Study On The Influencing Factors And Characterization Method Of Shale Brittleness Based On Meso-Macro Mechanics

Brittleness evaluation is the key link of shale reservoir compressibility evaluation.Reasonable and efficient shale brittleness evaluation system can provide important guarantee for high and stable production of shale oil and gas reservoirs.Aiming at the current situation that the accuracy of evaluation index of shale brittleness,the regularity and mechanism of scale effect of mechanical properties,the influencing factors of brittleness and the method of brittleness in mining field are vague and lack of in-depth study,Using RTR-1000 rock triaxial mechanical testing system produced by GCTS Company,600 KN servo testing machine,highresolution three-dimensional X-ray CT scanning system produced by Zeiss Company and D/Max 2600 powder X-ray diffractometer produced by Nippon Science Company and the other advanced instruments,In this paper,the mechanical properties,micro-defect scanning and mineral composition analysis of the downhole core and a large number of outcrops of Chang 7 continental shale reservoir in W block of southern Ordos Basin are tested.The influence factors of shale brittleness are qualitatively analyzed and quantitatively calculated by linear regression analysis and grey correlation method.The accuracy of common mechanical indexes for evaluating shale brittleness is obtained.Based on the cross-scale mechanical tests of shale samples under different loading modes,the scale effect of shale strength and deformation is preliminarily obtained by curve fitting of experimental data and theoretical analysis of rock mechanics.Based on the regularity and scale effect mechanism of rock strength,the influence of micro mineral components on macro-brittleness of shale is analyzed,based on finite element software COMSOL and grey mathematical fuzzy theory.The contribution of each mineral to brittleness is analyzed.A new brittleness evaluation method combining mineral components of shale and its brittleness in fluence coefficient is proposed,which is convenient for engineering.Cheng applied another new method of brittleness considering the mechanical environment of shale reservoir and the content of mineral components to realize rapid and accurate screening of high brittleness section in ore field.The results show that:(1)Research on evaluation index and accuracy of each evaluation index of shale brittleness(1)The confining pressure,Poisson's ratio and modulus of elasticity are positively correlated with shale brittleness,and the peak strain,peak stress,rupture time,residual stress,residual strain,differential stress,number of rupture strips are negatively correlated with shale brittleness;(2)peak strain has the highest accuracy for shale brittleness evaluation,while modulus of elasticity has the lowest accuracy for shale brittleness evaluation,and the middle order is when shale is broken.The accuracy ratios of residual strain,peak stress,differential stress,residual stress,number of cracked surfaces,confining pressure and Poisson's ratio were respectively 0.136:0.131:0.126:0.120:0.112:0.097:0.096:0.079:0.056:0.046.(2)Scale effect of mechanical properties of shale(1)Under uniaxial loading,the elastic modulus and Poisson's ratio increase with the increase of sample size,while peak strength,peak strain,residual strain,residual stress and fracture time decrease with the increase of sample size.The slope of stress-strain curve increases with the increase of rock sample size,and the curve after peak decreases rapidly,but it still has the ability to resist the damage of external load.Tensile-shear composite failure mainly occurs in rock samples of different scales,but the shear angle of shear failure occurs near 10 degrees.The larger the scale of rock samples,the more the number of fracture surface.(2)In triaxial loading test of shale,Poisson's ratio increases with the increase of sample size,peak strength and peak strain decreases with the increase of sample size,while the number of fractured surfaces,residual strain and residual stress do not change with sample size.The average values of the three factors are 2.2,2.93 and 36.17 MPa,respectively.Linear segment and yield segment exist in pre-peak deformation of shale at different scales.The larger the scale,the faster the decline rate of stress-strain curve in post-peak section is and the steeper the curve is.Tensile failure occurs mainly in small-scale rock samples,and shear failure occurs mainly with the increase of scale,and the shear angle mainly concentrates on 45.5% during shear failure.Near Degree.(3)In the Brazilian splitting test,the maximum load and the number of fractured surfaces increase with the increase of scale,while the tensile strength decreases with the increase of scale.The stress-strain curves of rock samples at different scales all undergo gentle,linear and rapid descent after peak.The failure modes of rock samples with different scales are single and all of them are tensile failure.The larger the scale of rock samples,the more the number of fracture surface.(4)The scale effect of strength and deformation of continental shale under uniaxial loading is the most obvious under three loading conditions,followed by Brazilian splitting test and triaxial loading.(5)The scale effect of rock strength is determined by both end effect and heterogeneity.For small-scale rock samples,end friction is the main factor of scale effect of rock strength.For large-scale rock samples,rock heterogeneity is the main factor of scale effect of rock strength,and Reasonable selection of rock sample size range is of great significance to the study of scale effect law and mechanism of rock strength.(3)Influencing factors of brittleness of continental shale and new characterization methods(1)The content and distribution characteristics of mineral components and sedimentary mechanical environment of shale are important factors affecting the macro-brittleness of shale.Marine shale siliceous minerals,carbonate minerals and other brittle minerals have a large content and a large burial depth,while continental shale clay minerals and other ductile minerals have a large content and shallow burial depth.Because of the significant differences in mineral composition and burial depth between marine and continental shales,the brittleness evaluation formula of marine shales is difficult to apply to continental shales.(2)The influence of dolomite,siderite,diopside,illite,chlorite,feldspar and quartz on shale brittleness in Chang 7 continental shale of block W is gradually weakened.The weight coefficients of brittleness influence are 0.136,0.117,0.138,0.173,0.145,0.136 and 0.157,respectively.The difference of weight coefficients is not more than 0.06.Therefore,the contribution of minerals to brittleness should be considered in shale brittleness evaluation.(3)A new method of brittleness evaluation based on the content of shale mineral components and its brittleness influence coefficient is proposed.The method is used to evaluate the brittleness of L well in W block,guide the fracturing design of the well,and realize the accurate characterization of the brittleness of shale reservoir.(4)A new method of brittleness evaluation considering shale mineral composition and mechanical environment is proposed.This method is used to guide Y well fracturing in W block.Fracturing results show that the newly established brittleness evaluation method has good correlation with reservoir fracturing operation and can effectively guide fracturing.The study of influencing factors and characterization methods of shale brittleness can not only simplify the formula of brittleness evaluation,but also improve the accuracy of shale reservoir brittleness evaluation,and provide some reference for shale reservoir evaluation and fracturing construction.