Mechanical Properties Of No.3 Coal Reservoir In Laochang Mining Area And Its Deformation Response To The Process Of Hydraulic Fracturing

The deformation of coal reservoir is accompanied by the whole process of fracturing,and affected by many factors,it's an important research topic in the field of coalbed methane(CBM)exploration and development,which is of great significance to reveal the deformation and failure mechanism of coal reservoir,evaluate the fracturing effect and optimize the fracturing scheme.This thesis took the No.3 coal seam in Laochang mining area of eastern Yunnan as the research object.Based on the analysis of the geological background of CBM and the physical and mechanical properties of target coal seam,through the physical simulation tests of uniaxial compression,triaxial compression and hydraulic fracturing,the relationship between the deformation characteristics of coal bedding,water saturation,stress state and fracturing displacement were analyzed,the concept and mathematical model of radial-axial strain ratio were put forward,the deformation and failure law of coal reservoir in the process of compression and fracturing were revealed.The No.3 coal seam in the study area is semi-dull to semi-bright coal,mainly vitrinite,R_o,max is between 2.14%and 3.28%,and the coal rank is between lean coal and anthracite.The pore types are mainly transitional pores and micropores.Microfissures are well developed,mainly shear fissures,most of which are filled with calcite,clay minerals,etc.,and the connectivity is poor.The permeability of coal reservoir is mostly less than 0.1m D,which is a medium-low permeability reservoir.The coal body structure is mainly primary structural coal,followed by fractured coal.The mechanical properties of the coal seam are quite different from those of the argillaceous siltstone in the roof and floor,indicating that the coal seam has good conditions for reservoir reconstruction.It is found that the compression deformation characteristics and mechanical properties of coal are controlled by bedding intersection angle,water saturation and confining pressure.When the bedding intersection angle increases from 0°to 90°,the compressive strength and peak strain of coal decrease at first and then increase.The tensile crack failure mainly occurs in the coal samples with low bedding intersection angle.The coal samples with medium bedding intersection angle crack along the bedding plane,and shear failure mainly occurs.The coal samples with large bedding intersection angle crack along the coal matrix,and shear failure mainly occurs.With the increase of water saturation,the complexity of cracks increases,the compressive strength decreases,the peak strain remains stable,and the fluctuation amplitude of stress-strain curve increases.With the increase of confining pressure(0?15 MPa),the compressive strength,residual strength,elastic modulus and strain in the elastic stage increase.The concept and mathematical model of radial-axial strain ratio are put forward,and it is used to characterize the relative relationship of strain along different directions in the process of deformation and failure of coal samples.its fluctuation amplitude and change trend can reflect the influence of specific geological background or fracturing conditions on the deformation behavior of coal matrix.When the confining pressure increases,the radial-axial strain ratio decreases,and there is a negative logarithmic relationship between them.The radial strain ratio is positively correlated with stress difference and fracturing displacement,showing a solitary peak in the process of hydraulic fracturing,and the radial strain starts slightly earlier than the axial strain,indicating that the coal sample is mainly deformed and destroyed along the direction of weak stress in the process of fracturing.According to the deformation characteristics of coal samples,the fracturing process was divided into four stages:?-stable stage,?-deformation start-up stage,?-unstable failure stage and?-post-failure stage.The deformation of coal sample appears inflection point during the rising period of water pressure and reaches the peak when the fracture pressure is reached,which mainly occurs in stages?and?,which means that the deformation curve of coal sample can be used to predict fracturing failure.The fracture pressure is negatively correlated with the stress difference and positively correlated with the fracturing displacement.There is a certain internal relationship among stress state,fracturing displacement,acoustic emission energy,radial axial strain ratio and fracture development.when the stress remains stable,the acoustic emission energy and radial axial strain ratio increase with the increase of fracturing displacement.coal samples are easy to form large-scale simple cracks along the direction of stress weakness.The coal matrix and internal fissures are regarded as a whole,and the deformation parameters can be used to evaluate the effect of fracturing.There are 78 figures,15 tables and 123 references in this thesis.