Monitoring pore-pressure from Vp/Vs ratio around the Costa Molina 2 wastewater disposal well in southern Italy

Monitoring conditions of the medium embedding the reservoir is strictly required for the hazard assessment in exploited areas.

Fluid injection/extraction operations cause a pressure perturbation into the volume hosting the reservoir which, in turn, may trigger new failures and induce changes in the elastic properties of rocks. Therefore, technologies are needed to reconstruct pore-pressure evolution around injection wells.

To test how the conditions of the reservoir can be monitored noninvasively by using induced micro-seismicity, here we show a rock physics approach aimed to reconstruct the pore-pressure temporal evolution from the changes in Vp/Vs ratio.  

We applied this strategy to the volume affected by the wastewater disposal activity of the Costa Molina 2 injection well, located in the High Agri Valley (Southern Italy) and belongs to the Val d’Agri oilfield, the largest productive onshore oil field in West Europe that produces hydrocarbons (oil and gas) from a fractured carbonate reservoir. We analyzed an enhanced seismic catalogue of the induced micro-seismicity, occurred between 2016 and 2018, that consists of 196 located earthquakes in the magnitude range − 1.2 ≤ Ml ≤ 1.2. For the same period, both seismicity recordings and fluid-injection data are available.

For the evaluation of Vp/Vs ratio with the Wadati formula, the accurate measure of arrival time is critical, especially in case of micro-events. So, we first refined with high precision the first P- and S-wave arrival times by using waveform cross-correlation and hierarchical clustering and selected the events with a high DD location quality; then, we calculated the Vp/Vs ratio for each source-station couple and averaged the ratio values for all the events at the stations nearest to the well (INS1, INS2, INS3) to reconstruct the elastic properties temporal evolution in the source region around the well. 

We found that the Vp/Vs ratio temporal evolution well correlates with injection operational parameters (i.e. injected volumes and injection pressures). With a rock physics model, by using the Pride approach of the Biot theory, we reconstruct the pore-pressure temporal variation starting with the Vp/Vs as known parameters, thus demonstrating the value of seismic velocity monitoring as a tool to complement a monitoring system.