Feasibility of Time-Lapse Seismic Methods in Monitoring CO2 Injection in the Fly Lake Field, Cooper Basin
Honours Degree, 2003
Unversity of Adelaide
Fluid pressures in the Tirrawarra Sandstone reservoirs of the Fly Lake Field have dropped from 4200 psi to less than 1700 psi due to 20 years of production. This has lead to an increase in effective stress on the rock fabric and a decline in oil recovery. Injecting carbon dioxide into this depleted reservoir will enhance oil recovery as it increases reservoir pressure and oil mobility. This study analyses the feasibility of using time-lapse seismic to observe, and hence monitor, the progress of a CO2 flood.
The success of time-lapse seismic methods to monitor the progress of a miscible CO2 flood is determined by the magnitude of the changes in elastic properties of the reservoir, and their visibility on a seismic profile. Seismic parameters of the Tirrawarra Sandstone were modelled for seven different fluid saturation and pressure scenarios including the virgin case and six hypothetical cases based on possible outcomes due to reservoir depletion or CO2 injection. Theoretical reflectivity calculations suggest that the percentage change in the reflection coefficient of the Tirrawarra Sandstone is approximately 5 % when a virgin reservoir model is pressure depleted or the depleted reservoir model is completely repressurised with CO2. An injection program resulting in a sweep efficiency of 100 % is in practice impossible. Therefore, the time-lapse seismic response of any realistic scenario will be smaller, and is unlikely to be detectable. Elastic wave synthetic seismograms reinforce this conclusion, particularly when wave propagation effects are included to mimic the complexities of field data. Multiples, mode converted waves and transmission losses mask the small time-lapse effects produced by the fluid and pressure variations, rendering the time-lapse seismic responses indistinguishable from each other.
Technical risk analysis performed on the Fly Lake Field suggests that current seismic and field parameters result in a failure of the criterion for successful time-lapse seismic analysis to be performed. The reservoir is simply not suitable for time-lapse seismic due to the depth of the Tirrawarra Sandstone and its poor reservoir quality.
This study suggests that repeated 3D seismic surveys for the purposes of time-lapse seismic reservoir monitoring are not required and would be an unnecessary expenditure in the Fly Lake Field.