application, improvements to the technical solutions of this application, combined with existing technologies, are obvious and all fall within the scope of protection of this application.

[0040] Unless otherwise specified, the raw materials and reagents used in the following examples are all commercially available products.

[0041] The dual-nano low-density thixotropic cement slurry system involved in the embodiments of this disclosure includes the following steps:

[0042] I. Preparation of Nano-Accelerating Agent

[0043] 1. Raw material composition and content: by weight percentage, soluble calcium salt: soluble silicate: dispersant: coupling agent in a weight ratio of 30-45: 25-45: 20-30: 5-10.

[0044] In the various embodiments of this disclosure, the soluble calcium salt is one or more of calcium nitrate tetrahydrate and calcium formate; the soluble silicate is one or more of sodium metasilicate pentahydrate and sodium metasilicate nonahydrate; the coupling agent is one or more of polyvinyl alcohol, polyvinylpyrrolidone, polyethylene glycol, and sodium dodecylbenzene sulfonate; and the dispersant is a comb-type polycarboxylic acid dispersant with an effective content greater than 95% and a molecular weight of around 40,000, exhibiting high water reduction rate, rapid plasticizing effect, and excellent early strength performance.

[0045] 2. Prepare a 10%-15% (w / w) solution of dispersant and deionized water, add it to a three-necked beaker, then add the coupling agent to the beaker and stir thoroughly. Add 30% sodium hydroxide solution dropwise to adjust the pH of the solution to between 11.0 and 13.5 to obtain the first solution.

[0046] 3. Prepare a second solution with a mass fraction of 10%-20% by mixing the soluble calcium salt with deionized water, and prepare a third solution with a mass fraction of 10%-20% by mixing the soluble silicate with deionized water; wherein, preferably, the molar ratio of calcium ions to silicate ions is 1:1-1.75:1.

[0047] 4. Continuously stir the first solution, and in a nitrogen atmosphere, slowly add the second and third solutions dropwise to the first solution simultaneously using a peristaltic pump to carry out the chemical reaction. The preferred dropwise addition rate is 1 mL / min-2 mL / min, the stirring speed is 700 rpm-7000 rpm, and the reaction temperature is 20℃-40℃. During the dropwise addition, use a 30% sodium hydroxide solution to control the pH value of the mixed solution in the dropwise addition system between 11.0 and 13.5. The preferred dropwise addition time is 3h-5h, and the second and third solutions are added simultaneously. After the dropwise addition is completed, continue stirring at a speed of 700 rpm-7000 rpm and keep the solution at this temperature for 1 day to obtain a suspension.

[0048] 5. After centrifuging the suspension to remove the supernatant, repeatedly filter and wash it three times with anhydrous ethanol. Place the filtered particles in a vacuum drying oven and dry them for 1 day, preferably at a temperature of 50℃-70℃. After removing them, place them in a desiccator and continue drying at room temperature for 2 hours. After the room temperature drying is completed, grind them thoroughly in a mechanical vibration mill to obtain nano-sized hydrated calcium silicate with a particle size of 10-100nm.

[0049] In the preparation method of the nano-early strength agent of the present invention, when preparing the first solution, the dispersant and coupling agent are used to make the hydrated calcium silicate particles uniformly dispersed and kept stable, and to prevent the particles from agglomerating and precipitating.

[0050] Since the calcium-to-silica ratio is an important factor affecting the preparation of suitable calcium silicate hydrate particle size, the method of this invention controls the molar ratio of calcium ions to silicate ions to be between 1:1 and 1.75:1. Under this ratio, the calcium silicate hydrate particle size is suitable and the size distribution is uniform.

[0051] Reaction temperature and reaction time are also factors affecting the preparation of nano-hydrated calcium silicate. At lower reaction temperatures, the reaction rate is slower, the calcium silicate hydrate particle size is smaller, and crystal growth is slower, but more uniform calcium silicate hydrate particles can be obtained. At higher reaction temperatures, the reaction rate is faster, the calcium silicate hydrate particle size is larger, and crystal growth is faster, but this may lead to the formation of uneven calcium silicate hydrate particles. 20℃-40℃ is the optimal reaction temperature. If the reaction time is too short, the calcium silicate hydrate particles will not form completely, and crystal growth will be insufficient; if the reaction time is too long, the calcium silicate hydrate particle size will be too large or the crystal morphology will be poor, affecting the performance of the calcium silicate hydrate particles. Therefore, controlling the reaction time to 3-5 hours, with continued stirring for 1 day, yields the best results. In addition, maintaining an alkaline environment will also help stabilize the calcium silicate hydrate particles; changes in pH value will affect the Ca2+ content. 2+ and SiO3 2- The complexing ability of SiO3 is affected, thus influencing the formation and growth of hydrated calcium silicate particles. Adjusting the alkaline pH value can promote the formation and growth of SiO3. 2- The hydrolysis and precipitation reactions cause it to form nano-hydrated calcium silicate particles.

