An in-situ coal pyrolysis downhole staged heating injection method

Abstract

This application relates to the fields of underground coal gasification and in-situ coal pyrolysis mining technology, and particularly to a downhole staged heating injection method for in-situ coal pyrolysis. The method includes: injecting ambient temperature water into a multi-stage heating coil string arranged within the injection wellbore; sequentially passing through four stages of heating along the longitudinal direction of the wellbore; the final stage heating device being located near the roof of the target coal seam or extending into the coal seam; and the heated injection fluid being directly injected into the coal seam through perforations or slotted channels for in-situ pyrolysis. The injection fluid, depending on the combination of system pressure and temperature, is in any of the following states: high-temperature liquid water, superheated steam, subcritical water, or supercritical water. Through the above steps, the heating process is transferred from the surface to the underground and four-stage heating is achieved, reducing heat loss to below 5%, completely eliminating the risk of water hammer, reducing the surface footprint by more than 80%, and providing fast control response and high thermal efficiency. This method is particularly suitable for in-situ pyrolysis mining of deep, oil-rich coal seams.

Description

Technical Field

[0001] This application relates to the fields of underground coal gasification and in-situ coal pyrolysis mining technology, and in particular to an underground staged heating and injection method for in-situ coal pyrolysis. Background Technology

[0002] In-situ coal pyrolysis mining technology refers to a new method of coal resource utilization that involves injecting high-temperature thermal fluids (such as superheated steam, subcritical water, or supercritical water) into underground coal seams to cause the organic matter in the coal to undergo pyrolysis reactions underground, generating oil and gas products, which are then extracted to the surface through production wells. This technology avoids the problems of surface damage and gangue discharge caused by traditional coal mining, while also enabling the clean and low-carbon utilization of coal resources.

[0003] The inventors discovered that in existing in-situ coal pyrolysis technologies, the heat source is primarily provided by a surface steam boiler system. This involves constructing a large boiler on the surface to heat water into high-temperature, high-pressure steam, which is then transported to the underground coal seam via long-distance injection wells. This technical solution has the following problems: (1) Ground equipment occupies a large area: Large steam boiler systems require the construction of supporting facilities such as fuel storage and transportation, water treatment, and steam pipelines, which can occupy tens of thousands of square meters. They are not suitable for mining areas with complex terrain and limited land.

[0004] (2) Significant heat loss during long-distance transport: During the transport of high-temperature steam through wells hundreds or even thousands of meters long, heat is continuously lost to the surrounding rock. Actual data shows that when steam with a wellhead temperature of 600°C reaches the bottom of the well, the temperature can drop by more than 100°C, and the thermal efficiency is only 60% to 80%.

[0005] (3) Risk of water hammer caused by gas-liquid two-phase flow: When the high-temperature steam generated by the ground boiler is transported downward, some of the steam condenses into water due to the temperature drop, forming a gas-liquid two-phase flow, which is very easy to generate water hammer effect and cause impact damage to pipelines and equipment.

[0006] (4) System adjustment response lag: The ground boiler is far from the target coal seam. After adjusting the heat injection parameters (temperature, flow rate, pressure), it takes several hours to affect the coal seam reaction zone, making it difficult to achieve precise control.

[0007] Existing underground heating solutions can only achieve single-stage heating or suffer from low heating efficiency and short equipment life, making it difficult to meet the high heat input requirements of deep coal seams. Summary of the Invention

[0008] To address the problems existing in the prior art, this application provides a downhole staged heating injection method for in-situ coal pyrolysis. By arranging a multi-stage spiral coil heating device along the longitudinal direction of the wellbore, the heating is carried out in stages underground, and the final stage heating device is set near the top of the coal seam, thereby realizing the cascade utilization of energy and near-zero distance injection of heat. This solves the problems of large footprint of surface equipment, large heat loss during long-distance transportation, high risk of water hammer, and lag in control response in the prior art.

[0009] To achieve the above objectives, this application provides the following technical solution: This application provides a downhole staged heating and injection method for in-situ coal pyrolysis, including: Step S1, Injecting ambient temperature water: Ambient temperature water is injected from the ground through injection pipelines into a multi-stage heating coil string installed inside the injection well. Unlike existing technologies that directly inject high-temperature steam, this invention injects ambient temperature water, thus avoiding the risks associated with the storage and transportation of high-temperature, high-pressure steam in ground equipment. The multi-stage heating coil string adopts a spiral coil structure, which, compared to straight pipes, extends the fluid heating path, increases the heat exchange area, and enhances turbulence disturbance, thereby significantly improving heating efficiency.

