Evaluation And Application Of Shale Brittleness Based On Tensile Action

Brittleness of shale is of vital significance to evaluate mechanical properties of oil and gas shale reservoir and has significant effect on hydraulic fracturing.The evaluation methods of shale brittleness are mainly based on Young’s modulus,brittle mineral component,or total stress-strain compression failure.These evaluation methods of brittleness mentioned above can hardly consistent the fracture characteristics of shale with hydraulic fracturing.In order to break out the limitations of characterizing the shale brittleness by mechanical parameters or stress attenuation degree after peak,this paper proposes the definition of brittle index based on tensile failure and establishes the corresponding evaluation method and analysis-forecast model,forming a non-continuous peak post evaluation system of shale brittleness.The research of Brazil split experiment and discrete element numerical simulation is carried out on shale samples from Longmaxi formation.The main contents are as follows:1 Experimental studies on brittle mechanics of shale under tensile are conducted.Combining with geological characteristics and the SEM microscopic imaging method shale stratification and fracture development characteristics are described.Mineral composition has been figured out by carrying XRD analyzing tests on shale of Longmaxi formation.The dynamic parameters are obtained by using ultrasonic tests.Based on different depths,angles of bedding,loading rates,and other factors,the experiment scheme has been developed,and a series of Brazilian splitting experiments have been carried out by GCTS rock mechanics testing machine.The tensile mechanical parameters of shale from Longmaxi formation are obtained.2 The mechanical evaluation model of shale brittleness under tension is established.The brittleness mechanism of shale based on stress-strain curve,fracture characteristics and strength criterion of the Brazil split experiment has been studied.The method of tensile brittle shale comprehensive evaluation has been established.This paper has also calculated brittleness index of shale,and analyzed the factors that influence of brittle strength,anisotropy,loading rate.In addition,through the comparative analysis of serval kinds of brittleness evaluation methods,the rationality of tensile comprehensive brittleness evaluation method is proved.3 The evaluation model of tensile brittleness of shale based on the discrete element method is established.We simulate the Brazil split experiment of shale by using PFC software,and the brittleness is evaluated through the fracture distribution characteristics.The numerical simulation parameters are calibrated by the Brazil splitting experiment.Through the numerical simulation of the shale specimen under tension,the distribution characteristics of the fracture are studied.The discrete element model consisting with shale characteristics is established by using PFC software.The effects of anisotropy and scale effect on brittleness are simulated and analyzed.From the view of numerical simulation,the accuracy of the evaluation model of shale brittleness under tension is demonstrated.