[0052] II. Construction of a dual-nano low-density thixotropic cement slurry system:

[0053] The dual-nano low-density thixotropic cement slurry system disclosed herein comprises, by weight percentage:

[0054] 40-60 parts of Grade G oil well cement, 14-25 parts of solid weight-reducing agent, 20-45 parts of solid admixture, 3-6 parts of solid thixotropic agent, 0.5-2 parts of nano stabilizer, 1-3 parts of nano early strength agent, 1-3 parts of water loss reducing agent, 0.5-1 part of dispersant, and 70-100 parts of water.

[0055] The present invention will be further described below with reference to specific preferred embodiments, but this does not limit the scope of protection of the present invention.

[0056] The comb-type polycarboxylic acid dispersant used in the following specific embodiments of this disclosure is SD-600P dispersant produced by Shanghai Sanrui Polymer Materials Co., Ltd.; the nano-sized silica is product 200 produced by Shandong Sailike New Materials Co., Ltd., with a particle size of 10-100nm; the attapulgite in the solid thixotropic agent is product 480 produced by Hebei Shijiazhuang Yuanjing Mineral Products Co., Ltd.; the water loss reducing agent is product 307A produced by Henan Weihui Chemical Co., Ltd.; and other raw materials / components are commercially available products well known to those skilled in the art.

[0057] The oil well cement test method involved in this invention shall be performed in accordance with GB / T 19139 "Test Methods for Oil Well Cement" and the setting time (final setting time) test method shall be performed in accordance with GB / T 1346 "Test Methods for Standard Consistency Water Requirement, Setting Time and Soundness of Cement".

[0058] Example 1

[0059] Preparation of nano-early strength agents:

[0060] Step 1: Weigh 17.6g of polycarboxylic acid dispersant, 8.7g of polyethylene glycol and 155.5g of water and add them to the reaction vessel. Stir thoroughly, set the stirring speed to 1000rpm, control the water bath temperature at 30℃, and add 30% sodium hydroxide solution to adjust the pH of the solution to 12.5 to obtain the first solution.

[0061] Step 2: Weigh 25.2g of calcium nitrate tetrahydrate and 113.5g of water to prepare the second solution, and weigh 15.3g of sodium metasilicate pentahydrate and 74.4g of water to prepare the third solution.

[0062] Step 3: Add the second and third solutions dropwise to the first solution simultaneously over a period of 3.5 hours. Stir at 5000 rpm and maintain a water bath temperature of 30°C. Adjust the pH of the mixed solution to 12.5 using a 30% sodium hydroxide solution. Control the dropping rate to ensure that the second and third solutions are added simultaneously. Continue stirring for 1 day to complete the process. The entire process is carried out under a nitrogen atmosphere to obtain a suspension.

[0063] Step 4: After centrifuging the suspension to remove the supernatant, repeatedly filter and wash with anhydrous ethanol. Place the filtered particles in a vacuum drying oven and dry them at 60°C for 1 day. After removing them, place them in a desiccator and continue drying at room temperature for 2 hours. After the room temperature drying is completed, add them to a vibratory mill and grind them thoroughly. The resulting particles are nano-hydrated calcium silicate early strength agent.

[0064] 1.20g / cm 3Preparation of a dual-nano low-density thixotropic cement slurry system:

[0065] Step 5: Weigh out 41.5 parts of Grade G oil well cement, 6 parts of hollow glass microspheres, 19 parts of vitrified perlite, 11 parts of fly ash, 4 parts of slag powder, 10 parts of microsilica, 2 parts of water loss reducing agent, 0.5 parts of dispersant, 1.5 parts of attapulgite, 1.5 parts of aluminum sulfate, 1 part of nano silica, 2 parts of the nano early strength agent as described above, and 85 parts of water. Mix thoroughly according to GB / T 19139 "Test Methods for Oil Well Cement" to obtain a cement strength of 1.20 g / cm³. 3 Dual-nano low-density thixotropic cement slurry.

