Fly ash composite low-density cement slurry and preparation method and application thereof

Abstract

The application relates to the technical field of oilfield chemical cementing, in particular to a fly ash composite low-density cement slurry as well as a preparation method and application thereof. The fly ash composite low-density cement slurry is prepared by multiple compounding of G-grade cement, fly ash-based floating beads as a weight-reducing agent, fly ash as an active filler, and micro-silicon powder, and by adding additives such as a fluid loss reducer, a retarder and a drag reducer, and by optimizing the material ratio, a 1.10-1.50 g / cm 3 continuous-density cement slurry system can be formed. The system has good slurry stability, low fluid loss, no obvious water separation, high compressive strength of the cured cement, excellent wellbore sealing effect, and the like, and the industrial solid waste fly ash is used to replace high-cost weight-reducing materials, so that the cementing cost is greatly reduced, and the solid waste is recycled.

Description

Technical Field

[0001] This invention relates to the field of oilfield chemical cementing technology, specifically to a fly ash composite low-density cement slurry, its preparation method, and its application. Background Technology

[0002] In oil and gas well cementing operations, low-pressure, easily leaking formations have long been a technical challenge hindering engineering implementation. These formations generally suffer from low pressure bearing capacity, susceptibility to well leakage, and reservoir contamination by cement slurry, requiring the cement slurry to possess both low density and low water loss characteristics. Currently, there are two main technical approaches in the industry to reduce cement slurry density, but both have inherent drawbacks that are difficult to overcome. One approach uses hollow glass microspheres or cenospheres as weight-reducing agents (such as Chinese patents with application numbers 202411506013.6 and 202011033444.7), which can reduce the slurry density to 1.20-1.50 g / cm³. 3 However, these materials often have strong surface inertness and low roughness, resulting in weak interfacial bonding with the cement matrix. When their dosage is high, they easily form weak interfaces within the cement paste, significantly inhibiting the development of cement paste strength and affecting cementing quality. Furthermore, their cost is typically 2-3 times that of conventional systems, making them economically unfeasible and limiting large-scale field applications. Another type is foamed cement systems (such as Chinese patent application number 201710701331.1), which can reduce the density to 1.00-1.30 g / cm³. 3 However, it has stringent requirements for foam stability, mixing process and on-site equipment. Under high temperature, high pressure and complex flow conditions downhole, it is prone to problems such as density fluctuation and slurry stratification, resulting in high construction risks.

[0003] Fly ash, as a major industrial solid waste emitted by coal-fired power plants, has advantages such as wide availability, low cost, light weight, and potential pozzolanic activity. Its bulk density is typically 0.6-1.0 g / cm³. 3 With a cost only 1 / 10 that of hollow glass microspheres, fly ash can not only serve as a weight-reducing material in cement slurry, but also participate in secondary hydration reactions under alkaline conditions and the activation of cement hydration products, generating hydration products with cementitious properties. Therefore, fly ash is an ideal material with the potential to reduce weight, fill, and enhance activity. However, directly applying fly ash to high-performance oil well cement systems still faces challenges. One challenge is that it is difficult to achieve a concentration below 1.30 g / cm³ when used alone. 3The ultra-low density of fly ash usually requires compounding with materials such as cenospheres, hollow glass microspheres, or expanded perlite. However, these weight-reducing agents also have the problem of poor interfacial bonding, and high dosage will inhibit the development of cement strength. In addition, excessive fly ash dosage can easily lead to insufficient early strength (for example, in Chinese patent application number 202111529222.9, when the dosage is >45%, the 48-hour compressive strength is only about 7MPa). Moreover, the solid phase content of the low-density system is reduced, which can easily lead to problems such as sedimentation, increased free liquid and increased water loss, affecting the integrity of cement and the interlayer sealing effect.

