Numerical Simulation of Gas–Water Two-Phase Seepage During Coalbed Methane Development in ZhengZhuang Block: A Case Study of Well Z29

Coalbed methane (CBM) wells with low production are widespread in China, and the influence of single-water-phase or -gas-phase seepage on CBM development was investigated. The influence of gas–water two-phase seepage on CBM development has rarely been studied. To study the controlling factors of gas–water two-phase seepage on CBM development, stress–strain relationship of coal reservoir, Darcy’s law of gas–water two-phases and the relationship between porosity and permeability were combined to establish a two-phase multi-physics coupling model. The feasibility and rationality of the established model was proven by comparing field CBM well data of Z29 in the ZhengZhuang Block and the simulation curve. Then, the coupling model was solved with COMSOL Multiphysics software (version 3.5), and the effect of Young’s modulus, initial permeability and the drainage system on the process of drainage was discussed. The results of numerical simulation show that the Young’s modulus of a reservoir has limited positive effects on CBM production. When the Young’s modulus of the reservoir increases by 80%, the gas production only increases by 10.71%. The initial permeability has a significant impact on CBM production. The reservoir with a permeability of 0.9 mD had the highest daily gas production of 2183 m3/d on the 162nd day, while the maximum daily gas production of the reservoir with a permeability of 0.1 mD was only 371 m3/d. In addition, a high pressure drop rate inevitably results in lower porosity and permeability, which limits the production of CBM. When the pressure drop reaches 0.1 MPa, the gas production drops sharply, with the daily gas production decreasing by more than 30%. Thus, a sudden change in bottom hole pressure should be avoided in the actual production scenario to extend the stable gas production stage. This simulation research quantifies the effects of Young’s modulus, initial permeability and the drainage system on CBM production, which could provide a basis for understanding CBM drainage and its controlling factor.