Lawrence Berkeley National Laboratory

A series of time-lapse seismic cross well and single well experiments were conducted in a diatomite reservoir to monitor the injection of CO2 into a hydrofracture zone, using P- and S-wave data. During the first phase the set of seismic experiments were conducted after the injection of water into the hydrofrac-zone. The set of seismic experiments was repeated after a time period of 7 months during which CO2 was injected into the hydrofractured zone. The issues to be addressed ranged from the detectability of the geologic structure in the diatomic reservoir to the detectability of CO2 within the hydrofracture. During the pre-injection experiment, the P-wave velocities exhibited relatively low values between 1700-1900 m/s, which decreased to 1600-1800 m/s during the post-injection phase (-5%). The analysis of the pre-injection S-wave data revealed slow S-wave velocities between 600-800 m/s, while the post-injection data revealed velocities between 500-700 m/s (-6%). These velocity estimates produced high Poisson ratios between 0.36 and 0.46 for this highly porous (~ 50%) material. Differencing post- and pre-injection data revealed an increase in Poisson ratio of up to 5%. Both, velocity and Poisson estimates indicate the dissolution of CO2 in the liquid phase of the reservoir accompanied by a pore-pressure increase. The results of the cross well experiments were corroborated by single well data and laboratory measurements on core data.