Method for displacing methane through cooperation of high-temperature water vapor and liquid carbon dioxide

Abstract

Strict control of methane concentration is a very important measure in coal mine engineering. Due to the fact that direct extraction efficiency is low and all methane resources are difficult to completely recover, a triaxial compression test is carried out on a sample collected from a target coal seam, compressive strength is obtained, the injection pressure of high-temperature water vapor and liquid carbon dioxide is determined, perforation is carried out on the coal seam through an insertion type perforating gun, and the methane resources are recycled. The injected high-temperature water vapor and liquid carbon dioxide penetrate into the coal seam to act. The injection amount of high-temperature water vapor is controlled by a methane concentration monitor, and the injection amount of liquid carbon dioxide is determined according to a Langmuir adsorption isotherm. The high-temperature water vapor, the liquid carbon dioxide and the thermal expansion and cold contraction reaction generated by cooperation of the high-temperature water vapor and the liquid carbon dioxide can increase the number of coal seam fractures, methane adsorbed on a coal matrix is promoted to be desorbed and released into fractures and pores, the methane is easier to displace, the displacement amount and collection amount of the methane are increased to a greater extent, and the coal seam quality is improved. And gas explosion accidents in coal mine engineering are prevented.

Description

Technical field:

[0001] The invention relates to the technical field of coal seam methane displacement methods in coal mine engineering, and in particular to a methane displacement method using high-temperature steam in coordination with liquid carbon dioxide. Background technology:

[0002] Coal, as a dual medium with a complex pore-crack structure, is a good place for coalbed methane to occur. The main component of coalbed methane is methane. Coal mine gas accidents, as one of the five major disasters in mines, pose a great threat to coal mine safety production. During coal mining, methane gas in the coal seam is released as it is mined. When the concentration of methane gas reaches a certain level, it will cause explosion risks, suffocation risks and environmental pollution. Moreover, as a potent greenhouse gas, methane's role in global climate change cannot be ignored.

[0003] Existing technologies for displacing coal seam methane include hydraulic fracturing, liquid carbon dioxide, high-temperature steam and nitrogen, but there are still many problems. Hydraulic fracturing displaces methane by increasing the permeability of coal seams, thereby increasing the recovery rate of methane. However, it consumes a lot of water resources and may cause groundwater pollution. In addition, the use of high-pressure liquids to inject into the formation may induce micro-earthquakes or small-scale earthquakes. When liquid carbon dioxide is used to displace methane, the production and transportation costs of liquid carbon dioxide are high. When high-temperature steam is used to displace methane, long-term high temperatures may affect the structure of the coal seams. When nitrogen is used to displace methane, the adsorption capacity of nitrogen is weak, the displacement effect is limited, and long-term injection may also have an adverse effect on the structure and permeability of the coal seams.

[0004] In view of the above problems, a method for displacing methane by high-temperature water vapor and liquid carbon dioxide is proposed, so as to avoid the influence of high temperature on coal seams caused by long-term high temperature when high-temperature water vapor is used alone for displacement and the high cost investment when liquid carbon dioxide is used alone. The desorption and flow efficiency of methane are improved through the synergistic effect of high-temperature water vapor and liquid carbon dioxide displacement technology. In addition, the injected high-temperature water vapor and liquid carbon dioxide will produce thermal expansion and contraction reactions in the coal seam, which can further increase the number of cracks in the coal seam, thereby releasing more methane, reducing the methane concentration in the coal seam before or during mining, and reducing the risk of explosion. At the same time, the methane displaced in the coal seam is collected by a methane collection device, which can effectively prevent methane from escaping from the mine into the atmosphere, thereby reducing greenhouse gas emissions and mitigating climate change, and helping to achieve sustainable development goals. Summary of the invention:

