High-surface tension and low-temperature waste heat synergetic coal seam fracturing and permeability increasing method

Abstract

The invention discloses a high-surface tension and low-temperature waste heat synergistic coal seam fracturing and permeability increasing method, which comprises the following steps: adding soluble inorganic salt (sodium chloride) into fracturing fluid to improve the surface tension of the fracturing fluid, adding a heat storage propping agent into the fracturing fluid, and accelerating liquid film evaporation through low-temperature waste heat to improve the permeability of the fracturing fluid. A primary crack of a coal body is expanded and a new crack is formed by utilizing huge capillary contraction force generated by high surface tension, and fracturing and permeability increasing are carried out on a coal seam. The preparation method comprises the following steps: mixing a prepared surface tension enhancer with a composite phase change heat storage propping agent, carrying out hydraulic fracturing crack forming, injecting industrial waste heat gas at the temperature of 30-150 DEG C after flowback, accelerating liquid film evaporation by hot air flow to generate dry shrinkage cracks, and releasing heat by the composite phase change heat storage propping agent to maintain the evaporation process when a heat source fluctuates.

Description

Technical Field

[0001] This invention belongs to the field of coal seam fracturing and permeability enhancement, specifically a method for coal seam fracturing and permeability enhancement synergistically using high surface tension and low temperature residual heat. Background Technology

[0002] Coalbed methane (CBM) is an unconventional natural gas and a clean energy source, primarily composed of methane. It exists mainly in an adsorbed state (over 90%) within coal seams. However, CBM is currently characterized by deep burial, low permeability, high density, high pressure, and strong adsorption, resulting in very low extraction efficiency. To extract CBM more effectively, reservoir modification is necessary to improve coal seam permeability.

[0003] Currently, commonly used reservoir stimulation methods include hydraulic fracturing, gas drive, and blasting. However, due to the strong adsorption capacity of coalbed methane (CBM), a large amount of CBM remains adsorbed in the coal and cannot be extracted in the later stages of CBM extraction. This leads to a decline in single-well production, making it difficult to effectively increase production. In this case, coalbed methane injection is a feasible method because CBM adsorption capacity is inversely correlated with reservoir temperature; for every 1°C increase in coal seam temperature, the adsorption capacity decreases by approximately 8%. When the coal temperature reaches a certain threshold, pyrolysis occurs within the coal body, creating new fractures and increasing coal seam permeability. Existing coal seam permeability enhancement technologies have the following problems:

[0004] 1. Existing coal seam thermal stimulation technologies (such as electric heating, microwave, and superheated steam) mainly rely on high temperatures (>300℃) to induce pyrolysis in the coal, thereby generating fractures. These methods consume enormous amounts of energy and have extremely high equipment requirements.

[0005] 2. Existing technologies generally reduce surface tension by adding surfactants, but this ignores the fact that surface tension can be converted into strong capillary contraction force in certain physical processes (such as evaporation and drying), which is theoretically sufficient to expand the original cracks in the coal body and create new cracks.

[0006] 3. Existing proppant only provides mechanical support, has low heat capacity and fast heat conduction, and cannot play a role in stabilizing ground temperature fluctuations.

[0007] 4. While industrial waste heat from surrounding mining areas (such as power plant flue gas and kiln exhaust) is low-cost, it has two major drawbacks: Firstly, its temperature is typically below 150°C, far below the pyrolysis temperature of coal, and traditional theory suggests it cannot effectively enhance permeability. Secondly, due to production operations, the supply of waste heat is intermittent; once injection stops, the underground temperature dissipates rapidly, causing the coal body to experience severe "thermal shock," and the effective duration is short. This makes it impossible to utilize industrial waste heat for enhancing the permeability of coalbed methane.