[0010] Step S2, Primary Preheating: Using a first-stage heating device installed in the upper part of the wellbore, the room-temperature water is heated to a first preset temperature (preferably 120℃~180℃) to obtain a preheated fluid. Under high-pressure conditions downhole, the boiling point of water increases significantly. For example, when the system pressure is 5MPa, the boiling point of water is approximately 264℃. Therefore, water at 120℃~180℃ is in a supercooled liquid state in the high-pressure downhole environment and will not boil or vaporize, thus avoiding the risks of gas-liquid two-phase flow and water hammer.

[0011] Step S3, Secondary Heating: Using a second-stage heating device located in the middle region of the wellbore, the preheated fluid is heated to a second preset temperature (preferably 250℃~350℃) to obtain a medium-temperature fluid. During this stage, the temperature range remains below the boiling point at the corresponding pressure, and the water continues to exist as a single-phase liquid. As the temperature increases, the water's solubility and diffusion capacity gradually increase, preparing for subsequent high-temperature injection.

[0012] Step S4, Third-stage high temperature: Using a third-stage heating device installed in the lower part of the wellbore, the medium-temperature fluid is heated to a third preset temperature (preferably 400℃~500℃) to obtain a high-temperature fluid. Within this temperature range, depending on the system pressure, the water can be in a subcritical liquid state (pressure higher than the corresponding saturated vapor pressure) or a supercritical state (pressure higher than 22.1MPa). The water's solubility and diffusion capacity are significantly enhanced, and some organic matter begins to undergo preheating decomposition.

[0013] Step S5, Fourth-stage final heating: Using a fourth-stage heating device located near the roof of the target coal seam or extending into the coal seam, the high-temperature fluid is further heated to a fourth preset temperature (preferably 550℃~650℃) to obtain the injected fluid. This injected fluid can be adjusted according to actual operating conditions by regulating the system pressure to present itself as high-temperature, high-pressure liquid water, supercritical water, or superheated steam. For example, when high solubility and high diffusion capacity are required, the pressure is maintained above 22.1MPa to keep the water in a supercritical state; when high latent heat exchange is required, the pressure can be appropriately reduced to convert the water into superheated steam. The final-stage heating device is located close to the injection point, minimizing heat loss.

[0014] It should be noted that this invention does not limit the specific phase of the injected fluid, as long as its temperature meets the pyrolysis requirements. The selection of system pressure depends on the coal seam depth, permeability, and pyrolysis process requirements, and those skilled in the art can adjust it flexibly according to the actual situation.

[0015] Step S6, injecting into the coal seam: the injection fluid at the outlet of the fourth-stage heating device is directly injected into the target coal seam through the perforation or slotted channel set at the injection end to perform in-situ pyrolysis of the coal.

[0016] The core feature of the above technical solution is that the energy injection process of heating water from room temperature to a high temperature state (reaching a supercritical state or superheated steam) is transferred from the ground to underground, and the temperature is gradually increased through a four-stage stepped heating method, which realizes the efficient utilization of heat; at the same time, the final stage heating device is set on the top plate of the coal seam, so that the high temperature fluid is injected into the coal seam almost immediately after production, minimizing heat loss.

[0017] Preferably, the first preset temperature is 120℃~180℃, more preferably 140℃~160℃; the second preset temperature is 250℃~350℃, more preferably 280℃~320℃; the third preset temperature is 400℃~500℃, more preferably 430℃~470℃; and the fourth preset temperature is 550℃~650℃, more preferably 580℃~620℃. The above temperature ranges take into account heating efficiency, material resistance, and pyrolysis effect. If the first-stage temperature is too low, the preheating effect is not significant; if the final-stage temperature is too high, it places excessive demands on the heat-resistant materials of the heating device and may lead to excessive coking of the coal seam.

[0018] Preferably, the fourth preset temperature is higher than the critical temperature of water (374°C), and the injection pressure is higher than the critical pressure of water (22.1 MPa), so that the injected fluid is in a supercritical water state. Supercritical water has a high diffusion coefficient similar to that of a gas and a high solubility similar to that of a liquid, which can more effectively penetrate into the microporous structure of coal, carry out pyrolysis products, and significantly improve oil and gas recovery rate.