[0005] Strictly controlling the methane concentration is a very important measure in coal mining engineering. The methane in the coal seam mainly exists in the coal matrix in an adsorbed state, and only a small part exists in the cracks and pores in a free state. Due to the low efficiency of direct extraction, it is difficult to completely recover all methane resources. By injecting high-temperature water vapor and liquid carbon dioxide, the methane adsorbed on the coal matrix is ​​desorbed and released into the cracks and pores, so that the methane is more easily displaced, and the displacement and collection of methane are increased to a greater extent. Therefore, a method for displacing methane with high-temperature water vapor and liquid carbon dioxide is proposed. It includes the following steps:

[0006] Step 1: Perform a triaxial compression test on the standard specimen and determine the injection pressure of high-temperature water vapor and liquid carbon dioxide based on the compressive strength obtained from the test.

[0007] Step 2: Drill a hole from the ground to the target coal seam. In the target coal seam, the liquid carbon dioxide injection well and the high-temperature steam injection well are connected. After the drilling is completed, the steel casing is lowered and cement is pumped in for cementing treatment.

[0008] Step 3: Lower the inserted perforating gun controlled by the cable to the predetermined position in the casing to perform perforations, penetrate the casing and the cement ring outside the casing, and penetrate into the coal seam to a certain depth, so that subsequent high-temperature water vapor and liquid carbon dioxide can penetrate into the coal seam.

[0009] Step 4: Use a bridge plug to plug the liquid carbon dioxide injection well. In addition, high-temperature water vapor is injected from the injection well into the target coal seam to increase the methane desorption and fluidity while displacing methane.

[0010] Step 5: When the methane volume fraction monitored by the methane monitor tends to be stable, stop injecting high-temperature water vapor, seal the high-temperature water vapor injection well with a bridge plug, and inject liquid carbon dioxide from the injection well into the target coal seam to allow the high-temperature water vapor and liquid carbon dioxide to work synergistically.

[0011] Step 6: The methane gas displaced by high-temperature water vapor and liquid carbon dioxide is transported to the methane gas collector through the gas production well. Description of the drawings:

[0012] Figure 1 It is a flow chart of the implementation of the method of the present invention;

[0013] Figure 2 It is the coal seam section and equipment layout diagram;

[0014] Figure 3 is the perforation gun perforation map;

[0015] Figure 4 Bridge plug placement Figure 1 ;

[0016] Figure 5Bridge plug placement Figure 2 .

[0017] In the figure: 1-sandstone; 2-gravel layer; 3-coal seam; 4-shale; 5-sandstone; 6-cement; 7-liquid carbon dioxide storage tank; 8-high-temperature steam generator; 9-pipeline; 10-methane concentration monitor; 11-methane gas collector; 12-cable; 13-insertion perforating gun; 14-bridge plug. Specific implementation method:

[0018] The present invention will be further described below in conjunction with the accompanying drawings.

[0019] Step 1: To determine the injection pressure of high-temperature water vapor and liquid carbon dioxide. Take samples from the target coal seam and make the coal samples into cylindrical specimens with a diameter of 50 mm and a height of 100 mm. Perform triaxial compression tests on the specimens. Take five groups of data x1, x2, x3, x4, and x5 as the control group. Detect the compressive strength P of the specimens under various confining pressure conditions. Calculate the average value of the five groups of test compressive strength P and record it as P0. Add a fluctuation range of 20% on the basis of the original P0, that is, take the pressure of P0-1.2P0 as the injection pressure of high-temperature water vapor and liquid carbon dioxide to ensure that the injection pressure of high-temperature water vapor is higher than the compressive strength of the target coal seam, prevent excessive injection pressure from causing formation rupture or wellbore damage, and prevent too low pressure from effectively displacing methane.

