Ultralow-density proppant for increasing yield of coal bed gas as well as preparation method and application of ultralow-density proppant

Abstract

The invention relates to the technical field of coalbed methane yield increase, in particular to an ultralow-density proppant for coalbed methane yield increase and a preparation method and application thereof. The preparation method comprises the following steps: S1, carrying out ball milling, drying and sieving on kaolin and ferric oxide to obtain powder; s2, dissolving polyether sulfone in N-methyl pyrrolidone to form a PES / NMP solution, adding the PES / NMP solution into the powder for multiple times, and performing ultrasonic dispersion and stirring to obtain slurry; s3, dropwise adding the slurry into deionized water through a 0.2 mm needle, rapidly curing and granulating, standing, and drying to obtain a ceramsite proppant sintered blank; and S4, sintering the ceramsite proppant sintering blank to obtain the ultralow-density proppant. The prepared ultralow-density proppant for increasing the yield of the coal bed gas is low in density, can be carried and conveyed by clear water or saline water, avoids chemical dosage consumed for preparing gel, does not contain residues, does not block strata, reduces pollution to underground water and environment, improves the fracturing efficiency and reduces the fracturing cost.

Description

Technical Field

[0001] This invention relates to the field of coalbed methane production enhancement technology, and in particular to an ultra-low density proppant for coalbed methane production enhancement, its preparation method, and its application. Background Technology

[0002] Coalbed methane (CBM), as an important unconventional natural gas resource, plays a crucial role in alleviating energy pressure through its efficient development. However, CBM reservoirs possess unique characteristics of "three lows and one high": low gas saturation, low permeability, low reservoir pressure, and high degree of metamorphism. This presents significant challenges to CBM extraction. To improve the productivity of coalbed methane wells, hydraulic fracturing technology has become the primary means of CBM reservoir stimulation. Its core principle is to create and maintain high-conductivity fractures in the reservoir to enhance the desorption and migration of CBM.

[0003] In conventional hydraulic fracturing, proppant (such as silica sand, ceramsite, or coated sand) is typically used to fill fractures and prevent them from re-closing under closure pressure, thereby maintaining the fracture's conductivity. However, these proppants generally suffer from high density and poor suspension properties, necessitating the use of high-viscosity proppant-carrying fluids (such as water-based polymer fracturing fluids like guar gum) for delivery during fracturing operations. While high-viscosity proppant-carrying fluids can improve the proppant's carrying capacity, they also bring a series of negative impacts: 1. Increased risk of reservoir damage: Coalbed methane reservoirs have a dual-pore structure (matrix pores and fracture pores), making them extremely sensitive to external conditions. High-viscosity sand-carrying fluids contain polymers and other organic additives that easily penetrate micropores and fractures, causing physical blockage or chemical adsorption, reducing reservoir permeability, and affecting the desorption and seepage of coalbed methane.

[0004] 2. Increased fracturing costs: The preparation and use of polymer-based proppant-carrying fluids increase material costs and construction complexity. At the same time, subsequent treatments (such as backflow and disposal) will further increase mining costs.

[0005] 3. Environmental and process issues: Polymer fracturing fluids are difficult to backflow and tend to remain in the reservoir, causing long-term pollution; in addition, high-viscosity fluids are difficult to completely backflow in low-pressure coal seams, further aggravating reservoir damage.

[0006] In existing technologies, although attempts have been made to improve the delivery performance of proppant by optimizing its particle size, shape, or surface modification, the high-density nature of proppant remains unchanged, and it still heavily relies on high-viscosity proppant-carrying fluids. Meanwhile, to reduce reservoir damage, fracturing fluid systems tend to simplify the use of additives, particularly avoiding organic polymers, which contradicts the need for proppant suspension delivery.

[0007] Therefore, achieving effective proppant delivery and maintaining long-term fracture conductivity without relying on high-viscosity proppant-carrying fluids has become a key challenge in coalbed methane hydraulic fracturing technology. There is an urgent need to develop a new type of low-damage, low-cost proppant material or fracturing process that can adapt to the special geological conditions of coal seams, in order to balance the relationship between reservoir protection and fracturing effectiveness, and to promote the development of coalbed methane extraction technology towards higher efficiency and environmental friendliness.

