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Detailed petrophysical and geophysical characterization of core samples from the potential caprock-reservoir system in the Sulcis Coal Basin (South-Western Sardinia - Italy).
S. Fais (1,2)
, P. Ligas (1)
, F. Cuccuru (1)
, Enrico Maggio (3)
, Alberto Plaisant (3)
and Alberto Pettinau (3)
(1) Dipartimento di Ingegneria Civile, Ambientale e Architettura (DICAAR), University of Cagliari, Via Marengo 2 – 09123 Cagliari, Italy. (2) Consorzio Interuniversitario Nazionale per l’Ingegneria delle Georisorse (CI.NI.Geo) Corso Vittorio Emanuele II – 00186 Roma, Italy.
(3) SOTACARBO S.p.A. (Grande Miniera di Serbariu ) 09010 – Carbonia, Italy.
Dipartimento Ingegneria Civile, Ambientale e Architettura University of Cagliari - Italy
Consorzio Interuniversitario Nazionale per l’Ingegneria delle Georisorse - Italy
Abstract: The need to minimize the amount of CO2 in the atmosphere is greater and greater and consequently sequestration of industrial emissions in adequate geologic formations is a present-day necessity. As is known, CO2 cannot be injected anywhere in the
subsurface, but geological formations with the appropriate requisites must be identified. The evaluation of the CO2 geologic storage site requires a robust experimental database especially with respect to spatial petrophysical heterogeneities. The integrated analysis of minero-petrographical, physical and geophysical parameters (e.g. longitudinal and transversal propagation velocity, Vp/Vs ratio, dynamic elastic moduli, etc.) of the rocks that make up a caprock-reservoir system can substantially reduce the geologic uncertainity in the storage site characterization and in the geological and numerical modelling for the evaluation of the CO2 storage capacity. In this study the Middle Eocene – Lower Oligocene Cixerri Formation made up of siliciclastic rocks and the Upper Thanetian - Lower Ypresian Miliolitico Carbonate Complex in the Sulcis Coal Basin (South-Western Sardinia - Italy) have been identified respectively as potential caprock and reservoir for the CO2 storage. The petrographical, physical and geophysical parameters of the above mentioned geological Formations (Cixerri and Milolitico) were investigated to improve the geological model aimed at verifying the geological CO2 storage capacity within the carbonate reservoir rocks, in order to guarantee an efficient use of the reservoir, and to improve the numerical simulation of CO2 behaviour in the short, medium and long term after its injection in single or multiple wells.
Fig. 1 – Location of the investigated area
Fig. 2 – Left: Stratigraphic scheme of the Tertiary succession of the
Sulcis Coal Basin (SW Sardinia – Italy), right: schematic set up of
the caprock-reservoir system.
CAPROCK-RESERVOIR SYSTEM FORMATIONS
The Middle Eocene – Lower Oligocene Cixerri Formation made
up of terrigenous continental unit of siltites, sandstones and
subordinated conglomerates and the Upper Thanetian - Lower
Ypresian Miliolitico Carbonate Complex in the Sulcis coal basin
(South-Western Sardinia - Italy) have been identified
respectively as potential caprock and reservoir for the CO2
storage. The carbonate reservoir (“Miolitico” Fm., saline aquifer)
and caprock (Cixerri Fm.) characterization carried out mainly by
petrophysical and seismic investigations was aimed to provide
experimental data useful for the subsequent geological
modelling.
Fig. 3 – Petrographical and physical analyses for the characterization of caprock-reservoir system.
CHARACTERIZATION OF CAPROCK-
RESERVOIR SYSTEM The petrographical characteristics of the caprock-
reservoir rocks were determined by optical and
SEM analyses of core samples representing the
different facies of the Cixerri Formation and of the
Miliolitico Carbonate Complex, provided by
Carbosulcis S.p.A.. Porosity analysis was
completed by mercury porosimeter
determinations which also provided quantitative
information on the permeability of the study rocks
and on the tortuosity of their pore system (Fig. 3).
Further physical properties, such as dry and
saturated density and porosity, and water
absorption were determined on the cylindrical
core samples of intact rocks (ISRM, 1979) from
wells drilled in the northern part of the Sulcis Coal
Basin (Nuraxi Figus area) (Fig.1). The
propagation velocity of longitudinal (Vp) and
transversal (Vs) waves was also determined on
the same samples by a portable ultrasonic non-
destructive digital indicating tester (P.U.N.D.I.T.
plus) (ISRM, 1978).
Starting from the P and S wave velocity, the
dynamic elastic moduli (Young modulus, bulk
modulus and Poisson’s ratio) were determined
using the well-known relationship involving the
longitudinal (Vp) and shear wave velocity (Vs) and
the rock bulk density.
Fig.5 – Comparative analysis between petrographic features and physical
properties obtained from laboratory and in situ measurements (mining tunnels
and reflection seismic data) on the caprock-reservoir rocks.
Concluding remarks: The physical characteristics of the possible caprock (Cixerri Fm.
Auct.) and reservoir (Miliolitico Fm. Auct.) have been analyzed in the light of their
petrographical characteristics and in particular of their texture. The primary porosity of the
reservoir is not very high. Therefore this rock takes on characteristics of reservoir to the
purposes of geological storage of CO2 particularly in light of the locally high
dolomitization and widespread fracturing. The seismic data interpretation allowed to
discriminate the carbonate unit (“Miliolitico” Formation) within the lower stratigraphical
series of the northern part of the Sulcis Coal Basin when combined with the well data and
petrophysical information. All the experimental data have made a significant contribution
to a realistic geological modelling and subsequent numerical modelling.
Fig. 4 – (a) NUFI 4 seismic section, (b) Interpretation of NUFI 4 seismic section. As an example in the tables
we report some of the calculated dynamic parameters along the seismic line: ζ (Fracture coefficient), Φdin
(Porosity), IQdin (Quality Index) and the thickness of the reservoir (Hb).
SEISMIC ANALYSIS INTEGRATED PETROGRAPHYCAL, PHYSICAL
AND
SEISMIC ANALYSES
Acknowledgements
We thank Carbosulcis S.p.A. for supplying the core samples used in this study.
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ELASTIC PROPERTIES
(Vp and Vs, Vp/Vs)
SEISMIC ATTRIBUTES (i.e. amplitude, frequency,
polarity, A.I.) “Miliolitico” limestone Well 6-79
Measurement of elastic properties of the intact rock samples of “Miliolitico” limestone and of the “Cixerri Formation” clastic rocks
Analysis of Western Seismic lines
Analysis of textures and of petrographic characteristics of intact rock samples
CARBONATE COMPLEX
POROSITY
Optical Microscopy ≤ 2%
Electron Microscopy (SEM)
1% – 4%
Mercury Porosimetry 0.3% – 6%
Laboratory measurements
(UNI 9724/7 guideline) 0.4% – 4.4%
Dynamic measurements
(in situ measurements: mining tunnels and seismic analysis)
7% – 25%
CARBONATE COMPLEX
Fracture coefficient (ζ)
Dynamic measurements
(in situ measurements: mining tunnels and seismic analysis)
0.16 – 0.45
NUFI – 4 LINE
INTERPRETATIVE SECTION
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