Gas blowout from shallow boreholes at Fiumicino (Rome): Induced hazard and evidence of deep CO2 degassing on the Tyrrhenian margin of Central Italy
Introduction
Central Italy, North and South of Rome, is characterized by the presence of two Quaternary volcanoes (e.g. Mts. Sabatini and Alban Hills, Fig. 1). This area, as the entire Tyrrhenian hinterland, has a thinned continental crust (20–25 km) and high heat fluxes (> 80 mW m− 2) (Gambardella et al., 2004). It is also characterized by a very strong degassing of CO2 of deep provenance, as estimated from the balance and the δ13C of carbon dissolved in regional aquifers (Chiodini et al., 2000, Chiodini et al., 2004, Gambardella et al., 2004 and references therein). It has also been suggested that the Apennine seismicity could be driven by high pressure CO2 sources at depth, i.e. crustal traps with overpressurized CO2 reservoirs (Miller et al., 2004, Chiodini et al., 2004).
The carbon dioxide rising from depth, whose origin is controversial, dissolves into aquifers, geothermal or not, hosted in buried Mesozoic limestones and in shallower cold aquifers hosted either in Neogene clastic sediments and in the Quaternary volcanic rocks. All these aquifers release CO2-rich gases toward the surface mostly along extensional fractures and faults, originating many discrete gas manifestations or zones of high CO2 diffuse emission from the soil. The quantity of CO2 released into the atmosphere is locally so high to represent a serious hazard to people and animals (Rogie et al., 2000, Chiodini and Frondini, 2001, Carapezza et al., 2003, Carapezza et al., 2005). In zones where there is no significant CO2 release at the surface because of the presence of an efficient cover of impervious rocks, the existence at various depth of pressurized CO2 pockets has been revealed by accidental and dangerous gas blowout during drilling (Carapezza and Tarchini, 2007).
In February 2005 a similar accident, that only by chance did not cause human casualties, occurred at Fiumicino, on the delta of Tiber river (Fig. 1), in a zone where no surface gas emission was known. The aim of this paper is twofold: i) to describe this accident, its related hazard and the activity carried out to control the gas emission and to restore safety conditions and ii) to use chemical and isotopic data of the emitted gas to show its endogenous deep origin, so enlarging the surface of Central Italy with evidence of strong CO2 Earth degassing.
Section snippets
Geological setting
Fiumicino is located at the mouth of Tiber river, SW of Rome (Fig. 1). Since Middle–Late Pliocene active extensional tectonic processes, with main fractures oriented NW–SE and NE–SW, controlled the evolution of the Tyrrhenian margin of Central Italy, generating structural highs and lows cut in Mesozoic limestones and their allochthonous flysch cover (Di Filippo and Toro, 1995). An extensive Quaternary alkaline–potassic volcanism characterizes the region, forming the so-called Roman Comagmatic
The Fiumicino accidental gas blowout and the remediation intervention
The borehole (F1 in Fig. 2) was located near an electric cabin. Drilling started on 9 February 2005 at 9 a.m., to install a conductive ground electrode inside a clay level to be found underneath sandy superficial layers. At 2 p.m., at about 27 m depth, an abrupt decrease of the electrical resistivity from 25 to 0.5 Ω m was observed, and the drillers presumed to have reached the clay. Drilling continued for another meter, and then a 2″ steel pipe was installed inside the borehole. At 3 p.m. the
Gas composition and origin
Four gas samples were collected from the Fiumicino boreholes. Gas mostly consisted of CO2 (97–98 vol.%) with minor N2 (1.6 vol.%) and CH4 (0.75–1.49 vol.%) and only a few ppm of H2S. Such a composition falls within the variation field of the cold gas manifestations of Central Italy and particularly to that of the gases emitted from the near volcanic areas of Alban Hills and Mts. Sabatini (Table 4). The latters have higher H2S content, frequently around 1 vol.%; hydrogen sulphide was likely
Conclusions
The accidental gas blowout occurred at Fiumicino boreholes in February 2005 significantly enlarges the region of Central Italy known to be characterized by a high level of deep CO2 degassing (Chiodini et al., 2004, Gambardella et al., 2004). The gas surface manifestations of this area, as well as the CO2 pressurized pockets encountered at various depths by wells (Carapezza and Tarchini, 2007), are mostly concentrated on structural highs, i.e. on buried horsts of the Mesozoic limestones or on
Acknowledgements
The Civil Protection Office of the Province of Roma provided the Draeger devises and the permanent CO2 flux station. The Fiumicino Municipality and ACEA Company are thanked for the field assistance. An intense technical collaboration was held with Ing. Alberto Bottai who supervised the squeezing operations. This study has been carried out with the support of the INGV-DPC programme (Project V5-Diffuse degassing in Italy — Research units coordinated by F. Barberi and M.L. Carapezza). The chemical
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