Research objectives and hypotheses

Seismic techniques provide the key monitoring capability for verifying site performance at the wide spatial scales required (tens of square kilometres and upwards) and will form the core of the proposal. Our industrial partners, BP and Statoil, have provided access to monitoring datasets from the three global large-scale CO2 storage operations: Sleipner, In Salah and Snøhvit. These include active (time-lapse 3D, multi-azimuth data) and passive seismics, ground displacements from satellite interferometry and reservoir pressure measurements. Monitoring results from these sites are outlined briefly below.

Sleipner has stored more than 14 million tonnes of CO2 in the Utsira Sand, a very large, shallow, saline aquifer that perhaps typifies the 'perfect' reservoir in terms of storage capacity and monitorability (Arts et al., 2008). The CO2 plume shows striking reflectivity (Figure 1a) and large time shifts. High resolution mapping of individual spreading CO2 layers has been history matched against both numerical (Chadwick and Noy, 2010), and analytical flow models (Bickle et al., 2007). More recently, statistical analysis of very small time shifts has been used to constrain pressure increase in the reservoir (Chadwick et al., 2012).

At In Salah, more than three million tonnes of CO2 have been stored in a complex fractured aquifer. Seismic time shifts are evident, aligned to likely fracturing, but diagnosis of these, in terms of fluid saturation or pressure changes, is difficult using conventional analysis. Passive seismics have shown evidence of induced micro-seismicity and satellite interferometry has revealed striking ground displacements above the injection wells (Figure 1b) that have been related to mechanical inflation and fracturing of the reservoir (e.g. Morris et al., 2011).

Snøhvit has stored 1.2 million tonnes of CO2 in a very deep (approx. 2800 m), faulted aquifer. However, during injection, downhole pressures rose significantly and injection was terminated in the preferred storage reservoir (Gilding et al., 2012). Intriguing time-lapse 3D seismic time shifts and reflectivity changes appear to show both CO2 saturation and pressure signatures in the reservoir (Figure 1c). Discriminating between these effects remains challenging.

Sleipner, In Salah and Snoøhvit


Seismic responses, essentially signal amplitude changes, frequency changes and time shifts, arise from a larger number of causative processes. Disentangling these, particularly fluid saturation and pressure effects, is notably challenging. Our research plans to improve discrimination between and characterisation of fluid changes, pressure changes and induced geomechanical effects, and rests upon four key hypotheses:

  1. hydromechanical modeling of CO2 injection can be used to predict changes in seismic properties, induced seismicity and ground displacements and fracture initiation
  2. statistical analysis of very small seismic time shifts can be used to detect and map pressure changes over a range of potential storage reservoir types
  3. azimuthal variation in seismic attributes can be used to map fracture patterns and fluid saturation changes within storage reservoirs
  4. the thickness, velocity and saturation properties of thin layers of CO2 can be constrained by simultaneous application of different analytical procedures