Negative pressure type underground in-situ pyrolysis method for residual roadway coal pillar oil-rich coal

Abstract

The invention relates to a method for extracting coal-based oil and gas resources by in-situ pyrolysis of underground residual roadway coal pillars, which comprises the following steps of: firstly, sealing the inside of a goaf by a grouting method to construct a goaf sealed space; then CO2 is injected into the closed space, horizontal directional drilling is conducted on the roadway coal pillar, fracture forming operation is conducted on the drilled coal pillar after well completion, and the horizontal drilled hole is closed after fracture forming is completed; vertical wells are constructed on the two sides of the inclined direction of the working face to serve as negative pressure extraction wells at the corresponding ground positions of the positions where the horizontal drill holes enter the coal seam; heating wells are constructed at the positions, close to the coal wall, of the two sides of a horizontal well end hole, and electric heaters are put into the hole for heating; an oil-gas extraction device is tripped into the extraction well, extraction operation is started, oil-gas resources are subjected to oil-gas separation on the ground, and redundant CO2 gas is injected into the goaf again. According to the technical scheme, the goaf-heating well-coal pillar-extraction well-goaf gas negative pressure circulation is constructed, the defect that pyrolysis products enter the goaf possibly caused by a conventional gas injection heating extraction process is overcome, and efficient recycling of the pyrolysis products is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of natural resource mining, in particular to a negative pressure underground in-situ pyrolysis method for oil-rich coal in residual roadway coal pillars. Background Art

[0002] Generally, after underground mining of coal resources is completed, a large amount of difficult-to-mine resources are often left underground and cannot be mined and utilized. The loss of these coal resources causes a large amount of coal resources to be wasted. For oil-rich coal, this waste is more prominent due to its good medium quality.

[0003] Underground in-situ pyrolysis of oil-rich coal refers to heating the coal seam through certain technical means under the original underground stratum conditions, so that it produces oil and gas resources after reaching the pyrolysis temperature, and then the oil and gas resources are extracted to the ground through a supporting extraction system.

[0004] However, underground in-situ pyrolysis technology for oil-rich coal is only applicable in intact coalfields. There is currently no corresponding technical method to effectively pyrolyze and recover the oil and gas resources in the residual coal pillars in the mined coal mines.

[0005] For conventional underground in-situ heating of coal seams, pyrolysis products will not enter the goaf due to the influence of the coal wall range, making it impossible to extract them. However, the coal pillar resources in the remaining space of the goaf may overflow through the fracture channels in the coal seam during the conventional steam injection convection heating process, carrying pyrolysis products into the goaf and ultimately becoming difficult to recover. Summary of the Invention

[0006] The purpose of the present invention is to provide a negative pressure underground in-situ pyrolysis method for oil-rich coal in residual tunnel coal pillars. Through the negative pressure extraction process, high-pressure gas in the goaf is "sucked" into the coal seam, and the coal seam is heated, finally achieving efficient recovery of pyrolysis products.

[0007] To achieve the above object, the present invention provides the following technical solutions: The negative pressure underground in-situ pyrolysis method for residual roadway coal pillar oil-rich coal includes the following steps: Step 1: A composite slurry containing a silicate cement matrix doped with aerogel particles with a particle size of 50-100 μm and carbon fiber particles with a length of 3-6 mm is injected into the goaf cracks at a pressure of 0.8-1.2 MPa using a high-pressure grouting pump. Distributed fiber optic sensors are embedded in the grout to monitor stress and strain data. Gas injection wells are drilled around the sealed goaf, and CO2 at a pressure higher than the surface atmospheric pressure is injected into the gas injection wells. A piezoelectric sensor array is installed on the goaf roof, and a time-difference inversion algorithm is used to construct a three-dimensional geomechanical model. Step 2: A horizontal borehole is drilled into the middle of the coal pillar in the closed goaf. The horizontal borehole is made of fiberglass casing with a temperature resistance of over 400°C. A fracturing fluid containing ferroferric oxide nanoparticles with a particle size of 20-50nm is injected into the horizontal borehole in sections, with an injection volume of 30-50m³ per section and a sand ratio of 15%-20%. An underground microwave generator is simultaneously activated to generate radial cracks in the coal body. The extension range of the fracturing fracture should not penetrate the coal wall. After the fracture is completed, the horizontal borehole is closed. Step 3: Drill a vertical hole at the end of the horizontal borehole, close to where the crack penetrates the coal wall, and lower an electric heater into the hole to serve as a heating well. The heating well adopts a double-tube structure, with a microwave antenna array arranged in the inner tube. Microwave energy is transmitted via a coaxial cable, and the annulus is filled with a magnesium oxide insulated resistance heater to maintain the heater heating temperature above 400°C. Step 4: Vertical wells are opened at the corresponding ground locations on both sides of the horizontal borehole where the well enters the coal seam, serving as negative pressure extraction wells. Oil and gas extraction equipment is lowered into the extraction wells and extraction operations are initiated. A vortex tube refrigeration device is installed at the front end of the extraction well to condense the high-temperature gas to recover heavy tar; a molecular sieve is used in the middle section to adsorb light aromatics; and a membrane separation component is configured at the end to achieve CO2 separation. Step 5: Install a Bi2Te3-based thermoelectric power generation module in the extraction well to directly supply heating to the heating well; after the extracted oil and gas resources are separated on the ground, the separated CO2 gas is re-injected into the goaf for recycling; after the pyrolysis is completed, the CO2 is sealed in a closed space.

