Stress-seepage mechanism of surrounding rocks of the coalbed methane well and the bottomhole pressure control during dewatering and depressurization

In coalbed methane (CBM) well production, “bottomhole flowing pressure control” is key to improving production. A reasonable bottomhole pressure range is essential for optimizing CBM well drainage. In this paper, the stress–permeability mechanism of the surrounding rocks of CBM wells and the method of controlling the bottomhole pressure were investigated through experimental study, theoretical analysis, and field applications. Their characteristics and correlations were analyzed. The research results indicate that the deformation and failure process of coal samples can be divided into four stages: initial compacting stage, elastic deformation phase, plastic deformation stage, and failure stage, revealing the stress–strain–permeability characteristics of coal rock during the deformation and failure process. The surrounding rock of a CBM well can be divided into elastic, plastic, and fracture zones based on elastic–plastic theory, and theoretical calculation models for stress seepage in three zones of surrounding rock of coalbed methane wells have been derived. The stress and range condition expressions for each zone are obtained, and analysis shows that the surrounding rock has the highest stress at θ=0∘ and the lowest stress at θ=90∘. A calculation model for bottomhole pressure was derived, and a method for determining the range of bottomhole pressure in CBM well drainage was established. The variation law of bottomhole pressure during coalbed methane well drainage was revealed. The theoretical model was validated using production data from six vertical wells in the Zhengzhuang block of the southern Qinshui Basin, and good application results were achieved.