Study On The Failure Process And Stability Of Surrounding Rock During Vertical Borehole Drilling

Vertical borehole technology is widely used in geotechnical and geological industry.The failure and instability of surrounding rock may cause engineering accidents such as borehole collapse,drill sticking,delaying the project schedule,and even lead to oil and gas resources leakage,resulting in serious economic losses.In order to explore the failure mechanism and process of surrounding rock during borehole drilling,firstly,the elastic-plastic mechanical analysis of surrounding rock was carried out,and the expressions of stress,displacement and plastic circle radius were obtained from the theoretical aspect.Then,a physical model with a size of 800 mm×800 mm×200 mm and a?160 mm circular cavern was established and a gradually increasing boundary load was applied to it.On this basis,considering the mechanical characteristics of surrounding rock,a borehole numerical model was established and solved by the strain-softening model in FLAC^3D.According to the results of physical and numerical simulation,the deformation and failure process of surrounding rock can be divided into five stages:initial,sensitive,stable,sudden and gradual increasing stage during drilling.In this study,it was shown as follows:when the load reached 3.88 MPa,the plastic zone of surrounding rock began to develop,and micro cracks appeared on the borehole wall;when the load reached 3.94 MPa,the plastic zone expanded rapidly,and cracks extended to the whole borehole wall;the growth rate of plastic zone slowed down and the failure of surrounding rock was relatively stable in the following load;when the load reached 4.44 MPa,the plastic and residual zone expanded rapidly,the cracks increased quickly,an obvious fracture zone appeared,and the surrounding rock was damaged significantly in a short time and load range;in the process of the load reaching 4.88 MPa,the plastic and residual zone expanded slowly,and no obvious damage occurred on the borehole wall.Due to the strain-softening characteristics,the deformation and failure of borehole surrounding rock do not simply increase linearly with the depth and load,but show obvious sudden and stable characteristics.Finally,an exploration borehole engineering of Qingdong mine in Huaibei was selected for borehole TV observation.The deformation and failure characteristics of the surrounding rock in this engineering are consistent with the results of physical and numerical simulation.The study results can not only provide theoretical basis and practical guidance for the site selection and drilling scheme design of exploration holes and hydrological holes in the area,but also provide reference for the stability analysis and evaluation of others non-horizontal borehole.