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.
Highlights
As a kind of fossil energy resources, coal plays an important role in the development of the world economy
We extend previous models and propose a fully coupled hydraulic stress damage (HSD) model with two-phase flow including coal deformation and damage, gas ad/desorption, diffusion and seepage, and water seepage governing equations
A fully coupled HSD model with two-phase flow was proposed for simulating the whole processes of hydraulic fracturing enhancing underground gas drainage
Summary
As a kind of fossil energy resources, coal plays an important role in the development of the world economy. Hydraulic fracturing enhancing underground gas drainage is a complex process involving the interaction of coal mass deformation and failure/damage, gas ad/desorption, water and gas migration (Xu et al, 2017). The variations of coal seam elastic modulus, permeability, and gas pressure are numerically investigated, as well as several influencing factors including elastic modulus, vertical stress, borehole spacing, and fluid injecting pressure, to better understand the rules of gas/water migration, coal deformation and damage evolution during hydraulic fracturing enhancing underground gas drainage process. (v) when the fracturing is finished, we set gas drainage duration and change the boundary condition, including non-flow condition around fracturing holes and drainage pressure condition around drainage holes, to simulate the gas drainage process According to this flow chart, the combined model is programmed using MATLAB and Comsol Multiphysics.
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