Timeliness Analysis of Coalbed Methane Workover for Reducing Damage to Coal Reservoirs

The workover operations are related to the efficiency of coalbed methane (CBM) and directly decide whether the CBM production target can be achieved. To reduce the damage to the coal reservoir caused by the workover duration, a series of experiments related to the damage to the coal seam by the workover duration, such as permeability experiment, stress-strain experiment, velocity sensitivity experiment, and stress sensitivity experiment, was carried out. The results show that the permeability of the coal sample increases first and then declines with the increase of the immersion time and the changed rate of coal samples is unbalanced. The permeability of the coal sample begins to decrease after 384 h of immersion. The mechanical properties of the coal sample did not change much when the water immersion time was short, but it showed a zigzag change when the water immersion time increased, and the stress-strain peak appeared earlier after 384 h of water immersion, and the elastic modulus became smaller. The consequences of stress sensitivity experiments show that the longer the immersion time, the weaker the recovery of permeability after pressure relief; especially after 384 h, the recovery of permeability is even weaker. Speed sensitivity test results show that the degree of damage to the sensitivity of the coal sample is moderate, but when the coal sample is immersed in water for more than 96 h, the permeability damage is generally greater than that of the dry coal sample. The experimental results prove that the effect of the workover duration has a significant impact on coal reservoir damage, and the workover duration is reasonably arranged. If we can make use of the phase characteristics of physical property changes of coal samples under different immersion times and complete the workover operation in the most appropriate time, then the coal reservoir can be well transformed, and it is possible to obtain good results. Both the production efficiency and production volume of CBM wells will increase.