[0008] In step 2 of the above method, a reformed zone is established at the end hole position of the horizontal drilling well corresponding to the inclined profile of the coal pillar in the roadway. The thickness of the reformed zone is not less than 50 cm, and the cracks in the reformed zone all penetrate the coal wall.

[0009] In the above method, the CO2 gas injected into the gas injection well is replaced by flue gas or nitrogen.

[0010] In step three of the above method, the PID algorithm is used to adjust the heating power, and the downhole infrared thermal imager is used to maintain the temperature at 450±10℃ in the area 0-5m away from the heating wellbore and 380±20℃ in the area 5-15m away from the heating wellbore.

[0011] In step 2 of the above method, when the crack extension speed exceeds 0.5 m / s through stress and strain data monitored by optical fiber, a temporary plugging agent containing a shape memory polymer with an expansion rate of more than 300% is automatically injected.

[0012] The beneficial effects of the present invention are: The present invention provides a negative pressure underground in-situ pyrolysis method for oil-rich coal in residual roadway coal pillars. By artificially constructing an underground "goaf → heating well → coal pillar → extraction well → goaf" gas circulation loop, the defect of pyrolysis products entering the goaf caused by conventional steam injection and extraction is avoided, and the efficient recovery of pyrolysis products is achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 This is a schematic diagram of the construction structure principle of the present invention; In the figure: 1 - goaf; 2 - coal pillar in the roadway; 3 - gas injection drilling well; 4 - horizontal drilling well; 5 - fracture creation area; 6 - transformation area; 7 - extraction well; 8 - heating well. DETAILED DESCRIPTION

[0014] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0015] refer to Figure 1 The present invention relates to a negative pressure underground in-situ pyrolysis method for oil-rich coal in residual roadway coal pillars, comprising the following steps: Step 1: Goaf 1 is sealed by grouting, creating an enclosed space within the combined area of goaf 1 and roadway coal pillar 2. A composite slurry containing a silicate cement matrix, doped with aerogel particles (50-100 μm in size) and carbon fibers (3-6 mm in length), is injected into the goaf's fissures at a pressure of 0.8-1.2 MPa using a high-pressure grouting pump. After curing, the slurry forms an insulating layer with a thermal conductivity of <0.05 W / (m·K) and a compressive strength exceeding 35 MPa. Distributed fiber optic sensors (DAS / DTS) are embedded in the grout, and a monitoring network is deployed at 2-meter intervals to provide real-time stress and strain data. Gas injection wells 3 are constructed within the enclosed space of goaf 1, and CO2 is injected into these wells to maintain a pressure above the surface atmospheric pressure. A 24-channel piezoelectric sensor array is installed in the goaf's roof, and a time-of-day inversion algorithm is used to construct a three-dimensional geomechanical model with a positioning accuracy of ±0.5 m.

