We report here, for the first time, evolution of the streaming potential coupling coefficient as liquid carbon dioxide infiltrates Berea sandstone. Using 125 Omega-m tap water, the coupling coefficient determined before and after each CO2 flood of five samples averaged approximately -30 mV/0.1 MPa. After liquid CO2 passed through the specimens displacing all mobile pore water, trapped water remained and the coupling coefficient was approximately -3 mV/0.1 MPa. A bound water limit of the coupling coefficient for liquid CO2 flow was foundusing an air-dried sample to be -0.02 mV/0.1 MPa. For initially water-saturated samples, bulk resistivity varied during CO2 invasion from 330 Ohm-m, to 150 Ohm-m during CO2/water mixing, to a final value of 380 Ohm-m. Results suggest that trapped and bound water control electrical conduction and the electrokinetic response. Applications include monitoring CO2 injectate in subsurface reservoirs using the self potential method.

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.

As part of a small scale sequestration test (about 1500 tons of CO2) in a saline aquifer, time-lapse borehole seismic surveys were conducted to aid in characterization of subsurface CO2 distribution and material property changes induced by the injected CO2. A VSP survey demonstrated a large increase (about 75 percent) in seismic reflectivity due to CO2 injection and allowed estimation of the spatial extent of CO2 induced changes. A crosswell survey imaged a large seismic velocity decrease (up to 500 m/s) within the injection interval and provided a high resolution image of this velocity change which maps the subsurface distribution of CO2 between two wells. Numerical modeling of the seismic response uses the crosswell measurements to show that this small CO2 volume causes a large response in the seismic reflectivity. This result demonstrates that seismic detection of small CO2 volumes in saline aquifers is feasible and realistic.