[0019] Preferably, before step S1, a downhole preheating step is included: before starting each stage of the heating device, each stage of the heating device is operated in a low-power mode (preferably 10% to 30% of the rated power) for 20 to 60 minutes to gradually establish a temperature gradient within the wellbore and prevent thermal stress damage to the heating device caused by cold fluid impact. This step is of great significance for protecting the heating device and extending its service life.

[0020] Preferably, the system further includes a temperature feedback control step: temperature sensors are installed between adjacent heating devices to monitor the fluid temperature in real time, and the heating power of each heating device is independently adjusted according to the deviation between the monitored value and the target temperature. Through closed-loop control, the outlet temperature of each stage can be kept stable within the set range (preferably with a control accuracy of ±3℃ to ±10℃ of the set value), ensuring a constant fluid temperature injected into the coal seam.

[0021] Preferably, the system also includes a pressure compensation step, in which pressure regulating valves or booster devices are installed between each heating device to keep the pressure at the inlet of each heating device stable (pressure fluctuations are preferably controlled within ±0.5 MPa), preventing cavitation or pressure fluctuations caused by fluid volume expansion. Since water may expand in volume during the heating process, pressure compensation can ensure the stable operation of the entire system.

[0022] Preferably, in step S1, the room-temperature water injected first flows through a heat exchanger located downhole before entering the first-stage heating device, where it exchanges heat countercurrently with the pyrolysis products returned from the production well to recover residual heat and preheat the injected water. The downhole heat exchanger is preferably a shell-and-tube or plate-fin type structure, with a heat exchange area preferably of 5–20 m² and a heat recovery rate preferably of 40%–70%. This scheme can further reduce the power consumption of the preheating stage, achieve closed-loop energy utilization, and improve overall energy efficiency.

[0023] Preferably, the heating devices at each stage employ electromagnetic induction heating, with the induction coil wound around the outer wall of the coil (or lined the inner wall of the coil). The operating frequency is preferably 1kHz to 100kHz, and the fluid being heated is inductively heated as it flows through the internal channels of the coil. Electromagnetic induction heating has advantages such as fast heating speed, high thermal efficiency, non-contact heating, and good insulation reliability, and is particularly suitable for high-temperature and high-pressure environments downhole.

[0024] Preferably, the outer wall of the multi-stage heating coil column is covered with a vacuum insulation layer or an aerogel insulation layer to reduce heat loss to the surrounding rock. The vacuum degree of the vacuum insulation layer is preferably below 0.01 Pa, and its thermal conductivity can be as low as 0.004 W / (m·K), which is 1 / 10 to 1 / 20 of conventional insulation materials; the thermal conductivity of the aerogel insulation layer is preferably below 0.02 W / (m·K). Both can significantly reduce heat loss along the heating path.

[0025] Preferably, each stage of the multi-stage heating coil column adopts a spiral coil structure, the spiral helix angle is preferably 10° to 20°, the ratio of coil diameter to wellbore diameter is preferably 0.3 to 0.6, and the ratio of the unfolded length of each stage coil to the straight pipe length is preferably 2:1 to 5:1.

[0026] Preferably, the process also includes a wellbore protection step after interruption or termination of mining: after stopping injection, nitrogen or inert gas is injected into the wellbore to purge residual fluid, and the heating devices at all levels are slowly cooled down. The cooling rate is preferably controlled below 50°C / h, more preferably 20-30°C / h, to prevent thermal damage. This step can prevent damage to the high-temperature heating devices due to thermal stress concentration after shutdown, thus extending the service life of the equipment.

[0027] Preferably, when the coal seam permeability is less than 0.5 mD, the injected fluid is supercritical water (pressure > 22.1 MPa, temperature > 374 °C); when the coal seam permeability is greater than 0.5 mD, the injected fluid is superheated steam (pressure may be lower than the critical pressure, but temperature may be higher than the boiling point at the corresponding pressure). Beneficial effects

[0028] (1) Significantly improved thermal efficiency: By arranging multi-stage heating devices along the longitudinal direction of the shaft and placing the final stage heating device near the top of the coal seam, near-zero distance heat injection is achieved. Compared with the traditional ground boiler scheme, the heat loss along the flow path is reduced from 20% to 40% to less than 5%, and the overall thermal efficiency can be improved by 15 to 30 percentage points.

[0029] (2) Enhanced heat transfer by coil structure: The heating devices at each stage adopt a spiral coil structure. Compared with traditional straight pipes, the fluid heating path is extended by 2 to 3 times and the heat transfer area is increased by more than 50%. At the same time, the centrifugal force generated by the spiral flow enhances the turbulent disturbance, which increases the heat transfer coefficient by 30% to 50%, resulting in more complete heating and more uniform temperature.

