Underground coal gasification strengthening method

Abstract

The invention discloses a coal underground gasification strengthening method which comprises the following steps: step 1, drilling a U-shaped well of which the horizontal section is positioned in a coal seam at a target coal seam, respectively arranging an injection well and a production well at two ends of the U-shaped well, and putting a multi-channel continuous pipe into the horizontal section through the injection well; 2, oxygen, fuel and an ignition agent are injected into the horizontal section through the multi-channel continuous pipe to ignite the coal seam, then oxygen and a gasification agent are continuously injected through the multi-channel continuous pipe to enable the coal seam to be gasified to generate raw gas, and a gasification cavity is formed in the coal seam in the gasification process; 3, fuel and oxygen are injected into the newly-formed gasification cavity through the multi-channel continuous pipe, combustible gas cloud is formed at an outlet by the fuel and the oxygen, then the combustible gas cloud is ignited through an ignition agent sprayed out of the multi-channel continuous pipe for deflagration, and raw gas is produced; step 4; repeating the step 3 until the gasification of the cavity body of the gasification cavity is completed; and 5, the multi-channel continuous pipe is dragged to retreat, the steps 2-4 are repeated till gasification transformation of the whole coal seam is completed, and the gasification efficiency is high.

Description

Technical Field

[0001] The present invention belongs to the technical field of underground coal seam gasification, and in particular relates to an underground coal gasification enhancement method. Background Art

[0002] Underground coal gasification (UCG) is the process of converting underground coal into combustible gas in situ through controlled combustion. This technology integrates well construction, gasification, and gas production, offering advantages such as high efficiency, safety, environmental friendliness, and high resource utilization. Currently, UGC technology for medium- and deep-layer coal is limited by the extremely underdeveloped porosity of the underground coal. The gasification process is prone to a series of problems, including obstructed gasification channels, poor coal seam permeability, difficulty in getting the gasifier into the blind pore structure of the coal seam, and difficulty in discharging the gasified gas. These issues hinder contact between the gasifier and the coal seam, leading to expansion of the gasification surface and slow gasification progress, affecting both gas quality and production. Intensifying the UGC process is a major challenge. Currently, the process of intensifying surface coal gasification is relatively mature and can be achieved through various means, such as introducing catalysts, modifying feedstock quantity and physical and chemical properties, and optimizing furnace configurations. However, UGC forms a furnace by ablation of the coal itself, making it difficult to achieve the heat and mass transfer effects of surface gasification. Furthermore, monitoring and control measures are even more challenging, making the current surface gasification intensification process unsuitable. By using fracturing technology to create cracks, increase seepage channels, and accelerate the contact between gasifiers and coal seams, underground coal gasification enhancement can be achieved. However, due to pump pressure limitations, the fracturing range is limited. As the gasification process proceeds and the gasification surface expands, the effect of cracks formed by fracturing on seepage enhancement gradually decreases, and the gasification enhancement effect will be greatly weakened. Summary of the Invention

[0003] In order to solve the above technical problems, the purpose of the present invention is to provide a method for enhancing underground coal gasification, which realizes periodic explosion and rapid production in the gasification chamber through injection and production control, thereby promoting disturbance of the gasification chamber and enhancing the gasification process.

[0004] In order to achieve the above-mentioned purpose, the technical solution of the present invention is as follows: A method for enhancing underground coal gasification comprises the following steps:

[0005] Step 1: Select a favorable gasification area and drill a U-shaped well with a horizontal section inside the target coal seam. The two ends of the U-shaped well are an injection well and a production well, and a multi-channel coiled tubing is lowered into the horizontal section through the injection well.

[0006] Step 2: Nitrogen is first purged into the horizontal section through the multi-channel coiled tubing, and then oxygen, fuel, and ignition agent are injected into the horizontal section through the multi-channel coiled tubing to ignite the coal seam. After the coal seam is ignited, the injection of ignition agent and fuel is stopped, and oxygen and gasification agent are continuously injected through the multi-channel coiled tubing to gasify the coal seam and produce raw coal gas. During the gasification process, a gasification cavity is formed in the coal seam, and the generated raw coal gas is discharged and collected outside through the production well.

[0007] Step 3: Fuel and oxygen are injected into the newly formed vaporization chamber through the multi-channel coiled tubing. The two together form a combustible gas cloud at the outlet. The ignition agent ejected from the multi-channel coiled tubing then ignites the combustible gas cloud, causing it to deflagrate within the vaporization chamber. The ignition agent injection is then stopped. Once the deflagration ends and the pressure within the vaporization chamber stabilizes, the produced gas flow rate is increased through production control to promote rapid discharge of the vaporized gas and reduce the pressure in the vaporization chamber.