[0016] Step 2: Starting from the surface, a horizontal directional borehole 4 is drilled toward the roadway coal pillar 2. After completion of the horizontal directional borehole 4, fractures are created within the drilled coal pillar 4 to form a fracture zone 5. This fracture creation can be accomplished using controlled shock waves, perforation, or fracturing, depending on the specific coal seam conditions. In this example, coiled tubing is used to drill the horizontal well in the middle of the coal pillar, and the wellbore is constructed with fiberglass casing, which is heat-resistant above 400°C. Fracturing fluid containing ferroferric oxide nanoparticles with a particle size of 20-50nm is injected in stages, with a volume of 30-50m³ per stage and a sand ratio of 15%-20%. Simultaneously, a 915MHz, 20kW microwave generator is activated downhole, leveraging the microwave thermal effect of the nanoparticles to achieve a localized temperature increase (ΔT) > 200°C / min, inducing radial fractures in the coal. Using strain data monitored by optical fiber, a temporary plugging agent containing a shape-memory polymer with a 300% expansion ratio is automatically injected when the extension rate of a particular fracture exceeds 0.5m / s, forcing the fracture to redirect. Based on a 30m coal pillar width, the resulting main fracture network is no longer than 12m, with a branch fracture density > 5 fractures / m. The fracture-forming area 5 ensures good gas permeability in the coal seam, but the main coal wall should not be damaged. At the end of the horizontal borehole 4, cracks within a range of at least 50 cm along the dip profile of the roadway coal pillar 2 should penetrate the coal wall, allowing the CO2 injected into the enclosed space in step 1 to penetrate through the coal seam and into the entire coal pillar. This is accomplished by establishing a reformed area 6 at the end of the horizontal borehole 4, corresponding to the dip profile of the roadway coal pillar 2. This reformed area 6 should be at least 50 cm thick, and all cracks in this reformed area 6 should penetrate the coal wall. The fracture-forming effect of the fracture-forming area 5 should be inspected, and if satisfactory, the horizontal borehole should be cement-sealed.

[0017] Step 3: At the end of the horizontal borehole, a vertical borehole was drilled close to the point where the fracture penetrated the coal wall, serving as a heating well. An electric heater was installed in the borehole. The downhole heater was activated and maintained at a temperature above 400°C. The heating well utilizes a 168mm x 89mm double-tube structure. A 2.45GHz microwave antenna array is arranged every meter of the inner tube, transmitting microwave energy via a coaxial cable. The annulus is filled with magnesium oxide insulated resistance heating tape with a power density of 3kW / m, forming a synergistic heating field with the microwaves. Ferrite powder containing at least 40wt% Fe3O4 is injected within a 20m radius around the wellbore to enhance microwave absorption efficiency. A PID algorithm is used to regulate the heating power, maintaining a temperature of 450±10°C within 0-5m of the wellbore and 380±20°C within 5-15m. Closed-loop control is achieved using a downhole infrared thermal imager with a sampling rate of 1Hz.

[0018] Step 4: At the surface position corresponding to the point where the horizontal borehole enters the coal seam, vertical wells 7 are constructed on both sides of the horizontal borehole as negative pressure extraction wells; an oil and gas extraction device is lowered into the extraction well and the extraction operation is started. A vortex tube refrigeration device is installed at the front end of the extraction pipeline to condense the high-temperature gas to recover heavy tar; ZIF-8 molecular sieve is used in the middle section to adsorb light aromatic hydrocarbons with an adsorption capacity of 1.2g / g; a membrane separation component is configured at the end to achieve CO2 purification with a purity of >90%.

[0019] Step 5: A Bi2Te3-based thermoelectric power generation module is installed in the extraction well, utilizing 300-400°C exhaust gas to generate electricity with a conversion efficiency of 7%-9%, which is directly supplied to the downhole heating system. A vehicle-mounted reforming unit uses a Pt-Sn / Al2O3 catalyst to convert tar into BTX (benzene, toluene, and xylene) components at 450°C and 2 MPa, with a yield exceeding 65%. The extracted oil and gas resources undergo oil and gas separation on the surface, and the separated CO2 gas is reinjected into the goaf for recycling. At this time, "goaf → heating well → coal pillar → extraction well → goaf" constitutes a negative pressure cycle. Under the action of the negative pressure in the extraction well, the gas begins to enter the coal pillar from the cracks in the heating well, and finally gathers in the extraction well; in the negative pressure circulation loop, the CO2 gas in the goaf is heated by the heater in the heating well and enters the coal pillar crack system. The oil-rich coal resources in the coal pillar undergo pyrolysis under high temperature and produce oil and gas resources, which are gradually gathered in the extraction well under the action of the negative pressure of the extraction well and finally carried to the ground; the oil and gas resources extracted to the ground are separated and purified, and the excess CO2 gas is re-injected into the goaf for recycling and reuse. After the pyrolysis is completed, the closed space can be used as a CO2 storage space.

