An Evaluation Method Of Brittleness Characteristics Of Shale Based On The Shale Unloading Experiment

With the exploration and development of unconventional oil and gas in China,it is particularly important to evaluate the brittleness of shale gas reservoirs.However,with the change of history,the reservoir has experienced a multi-period geological tectonic movement,and the stratum has undergone compaction,uplift,erosion,sedimentation,weathering,earthquakes,faults,etc.,that is,the regional reservoir has undergone a natural loading and unloading process.In-depth study of the loading and unloading mechanism and evaluation of shale mechanical properties are of great significance for the evaluation of reservoir tightness.In this paper,mineral component test and scanning electron microscope test,uniaxial and triaxial compression test,the axial compression loading and confining pressure unloading test,and simultaneous acoustic emission monitoring test are carried out to study the mineral composition of shale and the microstructure,the strength,deformation and fracture characteristics of shale during loading and unloading are discussed.The influence of unloading rate on its mechanical parameters is analyzed.The relationship between unloading rate and acoustic emission parameters is expounded,and the brittle characteristics of shale during loading and unloading is evaluated.This paper mainly achieved the following research results:?1?Mineral composition and microscopic pore structure of shale are analyzed by X-Ray Diffraction?XRD?and Scanning Electron Microscope?SEM?,whose results show that the mineral composition of shale is composed of brittle minerals such as quartz,albite and pyrite,which content is 64.73%-72.64%;the carbonate minerals mainly composed of calcite,dolomite and iron dolomite account for 6.11%-15.54%;the illite,montmorillonite and kaolinite are the main clay minerals accounted for17.71%-21.25%;compared with the Barnett shale in North America,the brittleness index is relatively good,and the shale pore size range is between 47.43 nm and 749.43 nm,which is beneficial to the compressibility of the reservoir.?2?On the MTS triaxial test machine,the uniaxial compression test of 50 mm^*100 mm cylindrical specimens and the triaxial compression tests with confining pressures of 20 MPa,40 MPa,60 MPa and 80 MPa are carried out to obtain the basic mechanical parameters of shale samples.The results show that the uniaxial compressive strength of shale samples is between 90MPa and 108MPa,and the failure mode contains dilatancy failure and X-shaped conjugate slope shear failure.Under triaxial compression conditions,the axial strain,peak strength,elastic modulus and other parameters of shale samples increased significantly with the increase of confining pressure.The maximum peak strength is 386 MPa under the confining pressure of 80 MPa.The failure mode is dominated by single-section oblique shear failure,and the shear band penetrated the whole shale sample;The Mohr-Coulomb strength criterion is used to fit the ?₁ and ?₃ under different confining pressures,the fitting coefficient is 0.97,indicating that the shale sample is homogeneous.?3?Aiming at the changes of reservoir geological structure,the geological influences of reservoir uplift and erosion are simulated in the laboratory by the axial compression loading and confining pressure unloading test.The shale failure characteristics under different unloading rates are discussed and the acoustic emission monitoring test is carried out simultaneously.The results show that the increase of unloading rate has weakening effect on some shale mechanical parameters,and there is a clear linear relationship between peak deviatoric stress,residual deviatoric stress,destructive confining pressure and unloading rate;Under the condition of low unloading rate,the damage characteristic is a single oblique section shear failure;Under the condition of high unloading rate,there are multiple shear zone symbiosis and associated with layered cracking failure mode;when the rock sample breaks,the energy of acoustic emission will be doubled.An increase in the order of magnitude indicates that a large amount of energy is released inside the shale sample at this time,and the crack initiation initiates an increase in the crack propagation.?4?Based on the analysis of more than 30 brittleness evaluation methods proposed by domestic and foreign scholars based on different theories,the brittleness characteristics of shale samples are discussed by using mineral component analysis and stress-strain curve analysis.The brittleness index B_d1 and B_d2 and feasibility in triaxial loading test are verified.Short board and drawbacks of them in the unloading test are pointed out.On this basis,the brittleness evaluation method based on the unloading elastic modulus E_u and total elastic modulus E_t are proposed,brittleness index B_{d 1} and B_d2 are further optimized,and the rock brittleness characteristics under different unloading rate tests are evaluated by the unloa ding brittleness evaluation parameters B_u1 and B_u2.The value of?and?in this method can fully consider the unloading rate and the unloading point.The selection of?and?can be used to measure the different effects of stress and strain on brittleness.The feasibility of the brittleness evaluation method is well verified.With the increase of the unloading rate,B_u1 and B_u2 show a rising trend,meaning the brittleness is getting stronger.Also,compared with the small fluctuations at high unloading rate of B_{u 2},the brittleness variation of shale samples can be better characterized by B_u1.