Case story: time-lapse seismic crosswell monitoring of CO2 injected in an onshore sandstone aquifer

We present a case study of time-lapse seismic monitoring of carbon dioxide (CO2) injection by traveltime delay tomography. Unlike standard tomography based on the ray theory, this tomographic method applies the 3-D finite-frequency wavefield theory which in seismology is known as the banana-doughnut theory. To monitor 4-D changes in the subsurface during CO2 injection, crosswell seismic data were measured before the injection was initiated and after injection of 3200, 6200 and 10 400 tons CO2 into a porous reservoir sandstone at 1100 m depth. The estimated tomographic velocity images compiled with the finite-frequency wave theory show a clear time-lapse velocity anomaly on the order of -18 per cent below the CO2 injection well head which is in agreement with 4-D sonic logging.