Analyzed the relationship between the micro-seismicity induced by the re-injection of layer waters associated with the extraction of hydrocarbons and the faults present in the Val d'Agri subsoil. The results of the study, conducted by INGV, have been published in Scientific Reports
For the first time in Italy, a team of seismologists from the National Institute of Geophysics and Volcanology (INGV) was able to study the relationships between the micro-seismicity induced by the re-injection of layer waters associated with the extraction of hydrocarbons and the faults present in the subsoil of Val d'Agri (Basilicata, province of Potenza). The objective of the research "Inversion of inherited thrusts by wastewater injection induced seismicity at the Val d'Agri oilfield (Italy)", published in Scientific Reports (editorial group of Nature), was to characterize the geological structure of the subsoil outlined by distribution of induced seismicity observed around the Costa Molina 2 re-injection well, in Val d'Agri, through the integration of a large amount of subsurface data for oil exploration (reflection seismic and deep well data) and of local micro-seismicity recorded from 2001 to 2014 by INGV permanent and temporary stations and by the ENI monitoring network. The analysis was conducted as part of an ENI-INGV research project, aimed at studying the seismicity of the Val d'Agri, and responds to a prescription from the Basilicata Region relating to the re-injection activities in the Costa Molina 2 well.
“Val d'Agri”, explains Mauro Buttinelli, INGV researcher and coordinator of this work, “is home to the largest European hydrocarbon field on dry land. Starting from June 2006, the stratum waters associated with the extraction of hydrocarbons are re-injected into a marginal and non-productive area on the south-eastern side of the oil field through the Costa Molina 2 well, in the same rocks that make up the origin of the extracted hydrocarbons, at a depth of about 4 km. The re-injection of layer waters induces seismicity with a maximum magnitude of 2.2 (Fig.1 of the article), which occurred from the first hours following the start of the injection activity”.
The study shows that the induced microearthquakes, located between 2 and 5 km deep in the immediate vicinity of the Costa Molina 2 reinjection well, are mostly concentrated in the injection reservoir (consisting of fractured carbonate rocks) on a blind reverse fault with a slope of about 50° towards the North-East, belonging to a pre-existing fault system (Fig. 2a of the article). The faults identified using the available dataset do not appear to have relationships with the normal faults delimiting the north-eastern edge of the Val d'Agri Quaternary basin.
"The physical mechanism that is proposed to explain the observed induced microseismicity", continues Buttinelli, "is that of the reactivation with extensional kinematics of small faults and fractures (with a maximum length of 100-200m) belonging to an area with high permeability of the fault outlined by the alignment of micro-earthquakes. Small faults and reactivated fractures are those favorably oriented within the current extensional stress field. Reactivation is induced by the increase in fluid pressure in the injection tank. In summary, with this study it was possible to define that the re-injection of fluids has reactivated some portions of a pre-existing fault".
Finally, the authors underline that in hydrocarbon exploitation areas, where fluid re-injection is in progress, the correct understanding of the relationships between the pre-existing tectonic discontinuities and any induced seismicity is a fundamental requirement for the safe management of field cultivation. "In this context", concludes the researcher, "the close collaboration between the world of research, the Control Bodies (MiSE, Regions, ARPA) and the oil companies is an element of great importance for the characterization and monitoring of seismicity, be it natural or induced.
The article is available in Open Access format starting from 11 am on 14 November 2016 on the website of the journal Scientific Reports (editorial group of Nature), at the addresses: link1 e link2.
Authors: Mauro Buttinelli, Luigi Improta, Samer Bagh and Claudio Chiarabba
Fig.1: Simplified geological diagram of the upper Val d'Agri (A) and summary geological section of the subsoil structure of the area (B). In evidence, the induced seismicity located around the CM2 re-injection well.
Fig. 2a: interpretation of a depth-converted reflection seismic profile passing through the CM2 injection well. The blue dots represent the hypocenters of induced seismicity between 2006 and 2014 projected onto the interpreted section. The green star represents the largest magnitude event (M2.2) that occurred on 26/10/2010. The focal mechanism calculated for that event is reported, consistent with most other events.
Extended
Inversion of inherited thrusts by wastewater injection induced seismicity at the Val d'Agri oilfield (Italy)
M. Buttinelli, L. Improta, S. Bagh & C. Chiarabba
Since 2006 wastewater has been injected below the Val d'Agri Quaternary basin, the largest on-land oilfield in Europe, inducing micro-seismicity in the proximity of a high-rate injection well. In this study, we have the rare opportunity to revise a massive set of 2D/3D seismic and deep borehole data in order to investigate the relationship between the active faults that bound the basin and the induced earthquakes. Below the injection site we identify a Pliocene thrusts and back-thrusts system inherited by the Apennine compression, with no relation with faults bounding the basin. The induced seismicity is mostly confined within the injection reservoir, and aligns coherently with a NE-dipping back-thrust favorably oriented within the current extensional stress field. Earthquakes spread upwards from the back-thrust deep portion activating a 2.5-km wide patch. Focal mechanisms show a predominant extensional kinematic testifying to an on-going inversion of the back-thrust, while a minor strike-slip compound suggests a control exerted by a high angle inherited transverse fault developed within the compressional system, possibly at the intersection between the two fault sets. We stress that where wastewater injection is active, understanding the complex interaction between injection-linked seismicity and pre-existing faults is a strong requisite for safe oilfield exploitation.