The initial formation pressure of coalbed methane（CBM） reservoir is a key parameter used for the evaluation of CBM reserves and productivity

which plays a guiding role in CBM production. Therefore

it is of great significance to accurately calculate the initial reservoir pressure of CBM reservoir. Based on the pressure potential superposition principle of the seepage mechanics theory

the pressure potential model in the process of fracturing and shut in after fracturing is established

and a method for determining the initial reservoir pressure of CBM reservoir is proposed by using the wellhead pressure drop data in the shut-in stage of hydraulically fractured gas well under two conditions: ignoring or considering the fracture network permeability change in the process of fracturing and shut in after fracturing. And then

a field application is carried out. From the fitting results of the case study

it can be seen that the linear relationships for both methods are obvious

indicating that the established methods are effective. From the perspective of interpretation accuracy

the method of considering the fracture network permeability change after fracturing involves more data points in fitting

and the fracture network permeability change during fracturing and after fracturing is an indisputable fact

so the interpretation result by using the method considering the fracture network permeability change is more reliable. If the change process of fracture network permeability of coal formation during shut in after fracturing is ignored

the interpreted stable permeability of fracture network and initial reservoir pressure will be high. Since this method can not only be used to determine the initial reservoir pressure

but also to determine the stable permeability of fracture network after fracturing and evaluate the change trend of fracture network permeability

it provides a basis for the classification of CBM reservoir types

CBM reserve calculation

fracturing effect evaluation

and optimization design of drainage and production system.