Development and application of gas leakage channel detection device for gas extraction borehole

The perimeter cracks of gas extraction borehole and the gap passage of sealing section are the main causes of the failure of borehole leakage. In order to detect the gas leakage channels in the borehole effectively, a gas leakage channel detection device is developed based on the theory of tube flow hydrodynamics and gas leakage detection method. Detecting gas sample parameters at different drilling depths and analyzing the distribution and abrupt changes are used to identify the cause of extraction borehole failure and the location of gas leakage channels. The detection device adopts highly stable piezoresistive pressure sensors, laser methane sensors and fluorescent oxygen sensors to detect extraction negative pressure, gas concentration and oxygen concentration. The device adopts 1.5 m/section quick-connect 25 mm thin-walled stainless steel pipe as the gas extraction fittings and the drilling depths is up to 30 m. The application results show that the variation of testing parameters in extraction pipe section is stable, the extraction pipe is not damaged, the interface is not leaking, and the sealing effect of the extraction pipe is good. In the sealing section, there are several abrupt change points of different degrees in the range of 9-18 m from the hole. The largest air leakage channel is in the range of 9-12 m from the hole, indicating that the original sealing depth is insufficient and the original sealing process cannot seal the air leakage channel effectively. In this study, the sealing depth is increased to 12 m and the 'two plugging and one injectio' pressure-injected grouting sealing technology is used for comparative tests. The results show that the overall extraction effect of the test hole is greatly improved, the volume fraction of gas at the hole is increased to more than 55%, the change of gas volume fraction is stable within the depth range of 12 m from the hole, the volume fraction of oxygen is almost 0, and the gas leakage channels are reduced. The test results verify that the gas leak channel detection device can effectively detect gas leakage channels, provide a basis for the targeted adjustment of sealing methods, related parameters and subsequent improvement.