Study On Mechanism Of Mechanical Borehole Wall Inatability In Coal Rock

In the process of coal bed methane drilling and conventional oil and gas reservoir drilling,the problem of coal rock borehole wall instability is generally faced,which greatly affects the safety and efficiency of drilling,and further affects the later production improvement.In the process of drilling,the instability of the coal rock borehole wall is mainly due to the weak structural planes,such as the cleats,which cause the coal rock brittle.The strong discontinuity and heterogeneity are presented in the mechanical properties,which is also the difficult point for the study of the problem of the coal rock borehole wall stability.In view of this,it is of great significance to further study the mechanism of mechanical borehole wall instability in coal rock.In this paper,based on the coal rock of Ordos Basin,the research was first carried out on the macro and micro structure properties,mechanical strength parameters and deformation characteristics of coal rock,then from the viewpoint of mechanics,based on two evaluation methods which are the maximum displacement and plastic zone radius normalized to evaluate the coal rock borehole wall stability,combined with the macro and micro structure,basic properties,mechanical strength parameters of coal rock in study area.The general software of discrete element numerical simulation was used to study the influence of mechanical borehole stability of coal rock and analyze the mechanism of coal rock wellbore instability.The research results could provide reference for the prevention and control of coal bed collapse during drilling.The following conclusions were obtained:(1)The coal rock structure characteristics of the study area were mainly native structure or cataclastic structure.The types and structure of coal rock had great influence on the mechanical and the basic physical properties.The brittler of coal rock was,the more developed the cleats were,the larger the acoustic travel time between longitudinal and transverse waves,the lower the elastic modulus,the higher poisson's ratio,and the lower mechanical strength.The development of cleats in coal rock was the fundamental reason for coal rock borehole wall instable(2)The triaxial mechanical characteristics and uniaxial mechanical characteristics of coal rock had obviously differences.Under uniaxial compression condition,which presented a compaction curve obviously,elasticity,yield and destroy,brittle failure characteristics was obvious,residual strength after peak was almost zero.With the increase of confining pressure,curve appeared elasticity,yield,destroy or softening stage after the peak;With the change of confining pressure,the change of brittleness to ductility of coal rock was obvious,with confining pressure increase,the increase of residual strength was relatively increased(3)Coal rock borehole stability was mainly affected by the coal cleats,pore pressure,stress and the like.In the isotropic stress condition,the displacement and plastic zone radius around borehole wall affected by the coal cleats angle was lesser,the borehole wall was relatively stable;With the increase of stress difference,the displacement and plastic zone radius around borehole wall was increased,wellbore stability becomes worse,When the angle of face cleat and the level of the minimum principal stress was 30°-45°,borehole wall stability was worse,When the angle of face cleat and the level of the minimum principal stress was 90°,borehole wall stability was well.(4)When the cleats spacing was larger than the borehole radius,cleats of coal rock had weak influence on borehole stability;When the cleat spacing was certain,with the borehole diameter increase,cleat traversed borehole wall also become more,the coal rock around borehole wall become brittler,the borehole wall was prone to instability.(5)With the increase of bottom hole pressure,the displacement and plastic zone radius around borehole wall was prone to decreased,borehole wall remained relatively stable.when the pressure was too large,the cleats and fractures of coal rock were opened,extended,leaded to borehole wall instable;When the coal rock cleats angle were different,the distance and degree of cleats opend and extended were different.With the increase of pore pressure,the bottom hole pressure needed to maintain the wellbore stability was gradually increasing,and the relationship between them were logarithmic.