A method for displacing methane using high-temperature steam and liquid carbon dioxide

Abstract

Strictly controlling methane concentration is a crucial measure in coal mining projects. Due to the low efficiency of direct extraction, complete recovery of methane resources is difficult. Triaxial compression tests are performed on samples taken from target coal seams to determine the compressive strength and determine the injection pressure for high-temperature steam and liquid carbon dioxide. The coal seams are then perforated with an insertion-type perforating gun, allowing the injected high-temperature steam and liquid carbon dioxide to penetrate deeply into the coal seams. The injection rate of high-temperature steam is controlled by a methane concentration monitor, while the injection rate of liquid carbon dioxide is determined based on the Langmuir adsorption isotherm. High-temperature steam, liquid carbon dioxide, and the resulting thermal expansion and contraction reactions of these two factors increase the number of fractures in the coal seam. Furthermore, these factors promote the desorption of methane adsorbed in the coal matrix and its release into fractures and pores, making it easier for methane to be displaced, maximizing methane displacement and collection, and preventing gas explosions in coal mining projects.

Description

Technical field:

[0001] The present 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 conjunction with liquid carbon dioxide. Background technology:

[0002] Coal, as a dual medium with a complex pore-fracture structure, is a prime reservoir for coalbed methane (CBM), the primary component of which is methane. Coal mine gas accidents, one of the five major mine hazards, pose a significant threat to safe coal mine production. During mining, methane gas is released from coal seams. When methane concentrations reach a certain level, it poses risks of explosion, asphyxiation, and environmental pollution. Furthermore, as a potent greenhouse gas, methane's role in global climate change cannot be ignored.

[0003] Existing technologies for displacing coalbed methane include hydraulic fracturing, liquid carbon dioxide, high-temperature steam, and nitrogen, but numerous problems remain. Hydraulic fracturing displaces methane by increasing the permeability of the coal seam, thereby improving methane recovery. However, this consumes large amounts of water resources and may cause groundwater contamination. Furthermore, the use of high-pressure liquids injected into the formation may induce microseismic or small-scale earthquakes. When displacing methane with liquid carbon dioxide, the production and transportation costs of liquid carbon dioxide are high. When displacing methane with high-temperature steam, long-term high temperatures may affect the structure of the coal seam. When displacing methane with nitrogen, nitrogen has a weak adsorption capacity and limited displacement effect. Long-term injection may also have an adverse effect on the structure and permeability of the coal seam.

[0004] In response to the above problems, a method for displacing methane using high-temperature water vapor in conjunction with liquid carbon dioxide is proposed to avoid the impact of long-term high temperature on the coal seam when using high-temperature water vapor for displacement alone and the high cost investment when using liquid carbon dioxide alone. The synergistic effect of high-temperature water vapor and liquid carbon dioxide displacement technology is used to improve the desorption and flow efficiency of methane. 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 methane concentration is a very important measure in coal mining engineering. Methane in coal seams mainly exists in an adsorbed state in the coal matrix, and only a small part exists in a free state in cracks and pores. 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, making it easier to displace methane, and greatly increasing the displacement and collection volume of methane. 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 drilling is completed, the steel casing is lowered and cement is pumped in for cementing.

[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 sheath 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 steam is injected from the injection well into the target coal seam to increase methane desorption and mobility while displacing methane.

[0010] Step 5: When the methane volume fraction detected 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 make the high-temperature water vapor and liquid carbon dioxide 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 map of the perforating gun;

[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 with reference to the accompanying drawings.

[0019] Step 1: To determine the injection pressure for high-temperature steam and liquid carbon dioxide, samples were collected from the target coal seam and formed into cylindrical specimens with a diameter of 50 mm and a height of 100 mm. Triaxial compression tests were then performed on the specimens. Five groups of data, x1, x2, x3, x4, and x5, were used as control groups for the tests. The compressive strength (P) of the specimens under each confining pressure was measured. The average of the five groups of compressive strengths (P) was calculated and recorded as P0. A 20% fluctuation range was added to the original P0, resulting in a pressure range of P0-1.2P0 as the injection pressure for high-temperature steam and liquid carbon dioxide. This ensured that the injection pressure of high-temperature steam exceeded the compressive strength of the target coal seam to prevent excessive injection pressure from causing formation fractures or wellbore damage, and prevented from effectively displacing methane due to excessively low pressure.

