Tracking fluid-induced seismicity: integrating Vp/Vs ratio variations and focal mechanism analysis for reservoir monitoring

Fluid injection activities cause pore pressure perturbations within the reservoir's rocks, which can potentially trigger fractures, faults failures, and alter the elastic properties of the surrounding rocks. Thus, monitoring stress conditions of the reservoir medium and the evolution of pore pressure around wells, is crucial for hazard assessment in injection areas.

We present a rock physics-based approach using induced micro-seismicity to track pore pressure temporal evolution from  Vp/Vs ratio variations. Additionally, focal mechanisms analysis (BISTROP, De Matteis et al., 2016; TESLA, Adinolfi et al. 2023) of microearthquakes revealed insights into local stress patterns within the host rocks, in relation to induced seismicity.

The method was applied to wastewater disposal-induced micro-seismicity detected near of the Costa Molina 2 injection well (High Agri Valley, Southern Italy) in the Val d’Agri oilfield, the largest onshore oil and gas field in Western Europe. We used as dataset the enhanced seismic catalogue obtained in the Costa Molina area by Stabile et al. 2021. It comprises 196 induced micro-earthquakes, occurred between 2016 and 2018 around the injection well. The catalogue has events magnitudes ranging between − 1.2 ≤ Ml ≤ 1.2.

Accurate arrival time measurements are essential for calculating the Vp/Vs ratio using the Wadati method. Therefore, we first refined the first P- and S-wave arrival times using waveform cross-correlation and hierarchical clustering method. Then, the Vp/Vs ratio was estimated for each source-station pair and averaged across events at the four nearest stations to the well. This allowed us to track the temporal evolution of elastic properties in the well’s surrounding region and compare it with injection parameters (i.e., injection volume and pressure). Our findings show that variations in the Vp/Vs ratio, especially for the station closest to the reservoir, closely correlate with injection parameters.

Additionally, the obtained focal mechanisms reveal strongly contrasting behaviors, ranging from strike-slip to reverse faulting. For the latter events, we identified highly anti-correlated seismic waveforms. The presence of anti-repeaters, as described by Cesca et al., 2024, has been observed in various settings and is often associated with transient stress perturbations. Since many of these phenomena have been attributed to fluid migration processes, they could provide valuable insights into subsurface fluid movements and help track their dynamics over time.

These findings demonstrate the potential of integrating accurate locations, seismic velocity monitoring and focal mechanism analysis to enhance reservoir monitoring systems. This method improves understanding of induced seismicity and offers a valuable tool for risk assessment and fluid injection management, applicable to various reservoir contexts.

References:

De Matteis R, Convertito V, Zollo A. 2016. Bayesian inversion of spectral-level ratios and P-wave polarities for focal mechanism determination. Seismol Res Lett. 87:944–954. https://doi.org/10.1785/0220150259.

Adinolfi G.M., Convertito V, De Matteis R. 2023. TESLA, A Tool for Automatic Earthquake Low‐Frequency Spectral Level Estimation: The Study of 2013 St. Gallen Earthquake Fault‐Plane Solutions. Seism ReS Lett.  94 (5): 2441–2455. https://doi.org/10.1785/0220230033

Stabile, T.A., Vlček, J., Wcisło, M. et al. Analysis of the 2016–2018 fluid-injection induced seismicity in the High Agri Valley (Southern Italy) from improved detections using template matching. Sci Rep 11, 20630 (2021). https://doi.org/10.1038/s41598-021-00047-6

Cesca, S., Niemz, P., Dahm, T. et al. Anti-repeating earthquakes and how to explain them. Commun Earth Environ 5, 158 (2024). https://doi.org/10.1038/s43247-024-01290-1