Experimental and applied analyses of particle migration in fractures of coalbed methane reservoirs

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This study investigated the behaviors of particle migration within fractures of coalbed methane (CBM) reservoirs using laboratory simulation technology. Two coal samples were collected from Qinshui Basin (Shanxi Province, China) and used in two sets of permeation experiments, which were designed to understand the particle migration process and to determine the critical flux of particle migration. Based on those experimental results, a mathematical model was built to calculate the maximum water production of a CBM well that can be tolerated without damage to the reservoir. Permeation experiments indicate that as upstream pressure increases, the instantaneous permeability of coal samples have different behaviors that represent three distinct stages of non-permeation, particle migration, and stable permeation/particle blockage. In the particle migration stage, particles in the fractures start to migrate and then get deposited in narrower parts of the fractures — this migrating-deposition process repeats a few times. The permeability of the sample also repeatedly increases and then decreases. The migration and deposition of particles will cause a major decline in permeability, and for a working CBM well, this will damage the CBM reservoir. The critical fluxes of the two samples are 0.17 and 0.29 mL/min, and their permeability are 0.038 and 0.043 mD. When a well near sample QSC2 is chosen as an example, the calculated maximum water production during CBM drainage is 3.98 m3/d. This can be a recommended water production value for a real CBM well in the region that can avoid the damage of CBM reservoir.