[0008] Therefore, a method for improving the permeability of coalbed methane by fracturing coal seams using industrial low-temperature preheating is proposed. Summary of the Invention

[0009] To address the aforementioned technical challenges, the inventors proposed a method for enhancing coal seam fracturing and permeability through the synergistic effect of high surface tension and low-temperature waste heat. This method breaks away from the high-temperature pyrolysis approach and utilizes a low-temperature evaporation + matrix shrinkage mechanism. By accelerating liquid film evaporation with low-temperature waste heat, the significant capillary contraction force generated by high surface tension expands existing fractures in the coal seam and creates new ones, thus enhancing the coal seam's fracturing and permeability. A high-surface-tension fracturing fluid mixed with a heat-storage proppant is prepared and hydraulically fracturing is performed. After flowback, industrial waste heat gas at 30℃-150℃ is injected. The hot gas flow accelerates liquid film evaporation, generating drying shrinkage cracks. Simultaneously, the heat-storage proppant releases heat during heat source fluctuations to sustain the evaporation process.

[0010] The present invention adopts the following technical solution:

[0011] A method for enhancing coal seam fracturing and permeability through the synergistic effect of high surface tension and low-temperature residual heat, comprising the following steps:

[0012] Step 1, Prepare fracturing fluid

[0013] Using pure water as the base fluid, a surface tension enhancer with a mass fraction of 5-20% is added to obtain a fracturing base fluid;

[0014] Add 10-20% thermal proppant to the fracturing base fluid to obtain the fracturing fluid;

[0015] Step 2, hydraulic fracturing

[0016] Hydraulic fracturing is performed by injecting fracturing fluid into the coal seam to construct a fracture network. Thermal proppant is then carried into the depth of the fractures by the fracturing fluid to provide support.

[0017] Step 3, Reverse runoff and heat injection

[0018] After fracturing is completed, flowback depressurization is performed to drain excess liquid, leaving only a liquid film on the surface of the fractures and pores.

[0019] Then, industrial waste heat gas at a temperature of 30-150℃ is injected;

[0020] The heat injection pressure is controlled below the coal seam closure pressure, so that the hot airflow can penetrate deep into the coal seam through the gaps supported by the heat storage proppant without damaging the existing gap structure.

[0021] Step 4, Evaporative Cracking and Thermal Buffering

[0022] As heating proceeds, the water in the liquid film evaporates due to heat, the gas-liquid interface shrinks to form a meniscus, and the capillary negative pressure generated by the surface tension enhancer in the fracturing fluid is enhanced. This tension acts on the pore wall, causing the primary fractures in the coal body to expand and create new fractures.

[0023] When the industrial waste heat source fluctuates or stops, the heat storage proppant releases internal heat to maintain the crack temperature and the liquid film continues to dry and crack.

[0024] In a preferred embodiment, the surface tension enhancer is a soluble inorganic salt.

[0025] In a preferred embodiment, the surface tension of the fracturing fluid at 20°C is 75-80 mN / m.

[0026] In a preferred embodiment, the thermal storage proppant has a particle size of 20-40 mesh and a density of 1.25-1.40 g / cm³. 3 The apparent porosity is 30-50%.

[0027] In a preferred embodiment, the preparation method of the thermal storage support agent includes the following steps:

[0028] Step 1, Preparation of porous framework

[0029] Using bauxite, kaolin, or coal gangue as raw materials, a pore-forming agent is added, and the mixture is ball-milled, granulated, and then sintered at high temperature to obtain porous ceramsite with an apparent porosity of 30%–50%. After sieving, particles with a particle size of 0.45–0.90 mm are selected as the proppant matrix.

[0030] Step 2, loading of phase change material

[0031] The porous ceramic particles are placed in a vacuum environment and impregnated in a molten phase change thermal storage core material, so that the core material fills the pores of the ceramic particles, and then cooled and solidified.

[0032] Step 3, Lazy encapsulation

[0033] A layer of heat-resistant thermosetting resin or hydrophobic ceramic glaze is coated on the surface of the loaded particles, and after curing, the heat storage support agent is obtained.

[0034] In a preferred embodiment, the phase change thermal storage core material is selected from industrial paraffin wax, microcrystalline wax, or low-melting-point polyethylene wax;

[0035] The melting point range of phase change thermal storage core material is 30-100℃.

[0036] In a preferred embodiment, the melting point of the phase change thermal storage core material is in the range of 50-80℃.

