Soil CO2 Monitoring Around Wells Discharging Methane

Soil(vadose zone) gas compositions were measured for about 3 days to suggest a method for monitoring and interpreting soil gas datacollected around wells from which methane(CH4) is outflowing. The vadose zone gas samples were collected within 1 m around twotest wells(TB2 and TB3) at Pohang and analyzed for CO2, CH4, N-2 and O-2 concentrations in situ. CO2 flux was measured beside TB2.In addition, gas samples from well head in TB2 and atmospheric air samples were collected for comparison. Carbon isotopes ofCO2(delta C-13(CO2)) of samples collected on the last day of the study period were analyzed in the laboratory. The two test wells (TB2 and 3)were 12.7 m apart and only TB3 was cemented to the surface. According to the bio-geochemical process-based interpretation, therelationships between CO2 and O-2, N-2, and N-2/O-2 of vadose zone gas were plotted between the lines of CH4 oxidation and CO2 dissolution. In addition, the CH4 concentrations of gas samples from the wellhead of the uncemented well (TB2) were 5.2 times higherthan the atmospheric CH4 concentration. High CO2 concentrations (average 1.148%) of vadose zone gas around TB2 seemed to beattributed to the oxidation of CH4. On the other hand, the vadose zone CO2 around the cemented well(TB3) showed a relatively lowconcentration(0.136%). This difference indicates that the vadose zone gas(including CO2) around the CH4 outflowing well werestrongly affected by well completion(cementing). This study result can be used to establish strategies for environmental monitoring ofsoil around natural gas sites, and can be used to monitor leakage around injection and observation wells for CO2 geological storage. Inaddition, the method of this study is useful for soil monitoring in natural gas storage and oil-contaminated sites.