[0030] (3) Completely eliminate the risk of water hammer: In the S2 and S3 stages, the present invention ensures that the water remains in a single-phase liquid state by maintaining high pressure conditions, thereby avoiding the existence of gas-liquid two-phase flow and fundamentally eliminating the risk of water hammer effect impact damage to the pipeline.

[0031] (4) Realize the cascade utilization of energy: The four heating devices correspond to different temperature ranges (120~180℃, 250~350℃, 400~500℃, 550~650℃). Each heating device can be optimized for its working temperature, avoiding the problems of thermal stress concentration and efficiency loss caused by single-stage large temperature difference heating.

[0032] (5) Fast control response: The heating devices at all levels are directly installed underground. After adjusting the heating power, the fluid temperature injected into the coal seam can be affected within a few minutes. Compared with the surface boiler scheme (response time of several hours), it has a significant advantage and facilitates precise process control.

[0033] (6) Ground equipment is greatly simplified: there is no need to build a large high-pressure steam boiler system, only conventional water injection pumps and power distribution facilities are needed, and the ground area can be reduced by more than 80%, which is especially suitable for mining areas with complex terrain and limited land use.

[0034] (7) High system safety: High temperature and high pressure steam is no longer stored and transported on the ground. The underground heating device adopts electric heating, which eliminates major safety hazards such as boiler explosion and steam leakage. Attached Figure Description

[0035] Figure 1 This is a schematic diagram of the overall process of the method of the present invention.

[0036] Figure 2 This is a schematic diagram showing the arrangement of heating devices at various levels in the wellbore. Detailed Implementation

[0037] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. The following embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0038] Example 1 This embodiment provides an underground staged heating injection method for in-situ pyrolysis of coal, applied to a coal seam with a thickness of 8m and a burial depth of 600m in a certain mining area.

[0039] The specific steps are as follows: (1) Four levels of electromagnetic induction heating devices are installed sequentially from top to bottom inside the injection wellbore. The first level heating device is located in the upper part of the wellbore (0-150m burial depth), the second level is located in the upper middle part of the wellbore (150-300m burial depth), the third level is located in the lower middle part of the wellbore (300-500m burial depth), and the fourth level is located in the lower part of the wellbore and extends to the top of the coal seam (500-600m burial depth). Temperature sensors and pressure regulating valves are installed between each level of heating device.

[0040] (2) Start the downhole preheating program and run each level of heating device at 20% of the rated power for 30 minutes to gradually establish the temperature gradient inside the well.

[0041] (3) Start the ground water pump and pump 15℃ ambient temperature water at a rate of 10m. 3 A flow rate of / h is injected into the multi-stage heating column.

[0042] (4) The first stage heating device heats the water to 150°C, the second stage heating device further heats the water to 300°C, the third stage heating device heats the water to 450°C, and the fourth stage heating device heats the water to 600°C. The system pressure is maintained at 25 MPa, so that the water is in a supercritical state.

[0043] (5) Supercritical water at 600℃ is directly injected into the coal seam through the perforation channel at the injection end to perform in-situ pyrolysis of the coal. The oil and gas mixture produced by pyrolysis is extracted to the surface through a nearby production well.

[0044] (6) During operation, the temperature sensor monitors the outlet water temperature of each stage in real time and feeds it back to the ground control system to independently adjust the power of each stage of heating device to ensure that the outlet temperature is stable within ±5℃ of the set value.

[0045] Field tests have verified that, using the method described in this embodiment, the temperature drop along the injection wellhead to the top of the coal seam is only 8°C, with a heat loss of about 1.5%. The coal seam pyrolysis temperature field is uniformly distributed, and the oil and gas recovery rate is about 25% higher than that of the traditional surface steam injection scheme.

[0046] Example 2 This embodiment is basically the same as Embodiment 1, except that a downhole heat exchanger is added between the water injection pipeline and the first-stage heating device. This heat exchanger is a shell-and-tube structure; the inner tube carries the high-temperature pyrolysis products (approximately 350°C) returned from the production well, while the annulus carries the injected room-temperature water. After counter-current heat exchange, the injected water is preheated to 80°C before entering the first-stage heating device.

[0047] Using the solution in this embodiment, the power consumption of the first-stage heating device is reduced by about 35%, and the overall energy efficiency of the system is further improved.