[0066] Example 2

[0067] The preparation of the nano-early strength agent is the same as in Example 1.

[0068] 1.30g / cm 3 Preparation of a dual-nano low-density thixotropic cement slurry system:

[0069] Weigh out 44.5 parts of Grade G oil well cement, 4 parts of hollow glass microspheres, 10 parts of vitrified perlite, 18 parts of fly ash, 7 parts of slag powder, 8 parts of microsilica, 2 parts of water loss reducing agent, 0.5 parts of dispersant, 1.5 parts of attapulgite, 1.5 parts of aluminum sulfate, 1 part of nano-silica, 2 parts of the aforementioned nano-early strength agent, and 90 parts of water. Mix thoroughly according to GB / T 19139 "Test Methods for Oil Well Cement" to obtain a cement strength of 1.30 g / cm³. 3 Dual-nano low-density thixotropic cement slurry.

[0070] Example 3

[0071] The preparation of the nano-early strength agent is the same as in Example 1.

[0072] 1.30g / cm 3 Preparation of a dual-nano low-density thixotropic cement slurry system:

[0073] Weigh out 55.5 parts of Grade G oil well cement, 5 parts of hollow glass microspheres, 10 parts of vitrified perlite, 9 parts of fly ash, 4 parts of slag powder, 8 parts of microsilica, 2 parts of water loss reducing agent, 0.5 parts of dispersant, 1.5 parts of attapulgite, 1.5 parts of aluminum sulfate, 1 part of nano silica, 2 parts of the aforementioned nano early strength agent, and 90 parts of water. Mix thoroughly according to GB / T 19139 "Test Methods for Oil Well Cement" to obtain a cement strength of 1.30 g / cm³. 3 Dual-nano low-density thixotropic cement slurry.

[0074] Comparative Example 1

[0075] Weigh out 65.5 parts of Grade G oil well cement, 15 parts of hollow glass microspheres, 14 parts of microsilica, 2 parts of fluid loss reducing agent, 0.5 parts of dispersant, 1 part of nano silica, 2 parts of nano early strength agent as described in Example 1, and 90 parts of water. Mix thoroughly according to GB / T 19139 "Test Methods for Oil Well Cement" to obtain a cement strength of 1.20 g / cm³. 3 Low-density cement slurry.

[0076] Comparative Example 2

[0077] Weigh out 46.5 parts of Grade G oil well cement, 4 parts of hollow glass microspheres, 10 parts of vitrified perlite, 18 parts of fly ash, 7 parts of slag powder, 8 parts of microsilica, 2 parts of water loss reducing agent, 0.5 parts of dispersant, 1.5 parts of attapulgite, 1.5 parts of aluminum sulfate, 1 part of nano-silica, and 90 parts of water. Mix thoroughly according to GB / T 19139 "Test Methods for Oil Well Cement" to obtain a cement concentration of 1.30 g / cm³. 3 Low-density cement slurry.

[0078] Comparative Example 3

[0079] Weigh out 45.5 parts of Grade G oil well cement, 4 parts of hollow glass microspheres, 10 parts of vitrified perlite, 18 parts of fly ash, 7 parts of slag powder, 8 parts of microsilica, 2 parts of water loss reducing agent, 0.5 parts of dispersant, 1.5 parts of attapulgite, 1.5 parts of aluminum sulfate, 2 parts of the nano-early strength agent as described above, and 90 parts of water. Mix thoroughly according to GB / T 19139 "Test Methods for Oil Well Cement" to obtain a cement strength of 1.30 g / cm³. 3 Low-density cement slurry.

[0080] The low-density cement slurries prepared in Examples 1-3 and Comparative Examples 1-3 were subjected to performance tests according to GB / T 19139 "Test Methods for Cement in Oil Wells" and GB / T 1346 "Test Methods for Standard Consistency Water Requirement, Setting Time and Soundness of Cement". The thixotropy was evaluated by the difference between the 10s and 30min gel strength measured by a six-speed rotational viscometer. The larger the difference, the better the thixotropy. The results are shown in Table 1.