[0004] Therefore, how to fully leverage the advantages of fly ash's light weight, low cost, and pozzolanic activity while solving the problems of dispersion stability, early strength development, and low water loss control in oil well cement slurry has become a key technical bottleneck that urgently needs to be overcome in the engineering application of fly ash-based low-density cement slurry systems. Summary of the Invention

[0005] To address the aforementioned problems, the present invention aims to provide a fly ash composite low-density cement slurry, its preparation method, and its application. This cement slurry uses fly ash as the main weight-reducing material. The particle size distribution is optimized through a multi-component compounding of fly ash, artificial fly ash cenospheres, and microsilica. Combined with additives such as fluid loss reducers, retarders, and drag reducers, a low-density cement slurry system is formulated. While ensuring the slurry's workability and curing strength, it significantly reduces cementing costs and is suitable for cementing operations in low-pressure, easily leaking formations and reservoirs requiring protection.

[0006] A fly ash composite low-density cement slurry includes solid raw materials and mixing water. By mass percentage, the solid raw materials include the following components: 50%-75% grade G cement, 8%-25% fly ash, 3%-25% artificial fly ash-based cenospheres, 2%-15% silica fume, 1.5%-6% water loss reducing agent, 0.2%-1.5% retarder, 0.1%-0.8% drag reducing agent, and a water-to-solid mass ratio (the mass ratio of mixing water to the above solid raw materials) of 0.70-0.95.

[0007] Furthermore, the density of the cement slurry is 1.10 g / cm³. 3 -1.50g / cm 3 .

[0008] The fly ash is Grade II or higher, with a fineness meeting the following requirements: ≤25% residue on a 45μm square-hole sieve, ≤8% loss on ignition, and ≤1% moisture content. It is used as an active filler and weight-reducing agent. Fly ash has a dual function: first, it reduces the overall density of the system by replacing part of the Grade G cement; second, under alkaline conditions and the activation of cement hydration products, it participates in secondary hydration reactions to generate cementitious products such as hydrated calcium silicate (CSH) gel and hydrated calcium aluminate, thereby improving the later-stage strength and density of the cement paste.

[0009] Furthermore, the particle size distribution of the artificial fly ash-based cenospheres is 200-2000 μm, and the bulk density is 0.25-0.40 g / cm³. 3 The breakage rate measured under 20MPa isostatic pressure is ≤5%.

[0010] Furthermore, the artificial fly ash-based cenospheres have a porous internal structure and a dense glaze layer on the surface; the glaze layer contains rod-shaped calcium silicate crystals, which are interspersed in the glaze layer to improve the strength of individual cenosphere particles and the surface roughness.

[0011] Furthermore, the microsilica powder is a white or light gray powder with a SiO2 content ≥92.2% and an average particle size of 0.1-0.5μm. It is used to fill the micropores between cement particles and fly ash particles to optimize particle size distribution and improve the density, impermeability and curing strength of cement stone.

[0012] Furthermore, the water loss reducing agent is a copolymer of 2-acrylamido-2-methylpropanesulfonic acid (AMPS) and acrylamide (AM) obtained by aqueous solution polymerization, wherein the molar ratio of AMPS to AM is (1.2-1.5):1.

[0013] Furthermore, the retarder is a compound of one or more of citric acid, sodium gluconate, or lignin sulfonate, used to adjust the thickening construction time.

[0014] Furthermore, the drag-reducing agent is a sulfonated acetone-formaldehyde condensate or a high water-reducing polycarboxylate superplasticizer.

[0015] A method for preparing fly ash composite low-density cement slurry specifically includes the following steps: S1. Weigh out G-grade cement, fly ash, artificial fly ash-based cenospheres, silica fume, water loss reducer, retarder and drag reducer according to the proportions. S2. Stir and mix the above solid raw materials evenly to obtain a mixed dry material; S3. Add mixing water to the mixed dry materials and stir at high speed for 30-120 seconds until the slurry is uniform and free of lumps, thus obtaining fly ash composite low-density cement slurry.

[0016] Furthermore, the artificial fly ash-based cenospheres described in step S1 are prepared using the following method: raw materials are weighed according to the specified ratio, including fly ash, glass powder, silicon carbide, and carboxymethyl cellulose (CMC). The above raw materials are mixed evenly and granulated to obtain a ceramic pellet body. After drying the ceramic pellet body, it is coated with a mixture of carbon black and quicklime. Then, the coated ceramic pellet body is placed in a rotary kiln and subjected to expansion treatment at 1150℃-1200℃ to obtain artificial fly ash-based cenospheres.

