Monitoring the Restart of a High-Rate Wastewater Disposal Well in the Val d'Agri Oilfield (Italy)

The Val d'Agri Quaternary basin in the Southern Apennine range of Italy hosts the largest inland oil field in Europe. Wastewater coming from the oil exploitation is re-injected by a high-rate disposal well into strongly fractured limestones of the hydrocarbon carbonate reservoir. Disposal activity has induced micro-seismicity since the beginning of injection in June 2006. Around 220 small magnitude events (ML < 2.3) were recorded between 2006 and 2013 by the trigger-mode monitoring local network managed by the oil company and by the National Seismic Network of Istituto Nazionale di Geofisica e Vulcanologia. The induced micro-seismicity illuminated a pre-existing high-angle fault located 1 km below the well. Since June 2006, wastewater has been re-injected with only short interruptions due acid stimulations. In January 2015 disposal activity was halted due to technical operations in the oil refinery and wastewater injection restarted after two weeks. We installed 5 short-period stations within 10 km of the disposal well to carefully monitor the re-start phase and the subsequent 3 months of disposal activity. This temporary network was complemented by stations of the National Seismic Network giving this final configuration:9 stations within 10 km of the well with the closest station 2 km apart, 13 stations within 20 km. Here we report on the preliminary analysis of the local earthquake recorded during the survey focusing on the events occurred in the injection area. The seismicity rate is compared with injection data.In spite of the dense network, we found that the rate of induced seismicity (both the number and energy of events) is very low when compared to the seismicity recorded during the first 5 years of injection activity carried out with comparable rate and pressure.