Coalbed methane mobility and primary drainage rate control strategy in different coal-rank blocks

The coalbed methane geological parameters of 20 blocks/mines in China are systematically collected and sorted to analyze the change laws of sorption time and permeability varying with coal-rank. In combination with the gas production curves of CBM wells in different rank blocks, this study reveals the reasons for gas production dynamic diversity in different rank blocks prior and posterior to peak production, in order to provide the basis for controlling primary drainage rate in different rank blocks. The studies show that (1) sorption time has a U-shaped variation with the increasing coal-rank. In the low and medium rank blocks/mines, the sorption time is generally less than 15 d; in high rank blocks/mines, the sorption time greatly varies from 0.1 to 85.54 d. (2) Coal reservoir permeability has not varied systemically with the increasing coal-rank between the blocks/mines, but shows 2-3 orders of difference within a block/mine. (3)Peak production time is correspondingly related to primary drainage rate and sorption time, but not evidently related to permeability. (4) In different rank blocks, the primary drainage rate of a well should be controlled according to the allocation relation between sorption time and permeability of coal seams in the well: in low and medium rank blocks, the sorption time is generally short, for the well/seam with low permeability, the primary drainage rate has to be strictly controlled. In high rank blocks, the sorption time varies from short to long, for the well/seam with long sorption time and high permeability, the primary drainage rate can be speeded up appropriately. © 2018, Editorial Office of ACTA PETROLEI SINICA. All right reserved.