Water-gas combined methane control in low permeability coal seam near fault: a case study

Coal and gas outburst (CGO) is one of the most devastating disasters in underground coal mines, and it always triggers substantial casualties and property losses. Therein, the coal seam permeability and gas extraction rate are usually low in tectonic coal reservoirs, threatening safe coal production activities. In this work, water-gas combined methane control technology was proposed, based on the merits of hydraulic fracturing (HF) and liquid CO2 phase change fracturing (LCPCF), to improve the permeability of tectonic coal reservoirs. A field experiment was then conducted in a coal seam near a fault to verify its technical practicality, and the tectonic coal seam gas governance effect was accordingly evaluated. The research results suggested that the maximal concentration and average concentration of a single drill hole after HF increased by 13 times and 3 times, respectively, while the maximal flow and average flow increased by 23.4 times and 24.7 times, respectively. On the other hand, the peak concentration and mean concentration of a single hole after LCPCF increase by 2.7 times and 3 times, respectively, and the peak flow and mean flow increase by 6 times and 6.6 times, respectively. Additionally, the destructive degree triggered by LCPCF can be intuitively quantified through grayscale analysis. Furthermore, through the application of water-gas combined methane control technology in engineering practice, the gross quantity of extracted gas is 11 times larger than when LCPCF or HF is individually conducted, and the mean concentration and flow are 1.9 times greater and 2.1 times greater, respectively, than when LCPCF or HF is separately performed, indicating that this technology is practical and has a superior tectonic coal seam permeability improvement effect. This study has profound implications for enhancing coalbed methane (CBM) recovery and for eliminating CGO disasters in tectonic coal reservoirs or complex geologic structure areas.