Otway Project: seismic monitoring
Seismic monitoring consists of generating artificial shock waves which propagate through the rock formations and reflect back to receivers where they register as a pressure pulse; these signals are digitised and processed to produce an acoustic image of the subsurface.
Seismic monitoring of the Otway Project is used to detect subtle changes associated with the presence of the injected CO2 and map the migration pathways.
Surface seismic monitoring maps the migration path of CO2 plume from injector to producer.
Geophones and source patterns are laid out and moved according to the subsurface target, yielding a 3D image of the distribution of the CO2. Subsequent surveys carried out after injection has started provide almost exactly the same image as the original survey. When one is subtracted from the other the differences become apparent and show the difference induced by the injection of CO2. This set of surveys is called 4D or time-lapse. The ability to repeat seismic surveys under similar conditions before, during and after CO2 injection is very important for showing the distribution of CO2 over time. These conditions include seismic source and receivers positioning, source signal, hardware, time of year and processing methods.
Borehole Seismic monitors changes in the reservoir over time and the vertical migration of CO2 in the vicinity of the observation well through different techniques.
High Resolution Travel Time (HRTT)
High Resolution Travel Time (HRTT) will enable us to monitor fine changes in fluid level and verify the volume of CO2 injected.
The injected CO2 is expected to rise and collect beneath the gas cap. Continuous injection will force the gas-water contact down. HRTT data will be acquired with permanently installed geophones strategically located above and below the GWC in the Naylor-1 monitoring well.
Vertical Seismic Profiling (VSP)
Vertical Seismic Profiling (VSP) provides a high resolution geological image in the immediate vicinity of the boreholes. Distinctly different applications can be achieved by varying the relative positions of the source and receivers (offset VSP, walkway VSP).
VSP is carried out at the Otway Project using seismic sources located at the surface and receivers positioned in the boreholes.
Microseismic surveys will check that the in-situ conditions of the reservoir have not lead to fractures or fault reactivation. When the subsurface is stressed natural earth tides or through the injection of CO2 it 'creaks'. Downhole geophones are installed just above the packer within the Naylor-1 well (subsurface) and geophones are placed near the top of a nearby well (surface).