[0008] Step 4: Repeat step 3 until the gasification of the gasification cavity is completed;

[0009] Step 5: Drag the multi-channel coiled tubing backward and repeat steps 2 to 4 until the gasification transformation of the entire coal seam is completed.

[0010] The target coal seam in step 1 of the above technical solution is a medium-deep coal seam with a burial depth of 800-1500m.

[0011] The ignition agent in the above technical solution is at least one of silane, phosphine, triethylborane and triethylaluminum.

[0012] The fuel in the above technical solution is at least one of methane, propane, heptane and diesel.

[0013] The gasifying agent in the above technical solution is at least one of air, oxygen-enriched air, water vapor and supercritical water.

[0014] In step 2 of the above technical solution, before ignition, the ambient fuel concentration in the horizontal section is monitored to be below the lower explosion limit to avoid explosion during ignition; in step 3, the fuel concentration in the combustible gas cloud is within the explosion limit range.

[0015] In the above technical solution, an igniter is installed at one end of the multi-channel coiled tubing located in the well, and the igniter is used to atomize the fuel for premixing with oxygen.

[0016] In the above technical solution, the ignition agent and fuel are transported through the same channel of the multi-channel continuous tube, and the ignition agent and fuel are injected in an alternating manner.

[0017] The produced gas increment in step 3 of the above technical solution matches the amount of deflagration fuel.

[0018] In step 3 of the above technical solution, during the gasification process of the gasification chamber, the amount of fuel injected by multiple explosions gradually increases.

[0019] The beneficial effects of the present invention are as follows: the underground coal gasification enhancement method provided by the present invention uses a biomimetic human breathing method to periodically and alternately change the gasification chamber pressure to promote gas replacement within the chamber, enhance gasification seepage, and accelerate the gasification reaction process. This can effectively solve the problem of the slow underground coal gasification process at this stage. The present invention rapidly increases the gasification chamber pressure by injecting fuel and deflagration, which has the advantages of high speed and high efficiency. The pressure wave generated by the deflagration can impact the gasification chamber, which is conducive to enhancing internal disturbance and accelerating the expansion of the gasification surface. In addition, the deflagration products can react with the coal seam to generate crude coal gas, thereby increasing the crude coal gas production in the production well. In the multi-chamber stacking process, the gasification chamber that has completed gasification no longer participates in the gasification process and does not need to enhance seepage. However, its large chamber structure will cause pressure fluctuations in the new gasification chamber and weaken the gasification seepage. The deflagration method adopted in this embodiment mainly causes rapid fluctuations in local pressure, promotes gas disturbance in the new gasification chamber, and is perfectly compatible with the multi-chamber stacking gasification process. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 Schematic diagram of the underground coal gasification enhancement method in an embodiment of the present invention;

[0021] Figure 2 Schematic diagram of the alternating injection of ignition agent and fuel in steps 2 and 3 in an embodiment of the present invention.

[0022] In the figure: 1, U-shaped well; 11, injection well; 12, production well; 13, horizontal section; 131, gasification cavity; 2, channel coiled tubing; 3, production tubing. DETAILED DESCRIPTION

[0023] The principles and features of the present invention are described below with reference to the accompanying drawings. The examples given are only used to explain the present invention and are not intended to limit the scope of the present invention. The following paragraphs describe the present invention in more detail by way of example with reference to the accompanying drawings. The advantages and features of the present invention will become more apparent from the following description and claims. It should be noted that the drawings are all in a very simplified form and are not in exact proportions. They are only used to facilitate and clearly illustrate the purpose of the embodiments of the present invention.

[0024] like Figure 1 As shown, this embodiment provides a method for enhancing underground coal gasification, comprising the following steps:

[0025] Step 1: Select a favorable gasification zone (need to be determined through geological exploration), drill a U-shaped well 1 with a horizontal section 13 in the target coal seam, with an injection well 11 and a production well 12 at both ends of the U-shaped well 1, complete cementing during the drilling process (cementing measures are to simultaneously run casing into the injection well and the production well during the drilling process), perform directional perforation and backward staged fracturing in the horizontal section 13 of the U-shaped well 1 (this can form fracturing cracks in the coal seam), A multi-channel coiled tubing 2 is lowered into the horizontal section 13 through the injection well 11 (the injection well needs to be sealed at the wellhead, and the upper end of the multi-channel coiled tubing 2 extends out of the injection well, while a production tubing 3 can be installed at the production well, and the wellhead of the production well also needs to be sealed, and the upper end of the production tubing 3 extends out of the production well), wherein the end of the multi-channel coiled tubing 2 located in the well can be located in the horizontal section and close to the production well 12;

