Lawrence Berkeley National Laboratory

Seismic surveys successfully imaged a small scale C02 injection (1,600 tons) conducted in a brine aquifer of the Frio Formation near Houston, Texas. These time-lapse bore-hole seismic surveys, crosswell and vertical seismic profile (VSP), were acquired to monitor the C02 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 1500 m. The crosswell survey provided a high-resolution image of the C02 distribution between the wells via tomographic imaging of the P-wave velocity decrease (up to 500 mls). 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 C02 saturations of 10-20 percent from the P-wave velocity change. The VSP survey resolved a large (-70 percent) change in reflection amplitude for the Frio horizon. This C02 induced reflection amplitude change allowed estimation of the C02 extent beyond the monitor well and on 3 azimuths. The VSP result is compared with numerical modeling of C02 saturations and is seismically modeled using the velocity change estimated in the crosswell survey.