Fracturing Mechanism And Damage Evolution Of Water/Gas Fracturing In Bedding Shale

Using gas,such as nitrogen and CO₂,to replace water as the main fracturing fluid in shale gas reservoir stimulation has the advantages of reducing water resources waste,increasing the flowback rate,eliminating the water lock and Jamin effect,and reducing environmental pollution.It can promote the extraction of shale gas,and protect the ecological environment.However,the density,compressibility and viscosity between water and gas have significant difference and the bedding structure is relatively developed in shale gas reservoir.It has signigicant impacts on the failure process,fluid permeation and damage process in shale gas reservoir.In order to further reveal the fracturing mechanism and damage evolution of hydarulic/gas fracturing in bedding shale reservoir,the means of laboratory test,theoretical analysis and numerical simulation are used.This paper mainly focuses on:a)the anisotropic characteristics of bedding shale under three-point bending load,b)the evolution of permeation boundary motion,c)the evolution of breakdown pressure and permeation area,d)difference of damage evolution between hydarulic and gas fracturing.The major advances and conclusions can be drawed as as follows:?1?This study revealed the anisotropic characteritics of mechanical parameters,energy evolution,and crack initiation and propogation during failure process of bedding shale.The three-point bending tests were conducted on the shale sample with differnt bedding angles.According to the testing results,the evolution of tensile strength,fracture toughness,acoustic emission characteristics,elastic bending modulus,fracture energy,crack initiation and propagation and fracture morphology with bedding angle was investigated.The relationship between tensile strength and fracture toughness,and the relationship between ealstic bending modulus and fracture energy were clarified.The experimental results have certain reference value to parameters selection of bedding shale.?2?This study established an anisotropic fluid-solid-moving boundary coupling model and clarified the internal mechanism of permeation boundary motion of hydraulic/gas fracturing.Based on the difference of compressibility and viscosity among incompressible fluid,ideal gas and gas at critical tempreture,a governing equation for anisotropic fluid flow were derived.A moving boundary method was proposed to describe the motion of permeation boundary.An anisotropic fluid-solid-moving boundary coupling model was established,and verified by previous experimental data.By this model,the impact of permeation boundary motion on pore pressure distribution,principal stress distribution and permeability evolution was investigated to reveal the importance and necessity of moving boundary method.The evolution of permeation boundary motion with permeability,viscosity,mobility,permeability anisotropy,elastic modulus anisotropy and the Possion's ratio anisotropy was explored.The difference of permeation boundary motion and pore pressure distribution among hydaulic,nitrogen and CO₂ fracturing was systematically analyzed.A theoretical method was proposed to fast calculate the permeation boundary motion.?3?This study proposed a breakdown pressure criterion for bedding shale and systematically analyzed the evolution of breakdown pressure and permeation area of hydaulic/gas fracturing.Based on the anisotropy of fracture toughness,and the impoacts of in-situ stress,injection pressure and pore pressure on the stress intensity factor of model-?crack,an anisotropic breakdown pressure criterion was established.The impacts of permebaility,viscosity,mobility,pressurization mobility,Biot's coefficient and confining pressure on breakdown pressure and permeation area druing hydaulic,nitrogen and CO₂ fracturing were investigated.A new approach was proposed to represent the combined impacts of permeability,visocsity and pressurization rate.According to the evolution of stress intensity factor,the effects of fluid compressibility and viscosity on failure mechanism of shale bedding were analyzed.?4?This study developed a solid-fluid-damage coupling model and revealed the damage evolution of hydraulic/gas fracturing in bedding shale.A fluid-solid-damage coupling model was proposed to simulate the hydraulic and gas fracturing in bedding shale.In this model,the Weibull distribution function was used to describe the heterogeneity of bedding shale.The node failure was judged by the maximal tensile stress criterion and D-P criteriion.The damage variable was determined by the linear elastic damage constitutive model.And then,the proposed model was verified by the nitrogen fracturing test on bedding shale.Through the verified model,this study investigate the difference of damage evolution among hydraulic,nitrogen and CO₂ fracturing in rock without bedding,bedding shale and bedding shale with natural flaw,respectively.The impacts of stress state,bedding angle,natural flaw and bedding strength on damage evoltuion were revealed.Furthermore,the evolution of permeability,pore pressure,crack initiation and propagation with damage evolution was explored.The damage node and shear percentage were introduced to quantitatively describe the damage degree and failure mode of shale bedding,respectively.