Injection and migration of CO2 within geologic formations and its reactions with reservoir rocks will cause changes in seismic velocities, density and attenuation, resulting in changes in time-lapse seismic scattering/diffraction. In this study, we use reverse-time migration of time-lapse walkaway vertical seismic profiling (VSP) data to study reservoir changes due to CO2 injection/migration. The data were acquired at the SACROC enhanced oil recovery (EOR) field in Texas using geophones at depths from 5000 to 5700 feet in a monitoring well. The CO2 injection started in October, 2008, and the time-lapse walkaway VSP data were acquired in July 2008 before the CO2 injection and April, 2009, six months after the CO2 injection. The objective of the project is to study the combined EOR and geologic carbon sequestration. We apply statics corrections to the time-lapse walkaway VSP data, and then use a spectrum analysis method to balance the amplitudes of the data to make migration images comparable. We conduct reverse-time migration using the upgoing waves of the baseline and balanced walkaway VSP data to study detailed changes in the reservoir. The results demonstrate that reverse-time migration of time-lapse walkaway VSP data can reveal reservoir changes due to CO2 injection and migration.