[0037] In a preferred embodiment, the temperature-resistant thermosetting resin is selected from phenolic resin or epoxy resin, and the layer thickness of the temperature-resistant thermosetting resin on the particle surface is 10-20 μm.

[0038] In a preferred embodiment, the pore-forming agent is carbon powder or wood chips, and the amount added is 10–15% of the raw material mass.

[0039] In a preferred embodiment, the high-temperature sintering temperature is 1250–1350℃.

[0040] Compared with the prior art, the present invention has the following beneficial effects:

[0041] 1. This invention creatively utilizes the shrinkage force of a high surface tension liquid film (with a soluble inorganic salt as the surface tension enhancer) during the drying process, converting it into a driving force for disrupting the coal seam structure. This force is used to create and expand existing cracks, significantly increasing the microfracture density of the coal seam without high-temperature chemical pyrolysis. Combined with low-temperature (<150℃) waste heat, it induces fracturing and improves permeability in the coal seam. This utilizes low-temperature waste heat, eliminating the need for high-energy-consuming electric heating equipment and significantly reducing gas extraction costs. Simultaneously, the effect of the heat storage proppant addresses the "intermittent" problem of industrial waste heat utilization. The proppant's heat storage and release capacity constructs an "underground thermal battery," locking the fracture temperature within the effective evaporation temperature range, mitigating ground temperature fluctuations caused by unstable industrial waste heat supply, and ensuring the continuity and stability of the permeability enhancement operation.

[0042] 2. This invention utilizes the soluble inorganic salts (preferably sodium chloride) within the fracturing fluid during the later stages of water evaporation. These soluble salts precipitate into regular cubic crystals within a confined space, generating pressure. This pressure, combined with capillary contraction, further expands the microcracks. The crystals naturally accumulate to form a porous framework, preserving intercrystalline pores while simultaneously expanding the microcracks, thus not obstructing gas migration pathways. This salt crystal framework possesses natural intercrystalline micropores and can be gradually dissolved during later gas extraction or formation water recirculation, exhibiting excellent self-cleaning properties and avoiding permanent damage to coal seam permeability. Attached Figure Description

[0043] Figure 1 The image shows the fractures and fracture parameters (number and width) of the coal sample in Example 1 of this invention.

[0044] Figure 2 The image shows the fractures and fracture parameters (number and width) of the coal sample in Example 2 of this invention.

[0045] Figure 3 The image shows the fractures and fracture parameters (number and width) of a coal sample, which is a comparative example of the present invention. Detailed Implementation

[0046] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0047] Example 1

[0048] A method for enhancing coal seam fracturing and permeability through the synergistic effect of high surface tension and low-temperature residual heat, comprising the following steps:

[0049] Step 1, Prepare fracturing fluid

[0050] Using pure water as the base fluid, a surface tension enhancer (soluble inorganic salt) with a mass fraction of 15% is added. The preferred soluble inorganic salt is sodium chloride. This results in a fracturing base fluid with a surface tension of 75-80 mN / m at 20℃, which provides a physical basis for generating strong capillary contraction force in the future.

[0051] Add 15% thermal proppant to the fracturing base fluid to obtain the fracturing fluid;

[0052] Step 2, hydraulic fracturing

[0053] Hydraulic fracturing is performed by injecting fracturing fluid into the coal seam to construct a fracture network. Thermal proppant is then carried into the depth of the fractures by the fracturing fluid to provide support.

[0054] Step 3, Reverse runoff and heat injection

[0055] After fracturing is completed, flowback depressurization is carried out to discharge most of the liquid, leaving only a liquid film on the surface of the fractures and pores.

[0056] Subsequently, industrial waste heat gas (such as waste heat from coal washing plants, mine return air heat, or waste heat from gas power generation) with a temperature of 30-150℃ is injected. The injection pressure is controlled below the coal seam closure pressure (e.g., 5-15MPa, depending on the specific well depth). The hot gas flow can penetrate deep into the coal seam through the gaps supported by the heat storage proppant without damaging the existing gap structure. At the same time, the heat storage proppant will...