[0017] Furthermore, a mixture of carbon black and slaked lime is used to coat the ceramsite blank, specifically including the following steps: First, carbon black and slaked lime are weighed according to a predetermined mass ratio and dry-mixed until they are uniformly mixed to obtain a coating material; then, the ceramsite blank is placed in a drum and rotated at a uniform speed, while water accounting for 2-6% of the mass of the ceramsite blank is sprayed into the drum to improve the wettability of the ceramsite blank surface and its adhesion to the coating material; then, the coating material is weighed according to a ratio of 1.5-4.5% of the mass of the ceramsite blank and added to the drum, and the coating material is uniformly adhered to the surface of the ceramsite blank by rolling and mixing, and the continuity of the coating layer is judged by the uniformity of the color of the ceramsite blank surface; finally, it is left to stand and dried to improve the stability of the coating layer, and the coated ceramsite blank is obtained.

[0018] Furthermore, when preparing artificial fly ash-based cenospheres, 69%-78% fly ash, 18%-27% glass powder, and 1%-5% silicon carbide are weighed by mass percentage, with an additional 0.01%-1% carboxymethyl cellulose (i.e., the mass of carboxymethyl cellulose accounts for 0.01%-1% of the sum of the masses of the above-mentioned fly ash, glass powder, and silicon carbide).

[0019] Furthermore, in the preparation of artificial fly ash-based cenospheres, the particle size of the ceramsite blank is less than 10 mesh, and the drying is carried out at 110°C for 24 hours; in the mixture of carbon black and slaked lime, the mass fraction of carbon black is 4% and the mass fraction of slaked lime is 96%.

[0020] Furthermore, in step S3, the liquid-to-solid mass ratio of the mixing water to the mixed dry material is 0.70-0.95, which is adjusted according to the target density. Furthermore, the mixing water mentioned in step S3 is obtained by dissolving the defoamer in water. The defoamer is a dimethyl silicone oil emulsion, and its dosage is 0.05%-0.2% of the water mass.

[0021] Furthermore, stir at a high speed of 3500 rpm.

[0022] The aforementioned fly ash composite low-density cement slurry is used for well cementing operations.

[0023] Compared with the prior art, the present invention has the following beneficial effects: (1) This invention uses fly ash as the main raw material to prepare artificial fly ash-based cenospheres, increasing the content of fly ash (including artificial fly ash-based cenospheres and virgin fly ash) in the slurry to 50%. The cost of artificial fly ash-based cenospheres is only one-tenth that of commercial hollow glass microspheres; at the same time, fly ash, as an industrial solid waste emitted by coal-fired power plants, is widely available and inexpensive. This technology realizes the high-value-added resource utilization of fly ash, combining significant economic benefits with the environmental value of solid waste disposal.

[0024] (2) This invention uses fly ash as the main raw material to prepare artificial fly ash-based cenospheres, and by modifying them, the fly ash content is increased, thereby reducing the density of cement paste. Unlike the traditional method of reducing density by reducing viscosity, the low-density cement paste prepared by this invention has higher stability.

[0025] (3) In preparing artificial fly ash-based cenospheres according to the present invention, a mixture of carbon black and slaked lime is used to coat the cenosphere blanks. On the one hand, the coating layer acts as an anti-sticking agent during the expansion process in the rotary kiln, preventing the blanks from sticking together and ensuring the quality of cenosphere molding. On the other hand, the slaked lime in the coating layer reacts with the melt on the surface of the blank at high temperature to generate rod-shaped calcium silicate. The rod-shaped calcium silicate is distributed intermittently in the dense glaze layer on the surface of the cenospheres, which not only plays a role in toughening and strengthening and improving the strength of single particles, but also significantly improves the surface roughness and surface activity of the cenospheres, thereby enhancing the interfacial bonding characteristics between the artificial fly ash-based cenospheres and the cement paste. This effectively overcomes the inherent defect of weak interfacial bonding caused by the strong surface inertia and low roughness of traditional hollow glass microspheres, so that the safe dosage of artificial fly ash-based cenospheres in cement paste can be greatly improved, and a weak interfacial transition zone will not be formed inside the cement matrix due to high dosage.