[0008] In recent years, microbial enhancement technology has attracted attention as an emerging green method for increasing coal production. This technology injects specific functional microorganisms into the reservoir, utilizing their metabolic activities to achieve the following production enhancement mechanisms: 1) bio-methane generation, converting recalcitrant organic matter in coal into methane, increasing gas supply; 2) bio-acidification and deblocking, where metabolically produced organic acids dissolve minerals in coal, expanding pore throats; 3) biosurfactants alter coal wettability, reducing methane adsorption energy and promoting desorption. However, existing microbial enhancement technologies face many challenges in application: the migration and colonization of microorganisms in low-permeability coal seams are difficult; most microorganisms are filtered near the wellbore during injection, making it difficult to penetrate deeper into the reservoir; the activity of microorganisms is difficult to maintain in high-temperature, high-pressure, and high-mineralization reservoir environments; the reach of microorganisms and nutrient solutions in the matrix is ​​limited, resulting in uneven production enhancement; and the biological processes are difficult to control, leading to unstable production efficiency.

[0009] In particular, when combining microbial production enhancement technology with hydraulic fracturing, existing technologies often employ pre-injection of microbial fluid or post-injection of nutrient solution, which has the following shortcomings: microorganisms are difficult to exist stably in the proppant-carrying fluid and are easily deactivated by shearing and chemical additives; microorganisms have weak adhesion to the fracture surface and are easily washed away by subsequent fluids; the distribution of microorganisms in the supporting fracture is uneven, making it difficult to form a stable biologically active area; and the long-term survival and metabolism of microorganisms lack a continuous nutrient supply mechanism.

[0010] In view of the shortcomings of existing technologies, there is an urgent need to develop a new method that can organically combine microbial production enhancement with hydraulic fracturing. Summary of the Invention

[0011] The purpose of this invention is to address the aforementioned shortcomings of the prior art by proposing an ultra-low density proppant for increasing coalbed methane production, its preparation method, and its application.

[0012] The first objective of this invention is to provide a method for preparing an ultra-low density proppant for enhancing coalbed methane production, comprising the following steps: S1. Kaolin and iron oxide are ball-milled, dried, and sieved to obtain powder; S2. Dissolve polyethersulfone in N-methylpyrrolidone to form a PES / NMP solution, and add it to the powder in multiple portions. Disperse and stir using ultrasound to obtain a slurry. S3. The slurry is dripped into deionized water through a 0.2mm needle, quickly solidified and granulated, left to stand, and dried to obtain ceramsite proppant sintered green body; S4. The sintered ceramsite proppant blank is heated to 400-500℃ in a muffle furnace at a heating rate of 10-20℃ / min and held for 1-2 hours; then heated to 1300-1500℃ at a heating rate of 5-10℃ / min and held for 1-2 hours. The sample is cooled with the furnace to obtain ultra-low density proppant.

[0013] Furthermore, in step S1, the mass of iron oxide is 2.5%-15% of the mass of kaolin.

[0014] Furthermore, in step S1, the sample is passed through a 200-mesh sieve.

[0015] Furthermore, in step S2, the mass ratio of polyethersulfone to N-methylpyrrolidone is 1:6.

[0016] Furthermore, in step S3, the needle is 10-20 cm above the water surface, and the droplet rate is 3 drops / s.

[0017] Furthermore, in step S3, the drying temperature is 60-80℃, and the drying time is 8-12 hours.

[0018] A second objective of this invention is to provide an ultra-low density proppant for increasing coalbed methane production prepared using the above-described preparation method.

[0019] A third objective of this invention is to provide a microcapsule for increasing coalbed methane production, comprising the aforementioned ultra-low density proppant for increasing coalbed methane production, a methanogenic agent loaded thereon, and a calcium alginate film coated thereon.

[0020] The fourth objective of this invention is to provide a method for preparing microcapsules of proppant for increasing coalbed methane production, as described above. The method involves immersing an ultra-low density proppant for increasing coalbed methane production in a methanogenic bacterial solution for a period of time, then removing it and placing it in a calcium lactate solution. Sodium alginate solution is then gradually added dropwise, and the microcapsules are dried after coating to obtain the proppant microcapsules for increasing coalbed methane production.

