Minimizing Environmental Impacts of CO2 Storage Projects: Time-Lapse Monitoring of Fault Reactivation for Safe Sequestration (Flexible Funding 2024)

Farshad Jafarizadeh, Heriot-Watt University, was awarded funding in the UKCCSRC’s Flexible Funding 2024 call to look at “Minimizing Environmental Impacts of CO2 Storage Projects: Time-Lapse Monitoring of Fault Reactivation for Safe Sequestration”.

Climate change mitigation at the gigatonne scale depends on safe and reliable Carbon Capture and Storage (CCS). One of the key questions is straightforward: how can we be sure that the CO₂ injected deep underground will remain securely trapped? It is not enough to inject and walk away. Achieving safe, large-scale storage requires robust measurement, monitoring, and verification (MMV) strategies that can identify potential risks before they escalate. One important risk is fault reactivation, where increases in reservoir pressure may induce small slips along faults, potentially creating leakage pathways.

In our recent UKCCSRC Flexible Fund project, we investigated how time-lapse (4D) seismic monitoring can be applied not only to track CO₂ plumes, but also to detect these subtle geomechanical events before they threaten containment.

Using 4D Seismic to “Listen” for Fault Movement

Time-lapse (4D) seismic surveys work much like an ultrasound of the Earth. By comparing repeated surveys (a base survey and subsequent monitor surveys), we can detect changes in pressure and fluid saturation caused by CO₂ injection in the subsurface.

Two key types of signals are considered:

• Amplitude changes are primarily sensitive to CO₂ saturation and fluid distribution.

• Time-shift signals are differences in seismic wave arrival times that reflect velocity changes caused by pressure and saturation changes.

For this study, time-shift signals are particularly valuable because they provide a clear geomechanical response in the overburden, unaffected by fluid saturation effects in the reservoir.

Modelling Fault Reactivation and Detectability of Time-Shift Signals

To evaluate the detectability of 4D seismic signals, we applied two analytical approaches to represent the effects of CO₂ injection:

• The Geertsma solution, which links reservoir pressure changes to strain in the reservoir and surrounding rock.

• The Okada solution, which relates fault slip to strain in the fault plane and its surroundings.

From the calculated vertical strain cubes, and by integrating both effects where pressure increase acts as the triggering mechanism for fault slip, we applied a rock physics model to estimate the corresponding time-shift signals for different earthquake magnitudes. We then evaluated the minimum event size (Richter scale) detectable by different monitoring systems. For three common acquisition methods, the minimum time-shift thresholds are as follows: Permanent Reservoir Monitoring (PRM), 0.06 ms; Ocean Bottom Nodes (OBN), 0.07 ms; and Towed Streamers, 0.24 ms. These thresholds show that PRM and OBN can detect seismic events as small as magnitude 2.65 Richter, a value that can be directly linked to maximum injection pressure and injection volume, which are key parameters for ensuring safe CO₂ sequestration (shown in Fig. 1).

Fig. 1: Relationship between moment magnitude and time-shift signals for different fault lengths

North Sea Case Study

We applied our approach to the 2001 Ekofisk K-22 incident in the Norwegian North Sea, where a leaking water injector led to an unexpected pressure build-up of around 20 MPa, triggering a magnitude 4.3 seismic event on the Richter scale. This case was selected because observed data from Towed Streamer were available for validation, and the overburden time-shift signal is unaffected by fluid type, providing a clearer geomechanical response.

Our modelling results indicate that:

• Pressure effects generated a time-shift of approximately +3.3 ms
• Fault slip produced a smaller but still detectable time-shift of 0.25–0.35 ms (Fig.2 shows the modelling workflow for the time-shift signal for a fault-slip event)

These findings demonstrate that 4D seismic monitoring can capture both pressure-induced responses and fault reactivation, even when using typical acquisition methods such as Towed Streamer.

Fig. 2 Workflow for post-stack time-shift estimation using Okada modelling

Conclusion and Future Directions

Our findings show that time-lapse seismic can hear the Earth’s smallest whispers of stress and strain before they grow into larger risks. This offers operators a practical way to ensure safe, long-term CO₂ storage.

The study highlights that:

• 4D seismic is not just for plume tracking; it is also a vital tool for geomechanical safety assurance and monitoring.
• Pressure and tectonic signals often overlap in observed data; however, careful analysis of angle-dependent time-shift data (pre-stack time-shift analysis) and horizontal velocity perturbations can improve their separation. Future research should explore these approaches further.

Read more on Farshad’s Flexible Funding 2024 project page.