Borehole Protection Technology of Screen Pipes for Gas Drainage Boreholes in Soft Coal Seams

Abstract

In this paper, research was carried out on borehole protection technology using screen pipes, aiming to address the problems of gas drainage borehole collapse and low gas drainage efficiency in soft coal seams, which have low gas permeability and high gas content. An experiment was conducted to study the compressive resistance and borehole protection effects of polyvinylchloride (PVC), polypropylene (PP), and acrylonitrile–butadiene–styrene (ABS) screen pipes with different sieve opening spacings. The optimal screen pipe material and sieve opening spacing were determined. The borehole protection effect and gas drainage efficiency using three borehole protection methods, namely, bare boreholes, traditional PVC screen pipes, and new ABS screen pipes, were compared and studied on site, and the influence of pipe diameter on borehole protection efficiency was revealed. The results show that the compressive resistance and borehole protection effects of the three types of screen pipes are in the following order: PVC < PP < ABS. ABS screen pipes can more effectively inhibit boreholes from collapse, and the best borehole protection is achieved with a sieve opening spacing of 50 mm. The field tests showed that the gas drainage concentration of the bare borehole in the creep deceleration stage was 113% higher than that of the borehole protected by the ABS screen pipe and 255% higher than that of the borehole with the PVC screen pipe. The screen pipe began to play a role in borehole protection at the beginning of the stable working stage, as the borehole wall began to deform and collapse. On the 30th day of gas drainage, the gas drainage concentration of the borehole installed with the ABS screen pipe was about 9 times that of the bare borehole due to the collapse and blockage of the bare borehole. The new ABS screen pipe with an outer diameter of 40 mm and a sieve opening spacing of 50 mm had a better borehole protection effect and gas drainage efficiency than the traditional PVC pipe. Proper sizing of the screen pipe diameter can improve borehole service life and gas drainage efficiency. The research results are of great significance for solving the problem of gas drainage borehole instability and for improving gas drainage efficiency in soft coal seams.

Gas Migration around Large Diameter Borehole with the Protective Screen Pipe in Fractured Soft Coal Seam

“…17,18 The grouting technology can enhance the strength of soft coal and prevent borehole col-lapse by detecting the strength and structure of coal rock mass, which may promote the development of gas drainage in soft coal seams. 19,20 In addition, numerous scholars have conducted research on parameters of the stress, deformation, damage in coal seams after drilling. The influence of large diameter borehole on the strength of surrounding rock was analyzed by numerical simulation, and the strength of surrounding rock after drilling the pressure relief borehole was obtained.…”

“…Numerical simulation on the stability of borehole wall was carried out to estimate the mechanical parameters of soft coal seams by Hoek–Brown criterion, and reveal the stress and displacement distribution around the borehole wall . A directional drilling technology with gas dynamic screw drill was developed and applied in soft coal seams. , The grouting technology can enhance the strength of soft coal and prevent borehole col-lapse by detecting the strength and structure of coal rock mass, which may promote the development of gas drainage in soft coal seams. , In addition, numerous scholars have conducted research on parameters of the stress, deformation, damage in coal seams after drilling. The influence of large diameter borehole on the strength of surrounding rock was analyzed by numerical simulation, and the strength of surrounding rock after drilling the pressure relief borehole was obtained. , Gas extraction using large diameter borehole was also carried out to control spontaneous combustion of residual coal in goaf .…”