[0057] Step 4, Evaporative Cracking and Thermal Buffering

[0058] As heating proceeds, the water in the liquid film evaporates due to heat, the gas-liquid interface shrinks to form a meniscus, and the capillary negative pressure generated by the soluble inorganic salts in the fracturing fluid is enhanced. This tension acts on the pore wall, causing the primary fractures in the coal body to expand and create new fractures.

[0059] When the industrial waste heat source fluctuates or stops (interruption or insufficient temperature), the heat storage proppant releases internal heat to maintain the crack temperature and the liquid film continues to dry and crack.

[0060] As moisture evaporates rapidly at the maintained temperature, the sodium chloride concentration in the residual liquid film quickly reaches saturation, and the surface tension remains high (>79 mN / m). This combination of "continuous drying" and "high tension" generates a strong capillary contraction force in the pore throat, causing the primary fractures in the coal seam to expand and create new fractures, generating a large number of drying shrinkage microcracks, which inducing fracturing and increasing permeability of the coal seam.

[0061] In this embodiment, the soluble inorganic salt can be one or more of sodium chloride, magnesium chloride, and calcium chloride.

[0062] To ensure that the gas migration pathways are not blocked while providing secondary support and self-cleaning, the preferred soluble inorganic salt is sodium chloride. This allows for the precipitation of regular cubic crystals in confined spaces (such as within fractures and proppant gaps) during the later stages of water evaporation, generating crystallization pressure. This pressure, in conjunction with capillary contraction, further expands the microcracks. Furthermore, the formed salt crystals have a regular cubic structure, naturally accumulating to form a porous framework. This provides secondary support without blocking the gas migration pathways, or allows the gas to dissolve in formation water during subsequent mining, thus preventing permanent blockage.

[0063] Example 2

[0064] Unlike Example 1, 20% by mass of a soluble inorganic salt (sodium chloride) was added.

[0065] Add 15% thermal support agent.

[0066] Comparative Example

[0067] Unlike Example 1, no soluble inorganic salts were added, the fracturing fluid was only pure water, and 15% of traditional ceramic proppant was added.

[0068] Depend on Figures 1 to 3 As shown, strain gauges were used to measure the deformation of the coal body, and micro-digital image correlation (micro-DIC) technology was used to systematically monitor the crack propagation process of the coal body.

[0069] As shown in the figure, the total number of cracks increased: the total number of cracks in the low concentration group increased by 87.5% compared with the baseline group, and the total number of cracks in the high concentration group increased by 156.25% compared with the baseline group. The increase in the total number of cracks in the high concentration group far exceeded that in the low concentration group.

[0070] The size of individual cracks increased: The size of individual cracks in all three groups showed an increasing trend from 0h to 15h. The base group increased by 42.4%, the low concentration group by 102.6%, and the high concentration group by 157.8%. The high concentration group had the highest increase in the size of individual cracks among the three groups.

[0071] It is evident that the use of soluble inorganic salts in this case, at a relatively optimal ratio of 20%, can effectively enhance crack expansion and the formation of new cracks, thereby promoting the transparency effect.

[0072] The preparation steps of the thermal storage proppant in the above scheme are as follows:

[0073] Step 1, Preparation of porous framework

[0074] Using bauxite, kaolin, or coal gangue as raw materials, a pore-forming agent is added, preferably carbon powder or wood chips, at a preferred addition amount of 15% of the raw material mass. After ball milling and granulation, the material is sintered at a high temperature of 1300℃ to obtain porous ceramsite with an apparent porosity of 45%. After sieving, particles with a particle size of 0.45–0.90 mm are selected as the proppant matrix. The breakage rate is <3.5% under a closure pressure of 52 MPa, which meets the requirements of the petroleum and natural gas industry standard SY / T 5108-2014.

[0075] Step 2, loading of phase change material

[0076] The porous ceramic particles are placed in a vacuum environment and impregnated in a molten phase change thermal storage core material, so that the core material fills the pores of the ceramic particles. Then, the material is cooled and solidified. The phase change thermal storage core material is selected from industrial paraffin wax, microcrystalline wax or low melting point polyethylene wax, preferably fully refined paraffin wax (58# paraffin wax).

