Study and application on influence mechanism of instability and collapse of drainage borehole on gas drainage

Abstract

The instability and collapse of drainage boreholes is one of the key problems that hinder the efficient extraction of coal-bed methane (CBM). In order to solve the problem of instability and collapse of extraction boreholes in soft coal seams, based on the laboratory tests and theoretical analysis of variable mass flow pressure drop, a coupled mathematical model is established, which comprehensively considers borehole deformation and instability, coal bed gas migration and dynamic attenuation of borehole negative pressure along the borehole length. The influence mechanism of instability and collapse of extraction boreholes on gas extraction is numerically analyzed, and the detection technology of instability and collapse characteristics of extraction boreholes based on the distribution test of extraction negative pressure is proposed. Research shows: ① The gas drainage effect of the complete borehole is very good, and the negative pressure loss of the borehole is very small. The total negative pressure loss of the100 m long borehole for one day only accounts for 0.04% of the negative pressure of the orifice. The gas distribution and effective drainage radius of the coal seam at the orifice and the bottom of the hole are basically the same. ② The influence of borehole bottom collapse on the gas drainage effect is limited, but the influence of borehole collapse near the hole opening on the gas drainage effect is significant. When 90% of the borehole is collapsed at the borehole bottom and near the borehole opening, the gas extraction volume after 90 days of extraction is 38.7% and 69.6% lower than that of the complete borehole. After 180 days of extraction, the effective extraction radius at the borehole bottom is 88.7% and 75.0% of the effective extraction radius of the borehole opening. The total negative pressure loss in the first day of extraction accounts for 5.1% and 28.4% of the negative pressure of the borehole opening. ③ Borehole plugging seriously affects the gas extraction effect. The blocked coal body in the hole forms a continuous medium with the surrounding coal body. The pressure in the borehole becomes the gas pressure of the coal seam, and the unstable section cannot extract gas. After 90 days of drainage, the gas drainage volume of the blocked hole at the bottom of the hole only accounts for 17.6% of the gas drainage volume of the complete borehole. ④ The negative pressure distribution of extraction drilling is sensitive to the instability and collapse characteristics of extraction drilling. A detection technology for the instability and collapse characteristics of extraction drilling based on the negative pressure distribution test of extraction drilling is proposed. Through the application in the 25030 working face of a mine, the instability and collapse area (65−80 m) and collapse degree (serious collapse borehole) of extraction drilling are determined. It is suggested that the mine should carry out a targeted grouting protection for the instability and collapse area of boreholes.

Keywords

• gas drainage borehole
• instability and collapse
• negative pressure distribution
• gas drainage volume
• effective drainage radius