Experimental Study on Effect of CO_2-Alkaline Water Two-Phase Gas Displacement and Coal Wetting

摘要

Permeability improvement through high-pressure CO2 displacement and outburst elimination through coal seam water injection are efficient engineering technologies in coal mine gas control. However, significant problems still exist in these technologies at present. For example, CO2 displacement is likely to cause gas enrichment, leading to outburst danger; besides, conventional coal seam water injection can hardly enter the micropores and fractures of coal, resulting in poor water injection effect. In this paper, gaseous CO2, the source of outburst danger in coal, was taken as a bridge. While weak alkaline water adsorbs gaseous CO2, it is closely integrated with coal to improve the wettability of coal. In this way, the purposes of improving permeability and eliminating outburst can be achieved. An experiment was performed to test the permeability and residual gas characteristics of coal after CO2 displacement, and then a CO2-alkaline water two-phase displacement simulation test was carried out to study CO2 adsorption characteristics of alkaline solution and its coal wetting effect. The results show that the residue and desorption amounts of CO2 in coal are both larger than those of CH4 under the same conditions. After adding a small number of alkaline materials, the amount of gaseous CO, dissolved in water solution rises greatly. The amount of alkaline water adsorbed by CO2-containing coal is twice that of distilled water adsorbed by it; compared with alkaline water, pure water injection into a coal seam produces a more serious pressure-building effect. Therefore, the adoption of CO2-alkaline water two-phase displacement not only greatly raises the moisture content of coal, but also notably increases gas drainage volume from coal seams, as the method combines the advantages of both gas-phase displacement and liquid-phase displacement. As a result, the coal seam outburst danger can be reduced by it.