[0021] Furthermore, the sodium alginate solution has a mass concentration of 25 mmol / L, the calcium lactate solution has a mass concentration of 3-11%, the coating is applied 3-5 times, and the coating time for each application is 1-5 minutes.

[0022] The ultra-low density proppant for coalbed methane production enhancement prepared by this invention has a low density and can be carried and transported by clean water or brine, eliminating the need for chemical dosages consumed in preparing gels. It contains no residue, will not clog the formation, reduces pollution to groundwater and the environment, and at the same time increases fracturing efficiency and reduces fracturing costs.

[0023] The microcapsules for coalbed methane production enhancement prepared in this invention load microorganisms onto the surface or interior of the proppant in a specific manner, allowing them to enter the fracturing fractures along with the proppant. This not only solves the challenges of microbial migration and colonization in the reservoir but also creates stable microbial enrichment zones within the fractures, providing a suitable environment for long-term microbial metabolic activities. This technical approach is expected to overcome the application bottlenecks of existing microbial production enhancement technologies, achieving synergistic enhancement of physical permeability and biological production, and providing an innovative solution for the efficient development of low-permeability coalbed methane reservoirs. Attached Figure Description

[0024] Figure 1 This is a process flow diagram for preparing the ultra-low density proppant for enhancing coalbed methane production according to the present invention. Figure 2 This is a photograph of the proppant sample prepared in Example 1; Figure 3 The XRD patterns of calcined samples with a PES:NMP:powder mass ratio of 5:30:10 under different temperatures and Fe2O3 addition conditions are shown. Figure 4 These are SEM images of the samples before and after calcination; Figure 5 The microstructure of the proppant cross section was prepared by sintering at different temperatures with a slurry ratio of PES:NMP:powder = 5:30:10 and an iron oxide addition of 10wt%. Figure 6 To investigate the relationship between different iron oxide addition amounts and temperatures in granulation with a 5:30:10 slurry ratio and the bulk density, apparent density, and breakage rate at 28 MPa of the proppant-calcined samples; Figure 7 This is a flowchart of the preparation process in Example 2; Figure 8 Scanning electron microscope image of the proppant microcapsules prepared in Example 2; Figure 9 These are the results of a sustained-release experiment of rhodamine in water at different pH values ​​under microcapsule conditions at 20°C. Detailed Implementation

[0025] The following are specific embodiments of the present invention, which are described in conjunction with the accompanying drawings to further illustrate the technical solutions of the present invention. However, the present invention is not limited to these embodiments.

[0026] Example 1 The process for preparing ultra-low density proppant for coalbed methane enhancement is as follows: Figure 1 As shown.

[0027] (1) Ingredients: Kaolin is the main raw material. Iron oxide is added in proportions of 0%, 2.5%, 5%, 7.5%, 10%, 12.5%, and 15% and poured into a corundum ball mill jar. Zirconia balls and anhydrous ethanol are used as the ball milling media. The mixture is ball milled in a planetary ball mill at a speed of 120 r / min for 12 h. It is then dried in an electric heating drying oven. The mixture is ground in a corundum mortar and passed through a 200-mesh sieve. (2) Preparation of slurry: Dissolve polyethersulfone in N-methylpyrrolidone at a mass ratio of 5:30. Add the powder to the PES / NMP solution in multiple batches on a multi-station magnetic stirrer. The liquid-solid ratio is 35:10 or 35:8. After ultrasonic dispersion for 30 min, continue stirring for 1 h to obtain the slurry. (3) Granulation: The slurry was dripped into deionized water through a 0.2mm needle, and the granules were rapidly solidified and allowed to stand for 12 hours. During the granulation process, the shape and size of the particles were observed. When the needle was 10-20cm above the water surface and the dripping rate was 3 drops / s, the particles exhibited better sphericity and roundness. The particles were dried in an electric heating oven at 60℃ for 12 hours to obtain the sintered ceramsite support material. (4) Sintering: In a muffle furnace, the temperature is increased to 500°C at a heating rate of 10°C / min and held for 1 hour to allow the polyethersulfone to burn completely and leave pores; then the temperature is increased to 1400°C at a heating rate of 5°C / min and held for 1 hour, and the sample is cooled with the furnace. (5) Performance testing and characterization: The performance of the proppant is tested according to national standards.

