Time-lapse crosswell seismic and VSP monitoring of injected CO2 in a brine aquifer

Seismic surveys successfully imaged a small scale CO(2) injection (1,600 ton) conducted in a brine aquifer of the Frio Formation near Houston, Texas. These time-lapse borehole seismic surveys, crosswell and vertical seismic profile (VSP), were acquired to monitor the CO(2) distribution using two boreholes (the new injection well and a pre-existing well used for monitoring) which are 30 m apart at a depth of 1,500 m. The crosswell survey provided a high-resolution image of the CO(2) distribution between the wells via tomographic imaging of the P-wave velocity decrease (up to 500 m/s). The simultaneously acquired S-wave tomography showed little change in S-wave velocity, as expected for fluid substitution. A rock physics model was used to estimate CO(2) saturations of 10-20% from the P-wave velocity change. The VSP survey resolved a large (similar to 70%) change in reflection amplitude for the Frio horizon. This CO(2) induced reflection amplitude change allowed estimation of the CO(2) extent beyond the monitor well and on three azimuths. The VSP result is compared with numerical modeling of CO(2) saturations and is seismically modeled using the velocity change estimated in the crosswell survey.