Production decline law and influencing factors of high-rank coal CBM wells

In order to reveal the production decline law of high-rank coal CBM wells, based on the development data of Fanzhuang block in southern Qinshui basin for more than 10 years, this paper studies the decline point, decline type and influencing factors of high-rank coal CBM wells by means of numerical simulation and statistical analysis. The results show that the decline point of the Fanzhuang block is 25% and the average decline point of the single well is 21%. Most wells begin to decline after drainage of 4 years. The decline point of CBM well is determined by matrix permeability and half length of fracture. The higher the matrix permeability is, the larger the effective control radius of single well is, the more effective control reserves are, and the larger the decline point is. The higher the matrix permeability, the greater the growth of decrease point caused by the increase of half length of fracture. Permeability is the main controlling factor of CBM well decline type. The higher the permeability is, the smaller the decline index is, and the slower the decline rate is. With the increase of permeability, the decline types are linear decline, exponential decline and hyperbolic decline successively. The reason is that for formation with high permeability, the average pore radius is large, which results in much weaker stress sensitivity and much greater permeability by shrinkage of the coal and rock, leading to much higher dynamic permeability and much slower decline rate.