Seismic Monitoring

There are several seismic techniques that can be used to monitor CO2 storage sites:

• Seismic Reflection Monitoring

  Seismic techniques measure the speed and energy adsorption of waves through rocks, which depends on the nature of the rock and fluids contained within the rock. This can be used to track the migration of CO2 within a reservoir over time by taking a series of surveys, provided that a background survey is undertaken before CO2 injection has been carried out. Small volumes of free phase CO2 can be detected in this way (as low as 5%) provided that there is a strong density contrast between the CO2 and the original fluids in the reservoir. If the CO2 density is similar to that of the original reservoir fluids then much less sensitive detection limits exist. Seismic can not detect CO2 dissolved in water.

• Passive Seismicity Monitoring

  This method uses micro seismic events (e.g. small earthquakes from cracks caused by CO2 injection) within the reservoir as the wave generating source. Waves from the mini earthquakes are picked up by an array of sensors located within the reservoir (often in old wells) and allows monitoring of stress and pressure changes within the reservoir and potentially gas movement.