Monitoring CO2 injection with cross-hole electrical resistivity tomography

In this study, the resolution capabilities of electrical resistivity tomography (ERT) in the monitoring of CO2 injection are investigated. The pole-pole and bipole-bipole electrode configuration types are used between two uncased boreholes straddling the CO2 plume. Forward responses for an initial pre-injection model and three models for subsequent stages of CO2 injection are calculated for the two different electrode configuration types, noise is added and the theoretical data are inverted with both L1-and L2-norm optimisation. The results show that CO2 volumes over a certain threshold can be detected with confidence. The L1-norm proved superior to the L2-norm in most instances. Normalisation of the inverted models with the pre-injection inverse model gives good images of the regions of changing resistivity, and an integrated measure of the total change in resistivity proves to be a valid measure of the total injected volume.