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