[0020] Step 2: If Figure 2 As shown, drilling is performed from the ground to the target coal seam, including a high-temperature water vapor injection well, a liquid carbon dioxide injection well, two horizontal wells and a gas outlet well. The drilling operation mode is vertical-horizontal-vertical segmented operation. After the drilling is completed, the steel casing is immediately lowered and cement is pumped in for cementing treatment to effectively prevent the wellbore from collapsing and ensure the stability of the wellbore structure. The air inlet end of the high-temperature water vapor injection well is connected to the high-temperature water vapor generating device through a pipeline, the air inlet end of the liquid carbon dioxide injection well is connected to the liquid carbon dioxide storage tank through a pipeline, and the air outlet end of the gas outlet well is connected to the methane collection device through a pipeline. In the target coal seam, the liquid carbon dioxide injection well and the high-temperature water vapor injection well are connected, and the drilling operations of the two horizontal wells are completed in the target coal seam. The horizontal wells are spaced one meter apart, and the air inlet end of the horizontal well is connected to the high-temperature water vapor and liquid carbon dioxide through-wellway, and the air outlet end is connected to the gas outlet well, to ensure that the displacement gas is smoothly injected into the target coal seam and methane is smoothly displaced.

[0021] Step 3: If Figure 3As shown in the figure, the inserted perforating gun controlled by the cable is lowered to the predetermined position in the casing for perforation. The predetermined positions of the perforating points are as follows: the initial position of the horizontal well inlet end is the first upper perforating point, and the subsequent upper perforating points are 50 decimeters away from the first upper perforating point. Each lower perforating point is located between the two upper perforating points. The perforating point arrangement of the two horizontal wells meets the above requirements. The inserted perforating gun penetrates the casing and the cement ring outside the pipe, and penetrates into the coal seam to a certain depth, so that the subsequent high-temperature water vapor and liquid carbon dioxide can penetrate into the coal seam.

[0022] Step 4: Figure 4 As shown in the figure, the bridge plug is lowered to the end of the vertical section of the liquid carbon dioxide injection well, and the bridge plug is activated so that the bridge plug expands and fits tightly against the well wall to form a seal, thereby preventing the subsequent high-temperature water vapor injected from leaking out of the liquid carbon dioxide injection well. The high-temperature water vapor is injected from the injection well into the target coal seam through a high-temperature water vapor generator. The high-temperature water vapor heats the free methane on the surface of the coal matrix, increases the molecular kinetic energy, and causes the methane to desorb from the solid surface and become free gas to enter the pore space. At the same time, after the high-temperature water vapor is injected into the coal seam, the holes shot from the perforating gun penetrate into the coal seam, and occupy a certain pore space in the coal seam and expand, generating new microcracks, expanding the pore and crack network in the coal seam, and further increasing the mobility of methane.

[0023] Step 5: Figure 2 As shown in the figure, after high-temperature steam is injected for a period of time, the methane concentration monitor can reflect that the methane volume fraction monitored will increase first because the desorbed methane begins to be displaced, but as time goes by, the methane volume fraction measured again will show a decrease and tend to be stable. When the methane volume fraction reaches a stable state, the bridge plug plugged in the liquid carbon dioxide injection well is removed, and a bridge plug is placed at the end of the vertical section of the high-temperature steam injection well for activation, as shown in the figure. Figure 5 As shown. Liquid carbon dioxide is injected into the target coal seam from the gas injection well. Due to the synergistic effect of high-temperature water vapor and liquid carbon dioxide, the temperature change will cause the coal body to expand and contract. This process will further cause microcracks to form inside the coal body or expand existing cracks. These newly formed and expanded channels provide more escape paths for methane, thereby promoting the release of methane from the coal seam. The injection amount of liquid carbon dioxide is determined by the Langmuir adsorption isotherm.

[0024] Among them, the Langmuir adsorption isotherm expression is: the injection amount of liquid carbon dioxide is determined by the Langmuir adsorption isotherm.

[0025]

[0026] Among them, θ is the methane surface coverage, which is defined as the ratio of the number of molecules to the total number of possible adsorbed molecules (0≤θ≤1); n is the fitting coefficient; k is the Langmuir adsorption constant; Q1 is the injection amount of high-temperature water vapor; Q2 is the injection amount of liquid carbon dioxide.

