Novel Discrete Fracture Network Model for Multiphase Flow in Coal

Multiphase flow intensely affects the movement and accumulation of other fluids in coal and plays an essential role in predicting the permeability of coal during coalbed methane (CBM) production. Fractures have played a decisive role in the transport of CBM after hydraulic fracturing has occurred. In this study, a multiscale pore network model (PNM) was constructed on the basis of focused ion beam scanning electron microscopy (FIB-SEM) image results. Additionally, a novel discrete fracture network model, fracture-pore network model (F-PNM), was proposed to investigate the effect of fracture density, fracture developing direction, and wettability on multiphase flow. The results reveal that the permeability of F-PNM increases with the increase of the fracture density, which could be the result of the predominance of snap off. The permeability decreases as the angle between the fracture and flow direction increases; initially, the permeability decreases steeply and then it tends to remain stable; and for angles between 0 & DEG; and 15 & DEG;, the permeability decreased by as much as 61.8%. Moreover, the wettability of coal has limited impact on its water relative permeability; however, it has a measurable effect on gas relative permeability, which could be owing to water accumulation on the coal surface under different wettability conditions. A good wetting performance would have a negative effect on the CBM production and reduce flowback efficiency.

Automated summary

In this study, a multiscale pore network model (PNM) and a novel discrete fracture network model (F-PNM) were constructed to investigate the effect of fracture density, fracture developing direction, and wettability on multiphase flow in coal. The results showed that the permeability of F-PNM increased with the increase of fracture density due to the predominance of snap off. The permeability decreased as the angle between the fracture and flow direction increased, with a maximum decrease of 61.8% between 0° and 15°. The wettability of coal had limited impact on its water relative permeability but had a measurable effect on gas relative permeability.