Damage Mechanics Model For Dual Porosity Medium And Its Application On Hydraulic Fracturing

It is of great significance to enhance coal seam gas and shale gas recovery,to increase the supply of clean energy,adjust the national energy consumption structure,and reduce greenhouse gas emission.For the purpose of this,studying the mechanism of rock-gas interaction and building a novel mechanical model for dual porosity medium,which is used to describe rock deformation and gas migration process,have great theoretical value.Gas adsorption-induced strain causes coal microstructure to be significantly changed through altering fracture effective strain,matrix effective strain,and pore effective strain.Gas adsorption-induced damage is relieved as three effective strains rearrange coal structure and result in coal mechanical property alterations and thus permeability change.The other hand,supercritical CO2 fracturing is viewed as a very promising method for permeability enhancement in coalbed and shale gas reservoirs.However,the mechanism of the permeability evolution caused by fracturing remains unclear.In view of this,we investigate the mechanism of rock-gas interaction and coal permeability evolution through both experimental and numerical studies.At the same time,supercritical CO2 fracturing and oriented perforations on fracture propagation and fracture complexity in hydraulic fracturing are studied.The main contents include:(1)Based on the concept of effective strain and rock-gas interactions,a novel damage mechanics model for dual porosity medium is proposed,as well as the finite element analysis code based on continuum damage mechanics.(2)Experimental and numerical works following the full stress-strain permeability-adsorption evolution of coals are conducted.The numerical modeling is validated against the experimental observations.The mechanism of gas adsorption-induced damage on coal mechanical property alterations and permeability changes are fully analyzed.(3)Permeability is continuously measured by the constant pressure differential method,together with axial and volumetric strains for Helium,Nitrogen and strongly adsorbing Carbon Dioxide(supercritical Carbon Dioxide).The evolutions of coal permeability under different stress states and pore pressures are obtained.The results show that different types of gases have different permeability evolution curves.(4)To explore the equilibrium process of fracture pressure,matrix pressure,and pore pressure during gas injection process,and analyze the time effect of local equilibrium and global equilibrium of coal.The influence of matrix diffusion coefficient on the permeability evolution is studied,as well as the impact of gas Langmuir adsorption strain.(5)Considering rock deformation,gas flow,and rock damage,the multiphysics field coupling model of supercritical CO2 fracturing is established to simulate supercritical CO2,liquid CO2 and hydro-fracturing based on the Span-Wagner state equation.We also explore the propagation of hydraulic fractures from oriented perforation using a fully coupled FEM-based hydraulic-geomechanical fracture model accommodating gas sorption and damage.