[0026] (4) This invention utilizes the synergistic effect of the pozzolanic activity of fly ash and the physical filling effect of silica fume to optimize the microstructure of cement stone: silica fume, with its ultrafine particle size (0.1-0.5 μm), fills the gaps between cement particles and fly ash particles, forming a dense packing and reducing matrix porosity; simultaneously, fly ash participates in secondary hydration reactions in the alkaline environment generated during cement hydration, continuously generating cementitious products such as hydrated calcium silicate (CSH) gel and hydrated calcium aluminate, improving the later-stage strength and density of the cement stone. The synergistic effect of these two factors significantly improves the 24-hour compressive strength, meeting the engineering requirements for rapid drilling and interlayer sealing after cementing.

[0027] (5) By optimizing the raw material ratio of fly ash (10-50μm), artificial fly ash-based cenospheres (200-1000μm), and microsilica (0.1-0.5μm), multi-level compact packing from submicron to millimeter scale is achieved, effectively improving the solid phase packing density and suspension stability of the slurry. Combined with the synergistic regulation of AMPS / AM copolymer water loss reducers, retarders, and drag reducers, the resulting low-density cement slurry system (1.10-1.50 g / cm³) 3 It features good slurry stability, low water loss, and little or no free liquid, solving construction problems such as sedimentation and increased water loss caused by the reduction of solid content in low-density systems, and ensuring the integrity of the cement ring and the quality of long-term sealing. Attached Figure Description

[0028] Figure 1 These are SEM images of the artificial fly ash-based cenospheres in the embodiments; Figure 2 This is Example 1 of the present invention (1.30 g / cm³). 3 Thickening time curve of fly ash cement slurry under heating conditions of 65℃×30MPa×30min; Figure 3 Example 2 of the present invention (1.46 g / cm³) 3 Thickening time curve of fly ash cement slurry under heating conditions of 70℃×40MPa×30min. Detailed Implementation

[0029] To better understand the content of this invention, it will be further described below with reference to specific embodiments and accompanying drawings. The following embodiments are based on the technology of this invention and provide detailed implementation methods and operating steps, but the scope of protection of this invention is not limited to the following embodiments.

[0030] The preparation method of artificial fly ash-based cenospheres in the embodiment is as follows: 72% fly ash, 25% glass powder, and 3% silicon carbide are weighed by mass percentage, and 0.2% carboxymethyl cellulose (i.e., the mass of carboxymethyl cellulose accounts for 0.2% of the total mass of the above fly ash, glass powder, and silicon carbide) are added as raw materials. The above raw materials are put into a mixer and mixed evenly. Then, a spray granulator is used for granulation to obtain a ceramic pellet body with a particle size of less than 10 mesh. Subsequently, the ceramic pellet body is dried at 110°C for 24 hours. After drying, the ceramic pellet body is coated with a mixture composed of carbon black and quicklime. The mass fraction of carbon black in the above mixture is 4%, and the mass fraction of quicklime is 96%. Finally, the coated ceramic pellet body is placed in a rotary kiln and expanded at 1180°C to obtain artificial fly ash-based cenospheres.

[0031] The obtained artificial fly ash-based cenospheres have a bulk density and particle size distribution of 200-2000 μm and a bulk density of 0.28 g / cm³. 3 The breakage rate measured under 20 MPa isostatic pressure was 1.29%.

[0032] The method of coating the ceramsite blank with a mixture of carbon black and quicklime includes the following steps: First, carbon black and quicklime are weighed according to a predetermined mass ratio and dry-mixed until they are uniformly mixed to obtain a coating material; then, the ceramsite blank is placed in a drum and rotated at a uniform speed, while water accounting for 3% of the mass of the ceramsite blank is sprayed into the drum to improve the wettability of the ceramsite blank surface and its adhesion to the coating material; next, the coating material is weighed according to a ratio of 2.5% of the mass of the ceramsite blank and added to the drum, and the coating material is uniformly adhered to the surface of the ceramsite blank by rolling and mixing, and the continuity of the coating layer is judged by the uniformity of the color of the ceramsite blank surface; finally, it is left to stand and dried to improve the stability of the coating layer, and the coated ceramsite blank is obtained.

