The Change Of Coal Pores And Fractures And Ultrasonic Anisotropy Characteristics Analysis Under Uniaxial Loading

The anisotropy of coal pore and fissure plays a controlling role in the occurrence and migration of coal seam,which has attracted wide attention.Geophysical exploration technology is an important method to study the physical information and structure of coal and rock mass losslessly.Ultrasonic testing technology and nuclear magnetic resonance?NMR?is important methods widely used in the study of coal and rock structure in recent years.In this paper,the research method of theoretical analysis combined with experiment is used to qualitatively and quantitatively study the anisotropy of coal pore and fracture structure.The ultrasonic and nuclear magnetic resonance experiments were carried out under the conditions of uniaxial loading at room temperature by using the coal pill of same kind of metamorphism,but different cutting direction in Pingdingshan N0.8mine,and its sound waves,nuclear magnetic characteristics are analysed.Firstly,a nuclear magnetic resonance experiment is performed in the initial state?no load stress?,to test the magnetic resonance T₂ relaxation spectrum,features of nuclear magnetic resonance imaging?MRI?.Secondly,the experiment of ultrasonic response of coal under uniaxial loading is carried out,to test wave velocity characteristics,quality factor characteristics,anisotropic characteristics.Then,the NMR experiment after the uniaxial loading is performed.Finally,the acoustic response characteristics are combined with the NMR characteristics,and use improved Biot theory to analyse anisotropy low of coal hole,crack structure change under uniaxial loading conditions.The research shows that:?1?In the initial state,there are anisotropy in the vertical and horizontal wave velocity of coal pillars in different bedding directions and quality factor.The T₂anisotropy of nuclear magnetic resonance in different bedding direction is obviously too.Theoretical analysis shows that,the main causes of coal pore and fracture anisotropy in initial state has a great relationship with different bedding direction,fracture development and development orientation.?2?Under uniaxial loading conditions,vertical and horizontal wave velocity of coal pill samples in different bedding direction rises first and then falls,the quality factor increases first and then decreases,which shows that the pillars have experienced dense closure,elastic deformation,plastic deformation,failure deformation stage during uniaxial loading,and its attenuation of the wave velocity decreases first and then increases.Nuclear magnetic resonance characteristics indicate that:After uniaxial loading,the orientation of the internal pores and cracks in the coal did not change in the elastic deformation stage,and only causes some of the pores and large scale cracks closed,which results in the increase of wave velocity and the decrease of attenuation;There is a big change in the distribution of pores and cracks inside the coal in the plastic deformation stage.Large scale fissures are further compacted into medium pores and new damage to the cracks inside the coal appears,which results in beginning to decline and the rapid increasing rapidly when the wave velocity is gradually stabilized.Theoretical analysis shows that,in the elastic deformation,plastic deformation stage,the structure of pores,cracks changes differently resulting in attenuation of the wave velocity is also different.?3?Under uniaxial loading conditions,the gap of wave velocity,quality factor and anisotropy factor is large in different bedding direction:Wave velocity,quality factor Anisotropy factor basically fluctuates around 0 value in cascade direction,and anisotropy is not obvious;Wave anisotropy factor continues to decrease in cascade and vertical bedding direction,indicating that the gap between the wave velocity decreases as the uniaxial pressure increases;Quality factor anisotropy is a bathtub curve in cascade and vertical bedding direction,first decreasing and then increasing,and because coal compressive strength in different bedding directions is different,the change characteristics of internal hole,fissure structure are also not the same,and quality factor is a more sensitive parameter for studying the internal structure of coal.?4?In the initial state,after uniaxial loading,the characteristics of coal pillar MRI are analyzed:In the initial state,the pore density in X direction is large,the distribution of cracks is staggered,fracture density is large in Y direction,and hole,fracture distribution characteristics are obvious in Z direction;After uniaxial loading,the coal experience elastic deformation,porosity in the direction of the bedding is compressed and closed,some fractures produce radial deformation,crack in the direction of vertical bedding is compressed and closed,and the influence of force on porosity is smaller.The coal undergo plastic deformation,and the crack expanding damage in the direction of bedding is obvious;Most of the cracks in the direction of vertical bedding are compacted into tiny pores.From the image point of view,MRI more intuitively reflects the variation characteristics of different strain stages of the hole,cracks inside the coal.In summary,in this paper through the ultrasonic test,nuclear magnetic resonance test means,the anisotropic deformation characteristics of coal pore and crack structures in the condition of uniaxial loading is studied,which provides some theoretical support for studying the physical properties of coal reservoirs by geophysical methods.