Numerical simulation of gas production for multilayer drainage coalbed methane vertical wells in southern Qinshui Basin

With the development of CBM (coalbed methane) exploitation, multilayer drainage of the CBM well has received wide attention. The production control technology is the key to ensure high and stable gas production of CBM well multilayer drainage. However, the complex geological conditions of multiple coal seams increase the difficulty of production control of multilayer drainage. Numerical simulation technology is an effective method to study the production control technology of CBM well multilayer drainage. The scientific and reliable simulation results can provide a basis for production control. In this study, considering the influence of temperature effect, shrinkage effect of coal matrix, and effective stress on the fluid migration law in the coal seam, permeability and other coal seam physical parameters, the multi-physical field coupling mathematical model for the dynamic process of CBM vertical well multilayer drainage was established. The coupled solution of the multiple physical fields was obtained by using the finite element method. Then, the gas production effect of CBM vertical well multilayer drainage at different drainage rates and the dynamic evolution characteristics of permeability and other coal seam physical parameters were discussed by simulating a multilayer drainage CBM well group in Zhengzhuang block, Qinshui Basin, and corresponding engineering proposals were also put forward. The simulation results show that the gas contents of No.3 and No.15 coal seams in Zhengzhuang block are relatively high, and the CBM well group has a great potential to increase production, causing an insignificant effect of increasing the drainage rate on improving CBM recovery. In the production process, the effect of coal matrix shrinkage on permeability is stronger than that of effective stress, which is the guarantee to improve the drainage rate of CBM wells. On this basis, an appropriate increase in the drainage rate can increase the production of CBM wells when the drainage rate does not exceed the upper limit of the seepage capacity of the coal seam. Based on the simulation results, it is suggested to adjust the drainage rate mainly by controlling the working fluid level or fluid column pressure. To increase gas production of multilayer drainage CBM wells in No.3 and No.15 coal seams, the drainage rate of vertical well multilayer drainage in Zhengzhuang block is decreased by 0.12− 0.20 MPa/d in the fluid column pressure or by 12− 20 m/d in the working fluid level in the water production stage and pressure holding stage, which can not only increase production but also avoid reservoir damage.