[0033] Figure 1 These are SEM images of the artificial fly ash-based cenospheres prepared in the examples. Figure 1 As can be seen, a dense glaze layer is formed on the surface of the artificial fly ash-based cenospheres. Rod-shaped calcium silicate crystals are distributed within the glaze layer, and these rod-shaped calcium silicates are interleaved and embedded within the glaze layer. This structure helps to enhance the individual particle strength of the cenospheres and improve their surface roughness.

[0034] The preparation method of fly ash composite low-density cement slurry in the embodiment is as follows: G-grade cement, fly ash, artificial fly ash-based cenospheres, silica fume, water loss reducer, retarder, and drag reducer are weighed according to the proportion. The raw materials are mixed at low speed for 120s to obtain a uniform dry mixture. The defoamer is pre-dissolved in water to obtain mixing water. The amount of defoamer is 0.05% of the mass of water. The mixing water is then added to the above-mentioned dry mixture and stirred at high speed at 3500rpm for 90s until the slurry is uniform and free of lumps to obtain fly ash composite low-density cement slurry.

[0035] The defoamer is a dimethyl silicone oil emulsion.

[0036] The fly ash is Class II fly ash, with a fineness that meets the following requirements: ≤25% residue on a 45μm square hole sieve, ≤8% loss on ignition, and ≤1% moisture content.

[0037] The microsilica powder is white or light gray powder with a SiO2 content ≥92.2% and an average particle size of 0.1-0.5μm.

[0038] The water loss reducing agent is a copolymer of 2-acrylamido-2-methylpropanesulfonic acid (AMPS) and acrylamide (AM) obtained by aqueous solution polymerization, wherein the molar ratio of AMPS to AM is 1.5.

[0039] The retarder is a mixture of one or more of citric acid, sodium gluconate, or lignin sulfonate.

[0040] The drag-reducing agent is a high water-reducing polycarboxylate superplasticizer.

[0041] Characterization methods for fly ash composite low-density cement slurry: The density of cement slurry, compressive strength of cement stone, thickening time of cement slurry, rheological properties of cement slurry, water loss and exudation are characterized according to the national standard GB / T 33294-2025 "Test Methods for Cement in Deepwater Oil Wells".

[0042] Example 1: In this embodiment, the retarder is citric acid.

[0043] The raw material composition of this embodiment is as follows (by mass percentage): Grade G cement 63%, fly ash 14%, artificial fly ash-based cenospheres 10%, silica fume 9%, water loss reducer 3.5%, citric acid 0.3%, high water-reducing polycarboxylate superplasticizer 0.2%, and water-to-solid ratio (i.e., the mass ratio of mixing water to the above solid raw materials) 0.95.

[0044] The properties of the fly ash composite low-density cement slurry prepared in this embodiment were tested and are as follows: density 1.30 g / cm³, fluidity 250 mm, water exudation 0.37%, settling stability (top / bottom density) 1.30 / 1.31 g / cm³, water loss 20 mL × 2, 24-hour compressive strength 5.79 MPa, thickening time (65℃ × 30 MPa × 30 min): 257 min (30 Bc) / 276 min (60 Bc). Rheological properties are detailed in Table 1.

[0045] Under high-temperature test conditions (70℃×30MPa×30min), the density of the cement slurry was 1.33g / cm³. 3 .

[0046] Table 1 Rheological properties of cement paste In Table 1, n is the flow index and k is the consistency coefficient.

[0047] Figure 2 This is Example 1 of the present invention (1.30 g / cm³). 3 The thickening time curve of fly ash cement slurry under the heating conditions of 65℃×30MPa×30min is shown in the figure. Figure 2 It can be seen that under the conditions of 65℃, 30MPa and 30min heating, the consistency of 1.30g / cm³ fly ash cement slurry gradually increased in the initial stage of heating, and then basically stabilized at about 10 Bc, maintaining small fluctuations throughout the test, without obvious premature thickening. Even after 5 hours of testing, the slurry consistency had not yet reached 100 Bc, indicating that its thickening time was greater than 5 hours, demonstrating good thickening stability and construction safety.

