Soil gas dynamics monitoring at a CO2-EOR site for leakage detection

Abstract

A comprehensive soil gas monitoring program was conducted at a CO2-enhanced oil recovery site in western Mississippi to validate near-surface monitoring technologies for CO2 leakage detection. The program included three main monitoring technologies: (1) using commercial sensors for real-time in situ monitoring of atmospheric and soil CO2 concentrations and of environmental parameters; (2) intermittent soil CO2 flux measurements with a portable CO2 flux survey system; and (3) on-site soil gas composition measurements in semipermanent soil gas wells with a portable gas chromatograph. Atmospheric and soil CO2 concentrations measured with the commercial sensors vary from 340 ppm in the atmosphere to ~12% (volume percentage) in soil at a depth of 3 m showed variations at different time scales over a period of 214 days. Atmospheric and soil CO2 concentrations also showed dependence on environmental parameters. Average CO2 flux measurements at the site were ~6 ± 5.2 g/m2/day, comparable to soil CO2 flux measurements reported at other sites. Grid soil CO2 flux measurements on the pad identified a point with CO2 flux up to 5200 ± 4000 g/m2/day, likely due to a focused leak from the underlying pipeline. Soil gas composition monitoring in the soil gas wells fell into two groups based on the relationship of the soil gas compositions, likely resulted from different biogeochemical processes. The field results provided valuable information on soil gas dynamics for validating numerical models of soil gas transport in the vadose zone and designing and implementing near-surface monitoring programs at other geological carbon sequestration sites.