[0077] Selection criteria: The melting point of this paraffin is approximately 58℃-60℃. This temperature range falls within the effective coverage of industrial waste heat (30℃-150℃) and ensures that the temperature at which it releases latent heat is significantly higher than the underground ground temperature (approximately 20℃-30℃), thus ensuring a continuous heat-driven drying effect.

[0078] Porous ceramic granules were placed in a vacuum autoclave and heated to 80°C to completely melt the paraffin wax. The mixture was then adsorbed under a vacuum of -0.09 MPa for 60 minutes and cooled to solidify.

[0079] Step 3, Lazy encapsulation

[0080] A layer of heat-resistant thermosetting resin or hydrophobic ceramic glaze is coated onto the surface of the loaded particles. The heat-resistant thermosetting resin is selected from phenolic resin or epoxy resin, preferably phenolic resin. After curing, the thickness of the cured heat-resistant thermosetting resin layer on the particle surface is 10-20 μm, and it is cured at 160℃. This resin layer is hydrophobic and serves a dual purpose: firstly, it prevents the internal liquid wax from leaking out during the 150℃ heating stage; secondly, it acts as an isolation layer to further isolate the external salt solution from contact with the internal structure, ultimately yielding the heat storage support agent.

[0081] Chemical stability verification: Sodium chloride solution is a highly polar electrolyte solution; while the fully refined paraffin core and resin coating prepared above are non-polar hydrophobic organic materials. According to the principle of "like dissolves like," the two are incompatible when in contact, ensuring that the proppant does not dissolve, swell, or react chemically after prolonged immersion in fracturing fluid, thus guaranteeing construction safety and rheological stability.

[0082] When the industrial waste heat supply is interrupted or the temperature is insufficient, the liquid paraffin solidifies and releases latent heat. This heat locks the ambient temperature inside the fracture at around 60°C for an extended period. Although this is below the boiling point of the fracturing fluid, the rate of water evaporation at this temperature is still several times that at ambient downhole temperature, ensuring the continuous and rapid drying of the liquid film.

[0083] As moisture evaporates rapidly at 60℃, the sodium chloride concentration in the residual liquid film quickly reaches saturation, and the surface tension remains high (>79 mN / m). This combination of "continuous drying" and "high tension" generates a strong capillary contraction force in the pore throat, causing the primary fractures in the coal seam to expand and create new fractures, generating a large number of drying shrinkage microcracks, which inducing fracturing and increasing permeability in the coal seam.

[0084] The performance indicators of the thermal storage proppant prepared above are as follows:

[0085]

[0086] As shown in the table above, the apparent density, bulk density, specific heat capacity, and latent heat performance of this project are significantly superior to those of traditional ceramsite proppant. Combined with a low-temperature heat source—which utilizes industrial waste heat (<150℃)—permeability enhancement can be achieved without the need for high-energy-consuming electric heating equipment, significantly reducing gas extraction costs and meeting the requirements of green mine construction. By using the low-temperature threshold (30℃-100℃) required for moisture evaporation to replace the high-temperature threshold (300℃) of coal pyrolysis, even low-grade waste heat (<150℃) has industrial application value, saving resources. Simultaneously, it mitigates ground temperature fluctuations caused by unstable industrial waste heat supply, ensuring the continuity and stability of permeability enhancement operations, significantly reducing the negative impact of thermal shock on the coal seam, and solving the "intermittent" pain point of industrial waste heat utilization.

[0087] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. The substitutions may be replacements of some structures, devices, or method steps, or they may be complete technical solutions. Equivalent substitutions or modifications made to the technical solutions and inventive concepts of the present invention should all be covered within the scope of protection of the present invention.

