Reservoir Structure and Wastewater‐Induced Seismicity at the Val d'Agri Oilfield (Italy) Shown by Three‐Dimensional V_p and V_p/V_s Local Earthquake Tomography

Abstract

Wastewater injection into a high‐rate well in the Val d'Agri oilfield, the largest in onshore Europe, has induced swarm microseismicity since the initiation of disposal in 2006. To investigate the reservoir structure and to track seismicity, we performed a high‐spatial resolution local earthquake tomography using 1,281 natural and induced earthquakes recorded by local networks. The properties of the carbonate reservoir (rock fracturing, pore fluid pressure) and inherited faults control the occurrence and spatiotemporal distribution of seismicity. A low‐Vp, high‐Vp/Vs region under the well represents a fluid saturated fault zone ruptured by induced seismicity. High‐Vp, high‐Vp/Vs bumps match reservoir culminations indicating saturated liquid‐bearing zones, whereas a very low Vp, low Vp/Vs anomaly might represent a strongly fractured and depleted zone of the hydrocarbon reservoir characterized by significant fluid withdrawal. The comprehensive picture of the injection‐linked seismicity obtained by integrating reservoir‐scale tomography, high‐precision earthquake locations, and geophysical and injection data suggests that the driving mechanism is the channeling of pore pressure perturbations through a high permeable fault damage zone within the reservoir. The damage zone surrounds a Pliocene reverse fault optimally oriented in the current extensional stress field. The ruptured damage zone measures 2 km along strike and 3 km along dip and is confined between low permeability ductile formations. Injection pressure is the primary parameter controlling seismicity rate. Our study underlines that local earthquake tomography also using wastewater‐induced seismicity can give useful insights into the physical mechanism leading to these earthquakes.

Large Earthquakes Driven by Fluid Overpressure: The Apennines Normal Faulting System Case

“…Increasing pore pressure is seen as a valid triggering mechanism for earthquakes (Raleigh et al, 1976; Stein, 1999), and fault lubrication is well documented in laboratory experiments (De Paola et al, 2011; Di Toro et al, 2011; Hainzl et al, 2006; Reches & Lockner, 2010; Sleep & Blanpied, 1992). Fluid diffusion can assist the onset and development of aftershocks and complex seismic sequences (Miller et al, 2004; Nur & Booker, 1972; Zhao et al, 2015) and is invoked at various scales to explain seismicity migration (Parotidis et al, 2003; Shapiro et al, 2003), changes in seismicity rate (Hainzl & Ogata, 2005; Llenos & Michael, 2016), and fault reactivation during fluid injection (Guglielmi et al, 2015; Improta et al, 2017; Peterie et al, 2018; Zoback & Harjes, 1997). High pore pressure in proximity of mainshock hypocenters is inferred from indirect imaging of the fault (Zhao et al, 1996; Chiarabba & De Gori, 2009), while in induced seismicity cases correlation with industrial data and modeling provide validation of fluid‐governed processes (Keranen et al, 2014; Yeck et al, 2016; Yu et al, 2019).…”

High Resolution Attenuation Images From Active Seismic Data: The Case Study of Solfatara Volcano (Southern Italy)

“…In that way we take into account both the resolution of single node (RDE and DWS) and the associated possible smearing effect (Sj). Therefore, for small to medium-sized linear or linearizable inverse problems, the calculation of resolution matrix is relatively straight forward, and has been frequently used as a measure of solution reliability in elastic and anelastic tomography (Rietbrock, 2001;Bisrat et al, 2013;Amoroso et al, 2014;Improta et al, 2017). Moreover, Rawlinson and Spakman (2016) clearly underlined that methods that quantify solution reliability explicitly, like calculation of full resolution matrix, should be preferred to sensitivity analysis in resolution assessment of a tomographic image.…”

How do Local Earthquake Tomography and inverted dataset affect earthquake locations? The case study of High Agri Valley (Southern Italy)

“…The main aims were to characterize the crustal structure of the area and the different kinds of seismicity involving the HAV. These goals were achieved by means of local earthquake tomographies (Valoroso et al 2011;Improta et al 2017), earthquake relocations, focal mechanisms and statistical analyses (Valoroso et al 2009;Stabile et al 2014aStabile et al , 2014bImprota et al 2015;Stabile et al 2015;Telesca et al 2015;Buttinelli et al 2016;Improta et al 2017) and joint Vp/Vs and attenuation studies (Wcisło et al 2018).…”