Characterization of Fractured Rocks for the Design of a Pilot-Scale CO2 Injection Site in the Sulcis Basin, Italy

Abstract

The naturally fractured carbonates have a great potential for Carbon Capture and Storage (CCS) purpose because they could offer the possibility for CO2 storage in areas where no suitable sandy reservoirs are available, as for example in the Mediterranean area, where most of the stratigraphic succession is composed by carbonate rocks. The Sulcis basin, located in the southwestern part of Sardinia island (Italy), is an ideal site for technology development. Although the site is not feasible for commercial-scale geological storage (mainly due to the lack of stationary CO2 sources), it represents an optimal location for pilot-scale experiments. Thanks to data available from several national and international projects, as well as from the mining activities, it has been possible to recognize a potential reservoir-caprock system, suitable for pilot-scale CCS development. A sequence of well bedded, about 50 m thick, mudstones and grainstones ("Miliolitico Fm.") has been identified as a potential reservoir; the caprock is represented by a thick succession of siltstones, sandstones and conglomerates ("Cixerri Fm.") and up to 900 m of Oligo-Miocene volcanic rock, ranging from basaltic to rhyolitic composition. In this work we present the results of several geophysical and geological surveys, including reflection seismic profiles, and geo-structural analysis. All the data were combined in a 3D model, defining the volumes of each formation. The evaluation of the storage capacity and the existence of a thick and impermeable caprock are essential to consider the site suitable for the experimental tests. With this aim, we measured primary porosity and permeability, whereas the fracture network was studied using scan lines and scan areas techniques on outcropping analogue of reservoir and seals. The measured linear parameters were used to build several Discrete Fracture Model (DFN) both of reservoir and caprock formations. In particular, DFN were constructed varying length and aperture values to evaluate their influence on the total secondary porosity.