Damage Model And Numerical Simulation For Gas Fracturingin Low-Permeability Coal Seams

Coalbed methane as a huge reserves of unconventional gas,has great prospects for development in China.Porosity and permeability ofunconventional natural gas reservoir are very low,resulting its difficult exploitation.At present,hydrofracturing is usually used to enhance the permeability of unconventional natural gas reservoir,thought this methord will lead to water pollution and be difficult to implement in water shortagearea.Water free fracturing,however,not only has many advantagescompared with traditional hydrofracturing,but also can avoid the problem of water pollution.Therefore,aim to damage evolution during water free fracturing,experimental test,theoretical modeling and numerical simulations are carried out in this thesis.The study includes:(1)High-pressure air blasting experiments under free and constrained boundary conditions are carried out.The stress wave and fracture distributions are obtained.Free boundary air blasting produces 5-6 main fractures,constrained boundary air blasting produces dozens of main fractures.The result of constraint boundary is better than the free boundary.(2)A mechanical model for high-pressure air blasting and gas frecturing are established with consideration of the rock heterogeneity and in-situ stress field,in which air blasting and gas frecture are considered as two consecutive stages,i.e.the stress wave and the blasting gas pressure or only static gas pressure.The seepage equation is used to describe the fracturing gas propagation in cracks and the mechanical effect of quasi-static pressure of fracturing gas near the crack tip is reflected based on seepage-mechanical coupling theory.The model is solved by Matlab programming with finite element software COMSOL Multiphysics.The hypotheses and simplification is also validated.(3)Numerical simulations on high-pressure air blasting experimentare conducted to study the impact of stress wave,quasi-static gas pressure and in-situ stress on fracture evolution.The gas drainage after blasting is also simulated to study the mechanism of high-pressure air blasting on gas drainage enhancement.Numerical simulations for gas fracturing are carried out to study the impact of rock homogeneity,rock types,dynamic viscosity of gas and gas flow rate on fracture evolution.The results show that the number of fractures decreases with model homogeneous degree increases.Gas flow rate increases could promote the formation of frctures.(4)Numerical model of coal seam with dirt bandbased on actual coal mine is established.Numerical simulations on gas fracturing and high-pressure air blasting are carried out to study the impact of dirt band fitness,distance,permeability and confining pressure on fracture initiation and propagation.Gas blasting could effectively avoid weakness of gas fracture and advantageous to the formation and expansion of frctures.