| Coalbed methane is an efficient and clean energy which has rich resources in our country.Its exploitation can reduce coal mine disasters,increase energy supply,meanwhile,relieve environmental pollution.Horizontal directional drilling technology has been widely used because it can keep the hole extended along the coal seam,increase the parse area and improve the efficiency of exploitation.However,with the deepening of the development,the problem of borehole instability is becoming increasingly outstanding,especially for drilling the horizontal part in coal seam,which has a seriously adverse impact on the drilling safety and economic benefits.Although there are many factors accounting for this,the relevant researches are restricted these days.So it’s imminent to explore the instability mechanism.The essay summarizes the factors affecting CBM drilling borehole stability from both geology and engineering.According to analyzing the data of 2014ZX-U-01H horizontal well,we can conclude that the horizontal part in the upper 3#coal seam is the most dangerous area.Dealing with the problem has a negative impact on the progress of the project.After transforming the stress coordinate,we used MIDAS GTS NX software to build a 3D finite element horizontal well model with the rock mechanics parameters,which is based on the Mohr-Coulomb strength criterion.Studied the influence rules of horizontal angle,thickness of mud cake and ground stress direction on borehole stress distribution characteristics,deformation,displacement,and the hole shape;evaluated the instability problem quantitatively;furthermore,found out the main factors influencing the stability of the horizontal well.The results shows that:(1)The Von Mises stress of borehole increases gradually as the horizontal well extends.But due to the poor property and vertical differentiation of 3#coal seam,the stability of borehole wall can also be influenced by formation lithology.(2)The horizontal well mainly deformed along the X axis direction.The displacement of bottom of well is larger than the top.And the cross-section of well is like an ellipse.While the borehole which is in the upper part of 3#coal seam is the maximum deformation zone.(3)Geostress has a greater effect on broken formation’s deformation and less effect on the complete formation;deviation angle mainly affects the radius of plastic deformation;mud cake can improve the borehole stress concentration phenomenon.(4)The order of factors from major to minor which influences the stability of horizontal well is:ground stress direction,deviation angle,mud cake thickness.The borehole stability enhance with the geostress angle increases,deviation angle decreases and mud cake thickness increases.(5)The safest drilling plan is the ground stress angle at 45°、deviation angle at 84°,and mud cake thickness at 1.2cm.On this occasion the maximum Von Mises stress is 0.52MPa,while the maximum deformation is 3.04cm.Not only does the study coincided with drilling phenomenon solve the shortcomings of traditional method such as wasting time and money,nonrepeatability and one-sided,but also have advantages in choosing calculation principle,load condition and material.The clear and visual results prove that it’s feasible and rational to study the borehole stability of CBM horizontal well by using finite element method.It also provides experiences for the similar borehole stability study. |
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