Research progress on variation pattern and monitoring control technology of negative pressure for gas extraction in directional long borehole

Significance Near-horizontal directional long borehole is an important technical approach and effective means for preventing and controlling the gas disasters of coal mine. Negative pressure is an important factor affecting the gas extraction effect of directional long borehole.

Progress  Herein, the research progress on gas extraction through near-horizontal directional long borehole was systematically summarized from three aspects: the attenuation mechanism and distribution law of negative pressure in the borehole, gas extraction parameter monitoring technology, and intelligent control system for gas extraction. Regarding the attenuation mechanism and distribution law of negative pressure in the borehole, the basic theoretical research results concerning the impact of negative pressure on gas extraction, negative pressure attenuation mechanism, and the distribution law of negative pressure were summarized. In terms of gas extraction parameter monitoring technology, the current status of different types of gas monitoring instruments in the borehole, including the ultrasonic, laser, orifice plate, and recirculating self-excited types, as well as the borehole gas monitoring methods based on the beam tube method, was elaborated. Regarding the intelligent control system for gas extraction, the achievements in the research on the concept and technical equipment of gas extraction intelligent control were introduced. With the extraction parameters such as negative pressure, flow rate, and concentration as the analysis and control objects, and based on intelligent algorithms and PLC control technology, preliminary achievements have been made in the intelligent control of underground pipeline networks, thus improving the efficiency of gas extraction.

Prospects The current deficiencies in the research were pointed out in the article, including the unclear variation patterns of negative pressure within directional long boreholes, the unclear mechanism of positive and negative pressure coupling effects, the lack of long-distance gas extraction parameter monitoring methods, and a shortage of multi-parameter gas monitoring methods, as well as the low intelligence level of the gas extraction control system. Meanwhile, the future research directions for gas extraction from directional long borehole were proposed, including the coupling mechanism of negative pressure extraction and positive pressure diffusion based on the special structure analysis of directional long borehole, the multi-parameter monitoring system of gas in borehole based on optical fiber sensing, and the fine control method of gas extraction parameters based on machine learning, which provides support for the design, construction, extraction and effect evaluation of gas control engineering in near-horizontal directional long borehole.