关键词: 节理煤层, 应力强度因子, 水平井, 井壁稳定, 评价模型

Abstract:

The analysis on wellbore instability mechanism shows that both the in-situ stress and the crack stress control the cracks near wellbore in coal seam,and the cracks get failure more easily than coal rock.The evaluation criterions for crack propagation and failure were presented according to the Drucker-Prage criterion and stress intensity factor used to describe the stress concentration of cracks.An evaluation model for the wellbore instability was developed on the above methods and criterion.This model can be used to assess the wellbore stability of coal seam in vertical or horizontal wells quantatively and may be helpful in other rocks.The effects of crack obliquity,bottom pressure,crack length and borehole radius on the stress intensity factor were analyzed.With the increase of bottom pressure,the stress intensity factors K_Ⅰ and K_Ⅱ decreased linearly,the wellbore stability was improved.With the increase of crack length,the factors K_Ⅰ and K_Ⅱ increased remarkably,and the wellbore stability got worse.As the crack obliquity increased,K_Ⅰ decreased and K_Ⅱ increased,the anti-tension capacity was improved while the anti-shear capacity was reduced and the stability of overall borehole got worse.With the increase of borehole diameter,K_Ⅱ increased and anti-shear capacity was reduced,the wellbore stability got worse.The size effect happened in the borehole of coal seam and was resulted from the cracks existing in the coal bed.When the ratio of crack length to borehole diameter is greater,the size effect will be more obvious.

Key words: coal seam, stress intensity factor, horizontal well, wellbore stability, evaluation model