Research On The Instability Mechanism Of Coal Reservoir Wall In The Drainage Well Of Agong Syncline Coalbed Methane Block In Zhijin, Guizho

As one of the unconventional natural gas resources,coalbed methane is of great significance to ensure national energy security.Guizhou is the largest coalbed methane enrichment area in southern China,and Zhina coalfield is one of the three major coalfields in Guizhou Province,accounting for 22.91%of the total geological resources of the province.However,in the process of coalbed methane exploitation,many problems are faced,such as the evolution law of wellbore instability and the unclear main control factors affecting the stability of wellbore.In view of the fact that many key scientific issues greatly affect the safety and efficiency of drilling and the gas production efficiency of coalbed methane in the process of coalbed methane mining,it is of great significance to study the mechanism of borehole instability of coalbed wells in Wenjiaba Block,Zhijin County.This paper takes the coal reservoir of Wenjiaba coalbed methane block in Zhijin County as the main research object.Firstly,the macro mesostructure,basic physical properties,mechanical parameters and strength deformation characteristics of coal rock are studied,and the physical simulation test study of the stability of the well wall is carried out,and finally according to the formation conditions and mechanical parameters,the three-dimensional mesh model of the main coal mining seam is established by ABAQUS finite element software,and the influence of different horizontal in-situ stress conditions,elastic modulus and drilling fluid column pressure on the stability of the well wall is simulated,and the following conclusions are mainly drawn.（1）The GSI quantitative model is established by logging curve,and the GSI value of coal structure of each coal seam is obtained by logging parameters such as natural gamma,compensation density,deep lateral resistivity,shallow lateral resistivity,acoustic time difference,and natural potential.The calculation results of the model have a good correspondence with the well diameter data in the actual project,and the model can better evaluate the stability of the well wall.（2）The multi-scale structural characteristics of coal were studied by in-situ observation,CT scan and nuclear magnetic resonance.Field observation showed that there were three main types of coal structures in the study area,with primary structure coal as the main coal,followed by granular structure and chyme structure coal.The CT scan results showed that a large number of surface cutting and end cutting were developed in the raw coal,all of which were distributed in the form of short cracks,and the extension range of surface cutting was longer than that of end cutting,and the end cutting density was large.The volume of the primary fracture in the sample was between 55.75 mm³-149.86 mm³,the average value was 82.93 mm³,the fracture surface area was between 316.50 mm²-838.71 mm²,the average value was 481.91 mm²,the fractal dimension of the three-dimensional fracture was between 1.75-1.85,and the average value was 1.78;the nuclear magnetic resonance test showed that the porosity of the coal rock in the study block was between 3.24%～5.14%,and the average value was 3.8%,T₂ The spectral morphology is basically similar,all of which show obvious three-peak morphology,among which the small pores corresponding to the left peak are the most developed,mainly reflecting the large amount of adsorption pores in the coal seam.（3）Mechanical tests such as single-and three-axis compression tests of coal rock and Brazilian splitting test were carried out.The results show that the strength,elastic modulus and Poisson’s ratio of coal rock show strong stratigraphic angle effects.The coal rock specimen at 90°was the highest,and the average uniaxial compressive strength reached20.20MPa;the smallest at 30°,the average uniaxial compressive strength was only 7.80MPa,and the change trend was distributed in a"U"shape with the inclination angle of the strata.The failure modes of 0°-90°coal rock are tensile-shear composite failure,shear-slip failure and split-tension failure.Through the triaxial mechanical test of coal rock,it is concluded that when the layer angle is 0°at low confining pressure,the failure mode of the specimen is tensile-shear composite failure,and when the layering angle is 30°,45°and 60°,the failure mode is mainly shear failure,and when the sandwich angle reaches 90°,the failure mode of the specimen is mainly tensile failure.The increase of confining pressure has obvious effect on the change of brittleness to ductility of coal rock,and the residual strength increases relatively with the increase of confining pressure.Through the tensile test of coal rock,the tensile strength of coal rock in Wenjiaba block is generally low,and the tensile strength is between1.19～1.99 MPa.According to the CT scan results of different inclination specimens before and after the uniaxial compression test,the evolution law of fracture surface area,volume and three-dimensional fractal dimension of fracture was analyzed.（4）Physical simulation tests of borehole instability of coal seam under different levels of principal stress were carried out.The test shows that different levels of in-situ stress conditions have a great influence on the stability of the wellbore,and with the decrease of the minimum level of principal stress,the wellbore of coalbed methane is damaged.The tension deformation of the well circumference is mainly used in the direction of the maximum horizontal principal stress,and the compression deformation is mainly used in the direction of the minimum horizontal principal stress.The main influencing factors of wellbore stability are horizontal in-situ stress conditions and primary fractures,and the fractures are mainly deformed along the primary fractures or approximately perpendicular to the coal rock specimen when they are damaged.With the decrease of the minimum level principal stress,the peak value of the coal rock sample at failure decreases,and the strain region and peak strain increase.（5）The numerical simulation study of borehole bore instability of coal seam under different levels of principal stress,drilling fluid column pressure and elastic modulus was carried out.The results show that during the drilling process of the vertical well of coalbed methane well,with the increase of the horizontal in-situ stress difference of the formation,the stress of the wellbore is more concentrated,and the more instability and failure of the wellwall is more likely to occur.With the increase of the elastic modulus value of the rock mass,the rupture pressure and collapse pressure of the surrounding rock of the well wall increase,and the less likely the surrounding rock of the well wall is to undergo instability failure.With the increase of drilling fluid injection pressure,the smaller the equivalent plastic strain of the surrounding rock of the well wall,the smaller the equivalent plastic strain area,the less stable the surrounding rock of the well wall,the more stable the well wall.