Claims

1. A method for synergistic fracturing and permeability enhancement of coal seams using high surface tension and low-temperature residual heat, characterized in that, The steps are as follows: Step 1, Prepare fracturing fluid Using pure water as the base fluid, a surface tension enhancer with a mass fraction of 5-20% is added to obtain a fracturing base fluid; Add 10-20% thermal proppant to the fracturing base fluid to obtain the fracturing fluid; Step 2, hydraulic fracturing Hydraulic fracturing is performed by injecting fracturing fluid into the coal seam to construct a fracture network. Thermal proppant is then carried into the depth of the fractures by the fracturing fluid to provide support. Step 3, Reverse runoff and heat injection After fracturing is completed, flowback depressurization is performed to drain excess liquid, leaving only a liquid film on the surface of the fractures and pores. Then, industrial waste heat gas at a temperature of 30-150℃ is injected; The heat injection pressure is controlled below the coal seam closure pressure, so that the hot airflow can penetrate deep into the coal seam through the gaps supported by the heat storage proppant without damaging the existing gap structure. Step 4, Evaporative Cracking and Thermal Buffering As heating proceeds, the water in the liquid film evaporates due to heat, the gas-liquid interface shrinks to form a meniscus, and the capillary negative pressure generated by the surface tension enhancer in the fracturing fluid is enhanced. This tension acts on the pore wall, causing the primary fractures in the coal body to expand and create new fractures. When the industrial waste heat source fluctuates or stops, the heat storage proppant releases internal heat to maintain the crack temperature and facilitate the continuous and rapid drying and cracking process of the liquid film.

2. The method for synergistic fracturing and permeability enhancement of coal seams using high surface tension and low-temperature residual heat as described in claim 1, characterized in that, The surface tension enhancer is a soluble inorganic salt.

3. The method for synergistic fracturing and permeability enhancement of coal seams using high surface tension and low-temperature residual heat as described in claim 2, characterized in that, The surface tension of the fracturing fluid at 20°C is 75-80 mN / m.

4. The method for synergistic fracturing and permeability enhancement of coal seams using high surface tension and low-temperature residual heat as described in claim 1, characterized in that, The thermal storage proppant has a particle size of 20-40 mesh and a density of 1.25-1.40 g / cm³. 3 The apparent porosity is 30-50%.

5. The method for synergistic fracturing and permeability enhancement of coal seams using high surface tension and low-temperature residual heat as described in claim 4, characterized in that, The preparation method of the thermal storage support agent includes the following steps: Step 1, Preparation of porous framework Using bauxite, kaolin, or coal gangue as raw materials, a pore-forming agent is added, and the mixture is ball-milled, granulated, and then sintered at high temperature to obtain porous ceramsite with an apparent porosity of 30–50%. After sieving, particles with a particle size of 0.45–0.90 mm are selected as the proppant matrix. Step 2, loading of phase change material The porous ceramic particles are placed in a vacuum environment and impregnated in a molten phase change thermal storage core material, so that the core material fills the pores of the ceramic particles, and then cooled and solidified. Step 3, Lazy encapsulation A layer of heat-resistant thermosetting resin or hydrophobic ceramic glaze is coated on the surface of the loaded particles, and after curing, the heat storage support agent is obtained.

6. The method for synergistic fracturing and permeability enhancement of coal seams using high surface tension and low-temperature residual heat as described in claim 5, characterized in that, The phase change thermal storage core material is selected from industrial paraffin wax, microcrystalline wax or low melting point polyethylene wax. The melting point range of phase change thermal storage core material is 30-100℃.

7. The method for synergistic fracturing and permeability enhancement of coal seams using high surface tension and low-temperature residual heat as described in claim 6, characterized in that, The melting point range of the phase change thermal storage core material is 50-80℃.

8. The method for synergistic fracturing and permeability enhancement of coal seams using high surface tension and low-temperature residual heat as described in claim 5, characterized in that, The thermosetting resin is selected from phenolic resin or epoxy resin, and the thickness of the thermosetting resin layer on the particle surface is 10-20 μm.

9. The method for synergistic fracturing and permeability enhancement of coal seams using high surface tension and low-temperature residual heat as described in claim 5, characterized in that, The pore-forming agent is carbon powder or wood chips, and the amount added is 10-15% of the raw material mass.

10. The method for synergistic fracturing and permeability enhancement of coal seams using high surface tension and low-temperature residual heat as described in claim 5, characterized in that, The high-temperature sintering temperature is 1250–1350℃.

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