[0027] Among them, when the water vapor injection amount Q1 is constant, the larger the liquid carbon dioxide injection amount Q2, the smaller the surface coverage, that is, the better the methane fluidity, but excessive injection of liquid carbon dioxide will significantly increase the operating cost, and may change the pressure state of the reservoir and cause changes in the formation structure, which may not only cause cracks in the coal seam or surrounding rock formations, but also may trigger micro-seismic activities, posing a threat to the safety of the mining area. Therefore, taking Q 2max =3Q1 is the maximum amount of liquid carbon dioxide.

[0028] Step 6: The methane displaced by high-temperature steam and liquid carbon dioxide is transported to the methane gas collector through the gas production well. During the collection process, various safety regulations must be strictly implemented, such as regular inspection of pipeline integrity and monitoring of gas composition changes, to prevent potential accidents.

Claims

1. A method for displacing methane using high-temperature steam in conjunction with liquid carbon dioxide, characterized in that: Specifically include the following steps: Step 1: Take samples from the target coal seam and conduct triaxial compression tests on standard samples. Determine the injection pressure of high-temperature steam and liquid carbon dioxide based on the compressive strength obtained from the test; Step 2: Drilling is performed from the ground to the target coal seam. After drilling is completed, a steel casing is lowered and cement is pumped in for cementing treatment. A perforating gun is used to perforate at a predetermined position of the perforating point in the target coal seam. Step three: Use a bridge plug to seal the liquid carbon dioxide injection well, and inject high-temperature water vapor from the injection well into the target coal seam. When the methane monitor detects that the methane volume fraction tends to be stable, stop injecting high-temperature water vapor, seal the high-temperature water vapor injection well with a bridge plug, and inject liquid carbon dioxide from the injection well into the target coal seam, so that the high-temperature water vapor and liquid carbon dioxide can work synergistically.

2. The method for displacing methane by high-temperature steam in cooperation with liquid carbon dioxide according to claim 1, characterized in that: In step one, a triaxial compression test is performed on the sample to detect the compressive strength P of the sample under five groups of confining pressure conditions of x1, x2, x3, x4, and x5. The average value of the five groups of test compressive strength P is calculated and recorded as P0, and a fluctuation range of 20% is added to the original P0, that is, the pressure of P0-1.2P0 is taken as the injection pressure of high-temperature water vapor and liquid carbon dioxide.

3. The method for displacing methane by high-temperature steam and liquid carbon dioxide according to claim 1, characterized in that: In step 2, drilling is performed from the ground toward the target coal seam, including a high-temperature steam injection well, a liquid carbon dioxide injection well, two horizontal wells and a gas outlet well.

4. The method for displacing methane by high-temperature steam in cooperation with liquid carbon dioxide according to claim 1, characterized in that: In step 2, the predetermined positions of the perforation points are as follows: the initial position of the horizontal well gas inlet end is the first upper perforation point, each subsequent upper perforation point is 50 decimeters away from the first upper perforation point, and each lower perforation point is located between the two upper perforation points.

5. The method for displacing methane by high-temperature steam in cooperation with liquid carbon dioxide according to claim 1, characterized in that: In step three, the methane concentration is monitored by a methane concentration monitor. The methane volume fraction will first increase and then decrease until it stabilizes, and the bridge plug sealed in the liquid carbon dioxide injection well is withdrawn.

6. The method for displacing methane by high-temperature steam in cooperation with liquid carbon dioxide according to claim 1, characterized in that: In step three, the injection amount of the liquid carbon dioxide is determined based on the Langmuir adsorption isotherm, and three times the injection amount of high-temperature water vapor is taken as the maximum injection amount of the liquid carbon dioxide.

7. The method for displacing methane by high-temperature steam in cooperation with liquid carbon dioxide according to claim 3, characterized in that: The drilling operation mode of the one high-temperature steam injection well, one liquid carbon dioxide injection well, two horizontal wells and one gas outlet well is: vertical-horizontal-vertical segmented operation, and in each segment operation, after the drilling is completed, the steel casing is immediately lowered and cement is pumped in for cementing treatment.

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