A detailed analysis of micro-seismicity induced by a high-rate wastewater disposal well in the Val d’Agri oilfield (Italy)

Abstract:

The Val d'Agri basin in the Apennines extensional belt hosts the largest oilfield in onshore Europe. High-quality recordings from a temporary dense network unravel a swarm of 111 small-magnitude events (M_L ≤ 1.8) occurred in June 2006 during the first stage of wastewater injection into a high-rate well. 3D absolute locations and high-precision relative locations define a high-angle fault located 1 km below the well inside fractured and saturated carbonates where wastewater is re-injected. Seismicity begins 3 hours after the initiation of injection. The seismicity rate strictly correlates with injection curves and temporal variations of elastic and anisotropic parameters. Seismicity is induced by rapid communication of pore pressure perturbations along a high permeability fault-zone favorably oriented with respect to the extensional stress field. The spatiotemporal distribution of events agrees with an isotropic hydraulic diffusivity of 0.8 m²/s, which corresponds to high permeability values on the order of 10^-13 m²/s. Such high permeability values are coherent with hydraulic well-tests in the very productive hydrocarbon reservoir and with the presence of a widespread system of open and conductive fractures in the carbonates that strike NW–SE parallel to the maximum horizontal stress of the extensional stress regime. Over the following 8 years, 235 events (M_L ≤ 2.2) were recorded within 5 km of the disposal well by permanent stations of the local operator network and of Istituto Nazionale di Geofisica e Vulcanologia. Accurate 3D locations concentrate on the fault and unravel that after June 2006 micro-seismicity migrated upwards and downwards along the fault measuring 5-km along dip. Subsurface data show that the fault is a pre-existing blind back-thrust of an inherited fold-thrust system developed in the carbonate reservoir during Pliocene-Early Pleistocene. The seismicity rate correlates with short-term increases in injection pressure of the disposal well.