[0028] Because kaolin has poor dispersibility in solution, it was found in actual operation that when the ratio of powder to PES / NMP solution is higher than 10:35, the viscosity of the slurry increases dramatically, and the droplets stick together when dripped from the needle, resulting in larger droplet volume and irregular shape, making granulation impossible.

[0029] Table 1 compares the performance of proppants prepared by molding slurries with different solid contents. In the table, I and II represent PES:NMP:powder mass ratios of 5:30:10 and 5:30:8, respectively, and the iron oxide content in the powder is 10wt%.

[0030] Based on the comparison of proppant performance results in Table 1, under the same temperature conditions, Group I samples had higher apparent density and bulk density, and lower breakage rate than Group II samples. This is due to the difference in porosity within the porous proppant. Furthermore, as the temperature increased, especially above 1400℃, the performance difference between the two proppant formulations decreased. This may be because the samples underwent more significant volume shrinkage at higher temperatures, and the porosity difference between the two decreased due to the different degrees of volume shrinkage. Finally, the breakage rate of both types of proppant at 28MPa was less than 9%, meeting the usage requirements of SY / T 5108-2014 and achieving a breakage grade of 4K.

[0031] Table 1 Comparison of proppant performance in slurries with different solid contents

[0032] Figure 2 This is a photograph of the proppant sample prepared in Example 1; The figures are as follows: Figure a represents 7.5 wt% at 1400℃; Figure b represents 10 wt% at 1400℃; Figure c represents 12.5 wt% at 1400℃; Figure d represents 7.5 wt% at 1450℃; Figure e represents 10 wt% at 1450℃; and Figure d d represents 12.5 wt% at 1450℃. Upon observation and comparison, the roundness and sphericity of the proppant are both above 0.9, meeting the requirements of SY / T 5108-2014.

[0033] Figure 3 The XRD patterns of calcined samples with a PES:NMP:powder mass ratio of 5:30:10, at different temperatures and with varying Fe2O3 addition amounts are shown. a: Samples calcined at different temperatures with 10% iron oxide; b: Samples calcined at 1400℃ with different iron oxide addition amounts. Kaolin is a low-grade aluminum mineral, and during high-temperature sintering, it transforms into the mullite phase, resulting in excess silica. The figures show that the main phases of the proppant are mullite and cristobalite. With increasing temperature or iron oxide content, the diffraction intensity of the cristobalite phase continuously decreases. The mullite phase diffraction peak intensity at 1400℃ initially increases with increasing iron oxide addition, then weakens after reaching 12.5 wt%. When the iron oxide content exceeds 10%, the hematite diffraction peak in the diffraction pattern begins to strengthen, indicating that excess iron oxide reduces the internal crystallinity of the proppant and negatively impacts its anti-fracture properties.

[0034] Figure 4 These are SEM images of the samples before and after calcination. Figure a shows the microstructure of the billet before sintering, with hexagonal flaky kaolinite adhering to PES. Figure b shows the microstructure of the billet after calcination at 500℃ for 1 hour; the PES has been completely burned, leaving large pores, and the flaky kaolinite is dispersed and connected together. Figure c shows the slurry ratio of PES:NMP:powder = 5:30:10 with 10wt% iron oxide added. Figure d shows the proppant sample calcined at 1400℃ with a slurry ratio of PES:NMP:powder = 5:30:8 with 10wt% iron oxide added; the proppant exhibits a porous internal structure. When the solid content of the molding slurry is high, the pore size inside the sample is uniform and evenly distributed. As the solid content decreases, due to the aggregation of materials during calcination, the original structure collapses, small pores aggregate, forming larger pores of varying sizes. The porous structure gives the proppant a lower bulk density.

