Monitoring a pilot CO2 injection experiment in a shallow aquifer using 3D cross-well electrical resistance tomography

Abstract:

Three dimensional electrical resistance tomography (ERT) was used to monitor a pilot CO₂ injection experiment at Vrøgum, Denmark. The purpose was to evaluate the effectiveness of the ERT method for monitoring the two opposing effects from gas-phase and dissolved CO₂ in a shallow unconfined siliciclastic aquifer. Dissolved CO₂ increases water electrical conductivity (EC) while gas phase CO₂ reduce EC. We injected 45kg of CO₂ into a shallow aquifer for 48 hours. ERT data were collected for 50 hours following CO₂ injection. Four ERT monitoring boreholes were installed on a 5m by 5m square grid and each borehole had 24 electrodes at 0.5 m electrode spacing at depths from 1.5 m to 13 m. ERT data were inverted using a difference inversion algorithm for bulk EC. 3D ERT successfully detected the CO₂ plume distribution and growth in the shallow aquifer. We found that the changes of bulk EC were dominantly positive following CO₂ injection, indicating that the effect of dissolved CO₂ overwhelmed that of gas phase CO₂. The pre-injection baseline resistivity model clearly showed a three-layer structure of the site. The electrically more conductive glacial sand layer in the northeast region are likely more permeable than the overburden and underburden and CO₂ plumes were actually confined in this layer. Temporal bulk EC increase from ERT agreed well with water EC and cross-borehole ground penetrating radar data. ERT monitoring offers a competitive advantage over water sampling and GPR methods because it provides 3D high-resolution temporal tomographic images of CO₂ distribution and it can also be automated for unattended operation. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344. Lawrence Livermore National Security, LLC. LLNL IM release#: LLNL-PROC-657944.