Leaking And Non-leaking Systems: Study Of Natural CO2 Accumulations For Geological Sequestration

San Francisco, California USA

The potential risks of geological CO2 storage must be understood and geologists are required to
predict how CO2 may behave once stored underground. As natural geological accumulations of
carbon dioxide occur in many basins in Italy and volcanic and seismically active areas allow CO2
rich fluids to migrate to the near surface, many of these areas have been investigated in order to
study long-term geochemical processes that may occur following geological storage of
anthropogenic CO2. A study representing an example of "leaking" system is the Solfatara crater
(Campi Flegrei, Southern Italy) characterised by the presence of both CO2 rich-waters and
fumarole. Soil gas flux measurements show that the entire area discharges between 1200 and 1500
tons of CO2 a day. Most part of analysed waters is the effect of a mixing between a shallow
meteoric water and a deep thermal Na-Cl end-member and/or seawater, resulting in sodiumchloride
waters. A high dissolved CO2 content (max value 566.28 cc/l) is also present. Furthermore,
the Campi Flegrei frequently undergo bradyseism related to the elastic response of the shallow crust
to increasing pressure within a shallow magma chamber. The study of this phenomenon could be
useful to detect ground deformation linked to geomechanical changes in a geological CO2 reservoir.
In contrast, an example of "non-leaking" system is the Pisticci oil and gas Field (Southern Italy)
where a great variety of hydrocarbons traps are formed by horst and tilted blocks in the Mesozoic
carbonate substratum covered by an almost continuous sequence of Lower Pliocene marls and
Middle Pliocene-Pleistocene marly blue clays. Soil gas surveys were performed after a MD 4.5
earthquake and two years later to test the permanence of the gas distribution pattern. CO2
distribution in soil gas seems not to be affected by changes in stress, as suggested by the average
values of both surveys. The principal aim of our research has been to evaluate and mitigate risks for
local populations as the studied areas are densely populated. To date, the obtained results suggest
that gas uprising is generally well localised around restricted areas, often controlled by local
tectonics (faults and/or fractures). This implies that, in the frame of geological CO2 sequestration, it
is necessary to carefully assess the presence of pathways (fault and/or fractures) that might allow
the migration of CO2 out of the reservoir.