A dual porous and fractures medium CBM numerical model development and analysis

Abstract

The drilling unloading will produce fractures in the plastic zone around the borehole. However, the fracture is the main seepage channel of coalbed methane. It is not appropriate to ignore the influence of fractures on gas flow, especially in the plastic zone. According to the stress redistribution characteristics after unloading, the fracture field in the plastic zone is considered in the coalbed methane numerical model. A two-dimensional random fracture field is introduced via the Monte Carlo method in the plastic zone, which is based on the central limit theory and box muller method. A dual porous and fracture medium numerical model is proposed, which considers not only gas adsorption and desorption, but also fracture distribution and creep deformation. A comparison between the proposed model and the single porous medium numerical model is carried. The results show that the proposed model can reflect not only the evolution characteristics of gas flow but also the anisotropic influence of fracture. It further explains the evolution mechanism and distribution law of gas flow. Finally, the influence of fracture field parameters (fracture density, fracture length, and fracture width) on gas flow is discussed, which is expected to provide a theoretical basis and reference for the development and research of coalbed methane.