[0035] Figure 5The microstructure of the proppant cross-section prepared by sintering at different temperatures with a slurry ratio of PES:NMP:powder = 5:30:10 and an iron oxide addition of 10wt% is shown. The bulk density and apparent density of the porous ceramic proppant are mainly related to the porosity and pore morphology inside the proppant particles; the higher the porosity, the lower the proppant density. SEM images show that a relatively dense shell forms on the outer layer of the proppant particles during calcination. This is due to uneven heating during high-temperature sintering, with the outer layer temperature being relatively higher than the inner layer temperature, resulting in a greater amount of molten phase. The internal substances of the proppant aggregate through the liquid phase, while the outer surface also contracts inward, forming a shell structure on the outer layer. Comparing a, b, c, and d, the shell thickness increases significantly with increasing sintering temperature (10μm→30μm→100μm→200μm). From 1450℃ onwards, the proppant collapses into a large cavity. From 1350℃ to 1400℃, the proppant volume shrinkage is small, and a large number of surface pores are filled, resulting in a slight increase in apparent density. From 1450℃ to 1500℃, the proppant volume shrinkage decreases, and both apparent and bulk densities tend to stabilize. Figure 5 As shown in e and f, the surface of the proppant sample at 1400℃ has a certain pore structure with a diameter of 5μm. When the calcination temperature is increased to 1450℃, the micron-sized pores on the outer surface completely disappear, forming a dense structure.

[0036] Effects of Fe2O3 content and sintering temperature on proppant properties Figure 6 The relationships between different iron oxide additions and temperatures in granulation with a 5:30:10 slurry ratio and the bulk density, apparent density, and breakage rate at 28 MPa of the proppant-calcined samples are presented. The apparent density and bulk density of the proppant increase with increasing sintering temperature, while the breakage rate decreases. The calcined samples of pure kaolin without added iron oxide exhibit high breakage rates and do not meet the application requirements. This is due to the high porosity and low densification of the samples. The addition of iron oxide increases the proppant density and significantly reduces the breakage rate. This is because iron oxide, as a solid solution additive, lowers the sintering temperature of kaolin, promotes mullite phase formation, and generates a large amount of liquid phase during calcination, increasing the mass transfer rate. The proppant-treated samples undergo significant volume shrinkage, improving the densification of the sample structure. The apparent density of the proppant sample with 10 wt% iron oxide addition after calcination at 1400℃ for 1 h was 2.13 g / cm³. 3 Its bulk density is 1.15 g / cm³. 3 The breakage rate at 28 MPa was 9.2%.

[0037] Example 2 Simulated preparation of proppant microcapsules like Figure 7As shown, the ultra-low density proppant for increasing coalbed methane production prepared in Example 1 was placed in a beaker, and 200 mg / L Rhodamine 6G solution was added to fully soak it. The mixture was ultrasonically vibrated for 30 min, filtered, and dried for 12 h. The above drug-loaded proppant was added to 90 mmol / L calcium lactate solution, and then 25 mmol / L sodium alginate solution was gradually added dropwise to prepare proppant microcapsules.

[0038] like Figure 8 The image shown is a scanning electron microscope (SEM) image of the microcapsules. It can be seen that the dried calcium alginate film coats the surface of the proppant particles in a shrunken state. During the coating process, some proppant particles adhere together under the action of the gel. Some gel surfaces after a single coating show defects such as cracking, possibly due to insufficient gel strength. To improve the stability of the coating process, the concentration of the sodium alginate solution needs to be increased or multiple coating processes should be used. When the microcapsules come into contact with water, the dried calcium alginate gel swells, opening its internal pores. The drug loaded inside the proppant is then released into the solution through these pores driven by the concentration gradient.

[0039] Table 2 shows the microcapsule encapsulation rates at different encapsulation reaction times at 20℃. The longer the reaction time, the more severe the drug loss during microencapsulation, and the lower the encapsulation rate.

[0040] Table 2 Encapsulation efficiency of microcapsules at different reaction times

[0041] To investigate the relationship between the release rate and time of microcapsules under different conditions, the microcapsules were coated once, twice, and three times, and sustained-release experiments were conducted in deionized water at different temperatures and pH values ​​at 20, 35, and 50°C.

[0042] Table 3 shows the time required for complete release of Rhodamine 6G in neutral deionized water. The table shows that at 50°C, monolayer microcapsules are completely released in 35 minutes, while triple-layer microcapsules extend this time to 120 minutes. At 20°C, the complete release time of triple-layer coated microcapsules in pure water can reach 200 minutes.