[0026] Step 2 (equivalent to the ignition stage): nitrogen is purged into the horizontal section 13 through the multi-channel coiled tubing 2 to create a reliable underground ignition environment (i.e., nitrogen carries the bottom hole liquid out of the production well, and a relatively dry and reliable underground ignition environment is created by gas lift drainage). Then, the nitrogen purge is stopped, and oxygen, fuel, and ignition agent are injected into the horizontal section 13 through the multi-channel coiled tubing 2 to ignite the coal seam (the ignition agent spontaneously ignites the fuel when it encounters oxygen, and the continuous injection of oxygen and fuel maintains the combustion flame and ignites the coal seam). After the coal seam is ignited, the injection point is stopped. Fire agent and fuel are injected, and oxygen and gasifying agent are continuously injected through the multi-channel continuous pipe 2 to gasify the coal seam to produce raw coal gas (the high-temperature coal seam reacts with the gasifying agent to produce raw coal gas rich in gases such as H2, CO, CH4, CO2, etc., which is collected and produced through the production well). During the gasification process, the coal seam is fractured and ablated, gradually forming a gasification cavity 131 (during the gasification and combustion process of the coal seam, the underground coal gradually fractures and abslates along the direction of the coal seam fractures, forming a gasification cavity in the horizontal section). The generated raw coal gas is discharged and collected through the production well 12.

[0027] Step 3 (equivalent to the deflagration stage): Fuel and oxygen are injected into the newly formed vaporization chamber 131 through the multi-channel coiled tubing 2. These two fuels form a combustible gas cloud at the outlet, and the fuel concentration in the combustible gas cloud is controlled to be within the explosive range. Subsequently, an ignition agent ejected from the multi-channel coiled tubing 2 ignites the combustible gas cloud, causing a deflagration within the vaporization chamber 131. The deflagration enhances the disturbance within the chamber and increases the pressure in the vaporization chamber 131. The injection of the ignition agent is then stopped. After the deflagration ends and the pressure in the vaporization chamber 131 stabilizes, the produced gas flow rate is increased through production control to promote rapid discharge of the gasified gas and reduce the pressure in the vaporization chamber 131. (Increasing the produced gas flow rate promotes rapid discharge of the gasified gas and reduces the pressure in the vaporization chamber. The produced gas increment is calculated based on the volume of the gasification products of the deflagration fuel combustion products and is adjusted through bottomhole pressure monitoring to ensure that the bottomhole pressure after the deflagration and enhanced gas production is close to the initial pressure.)

[0028] Step 4: Repeat step 3, periodically implement deflagration and increase the extraction volume until the gasification of the gasification cavity 131 is completed (through periodic deflagration and extraction enhancement, the gasification cavity pressure is changed, the gasification surface is expanded, and the gasification enhancement process is achieved; the ignition agent and fuel required for deflagration are adjusted according to the Figure 2 The scheme shown is arranged according to the fuel and ignition agent cycle, and the amount of fuel injected in each stage increases step by step to match the expanding gasification cavity);

[0029] Step 5: Drag the multi-channel coiled tubing 2 backward, repeat steps 2 to 4, carry out the backward gasification process, and realize the explosive gasification enhancement in the new gasification cavity 131 until the gasification transformation of the entire coal seam is completed (carry out the backward gasification process to realize the gasification enhancement mining of the entire coal seam; in the multi-cavity superimposed gasification process, since the explosion mainly causes local pressure fluctuations, the explosion injection control of the initial cavity can meet the gas disturbance inside the new cavity with a cavity size similar to that of the cavity, and meet the gasification enhancement requirements, so its explosion injection control does not need to be significantly adjusted).

[0030] The target coal seam in step 1 of the above technical solution is a medium-deep coal seam with a burial depth of 800-1500m (mainly because medium-deep coal seams are rich in resources, have high calorific value of produced gas, and have low environmental pollution, and have advantages in underground coal gasification development. In this embodiment, a coal seam with a burial depth of 800-1500m is selected as an advantageous reservoir for transformation and underground coal gasification); the ignition agent is at least one of silane, phosphine, triethylborane and triethylaluminum; the fuel is at least one of methane, propane, heptane and diesel; and the gasifying agent is at least one of air, oxygen-enriched air, water vapor and supercritical water (which is used to produce combustible gas by oxidation reaction with coal).

[0031] In the above technical solution, in step 2, before ignition, the ambient fuel concentration in the horizontal section is monitored to ensure it is below the lower explosion limit (LEL) to prevent explosion during ignition. In step 3, the fuel concentration in the combustible gas cloud is within the explosive limit range (for example, heptane, whose LE range is 1.2% to 6.7% by volume, increases in temperature and pressure lower its LEL, increase its LEL, and widen its explosion range. A concentration of 2% by volume, close to the theoretical mixing ratio, is selected as the combustible gas cloud concentration to achieve rapid deflagration). An igniter is mounted at one end of the multi-channel coiled tubing 2 within the wellbore, which atomizes the fuel for premixing with oxygen. The igniter and fuel are delivered through the same channel of the multi-channel coiled tubing 2, and the igniter and fuel are injected alternately (in step 2, the igniter is ejected first and spontaneously ignites upon encountering oxygen, immediately igniting the subsequent fuel; in step 3, the fuel is ejected first, forming a combustible gas cloud, which is then delayed in ignition by the later ejected igniter, resulting in deflagration).