[0048] Example 2: In this embodiment, the retarder is sodium gluconate.

[0049] The raw material composition of this embodiment is as follows (by mass percentage): Grade G cement 68%, fly ash 16%, artificial fly ash-based cenospheres 7%, silica fume 4%, water loss reducing agent 4%, sodium gluconate 0.7%, high water-reducing polycarboxylate superplasticizer 0.3%, and water-to-solid ratio (i.e., the mass ratio of mixing water to the above solid raw materials) 0.76.

[0050] The properties of the fly ash composite low-density cement slurry prepared in this embodiment, as tested, are as follows: density 1.46 g / cm³. 3 Flowability 260 mm, water separation rate 0.32%, settling stability (top / bottom density) 1.46 / 1.47 g / cm³ 3 Water loss 10 mL × 2, 24h compressive strength 8.62 MPa, thickening time (70℃ × 40 MPa × 30 min): 310 min (30 Bc) / 324 min (60 Bc). Rheological properties are detailed in Table 2.

[0051] Under high-temperature test conditions (75℃ × 40MPa × 30min), the density of the cement slurry was 1.48 g / cm³. 3 .

[0052] Table 2 Rheological properties of cement paste In Table 2, n is the flow index and k is the consistency coefficient.

[0053] Figure 3 Example 2 of the present invention (1.46 g / cm³) 3 The thickening time curve of fly ash cement slurry under the heating conditions of 70℃×40MPa×30min is shown in the figure. Figure 3 It can be seen that under the conditions of 70℃, 40MPa and 30min heating, the consistency of 1.46g / cm³ fly ash cement slurry gradually increased in the early stage of heating, and then remained stable at about 8Bc for a long time with small fluctuations. Although the consistency increased rapidly in the later stage of the test, it did not reach 100Bc at the end of the test, indicating that the cement slurry has good thickening stability and construction safety, and its thickening time is longer than the test duration.

[0054] Example 3: In this embodiment, the retarder is citric acid.

[0055] The raw material composition of this embodiment is as follows (by mass percentage): 60% Grade G cement, 14% fly ash, 13% artificial fly ash-based cenospheres, 9% silica fume, 3.5% water loss reducer, 0.3% citric acid, 0.2% high water-reducing polycarboxylate superplasticizer, and a water-to-solid ratio (i.e., the mass ratio of mixing water to the above solid raw materials) of 0.85.

[0056] The properties of the fly ash composite low-density cement slurry prepared in this embodiment, as tested, are as follows: density 1.25 g / cm³. 3 Flowability 241 mm, water separation rate 0.31%, settling stability (top / bottom density) 1.25 / 1.26 g / cm³ 3 Water loss 18 mL × 2, 24h compressive strength 5.21 MPa, thickening time (65℃ × 30 MPa × 30 min): 230 min (30 Bc) / 248 min (60 Bc). Rheological properties are detailed in Table 3.

[0057] Under high-point test conditions (70℃×30MPa×30min), the density of cement slurry was 1.26g / cm³. 3 .

[0058] Table 3 Rheological properties of cement slurry In Table 3, n is the flow index and k is the consistency coefficient.

[0059] Example 4: In this embodiment, the retarder is sodium gluconate.

[0060] The raw material composition of this embodiment is as follows (by mass percentage): 70% Grade G cement, 15% fly ash, 6% artificial fly ash-based cenospheres, 4% silica fume, 4% water loss reducing agent, 0.7% sodium gluconate, 0.3% high water-reducing polycarboxylate superplasticizer, and a water-to-solid ratio (i.e., the mass ratio of mixing water to the above solid raw materials) of 0.76.

[0061] The fly ash composite low-density cement slurry prepared in this embodiment has the following properties after testing: density 1.49 g / cm³. 3 Flowability 278 mm, water separation rate 0.28%, settling stability (top / bottom density) 1.49 / 1.50 g / cm³ 3 Water loss: 8 mL × 2; 24-hour compressive strength: 9.24 MPa; Thickening time (70℃ × 40 MPa × 30 min): 335 min (30 Bc) / 350 min (60 Bc). Rheological properties are detailed in Table 4.