[0020] The CO2 gas injected in this solution can also be replaced by other gases, such as flue gas, nitrogen, etc.

[0021] The above is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with this technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solutions and inventive concepts of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A negative pressure underground in-situ pyrolysis method for residual roadway coal pillars rich in oil, characterized in that: The steps include: Step 1: A composite slurry containing a silicate cement matrix doped with aerogel particles with a particle size of 50-100 μm and carbon fiber particles with a length of 3-6 mm is injected into the goaf cracks at a pressure of 0.8-1.2 MPa using a high-pressure grouting pump. Distributed fiber optic sensors are embedded in the grout to monitor stress and strain data. Gas injection wells are drilled around the sealed goaf, and CO2 at a pressure higher than the surface atmospheric pressure is injected into the gas injection wells. A piezoelectric sensor array is installed on the goaf roof, and a time-difference inversion algorithm is used to construct a three-dimensional geomechanical model. Step 2: A horizontal borehole is drilled into the middle of the coal pillar in the closed goaf. The horizontal borehole is made of fiberglass casing with a temperature resistance of over 400°C. A fracturing fluid containing ferroferric oxide nanoparticles with a particle size of 20-50nm is injected into the horizontal borehole in sections, with an injection volume of 30-50m³ per section and a sand ratio of 15%-20%. An underground microwave generator is simultaneously activated to generate radial cracks in the coal body. After the fracture formation is completed, the horizontal borehole is closed. Step 3: Drill a vertical hole at the end of the horizontal borehole, close to where the crack penetrates the coal wall, and lower an electric heater into the hole to serve as a heating well. The heating well adopts a double-tube structure, with a microwave antenna array arranged in the inner tube. Microwave energy is transmitted via a coaxial cable, and the annulus is filled with a magnesium oxide insulated resistance heater to maintain the heater heating temperature above 400°C. Step 4: Vertical wells are opened at the corresponding ground locations on both sides of the horizontal borehole where the well enters the coal seam, serving as negative pressure extraction wells. Oil and gas extraction equipment is lowered into the extraction wells and extraction operations are initiated. A vortex tube refrigeration device is installed at the front end of the extraction well to condense the high-temperature gas to recover heavy tar; a molecular sieve is used in the middle section to adsorb light aromatics; and a membrane separation component is configured at the end to achieve CO2 separation. Step 5: Install a Bi2Te3-based thermoelectric power generation module in the extraction well to directly supply heating to the heating well; after the extracted oil and gas resources are separated on the ground, the separated CO2 gas is re-injected into the goaf for recycling; After pyrolysis is completed, CO2 is sealed in a closed space.

2. The negative pressure underground in-situ pyrolysis method for residual roadway coal pillar oil-rich coal according to claim 1, characterized in that: In step 2, a reformed zone is established at the end hole position of the horizontal drilling well corresponding to the inclined profile of the coal pillar in the roadway. The thickness of the reformed zone is not less than 50 cm, and the cracks in the reformed zone all penetrate the coal wall.

3. The negative pressure underground in-situ pyrolysis method for residual roadway coal pillar oil-rich coal according to claim 1, characterized in that: The CO2 gas injected into the gas injection well is replaced by flue gas or nitrogen.

4. According to the negative pressure underground in-situ pyrolysis method of residual tunnel coal pillar oil-rich coal in claim 1, in step three, a PID algorithm is used to adjust the heating power, and an underground infrared thermal imager is used to maintain the temperature at 450±10°C in the area 0-5m away from the heating wellbore and at 380±20°C in the area 5-15m away from the heating wellbore.

5. The method for negative pressure underground in-situ pyrolysis of oil-rich coal in residual roadway coal pillars according to claim 1, wherein in step 2, when the crack extension rate exceeds 0.5 m / s using stress and strain data monitored by optical fiber, a temporary plugging agent containing a shape memory polymer with an expansion rate of 300% or more is automatically injected.

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