[0020] Step 2: If Figure 2 As shown, drilling into the target coal seam from the surface involves one high-temperature steam injection well, one liquid carbon dioxide injection well, two horizontal wells, and one outlet well. The drilling operation utilizes a vertical-horizontal-vertical segmented approach. After drilling is completed, steel casing is immediately lowered and cement is pumped in for cementing to prevent wellbore collapse and ensure structural stability. The inlet of the high-temperature steam injection well is connected to a high-temperature steam generator via a pipeline. The inlet of the liquid carbon dioxide injection well is connected to a liquid carbon dioxide storage tank via a pipeline. The outlet of the outlet well is also connected to a methane collection device via a pipeline. The liquid carbon dioxide injection well and the high-temperature steam injection well intersect within the target coal seam. Both horizontal wells are drilled within the target coal seam, spaced one meter apart. The inlet of the horizontal wells is connected to the high-temperature steam and liquid carbon dioxide wellbore, and the outlet is connected to the outlet well, ensuring smooth injection of displacement gas into the target coal seam and successful methane displacement.

[0021] Step 3: If Figure 3As shown in the figure, a cable-controlled perforating gun is lowered into the casing to a predetermined location for perforation. The predetermined perforation locations are as follows: the initial point at the horizontal well's gas inlet is the first upper perforation point, with each subsequent upper perforation point spaced 50 centimeters from the first, and each lower perforation point located between the two upper perforation points. The perforation point arrangement for both horizontal wells meets these requirements. The perforating gun penetrates the casing and the cement sheath around the casing, penetrating to a certain depth into the coal seam, allowing subsequent high-temperature steam and liquid carbon dioxide to penetrate deep into the coal seam.

[0022] Step 4: If Figure 4 As shown, a bridge plug is lowered to the end of the vertical section of a liquid CO2 injection well and activated, causing it to expand and adhere to the wellbore wall, forming a seal and preventing subsequent high-temperature steam from escaping the liquid CO2 injection well. High-temperature steam is injected from the injection well into the target coal seam via a high-temperature steam generator. This high-temperature steam heats the free methane on the coal-based surface, increasing its molecular kinetic energy and causing it to desorb from the solid surface, transforming into free gas and entering the pore space. Simultaneously, after high-temperature steam is injected into the coal seam, the perforating guns penetrate deep into the coal seam, occupying a certain amount of pore space within the coal seam and expanding, creating new microfractures. This expands the pore and fracture network within the coal seam, further increasing methane mobility.

[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 desorbed methane begins to be displaced, and the monitored methane volume fraction will first increase. However, 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 sealed 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 in Figure 2, liquid CO2 is injected into the target coal seam from a gas injection well. Due to the synergistic effect of high-temperature steam and liquid CO2, the temperature change causes the coal to expand and contract. This process further creates microcracks within the coal or widens existing cracks. These newly formed and expanded channels provide more escape pathways for methane, thereby promoting its release from the coal seam. The injection rate of liquid CO2 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] Where θ is the methane surface coverage, defined as the ratio of the number of methane 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; and Q2 is the injection amount of liquid carbon dioxide.

[0027] Among them, when the water vapor injection volume Q1 is constant, the larger the liquid carbon dioxide injection volume Q2, the smaller the surface coverage rate, that is, the better the methane mobility. However, excessive injection of liquid carbon dioxide will significantly increase operating costs and may change the pressure state of the reservoir and cause changes in the stratum structure, which may not only cause cracks in the coal seam or surrounding rock formations, but may also 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 a methane gas collector via a gas production well. During the collection process, strict safety procedures must be followed, such as regular inspections 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 tests. Step 2: Drill a hole into the target coal seam from the ground. After drilling is completed, lower the steel casing and pump cement into the well for cementing. Use a perforating gun to perforate the target coal seam at the predetermined perforation point. Step 3: 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 work synergistically. Among them, in step 1, 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 compressive strength P of the five groups of tests 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; In step 2, drilling is performed from the ground into 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. The predetermined perforation points are as follows: the initial point at the gas inlet end of the horizontal well is the first upper perforation point, and each subsequent upper perforation point is 50 decimeters away from the first upper perforation point. Each lower perforation point is located between the two upper perforation points. In step 3, the injection amount of the liquid carbon dioxide is determined by the Langmuir adsorption isotherm, and 3 times the injection amount of high-temperature water vapor is taken as the maximum injection amount of liquid carbon dioxide. The Langmuir adsorption isotherm expression is shown in formula (1): Wherein, in formula (1), 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.

2. The method for displacing methane by using high-temperature steam in conjunction 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, at which time the bridge plug sealed in the liquid carbon dioxide injection well is removed.

3. The method for displacing methane by high-temperature steam in conjunction with liquid carbon dioxide according to claim 1, characterized in that: The drilling operation mode of the high-temperature steam injection well, the liquid carbon dioxide injection well, the two horizontal wells and the gas outlet well is: vertical-horizontal-vertical segmented operation, and in each segment, the steel casing is immediately lowered and cement is pumped in for cementing treatment after the drilling is completed.

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