[0081] Table 1 Performance test data of low-density cement paste system

[0082]

[0083] As shown in Table 1, the cement slurry density formed in Examples 1, 2, and 3 according to the specific implementation method is 1.20 g / cm³. 3 With 1.30g / cm 3The test results in the table above show that, compared with Comparative Example 1, Example 1 contains a low-density cement slurry without vitrified perlite, fly ash, slag, and thixotropic agent; Comparative Example 2 contains a low-density cement slurry without nano-early strength agent; and Comparative Example 3 contains a low-density cement slurry without nano-silica stability. Example 1, containing thixotropic agent, showed a 10s-30min difference in cementitious strength of 40.2 Pa (comparative Example 1 showed 7.4 Pa). This indicates that introducing thixotropic agent can improve the thixotropic recovery speed and structural stability of the cement slurry system, which is beneficial for preventing leakage. Furthermore, Example 1, containing vitrified perlite, fly ash, and slag, had a 24h compressive strength of 5.0 MPa, lower than the 14.2 MPa of Comparative Example 1. This is because vitrified perlite has lower strength than hollow glass microspheres, and the reaction rate of fly ash and slag is lower than that of cement clinker hydration, resulting in a lower early 24h compressive strength. The addition of vitrified perlite, fly ash, and slag facilitates drilling after plugging, preventing excessive cement stone strength from causing the plug to deviate. Similarly, Example 2, compared to Example 3, shows an increase in the content of fly ash and slag powder in the solid admixtures, reducing the 24-hour compressive strength from 7.5 MPa to 5.5 MPa. Comparing Comparative Example 2 with Example 2, due to the absence of nano-early strength components, the setting time reaches 600 minutes, while the setting time of Example 2 is shortened to 330 minutes, demonstrating that nano-early strength agents can shorten the slurry gelation time, allowing for earlier plugging and saving drilling time. Furthermore, Comparative Example 3, lacking nano-silica stabilizers, exhibits a longitudinal density difference as high as 0.21 g / cm³. 3 This leads to poor stability in the low-density cement slurry system, severe sedimentation of the cement slurry, and weakened thixotropy of the slurry, affecting the safety of leak sealing construction and reducing the success rate of leak sealing. In summary, the dual-nano low-density thixotropic cement slurry system exhibits an even lower density (cement slurry density 1.20-1.30 g / cm³). 3 Cement plugs have the advantages of effectively retaining themselves in the wellbore, rapidly forming a gel structure due to their high thixotropy, having relatively low compressive strength which is beneficial for drilling plugs, and shortening the setting time to save on production costs.

[0084] The cement slurry system of this invention uses hollow glass microspheres and vitrified perlite as weight-reducing agents, forming a weight of 1.20-1.30 g / cm³. 3The lower-density cement slurry system, due to the lower cost of vitrified perlite, not only reduces costs but also facilitates the formation of a longer cement plug within the wellbore. External admixtures reduce the strength of the cement stone, while nano-hydrated silica can shorten the setting time while maintaining lower cement stone strength, thus saving plugging time and preventing plug deviation. Nano-silica can effectively improve the stability of the cement slurry and has a certain thixotropic effect. The composite thixotropic agent can form a network structure through chemical bonds, giving the cement slurry good pseudoplasticity and shear dilution properties. When at rest, the chemical bonds reconnect and interact with the charge of the cement particles to form a network structure, rapidly developing a certain gel strength, thereby achieving the production goals of efficient plugging, improved quality, and increased efficiency.

[0085] Those skilled in the art will recognize that the embodiments described herein are intended to help the reader understand the implementation methods of the present invention, and should be understood that the scope of protection of the present invention is not limited to such specific statements and embodiments. Those skilled in the art can make various other specific modifications and combinations based on the technical teachings disclosed in this invention without departing from the spirit of the invention, and these modifications and combinations are still within the scope of protection of the present invention.

Claims

1. A dual-nano low-density thixotropic cement slurry system, characterized in that: Its composition, by weight, includes: 40-60 parts of Grade G oil well cement, 14-25 parts of solid weight-reducing agent, 20-45 parts of solid admixture, 3-6 parts of solid thixotropic agent, 0.5-2 parts of nano stabilizer, 1-3 parts of nano early strength agent, 1-3 parts of water loss reducing agent, 0.5-1 part of dispersant, and 70-100 parts of water.

2. The dual-nano low-density thixotropic cement slurry system according to claim 1, characterized in that: The solid weight-reducing agent includes hollow glass microspheres and vitrified perlite; The weight ratio of the hollow glass microspheres to the vitrified perlite is 20-40:60-80; The hollow glass microspheres have a density of 0.39-0.42 g / cm³. 3 Specific surface area 0.2-2m² 3 / g; The vitrified perlite has a density of 0.80-1.20 g / cm³. 3 Specific surface area 1-3m² 3 / g.

