Drainage engineering

Coalbed methane drainage is a process of drainage and pressure reduction. By reducing the pressure of the coal reservoir, the coalbed methane is desorbed and migrated. At the same time, this is also the process of generating and transporting pulverized coal. During the drainage process, the liquid flow will erode the coal seam and transport the native coal powder; when the reservoir pressure drops to the critical desorption pressure, the coal bed methane begins to desorb, and the coal matrix shrinks to produce micro-cracks. Under the erosion of the gas-liquid-solid three-phase flow, Powdered coal is produced; due to the existence of production pressure difference, changes in reservoir pressure can easily cause the production of pulverized coal. Therefore, drainage gas production intensity and bottom hole pressure fluctuation are important factors affecting pulverized coal output.

In the process of coalbed methane drainage and decompression, the formation fluid will produce a certain viscous force on the inner wall of the coal rock fracture during the seepage process, and the damage to the coal rock will mainly be shearing. destroy. When the fluid viscosity is greater than the shear strength of the coal seam itself, the coal rock will be destroyed and coal powder will be generated. If greater production intensity is used for drainage and gas recovery, the production pressure difference will inevitably increase. For plane radial flow, the flow velocity of fluid in the near-wellbore zone is significantly greater than the flow velocity in the far-wellbore zone, resulting in a large amount of pulverized coal being produced in the coal reservoir near the wellbore zone. With the migration of formation fluids, part of the pulverized coal migrates to the area near the wellbore of the coalbed methane well, which can easily cause the effective cracks in the coal seam near the wellbore to be blocked, resulting in a reduction in the permeability of the coal reservoir near the wellbore, seriously affecting the drainage and pressure reduction effect of the coalbed methane well. The rest of the pulverized coal is carried by the fluid to the bottom-hole drainage equipment, which can easily cause underground faults such as pump jamming or oil pump leakage.

In addition, usually when a rock is stressed many times, even if the force applied again is relatively small, it can easily cause the rock to break. This is because the effect of repeated force will cause fatigue in coal and rock, resulting in a reduction in its rupture stress. Therefore, the production phenomenon of discontinuous coalbed methane well drainage or unstable parameters will also increase the output of pulverized coal. When the bottom hole pressure fluctuates periodically, it is easy to cause fatigue in the coal and rock near the well and reduce the failure stress. From this perspective, sudden changes in the coalbed methane drainage system and frequent underground operations will induce the production of pulverized coal.

The decrease and rise of the liquid level in the oil casing annulus of coalbed methane wells and the rise and fall of casing pressure will cause irregular fluctuations in bottom hole pressure, making it impossible to form a continuously expanding and extending pressure drop funnel around the wellbore. Sudden changes in water and gas production during the drainage process will cause sudden changes in bottom hole pressure, which will particularly seriously disturb the effective stress of the coal reservoir. This will cause the coal rock to gradually lose its original stress balance, causing the effective stress of the coal reservoir to increase, causing shear failure of the coal body skeleton, causing the pulverized coal particles to fall off and separate from the coal body skeleton, and then causing the pulverized coal to move into the coal reservoir. Migration within the fracture system and coalbed methane drainage system (Fu Xuehai, 2001).

1. Rapid drainage

Gas and water in the coal seam migrate along the fissure channels and are produced. The stability of the wellbore directly affects the flow rate of gas and water during the coal bed methane drainage process. . Excessively fast drainage, rapid fluid production, and rapid increase in effective stress will not only produce a large amount of pulverized coal, but also easily cause wellbore collapse.

Formation water flowing in fractures will carry a certain amount of solid particles. Under the same conditions, as the fluid speed increases, its carrying capacity also increases. If the drainage speed is too fast, the unit pressure gradient will increase, the liquid flow rate will accelerate, a large amount of coal powder will be produced, and the coal powder will be carried to the wellbore. Some large particles of coal powder will sink to the bottom of the well, and some will enter the drainage equipment, reducing the pump efficiency. It may even lead to pump jamming and buried pump.

2. Discontinuous drainage

Well workover shutdowns caused by oil pipe leakage, pump jam, etc. will interrupt the continuity of coalbed methane well drainage, causing coal seam agitation and causing certain amount of coal powder.

3. Matrix shrinkage effect

As the coalbed methane well drainage and depressurization progress, the coalbed methane continues to be desorbed from the surface of the coal matrix, the surface free energy of the matrix micropores increases, and the matrix shrinkage effect occurs , new cracks and cleats are constantly generated (Li Peichao et al., 2002), and micro-fractures occur in the coal rock, resulting in the production of pulverized coal particles.