[0043] Table 3. Complete release time of R6G

[0044] Calcium alginate gel successfully coated the outer surface of the proppant, tightly shrinking onto the proppant particles. The microcapsule encapsulation rate was high, but decreased with increasing reaction time. The microcapsule encapsulation rate was 91.6% after 1 minute of reaction. Multiple encapsulation processes can be used to improve the thickness and strength of the coating layer. Figure 9This is the result of a sustained-release experiment of rhodamine in water at different pH values ​​and at 20℃ using microcapsules. The figure shows that the number of microcapsule coating layers, temperature, and pH value all affect the release performance of the microcapsules. The overall trend of the release rate over time is the same, showing a process of first increasing and then decreasing. In acidic solutions, the drug release rate of the microcapsules is significantly higher than in pure water; the time required for complete release of rhodamine decreases from 200 minutes in neutral deionized water to 140 minutes. In alkaline environments, the release rate of rhodamine is weakened, and the time required for complete release is significantly prolonged to over 300 minutes. This is because the network structure of the gel is altered during the swelling process of the calcium alginate dry gel. + The disruption weakened the gel's ability to impede the diffusion of rhodamine.

[0045] For any points not covered above, existing technologies shall apply.

[0046] Although specific embodiments of the present invention have been described in detail by way of examples, those skilled in the art should understand that the above examples are for illustrative purposes only and are not intended to limit the scope of the invention. Those skilled in the art can make various modifications or additions to the described specific embodiments or use similar methods to replace them, without departing from the direction of the invention or exceeding the scope defined by the appended claims. Those skilled in the art should understand that any modifications, equivalent substitutions, improvements, etc., made to the above embodiments based on the technical essence of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing an ultra-low density proppant for enhancing coalbed methane production, characterized in that, Includes the following steps: S1. Kaolin and iron oxide are ball-milled, dried, and sieved to obtain powder; S2. Dissolve polyethersulfone in N-methylpyrrolidone to form a PES / NMP solution, and add it to the powder in multiple portions. Disperse and stir using ultrasound to obtain a slurry. S3. The slurry is dripped into deionized water through a 0.2mm needle, quickly solidified and granulated, left to stand, and dried to obtain ceramsite proppant sintered green body; S4. The sintered ceramsite proppant blank is heated to 400-500℃ in a muffle furnace at a heating rate of 10-20℃ / min and held for 1-2 hours; then heated to 1300-1500℃ at a heating rate of 5-10℃ / min and held for 1-2 hours. The sample is cooled with the furnace to obtain ultra-low density proppant.

2. The preparation method according to claim 1, characterized in that, In step S1, the mass of iron oxide is 2.5%-15% of the mass of kaolin.

3. The preparation method according to claim 1, characterized in that, In step S1, the sample is passed through a 200-mesh sieve.

4. The preparation method according to claim 1, characterized in that, In step S2, the mass ratio of polyethersulfone to N-methylpyrrolidone is 1:6; and the mass ratio of polyethersulfone to PES / NMP solution is 8-10:

35.

5. The preparation method according to claim 1, characterized in that, In step S3, the needle is 10-20cm above the water surface, and the droplet rate is 3 drops / s.

6. The preparation method according to claim 1, characterized in that, In step S3, the drying temperature is 60-80℃, and the drying time is 8-12 hours.

7. An ultra-low density proppant for increasing coalbed methane production, prepared by the preparation method according to any one of claims 1-6.

8. A microcapsule for increasing coalbed methane production, characterized in that, It includes the ultra-low density proppant for increasing coalbed methane production as described in claim 7, a methanogenic agent loaded thereon, and a calcium alginate film coated on its surface.

9. A method for preparing microcapsules for enhancing coalbed methane production as described in claim 8, characterized in that, After impregnating the ultra-low density proppant for coalbed methane production enhancement in methanogenic bacteria solution for a period of time, it was removed, placed in calcium lactate solution, and sodium alginate solution was gradually added dropwise. After the coating was completed, it was dried to obtain microcapsules of proppant for coalbed methane production enhancement.

10. The preparation method according to claim 9, characterized in that, The sodium alginate solution has a mass concentration of 25 mmol / L, the calcium lactate solution has a mass concentration of 90 mmol / L, the coating is performed 3-5 times, and the coating time for each coating is 1-5 minutes.

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