Physical-chemical characteristics of CO2 altered rocks

Carbon capture and storage (CCS) is considered a valid option to reduce the CO2 concentration in atmosphere. The gas is collected from industrial plants, then injected and stored underground in geological reservoirs. A very wide range of information about chemical-physical characteristics of the sequestration site is therefore needed and, among them, data on porosity microstructure are crucial. Part of the processes which affect micro-porosity and act during and after the rock formation can be reconstructed by studying micro-porosity features. In this work, different experimental approaches have been combined to reconstruct the mineral association, bulk chemistry and microporous structure of rocks from a natural CO2 reservoir (which represents a natural analogue of a CCS site). The fluid reservoir, i.e. Caprese Reservoir, was discovered while drilling PSS1 borehole 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 bars and 120 °C respectively, with a calculated density for CO2 of 860 Kg·m-3. The study was conducted on the volcanic rocks altered by CO2 from the PSS1 wellbore drill cores 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. Drill core samples corresponding to the depth 3,864-3,871 m with respect to PSS1 log, have been investigated with SEM-EDS, XRD, EMP, ICP-MS and LOI and for the mineralogical and chemical characterization. The SANS/VSANS experiments at LLB (Saclay, France) served for microporous structure investigation of PSS1 rocks, volcanic rocks from Eastern Alps (IG1) and the Albani Hills (IG2 and IG3) unaffected by CO2 and Burano Formation.