Abstract
The generation of complex fracture networks by hydraulic fractures activating and connecting natural cleats and fractures during hydraulic fracturing is of great significance to the development of coalbed methane (CBM) reservoirs. Hydraulic fracturing physical simulation experiments of shallow bright-type coal seams in Hancheng block were conducted. Based on the experiment results, fluid-solid interaction theory, and discrete fracture network model, a 3D fracture network model of bright-type coal seams was developed. Fracture network propagation behaviors under different rock mechanical properties and fracturing operation parameters were investigated. Sensitivity analysis of the influencing factors of fracture network propagation was performed with the grey correlation method. The results show that a complex fracture network dominated by the main fracture is formed after hydraulic fracturing in bright-type coal seams. Fracturing operational parameters (fracturing fluid displacement and viscosity and perforation location) are the main controlling factors of fracture network propagation, whereas Poisson’s ratio of the coal seam has a relatively weak effect on the fracture network. As the elastic modulus difference between the roof and coal seam increases, the fracture network conductivity and simulated reservoir volume (SRV) first increase and then decrease. When the elastic modulus difference between layers is 10 GPa, the conductivity and SRV achieve their maximum values. With increasing displacement, the length, height and SRV of the fracture network increase linearly, but the conductivity decreases. Large displacement is beneficial to the formation of a complex fracture network in bright-type coal seams. The optimized fracturing fluid viscosity is 20 mPa·s. The optimal perforation location is near the formation interface. The results could provide theoretical guidance for the design and optimization of efficient fracturing of CBM reservoirs.
