Mineralogy and microporous structure of rocks from a natural CO2 reservoir

Different experimental approaches have been combined to reconstruct the mineral association and microporous structure of rocks from a natural CO2 reservoir. The fluid reservoir (Caprese Reservoir), was discovered while drilling PSS1 (Pieve Santo Stefano 1) wellbore in San Cassiano Basin (Eastern Tuscany, Central Italy, and consists of sedimentary rocks (Burano Fm.) interbedded with altered volcanic rocks, its depth being about 3,300 m with respect to the land surface. At 3,700 m depth fluid pressure and temperature are 700 bar and 120 °C respectively, with a density for the supercritical CO2–rich fluid of 840 Kg·m-3. The study was conducted on the volcanic rocks altered by CO2 from the PSS1 wellbore drillcores and on some volcanic rocks unaffected by the presence of CO2. Lastly, rocks from the Burano Formation, unavailable from PSS1, have been sampled on outcrop. Focus is on rocks samples corresponding to the depth 3,864-3,871 m with respect to PSS1 log, which have been investigated with SEM-EDS and XRD for mineralogical characterization. Moreover, Small Angle Neutron Scattering (SANS) experiments at LLB (Saclay, France) served for microporous structure investigation of PSS1 rocks, and other volcanic rocks from Eastern Alps (IG1) and the Albani Hills (IG2 and IG3) unaffected by CO2, as well as Burano Formation rocks from outcrop.