Investigation on the Influence of Cleat Structure on the Coalbed Methane Borehole Stability in WW Basin
As a clean unconventional energy,coalbed methane is abundant in China and has become an important supplementary source for increasing reserves and production.The exploitation of coalbed methane can effectively prevent gas leakage and explosion.However,the elastic modulus and strength of coal rock are low,the cleat struc-ture is developed,and pipe sticking is very common in coalbed methane well due to borehole collapse,which re-stricts the efficient development of coalbed methane.In this paper,the microstructure of coal rock is studied first-ly,and the strength criterion of coal rock established,considering the effects of face cleat and butt cleat,and the influence of cleat structure on the wellbore stability in WW basin is studied in combination with the stress model around dual porosity and dual permeability wells.The results show that the content of clay minerals of coal rock in this area is less,and the hydration is weak;face cleat,butt cleat and face cleat cut coal rock vertically and cross each other,seriously weakening the strength of coal rock;the cleat structure has a significant influence on the col-lapse pressure of coal rock wellbore.If the cleat effect is not considered,the wrong change rule of collapse pres-sure with wellbore trajectory will be obtained;the study on the influence of cleat occurrence on the collapse pres-sure of three types of wells shows that the collapse pressure of vertical wells does not change with the change of cleat angle,but is greatly affected by the cleat tendency;When the cleat angle is large,the horizontal well along the horizontal minimum in-situ stress has better stability;under the condition of specific cleat orientation,the hori-zontal well along the direction of horizontal maximum in-situ will not generate shear slip failure along the cleat.
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