Study On The Permeability Evolution Of Deep Low Permeability Sandstone Under Cyclic Loading

With the advancement of technology and the depletion of shallow mineral resources,the development and utilization of deep mineral resources will become the norm.The mechanical behaviors in deep underground is more complex and the principle physical parameters changes with the depth increase,which in the meantime,more dangerous for engineering construction.As one of the most common rocks in engineering,the study of sandstone including mechanical behavior and hydraulic permeability is vital important for coal mining engineering and deep space development.The specimens studied in this paper take the sandstone from the roof in about 1050 m depth of Pingdingshan No.12 coal mining company and artificial sandstone manufactured by quartz sand and clay under same crustal stress.To study the evolution of the permeability under the cycle loading of low-permeability sandstone in deep underground,the confining pressure cyclic loading test and axial compression cyclic loading test and CT scanning experiment are performed.Based on the theories of dissipated energy density,fractal,rock damage,the permeability evolution of deep low permeability sandstone is systematically studied.And establish the seepage model of stress and damage.The research works of this dissertation are mainly carried out under the six aspects as follows:(1)Manufactory of artificial sandstone and comparative analysis.The artificial sandstone have the advantage that the material recipe,mechanical properties and pore structure properties are all manipulable,it's effortless to make the rock sample as designed.In this dissertation,the artificial sandstones are cemented under high pressure.The main ingredients are montmorillonite clay,illite clay and fine,coarse and very coarse quartz sands with grain mesh size of 100-200,70-140 and 60-70 respectively.The adhesive used in this test is epoxy resin and the confining pressure set to 40 MPa,which is the maximum crustal stress of the nature sandstone underground.The samples are divided by the portion of clay as 5%,10%,15%,20%,25%and 30%by mass.(2)Study on mechanical properties and pore structure characteristic.The mechanical properties and pore structures of the rock samples have significant influence on the permeability evolution under cycling loads.The components of the roof sandstone can be obtained by X-ray diffraction test.The mechanical behavior of rock samples can be obtained through axial and triaxial compression test.The pore ratio and permeability could be obtained by overburden pressure permeameter.The grayscale distribution and pore characterize could be obtained by CT scan test.The pore size could be obtained by high pressure mercury test.All these could give required parameters for the permeability study of sandstone under cycling loads.(3)The triaxial confining loading and unloading permeability test of artificial sandstone.The confining loading and unloading air permeability test is performed by TOP triaxial test system under same stress path.The test permeate fluid is nitrogen,and the osmotic pressure,axial pressure and confining test are IMPa,IMPa and 4?15?4MPa respectively.Under this condition,the mechanical behavior and the permeability evolution feature of artificial rock samples with different clay proportions under different confining pressure are obtained.The relation of permeability and confining pressure are studied based on Darcy's Law and Poiseuille law.The test results show that the permeability and confining pressure follow a power-law distribution when the clay percentage is lower,in the meantime when the clay proportion are higher,the permeability and confining pressure follows a negative exponential distribution.The compression effect caused by confining pressure could transfer the stress-strain to linear relation that could be observed as the relation between permeability and confining pressure transforming from negative exponential to power law.(4)The triaxial compression cyclic loading and unloading test of roof sandstone in deep underground acoustic emission permeability study.The cyclic loading and unloading acoustic emission permeability experiment is an effective method to study the evolution law of rock permeability under stress.In this paper,the MTS815 mechanical experiment system is used to carry out the triaxial cyclic loading and unloading acoustic emission infiltration experiment on the deep roof sandstone.The results show that the effect of stress on rock can be divided into compaction and fracturing.In the cycle with lower stress level,it mainly plays the role of compaction,and the permeability decrease with the increase of stress.When increasing to more than 60%of the yield strength,the stress mainly acts as a fracturing,and the permeability increases slightly with the increase of the stress.The permeability and stress-strain characteristics,the proportion of dissipated energy density in the cyclic loading and unloading process,and the cumulative number of acoustic emission events have significant evolution laws during cyclic loading and unloading.The permeability has four characteristics under cyclic loading.The permeability is firstly reduced by the compaction of lower stress,and then the fracturing effect by large stress is slightly increased,and it is suddenly increased in the failure stage.The compaction effect is then reduced.(5)Study on evolution law of rock damage and permeability under cyclic loading.The evolution of damage and permeability defined by the fractal characteristics of acoustic emission and the compression parameters of rock are studied.The experimental results show that the acoustic emission of rock under cyclic loading has fractal characteristics,and the variation of acoustic emission b value and correlation dimension can characterize rock damage.When the fractal value of acoustic emission increases,it indicates that the rock is in the compaction stage,the damage is not obvious,and the permeability is reduced at this time;the sudden drop of acoustic emission fractal value indicates that the rock is damaged,and the permeability shows a sharp rise.The compression parameters defined by the elastic modulus are introduced to characterize the compaction and damage state of the rock,and the relationship model between permeability and compression parameters is established.(6)CT image reconstruction and analysis of roof sandstone in deep working face.Based on CT scanning and image processing,the permeability of artificial sandstone and roof sandstone under fractal seepage model is obtained.And the fractal seepage model is verified and studied.The results show that the permeability and confining pressure evolution law obtained from the fractal seepage model of pore fracture structure is consistent with the experimental results.The permeability of roof sandstone calculated by the fractal model of pore structure is close to the experimental value,which indicates that it is feasible to use the CT image processing method to study the rock permeability by using the fractal seepage model of the pore fracture structure.