3. The dual-nano low-density thixotropic cement slurry system according to claim 1, characterized in that: The solid admixtures include fly ash, slag powder, and microsilica; the weight ratio of fly ash: slag powder: microsilica is 40-60: 15-25: 20-50.

4. The dual-nano low-density thixotropic cement slurry system according to claim 3, characterized in that: The fly ash is grade II fly ash with a specific surface area greater than 300 m². 3 / kg, loss on ignition less than 8%; The slag powder is of grade S95, with a specific surface area of ​​400-500 m². 3 / kg; The density of the microsilicon is 1.20 g / cm³. 3 The SiO2 content is greater than 85%, and the particle size is 0.1-0.3μm.

5. The dual-nano low-density thixotropic cement slurry system according to claim 1, characterized in that: The solid thixotropic agent comprises attapulgite and sulfate, wherein the weight ratio of attapulgite to sulfate is 40-50:50-60.

6. The dual-nano low-density thixotropic cement slurry system according to claim 5, characterized in that: The attapulgite clay has a colloidal value of 55-60 ml / 15 g and a specific surface area of ​​400-500 m². 3 / g; The sulfate is one or more of aluminum sulfate, sodium sulfate, and calcium sulfate.

7. The dual-nano low-density thixotropic cement slurry system according to claim 1, characterized in that: The nano stabilizer is hydrophilic nano silica with a particle size of 10-100 nm; the water loss reducing agent is an AMPS-type water loss reducing agent; and the dispersant is a comb-type polycarboxylic acid dispersant.

8. The dual-nano low-density thixotropic cement slurry system according to claim 1, characterized in that: The nano-early strength agent is a nano-sized hydrated calcium silicate type early strength agent with a particle size of 10-100nm. Its composition by weight ratio is: soluble calcium salt: soluble silicate: dispersant: coupling agent in a weight ratio of 30-45: 25-45: 20-30: 5-10.

9. The dual-nano low-density thixotropic cement slurry system according to claim 8, characterized in that: The soluble calcium salt is at least one of calcium nitrate tetrahydrate and calcium formate; the soluble silicate is at least one of sodium metasilicate pentahydrate and sodium metasilicate nonahydrate.

10. The dual-nano low-density thixotropic cement slurry system according to claim 8, characterized in that: The coupling agent is one or more of polyvinyl alcohol, polyvinylpyrrolidone, polyethylene glycol, or sodium dodecylbenzenesulfonate; the dispersant is a comb-type polycarboxylic acid dispersant.

11. The dual-nano low-density thixotropic cement slurry system according to claim 8, characterized in that: The preparation method of the nano-sized hydrated calcium silicate type early strength agent includes the following steps: The dispersant and coupling agent are prepared into a first solution; A second solution is prepared from soluble calcium salts, and a third solution is prepared from soluble silicates. Under continuous stirring and nitrogen atmosphere, the second solution and the third solution are simultaneously added dropwise to the first solution. The dropping system is controlled under a set temperature condition and a set time is added. Then, under the same temperature condition, the dropping system is stirred for a set time to obtain a hybrid suspension. After centrifugation, filtration, and drying of the hybrid suspension, the resulting particles are ground into solid nano-sized hydrated calcium silicate.

12. The dual-nano low-density thixotropic cement slurry system according to claim 11, characterized in that: After preparing a dispersant solution with a mass concentration of 10%-15%, add the coupling agent, stir evenly, and adjust the pH value to between 11.0 and 13.5 to obtain the first solution; The second solution is a soluble calcium salt solution with a mass concentration of 10%-20%, and the third solution is a soluble silicate solution with a mass concentration of 10%-20%, wherein the molar ratio of calcium ions to silicate ions is 1:1-1.75:

1.

13. The dual-nano low-density thixotropic cement slurry system according to claim 11, characterized in that: The stirring speed is 700 rpm - 7000 rpm, the temperature of the dropping system is set at 20℃ - 40℃, the dropping time is set at 3h - 5h, and the second solution and the third solution are dropped simultaneously; during the dropping process, the pH value of the dropping system is controlled between 11.0 and 13.5, and the stirring time of the dropping system is set at 1 day; The drying process involves drying in a vacuum drying oven at a temperature of 50℃-70℃ for 1 day, followed by continued drying at room temperature in a desiccator for 2 hours.

14. The application of the dual nano-low density thixotropic cement slurry system according to any one of claims 1-13 in plugging operations for severe and malignant leakage during drilling.

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