[0062] Under high-point test conditions (75℃×40MPa×30min), the density of cement slurry was 1.51g / cm³. 3 .

[0063] Table 4 Rheological properties of cement paste In Table 4, n is the flow index and k is the consistency coefficient.

[0064] Comparative Example 1: Patent publication number CN116262656A discloses a low-density cement slurry for well cementing and its preparation method. Example 2 in the patent specification is used as Comparative Example 1 of this invention. This comparative example, by weight, has the following formulation: 25 parts oil well cement, 45 parts fly ash, 20 parts slag, 10 parts silica fume, 4 parts stabilizer, 3.5 parts activator, 2.5 parts fluid loss reducer, 0.1 parts retarder, and 137 parts water (water-to-solid ratio of 1.27). The density of the prepared cement slurry is 1.30 g / cm³. 3 The cement slurry density is the same as that of Example 1 of this invention, and the performance comparison results are shown in Table 5.

[0065] Table 5 Performance Comparison The formulation systems of each density in Examples 1-4 of this invention have stable performance. Compared with traditional low-density cement slurry, they have significantly reduced costs and have excellent overall performance, which can meet the technical requirements of cementing in various low-pressure formations.

[0066] Comparative Example 1 had a water-to-solid ratio as high as 1.27, relying on a large amount of free water to dilute the solid phase and reduce density. Excessive free water not only severely weakened the mechanical framework of the cement paste but also created numerous capillary pores after hardening, resulting in a 24-hour compressive strength of only 3.6 MPa. In contrast, this invention achieves physical density reduction by introducing artificial fly ash-based cenospheres, controlling the water-to-solid ratio at 0.95, achieving the same density without sacrificing the solid phase bonding quality. The low water-to-solid ratio reduces the capillary volume in the hardened paste from the source, ensuring the density of the cement paste. The 24-hour compressive strength reaches 5.79 MPa, an increase of 60.8% compared to Comparative Example 1; the water separation rate is reduced to below 0.37%, significantly improving the sedimentation stability of the paste.

[0067] Regarding rheological properties, the slurry of Example 1 of this invention has a flow index n = 0.60 and a consistency coefficient k = 0.37 Pa•s. n In contrast, in Comparative Example 1, n=0.85 and k=0.77 Pa•s nThe lower n-value indicates that the slurry of this invention has stronger shear dilution characteristics—low viscosity and low flow resistance under high shear pumping conditions, and rapid viscosity recovery after settling, which is beneficial for suppressing annular air channeling; the k-value is only half that of Comparative Example 1, with lower overall viscous resistance, providing good construction adaptability for deep-water long-distance pumping. Furthermore, the thickening time (30Bc) of Example 1 of this invention is 257 min, which is 49 min longer than the 208 min of Comparative Example 1, providing a more ample safety window for deep-water cementing and displacement operations.

[0068] In summary, under the same density conditions, the present invention, by using artificial fly ash-based cenospheres for physical densification instead of high water-to-solid ratio dilution and densification, demonstrates significant advantages in compressive strength, slurry stability, rheological properties, and construction safety.

[0069] The above description is merely an embodiment of the present invention and is not intended to limit the present invention in any way. The present invention can also have other embodiments based on the above structure and function, which will not be listed hereafter. Therefore, any simple modifications, equivalent changes, and alterations made by those skilled in the art to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A fly ash composite low-density cement slurry, characterized in that, The mixture includes solid raw materials and mixing water. By mass percentage, the solid raw materials include the following components: 50%-75% G-grade cement, 8%-25% fly ash, 3%-25% artificial fly ash-based cenospheres, 2%-15% silica fume, 1.5%-6% water loss reducer, 0.2%-1.5% retarder, and 0.1%-0.8% drag reducer. The mass ratio of mixing water to the above solid raw materials is 0.70-0.

95.

