Abstract
Hydraulic fracturing and permeability enhancement are effective methods to improve low-permeability coal seams. However, few studies focused on methods to increase permeability, and there are no suitable prediction methods for engineering applications. In this work, PFC2D software was used to simulate coal seam hydraulic fracturing. The results were used in a coupled mathematical model of the interaction between coal seam deformation and gas flow. The results show that the displacement and velocity of particles increase in the direction of minimum principal stress, and the cracks propagate in the direction of maximum principal stress. The gas pressure drop rate and permeability increase rate of the fracture model are higher than that of the non-fracture model. Both parameters decrease rapidly with an increase in the drainage time and approach 0. The longer the hydraulic fracturing time, the more complex the fracture network is, and the faster the gas pressure drops. However, the impact of fracturing on the gas drainage effect declines over time. As the fracturing time increases, the difference between the horizontal and vertical permeability increases. However, this difference decreases as the gas drainage time increases. The higher the initial void pressure, the faster the gas pressure drops, and the greater the permeability increase is. However, the influence of the initial void pressure on the permeability declines over time. The research results provide guidance for predicting the anti-reflection effect of hydraulic fracturing in underground coal mines.
Highlights
Research has shown that hydraulic fracturing increases coal seam permeability, which is conducive to the exploitation of coalbed methane and the prevention of coal and gas outburst (Cheng et al 2018; Wang et al 2014; Zhang 2014)
The results show that the gas pressure decreased rapidly in the first few days, but the decrease rate decreased slowly with an increase in the drainage time
A coupled mathematical model of the interaction between coal seam deformation and gas flow was established, and simulations were conducted to determine the effects of hydraulic fracturing on gas drainage
Summary
Research has shown that hydraulic fracturing increases coal seam permeability, which is conducive to the exploitation of coalbed methane and the prevention of coal and gas outburst (Cheng et al 2018; Wang et al 2014; Zhang 2014). Many studies were conducted on the influence of coal seam hydraulic fracturing on the gas drainage effect. The field application results of pulse hydraulic fracturing (Xu et al 2017) showed that the proportion of micropores decreased by 7.7%, the proportion of mesopores increased by 23.1%, and the proportion of macropores increased by 2.9%, significantly improving the permeability of the coalbed methane reservoir. The connected fractures are extracted and imported into COMSOL Multiphysics numerical analysis software for gas drainage simulation to evaluate the permeability enhancement caused by hydraulic fracturing of the coal seam. The research results can be used to predict the anti-reflection effect of hydraulic fracturing in underground coal mines
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