SANS/VSANS investigation of the porosity microstructure in rocks from a natural CO2 reservoir

Sequestration of CO2 in deep geological reservoirs represents one of the potential methods to reduce anthropogenic emissions into the atmosphere. In the long term the injected CO2 dissolves into the local formation of rocks and, when present, in saline deep aquifers, participating to a variety of geochemical reactions. The overall impact of these processes produces changes in mineralogy, texture, permeability and porous structure of the rocks, to a level which depends on the different lithologies present in the rocks. Mineralogical changes can be investigated by considering what occurs in rocks and minerals with natural CO2 accumulations, as analogues for geological sequestration. Also computer simulations, based on thermodynamics, kinetics and geochemical modeling [1, 2], can be beneficial. On the other hand, reliable data concerning the porous structure, which is so important to trap CO2, can be hardly extracted from computer simulations. In this context a valuable help can be provided by Small and Very Small Angle Neutron Scattering techniques (SANS and VSANS, respectively): they have been indeed demonstrated to be powerful tools for the determination of the microstructure and porosity of rocks from Ångstrom to millimeter sizes [3], proofing also the existence of fractal dimensions for the volume and surfaces interfaces between pores and rocks. Here we present the preliminary results of a SANS/VSANS investigation on rocks pertaining to a geological context (located in Eastern Tuscany, Central Italy) featured by a deep geological gas reservoir. The former reservoir was intercepted by a bore-well drilled in ’80 by ENI and is presently hosting 700 bar of supercritical CO2. Texture and mineralogy of volcanic rocks samples, from drill cores corresponding to the top of the reservoir, were found to be heavily modified by the interaction with CO2-rich fluids. The combined neutron experiments, performed using the PAXE and G5bis diffractometers at LLB (Saclay, F), allowed to approximately investigate pore sizes ranging from ∼ 60 nm to ∼ 6 μm. Samples of host rocks (from drill core) and a selection of possible analogues of the same volcanic rocks, unaffected by CO2 presence, were chosen for the experiments together with samples of rocks, from outcrops, corresponding to the geological layers the characteristics of geological layers overlying the reservoir. Globally these rocks are representative of a wide spectrum of different lithologies (as limestones, marls, evaporitic deposits bearing gypsum and volcanic rocks with intermediate-acid composition). The information provided by these experiments must still be fully analyzed and integrated with other data (e.g. chemical composition of rocks and fluids [4]) in order to get a better understanding of: i) the role and the possible effects of CO2 in determining the micro-porosity of these host rocks, and ii) the sealing effect of the sedimentary rocks overlying the reservoir which act as a barrier with respect to the CO2-rich gases. References [1] Zhu C. and Anderson G., 2002 Environmental Applications of Geochemical Modeling, Cambridge University Press, Cambridge, UK, 284 pp. [2] Cantucci B, Montegrossi G., Vaselli O., Tassi F., Quattrocchi F., and Perkins E.H., 2009 Geochemical modelling of CO2 storage in deep reservoirs: The Weyburn Project (Canada) case study. Chemical Geology, 265 (1), 181-197. [3] A.P. Radlinski, 2006 and reference therein, in Neutron Scattering in Earth Sciences, Reviews in Mineralogy & Geochemistry, 63, pp. 363-397. [4] Bicocchi G., Montegrossi G., Ruggieri G., Buccianti A. and Vaselli O. (2011). Modeling composition of Ca-Fe-Mg carbonates in a natural CO2 reservoir. In: Egozcue, J.J., Tolosana-Delgado, R. and Ortego, M.I. (eds.). Codawork11 Proceedings, 16 p.