Research status of borehole instability characteristics and control technology for gas extraction in soft coal seam

This paper systematically summarises the instability and failure mechanism of boreholes drilled in soft coal seam, indicating that the instability of coalbed borehole is mainly affected by the basic mechanical properties of coal, the stress environment around borehole, the pore pressure and the drilling method. It is suggested that increasing the mechanical strength of surrounding rock, reducing the extent of plastic zone and radial displacement of borehole are the keys to improve the stability of borehole and the rate of borehole formation. The application conditions, advantages and disadvantages of the existing borehole instability control technology are analysed. It is considered that the development direction of borehole instability prevention and control is to develop the integrated technology of borehole protection and blowout prevention while drilling and visual repair. The research status of traditional grouting reinforcement materials is systematically reviewed. It is concluded that the development and application of nano-sized slurry foam materials, which can reinforce boreholes, inhibit blowout and release gas pressure, thereby extending borehole service period and improving gas recovery efficiency, with a promising application prospect.