[0032] In the above technical solution, the incremental amount of produced gas in step 3 matches the amount of deflagration fuel; in the gasification chamber 131 in step 3, during the gasification process, the amount of fuel injected by multiple deflagrations gradually increases.

[0033] In this embodiment, the multi-channel coiled tube 2 may have three injection channels, wherein the fuel and ignition agent share one injection channel. During the nitrogen purge phase, nitrogen needs to be introduced into each injection channel for purge. Oxygen and the gasifying agent may each use an independent injection channel. The multi-channel coiled tube may be directly composed of three composite pipes, which will not be described in detail here.

[0034] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Any ordinary technician in this industry can smoothly implement the present invention as shown in the drawings and described above. However, any equivalent changes, modifications and evolutions made by technicians familiar with this profession without departing from the scope of the technical solution of the present invention using the technical content disclosed above are all equivalent embodiments of the present invention. At the same time, any equivalent changes, modifications and evolutions made to the above embodiments based on the essential technology of the present invention are still within the scope of protection of the technical solution of the present invention.

Claims

1. A method for enhancing underground coal gasification, characterized in that: The steps include: Step 1: Select a favorable gasification area and drill a U-shaped well (1) at the target coal seam with a horizontal section (13) located within the coal seam. The two ends of the U-shaped well (1) are an injection well (11) and a production well (12), and a multi-channel coiled tubing (2) is lowered into the horizontal section (13) through the injection well (11); Step 2: nitrogen is first purged into the horizontal section (13) through the multi-channel continuous pipe (2), and then oxygen, fuel and ignition agent are injected into the horizontal section (13) through the multi-channel continuous pipe (2) to ignite the coal seam. After the coal seam is ignited, the injection of ignition agent and fuel is stopped, and oxygen and gasification agent are continuously injected through the multi-channel continuous pipe (2) to gasify the coal seam to produce raw coal gas. During the gasification process, a gasification cavity (131) is formed in the coal seam, and the raw coal gas produced is discharged and collected through the production well (12); Step 3: injecting fuel and oxygen into the newly formed gasification chamber (131) through the multi-channel continuous tube (2), and the two together form a combustible gas cloud at the outlet. Subsequently, the ignition agent ejected from the multi-channel continuous tube (2) ignites the combustible gas cloud, causing it to deflagrate in the gasification chamber (131). Then, the injection of the ignition agent is stopped. After the deflagration ends and the pressure in the gasification chamber (131) stabilizes, the produced gas flow rate is increased through the production control, so as to promote the rapid discharge of the gasified gas and reduce the pressure in the gasification chamber (131); Step 4: Repeat step 3 until the gasification of the gasification cavity (131) is completed; Step 5: Drag the multi-channel coiled tubing (2) backward and repeat steps 2 to 4 until the gasification transformation of the entire coal seam is completed.

2. The underground coal gasification enhancement method according to claim 1, characterized in that: The target coal seam in step 1 is a medium-deep coal seam with a burial depth of 800-1500m.

3. The underground coal gasification enhancement method according to claim 1, characterized in that: The ignition agent is at least one of silane, phosphine, triethylborane and triethylaluminum.

4. The underground coal gasification enhancement method according to claim 1, characterized in that: The fuel is at least one of methane, propane, heptane and diesel.

5. The underground coal gasification enhancement method according to claim 1, characterized in that: The gasifying agent is at least one of air, oxygen-enriched air, water vapor and supercritical water.

6. The underground coal gasification enhancement method according to claim 1, characterized in that: In step 2, before ignition, the ambient fuel concentration in the horizontal section is monitored to be below the lower explosion limit to avoid explosion during ignition; in step 3, the fuel concentration in the combustible gas cloud is within the explosion limit range.

7. The underground coal gasification enhancement method according to claim 1, characterized in that: An igniter () is installed at one end of the multi-channel continuous tube (2) located in the well, and the igniter () is used to atomize the fuel for premixing with oxygen.

8. The underground coal gasification enhancement method according to claim 1, characterized in that: The ignition agent and the fuel are transported through the same channel of the multi-channel continuous tube (2), and the ignition agent and the fuel are injected in an alternating manner.

9. The underground coal gasification enhancement method according to claim 1, characterized in that: In step 3, the produced gas increment matches the amount of the deflagration fuel.

10. The underground coal gasification enhancement method according to claim 1, characterized in that: In the gasification chamber (131) in step 3, during the gasification process, the amount of fuel injected by multiple explosions gradually increases.

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