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Coal is made up of coal matrix, cleats and pores with gas and water. The coal matrix is ​​the main body of coal, which is generally in the form of a block. There are many pores that are invisible to the naked eye, some are connected to each other, and some are closed "dead hutongs". Cleats are larger coal seam cracks, which are called cleats due to the staggered texture (see Figure 1).
Figure 1 Surface structure of coal seam
The free-state coalbed methane is distributed in the cleats, cracks and pores of the coal seam in a free gas state, and can move freely under the action of the pressure difference, and the free gas accounts for 10% to 20% of the coalbed methane. The gas dissolved in the coal seam water is called dissolved gas, and the amount is very small, within 5%. A large amount of coalbed methane is adsorbed on the inner surface of coal in an adsorbed state. This adsorption is physical adsorption, and this part of coalbed methane accounts for 70% to 95%.
In addition to the adsorption and free coalbed methane, coal seam cleats, tiny cracks, and pores are filled with water. For this reason, it is necessary to discharge the water first, and then the natural gas can be extracted.
Since coalbed methane is present in the coal seam, in order to develop and utilize coalbed methane, it is necessary to screen out areas where coalbed methane is large and easy to be mined from the coal producing area. It is one of the work of the China Geological Survey by geologists to analyze the areas that are more favorable after the various properties of the coal seam. Later, the enterprises will mine the coalbed methane.
Figure 2 Drilling process
The way of mining is mainly through underground drilling from the ground, that is, excavating a channel or wellbore in the ground, facilitating the flow of natural gas into the well, and then pumping it from the well to the ground. The process of forming the oil and gas flow channel is called drilling.
Figure 2 shows the drilling process. The driving force provided by the drilling machine drives the drill pipe and the drill bit to rotate. The drill bit successively breaks down the rock to form a wellbore. While the drill bit is crushing the rock, a hollow rod is injected into the bottom of the well into a liquid filled with a large amount of chemical material (ie, drilling fluid), and the broken rock debris of the drill bit is carried from the space of the drill pipe and the wellbore to the ground. Therefore, the drilling process is a process in which the drill bit breaks down the rock and the drilling fluid continuously carries the rock debris through the circulating flow to form the wellbore. After the drill bit reaches its destination, a seamless steel pipe (casing) needs to be lowered into the wellbore; cement is injected into the annular space of the casing and the wellbore to fix the casing on the well wall; the last process is The location of the coal seam, the casing and the cement outside the casing and the formation are opened by shells or other means, artificially forming a connecting formation and a wellbore to facilitate the passage of coal seam gas into the well (see Figure 4).
Figure 4 Coal seam gas flow into the wellbore process
To allow coalbed methane to flow from the coal into the well, three processes are required: drain desorption, gas diffusion, and flow into the wellbore (see Figure 3).
Figure 3 Process of desorption and diffusion of coalbed methane
The first step is to drain and desorb. Since the coalbed methane is adsorbed on the surface of the coal, and the pores and cracks of the coal are filled with water, the coalbed methane cannot be separated from the surface of the coal. Therefore, it is necessary to first discharge the water filled in the coal, and the gas molecules are free of water. ", it can be detached from the surface of the coal. This is called "desorption". After the coal is removed, the coalbed methane enters the pores of the coal.
The second step is diffusion. As more and more coalbed methane is detached from the coal surface, the concentration of coalbed methane in many small holes and seams is getting higher and higher, and it will spread from a high concentration to a low concentration. Finally, the cracks and pores are filled with coalbed methane.
The third step is that coalbed methane flows from the cracks in the coal into the wellbore. Due to the diffusion of the previous step, the coalbed methane eventually merges into the large crack and finally flows from the large crack into the wellbore connected to it.
Hydraulic fracturing is the main method for increasing coalbed methane production, that is, the coal seam and the mudstone sandstone above and below it are crushed by the force and pressure of water to cause cracks and communicate the coalbed methane gas reservoir farther away from the wellbore. Because through the drilling, the wellbore and the coal seam are connected, the coalbed methane near the wellbore is easy to flow into the wellbore and draw out of the ground. However, after all, the gas storage is limited, and the coalbed methane desorption and migration is difficult due to the narrow and narrow cracks and pores in the distance, resulting in limited production. . Therefore, it is necessary to use hydraulic fracturing to increase the original cracks, communicate narrow channels, form a complex network of fractures, communicate the gas reservoirs in the distance, so that the coalbed methane can run into the wide cracks and finally flow into the wellbore. All roads lead to Rome". In short, the smoother the road, the larger the gas storage, the greater the gas flow, and the effect of hydraulic fracturing is as shown in Figure 5 (Figure 5).
Figure 5 Schematic diagram of fracturing fracture network
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