Numerical Research And The Improvement Of Evaluation Method For Rock Brittleness

The failure characteristics of brittle rock and its mechanical behavior under the action of external environment are the two important parts of rock mechanics and engineering.However,because of the complication of rock types and its environments,the brittleness evaluation is very difficult.In recent years,with the large-scale developing of oil and gas in shale reservoir,the evaluation of brittleness index has become the key technology in the process of hydraulic fracturing.Therefore,the study of evaluation methods on rock brittleness is very significant.From this view,the advantages and disadvantages as well as their applying conditions are analyzed firstly based on the existing rock brittleness evaluation methods.At the same time,the new method for the evaluation of rock brittleness according to stress-strain curves was proposed.Besides,the correction of the new method was verified by the different confining pressures' rock specimens which drilled from the oil layer and interlayer of Shengli Oil Field.After that,the mechanical parameters and acoustic emission modes of specimens related to brittleness index are anlyzed,the main researches are listed as follows:(1)The common used evaluation methods for rock brittleness are summarized,and the applicable conditions as well as the advantages for each method are analyzed.The results shows that it is effective to evaluate rock brittleness by the stress-strain curve.After that,a new brittleness evaluation method was proposed based on the dropping rate of peak stress and energy ratio.In addition,this method was varifed by the specimens which drilled from the oil layer and interlayer of Shengli Oil Field under different confining pressures.The verification results not only proved the validity of the rock brittleness evaluation method,but also illustrate the effectiveness of the method for rock specimens which have the similar post stress-strain relationship.(2)The relationship between brittleness index Bi and the main mechanical parameters was analyzed through the sandstone specimens of the reservoir.In additon,the effection of the confining pressure to the brittleness was discussed.The results show that there is a good relationship between the rock parameters and Bi by lines fitting.At the same time,the brittle behavior for rocks under the conditon of constant parameter values such as elastic modulus,poisson's ratio,compression to tensile strength,residual strength coefficient,internal friction angle,homogeneity index and loading rate were studied.The results show that the realtionship between the brittleness and uniaxial compressive strength is not sensitive.It suggests that it is not accurate to evaluate the rock brittlenss just by uniaxial compressive strength.Moreover,the brittleness of specimen declined with the increasing of residual strength coefficient and Possion's ratio,but increased with the increasing of elastic modulus,C/T ratio,internal friction angle,homogeneity index and loading rate.As a result,during the evaluation for brittleness indexes,it is important to consider the variation of brittle for rock which caused by the changeable of main parameters.(3)The relationship between acoustic emission modes and brittleness index Bi was summarized.The research results shows that acoustic emission modes for the interlayer and oil layer sandstones are dominated by pre-main-after shocks and swarm shocks.Besides,the relationship of rock specimens between brittlenss index Bi and acoustic emission modes during the process of failure was discussed according to the laboratory experiments and numerical simulations.Under the condition of the same loading rate,the acoustic emission modes for the low brittleness zone,intermediate brittleness zone and high brittleness zone are swarm shocks,pre-main-after shocks and main shock,respectively.Furthermore,the validity of the mechanism was verified by the acoustic emission modes under different kinds of limestone specimens and coal cores.(4)Based on the numerical and laboratory test results,the fracture characteristics of the shale specimens were analyzed under the condition of different bedding angles.At the same time,the variation of anisotropic characteristics and indexes for main mechanical parameters such as peak strength,elastic modulus and Possion's ratio at each bedding angle were summarized.Besides,the anisotropic of brittleness indexes at each bedding angle for the rock specimens which under different confining pressures were analyzed,the results showed that the brittle degree of the weak layer in the rock determines the overall brittle of the entire rock especially the bedding angle is nearby 45°.(5)Throuth the numerical simulation,the fracuring influenced by the thickness of the mud-sand interbeded layer was studied,and the crack propagation types for each layer were summarized.Besides,the variation of the fracture density in the sand layer which infuenced by the residual strength coefficient and brittleness index of mudstone was analyzed.Furthermore,the hydraulic fracturing and instable stress which caused by the residual strength coefficient and brittleness index Bi during the process of fracture formation and development were studied by the simulation.