2. The fly ash composite low-density cement slurry as described in claim 1, characterized in that, The fly ash is grade II or above; the silica fume is white or light gray powder with a SiO2 content ≥92.2% and an average particle size of 0.1-0.5μm; the water loss reducing agent is a copolymer of AMPS and AM obtained by aqueous solution polymerization, wherein the molar ratio of AMPS to AM is (1.2-1.5):

1.

3. The fly ash composite low-density cement slurry as described in claim 1, characterized in that, The retarder is one or more of citric acid, sodium gluconate, or lignin sulfonate; the drag-reducing agent is sulfonated acetone-formaldehyde condensate or high water-reducing polycarboxylate superplasticizer.

4. The fly ash composite low-density cement slurry as described in claim 1, characterized in that, The density of the cement slurry is 1.10 g / cm³. 3 -1.50g / cm 3 The artificial fly ash-based cenospheres have a particle size distribution of 200-2000 μm and a bulk density of 0.25-0.40 g / cm³. 3 The breakage rate measured under 20MPa isostatic pressure is ≤5%; the artificial fly ash-based cenospheres have a porous internal structure and a dense glaze layer on the surface; the glaze layer contains rod-shaped calcium silicate crystals, which are interspersed in the glaze layer.

5. A method for preparing fly ash composite low-density cement slurry as described in any one of claims 1-4, characterized in that, Specifically, the following steps are included: S1. Weigh out G-grade cement, fly ash, artificial fly ash-based cenospheres, silica fume, water loss reducer, retarder and drag reducer according to the proportions. S2. Stir and mix the above solid raw materials evenly to obtain a mixed dry material; S3. Add mixing water to the mixed dry materials and stir at high speed until the slurry is uniform and free of lumps, thus obtaining fly ash composite low-density cement slurry.

6. The method for preparing fly ash composite low-density cement slurry as described in claim 5, characterized in that, The artificial fly ash-based cenospheres described in step S1 are prepared by the following method: raw materials, including fly ash, glass powder, silicon carbide, and carboxymethyl cellulose, are weighed according to the formula. The raw materials are mixed evenly and granulated to obtain ceramsite blanks. After drying the ceramsite blanks, they are coated with a mixture of carbon black and quicklime. The coated ceramsite blanks are placed in a rotary kiln and subjected to expansion treatment at 1150℃-1200℃ to obtain artificial fly ash-based cenospheres.

7. The method for preparing fly ash composite low-density cement slurry as described in claim 6, characterized in that, When preparing artificial fly ash-based cenospheres, 69%-78% fly ash, 18%-27% glass powder, and 1%-5% silicon carbide are weighed out by mass percentage, with 0.01%-1% carboxymethyl cellulose added; the particle size of the ceramsite body is less than 10 mesh, and the drying is carried out at 110℃ for 24 hours; in the mixture composed of carbon black and quicklime, the mass fraction of carbon black is 4% and the mass fraction of quicklime is 96%.

8. The method for preparing fly ash composite low-density cement slurry as described in claim 6, characterized in that, In preparing artificial fly ash-based cenospheres, a mixture of carbon black and slaked lime is used to coat the ceramsite blank. The specific steps include: First, carbon black and slaked lime are weighed according to a specific ratio and dry-mixed until they are uniformly mixed to obtain a coating material. Then, the ceramsite blank is placed in a drum and rotated at a uniform speed, while simultaneously spraying 2-6% of the ceramsite blank's mass of water into the drum. Next, the coating material is weighed according to a ratio of 1.5-4.5% of the ceramsite blank's mass and added to the drum. The coating material is then rolled and mixed to ensure uniform adhesion to the surface of the ceramsite blank. Finally, the mixture is allowed to stand and dried to obtain the coated ceramsite blank.

9. The method for preparing fly ash composite low-density cement slurry as described in claim 5, characterized in that, In step S3, the liquid-to-solid mass ratio of the mixing water to the mixed dry material is 0.70-0.95; the mixing water is obtained by dissolving the defoamer in water, and the defoamer is a dimethyl silicone oil emulsion, the amount of which is 0.05%-0.2% of the water mass.

10. The application of fly ash composite low-density cement slurry as described in any one of claims 1-4 in well cementing operations.

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