Experimental Study On Borehole Deformation Of Coalbed Methane Horizontal Well Under Stress Disturbance

Coalbed methane,also known as gas,is a kind of unconventional natural gas resource formed and stored in coal seams during coal-forming process.It has the characteristics of cleanliness and high calorific value.China's shallow coal-bed methane geological resources within 2000 m depth rank third in the world.Meanwhile,China's high-quality energy such as oil and natural gas production capacity is insufficient.Developing and utilizing coalbed methane resources can not only effectively curb coal mine gas disasters and reduce the greenhouse effect of methane on the atmospheric environment,but also optimize China's energy structure and alleviate energy shortage.Coalbed methane is a complex environment in which coal and rock are significantly affected by stress and seepage field.The problem of borehole deformation and stability is becoming more and more prominent in CBM surface well exploitation,which not only affects drilling speed,but also damages downhole tools and increases downhole accidents.The instability failure of coalbed methane wells is a complex dynamic problem which is affected by stress,borehole diameter and drilling fluid column pressure under stress disturbance.In this paper,taking coal and rock around wells in Ningwu Basin,Shanxi Province as the research object,using the methods of theoretical analysis,device design,model test and numerical simulation,the borehole deformation and stability of coalbed methane horizontal wells under stress disturbance are deeply studied,the failure forms and influencing factors of coal and rock are analyzed,and the similar parameters are calculated by combining the similarity theory with the parameters of Ningwu Basin.Based on similar parameters,a set of triaxial mechanical experimental device for coal and rock around CBM wells is designed.The simulation tests of deformation and failure of coal and rock around wells under different triaxial stresses and borehole diameters are carried out with the device,and the deformation and failure laws of coal and rock around wells under corresponding conditions are analyzed.By using COMSOL Multiphysics finite element software,the stress distribution and deformation of surrounding rock during under-balanced and near-balanced drilling are simulated,and the deformation and failure law of surrounding coal and rock under corresponding conditions are analyzed.Through experiment and numerical simulation,the deformation and failure law of coal and rock around horizontal well of coalbed methane under stress disturbance is obtained.The following results have been achieved:(1)Keeping the minimum horizontal principal stress unchanged and increasing the maximum horizontal principal stress for vertical loading,the strength of coal and rock around the well first increases and then decreases;keeping the maximum horizontal principal stress unchanged and increasing the minimum horizontal principal stress for vertical loading,the strength of coal and rock around the well gradually increases,and the influence of the minimum horizontal principal stress on the strength of coal and rock around the well is greater than that of the maximum horizontal principal stress.When the maximum and minimum horizontal principal stresses are loaded vertically,the strength of coal and rock around the well increases gradually,and the influence of the simultaneous change of the maximum and minimum horizontal principal stresses on the strength of coal and rock around the well is greater than that of the single horizontal principal stress.(2)Keep the maximum and minimum horizontal principal stress unchanged,with the decrease of borehole diameter,the proportion of coal and rock deformation around the wellbore increases during the instability failure stage,and the plasticity of coal and rock around the wellbore increases;keep the maximum and minimum horizontal principal stress unchanged,reduce the diameter of the wellbore for vertical loading,the strength of coal and rock around the wellbore increases gradually,and the increment increases gradually.(3)The circumferential extrusion force is very small and the radial force is relatively large near the 45 degree azimuth of the borehole,along which a relatively large deformation occurs and shear failure occurs.Vertical deformation of wellbore is more obvious.Coal and rock around the wellbore produce micro-cracks during the deformation process.It is not easy to develop after the formation of side micro-cracks,and it develops faster after the formation of top micro-cracks,forming macro-cracks and wellbore damage,and easy to collapse and damage in the vertical side.Underbalanced drilling fluid string has lower pressure than near-balanced drilling fluid string,greater pressure difference in radial direction,and greater periwellbore deformation along the 45 degree direction and vertical direction of borehole.When underbalanced drilling is used,the drilling fluid column pressure should be focused on meeting the stress balance of the borehole near the vertical direction and 45 direction,so as to avoid borehole collapse and shear failure.