Numerical simulation of hydraulic fracturing in coal seam for enhancing underground gas drainage

Abstract

Hydraulic fracturing is an efficiency approach to improve underground gas drainage. Although the interaction of fluid and coal has been comprehensively investigated in fracturing process and gas drainage process, fewer scholars have combined these two processes together and taken the gas–water two-phase flow into account, which brought a large deviation for design of hydraulic fracturing enhancing underground gas drainage. In this paper, we proposed a fully coupled hydraulic stress damage mathematical model considering gas–water two-phase flow, which can be used to simulate the whole process of hydraulic fracturing enhancing underground gas drainage. The coal seam is simplified as a dual-porosity single-permeability elastic media with elastic modulus reduce and permeability increase when encountered damage. The permeability and porosity serving as the coupling term is a function of stress, water/gas pressure, gas ad/desorption, and damage value. The proposed model was first verified by showing that the modeled gas flux agrees with the field data. The evolution laws of permeability and gas pressure during hydraulic fracturing enhancing underground gas drainage were studied and several influence factors were analyzed by accomplishing a series of simulations. Gas drainage can be effectively enhanced only when the hydraulic fracturing induced damage zone is breakthrough at drainage hole. After the coal seam is effectively fractured, the gas flux has a decline–incline–decline tendency with increasing of drainage time. The breakthrough time of damage zone increases linearly with coal seam elastic modulus, increases exponentially with vertical stress and borehole spacing, and decreases exponentially with injecting pressure.