[0048] Comparative Example A traditional surface steam boiler was used to conduct in-situ pyrolysis tests under the same coal seam conditions. The boiler outlet steam temperature was 600℃ and the pressure was 25MPa, which was transported to the coal seam through a 600m long heat injection well.

[0049] Actual measurement data shows that the temperature of steam drops to 480℃ when it reaches the bottom of the well, with a temperature drop of 120℃ along the pipeline and a heat loss of about 20%. At the same time, water hammer occurred multiple times during operation, causing pipeline vibration and damage to seals.

[0050] This specific embodiment is merely an explanation of this application and is not intended to limit it. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.

Claims

1. A downhole staged heating injection method for in situ pyrolysis of coal, characterized by, include: Step S1, Injecting room temperature water: Inject room temperature water from the ground through the water injection pipeline into the multi-stage heating coil string installed inside the heating well; Step S2, First-stage preheating: Using a first-stage heating device installed in the upper part of the wellbore, the room temperature water is heated to a first preset temperature to obtain preheated fluid; Step S3, Secondary heating: Using a second-stage heating device installed in the middle area of ​​the wellbore, the preheated fluid is heated to a second preset temperature to obtain a medium-temperature fluid; Step S4, Level 3 High Temperature: Using a third-level heating device installed in the lower part of the wellbore, the medium-temperature fluid is heated to a third preset temperature to obtain a high-temperature fluid; Step S5, fourth-stage final heating: using a fourth-stage heating device installed near the roof of the target coal seam or extending into the interior of the coal seam, the high-temperature fluid is further heated to a fourth preset temperature to obtain the injected fluid; Step S6, injecting into the coal seam: the injection fluid at the outlet of the fourth-stage heating device is directly injected into the target coal seam through the perforation or slotted channel set at the injection end to perform in-situ pyrolysis of the coal. The injected fluid can be in any of the following states, depending on the combination of system pressure and temperature: high-temperature liquid water, superheated steam, subcritical water, or supercritical water.

2. The downhole staged heating injection method for coal in situ pyrolysis of claim 1, wherein, The first preset temperature is 120℃~180℃, the second preset temperature is 250℃~350℃, the third preset temperature is 400℃~500℃, and the fourth preset temperature is 550℃~650℃.

3. The underground staged heating and injection method for in-situ coal pyrolysis according to claim 1 or 2, characterized in that, Each stage of the multi-stage heating coil column adopts a spiral coil structure to extend the fluid heating path and increase the heat exchange area.

4. The underground staged heating and injection method for in-situ coal pyrolysis according to claim 1, characterized in that, Before step S1, a downhole preheating step is also included: before starting each level of heating device, each level of heating device is first run in low power mode to gradually establish the temperature gradient in the wellbore and prevent thermal stress damage to the heating device caused by cold fluid impact.

5. The underground staged heating and injection method for in-situ coal pyrolysis according to claim 1, characterized in that, It also includes a temperature feedback control step: temperature sensors are installed between each adjacent heating device to monitor the fluid temperature in real time, and the heating power of each heating device is independently adjusted according to the deviation between the monitored value and the target temperature.

6. The underground staged heating and injection method for in-situ coal pyrolysis according to claim 1, characterized in that, It also includes a pressure regulation step: pressure regulating valves are installed between each heating device to keep the pressure at the inlet of each heating device stable and prevent cavitation or pressure fluctuations caused by fluid phase change.

7. The underground staged heating and injection method for in-situ coal pyrolysis according to claim 1, characterized in that, Before entering the first-stage heating device, the room-temperature water injected in step S1 flows through a heat exchanger installed in the well, where it exchanges heat countercurrently with the pyrolysis products returned from the production well to recover residual heat and preheat the injected water.

8. The underground staged heating and injection method for in-situ coal pyrolysis according to claim 1, characterized in that, The heating devices at each stage adopt electromagnetic induction heating, with their induction coils wound around the outer wall of the wellbore. The fluid being heated is induced to heat as it flows through the central channel of the tubing string.

9. The underground staged heating and injection method for in-situ coal pyrolysis according to claim 1, characterized in that, The outer wall of the multi-stage heating column is covered with a vacuum insulation layer or an aerogel insulation layer to reduce heat loss to the surrounding rock.

10. The underground staged heating and injection method for in-situ coal pyrolysis according to claim 1, characterized in that, It also includes wellbore protection steps after the mining process is interrupted or terminated: after stopping injection, nitrogen or inert gas is injected into the wellbore to drain the residual fluid, and the heating devices at all levels are slowly cooled to prevent thermal damage.

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