Effects and influencing factors of the CO2-alkaline-water two-phase displacing gas and wetting coal

Abstract

To prevent the risk of coal and gas outburst and improve the coalbed methane (CBM) recovery, the CO2-alkaline-water two-phase displacing gas and wetting coal technique (CADW) was researched. Effects of the CADW in the residual CH4 and CO2, pore pressure, moisture content and permeability in coal under different conditions were studied detailedly. The results showed that, compared with the CO2 displacing CH4, after adopting the CADW, the amount of residual gas in the Qincheng (QC), Liangshun (LS) and Zhongxing (ZX) coals increased significantly, with the increment of 203.99%, 75.15%, 61.35%. The desorbed CH4 and CO2 of the three coals accounted for 86.37%, 69.12%, 64.60% and 4.30%, 3.38%, 13.43% of the injected CH4 and CO2 respectively. Compared to the conventional water injection, after using the CADW, the pore pressure, water injection volume and moisture content in coal all increased obviously. The maximum pore pressure of QC, LS and ZX coals increased by 2.05–6.96%, 4.37–5.44% and 2.88–9.25% respectively. The water injection volume of QC, LS and ZX coals increased by 10.69%, 33.51%, 43.45% respectively. The moisture content increased by 2.81–13.01%, 20.00–42.58%, 16.69–114.34% radially, and its increment increased with distancing the borehole. Besides, the permeability in three coal samples all decreased by one order of magnitude, with the reduction of 84.21%, 86.49% and 89.47%. Additionally, increasing the water injection pressure and time can distinctly improve effects of the CADW, which will reduce with rising ground stress and reducing temperature. The results can provide important reference for better preventing the coal and gas outburst risk and increasing the CBM recovery.