Roma Tre University

In this study numerical simulations of reactive transport in an off-shore deep saline aquifer for the geological sequestration of carbon dioxide are presented and discussed. The main goals are to assess: i) the CO2 injection impact in the reservoir and ii) the cap-rock stability, both being strategic requisites for feasibility studies that are about to be started in Italy. The stratigraphic succession is characterized by a sedimentary succession: from Triassic anhydrites to Jurassic Tuscan calcareous units, up to Cretaceous calcarenites, belonging to the Liguride units, and Quaternary shallow marine sediments. Stratigraphic data from a deep well indicate that below an 1,800 m thick cap-rock, constituted by allochtonous marly calcarenites and clay marls, a regional deep saline aquifer is present. This aquifer, hosted in six Late Triassic to Early Jurassic formations, belonging to the Tuscan Nappe units, consists of porous limestone (mainly calcite) and marly limestone deposits at 1900-3100 m b.s.l. A common problem working with off-shore closed wells, where only the well-log information are available, is that to obtain reliable physico-chemical parameters (e.g. petrophysical and mineralogical) to be used for numerical simulations. Available site-specific data include only basic physical parameters such as temperature, pressure, and salinity of the formation waters. Bulk and modal mineralogical composition were obtained after sampling each formation in contiguous on-shore zones. Mineralogy was determined by X-Ray diffraction analysis coupled with Rietfield refinement. The latter was performed using Maud v2.2. The surface reactive area of minerals was assumed as geometric area of a truncated sphere calculated on the basis of Scanning Electronic Microscopy analysis. Porosity and permeability were inferred by the well-log data along with the use of boundary conditions such as surficial measurements and temperature profiles. The chemical composition of the aquifer pore water is unknown. As a consequence, this was calculated by batch modeling, assuming thermodynamic equilibrium between minerals and a NaCl (0.45 M) equivalent brine at reservoir conditions (up to 118 °C and 300 bars). The reconstructed dataset represented the base of numerical simulations to evaluate the potential geochemical impact of CO2 storage and to quantify water-gas-rock reactions. Three dimensional simulations were performed by the TOUGHREACT code via the implementation to the source code and the correction of the chemical parameters at the theoretical CO2 injection pressure. A re-interpretation of the available seismic reflection data was carried out to: i) define the 3D geometry, and ii) evaluate the volume of the geological structure potentially suitable for CO2 storage. In particular the main surfaces where physicochemical modeling was applied, i.e. the top and the bottom of the cap-rock units and the spill point surface, to better define the 3D geometry of the potential injection reservoir, were reconstructed. Reactive transport simulations were conducted under multiphase advection, aqueous diffusion, gas phase participation in multiphase fluid flow and geochemical reaction in non-isothermal conditions. Feedbacks between flow and geochemical processes were taken into account to evaluate changes in porosity and permeability as kinetic reactions were proceeding. Twenty years of CO2 injection at the rate of 1.5 Mt/year were simulated, whereas water-gas-rock interactions between CO2-rich brines and minerals over a period of 100 years were performed. Preliminary results suggest that injected CO2 can safely be retained in the reservoir by mineral trapping and that the cap-rock can be considered as efficient barrier.

CO2 Capture & Storage in saline aquifers is presently one of the most promising technologies for reducing anthropogenic emissions of CO2. In these sites the short-longterm consequences of CO2 storage into a deep reservoir can be predicted by numerical modelling of geochemical processes. Unfortunately a common problem working with off-shore closed wells, where only the well-log information are available, is to obtain physico-chemical data (e.g. petrophysical and mineralogical) needed to reliable numerical simulations. Available site-specific data generally include only basic physical parameters such as temperature, pressure, and salinity of the formation waters. In this study we present a methodological procedure that allows to estimate and integrate lacking information to geochemical modelling of deep reservoirs such as: i) bulk and modal mineralogical composition, ii) porosity and permeability of the rock obtained from heat flow measurements and temperature, iii) chemical composition of formation waters (at reservoir conditions) prior of CO2 injection starting from sampling of analogue outcropping rock formations. The data sets in this way reconstructed constitute the base of geochemical simulations applied on some deep-seated Italian carbonatic and sandy saline aquifers potentially suitable for geological CO2 storage. Numerical simulations of reactive transport has been performed by using the reactive transport code TOUGHREACT via pressure corrections to the default thermodynamic database to obtain a more realistic modelling. Preliminary results of geochemical trapping (solubility and mineral trapping) potentiality and cap-rock stability as strategic need for some feasibility studies near to be started in Italy are here presented and discussed.

CO2 Capture & Storage (CCS) is presently one of the most promising technologies for reducing anthropogenic emissions of CO2. The numerical modeling procedures of geochemical processes are one of the few approaches for investigating the short-long-term consequences of CO2 storage into a deep reservoir. We present the results of a new approach for the reconstruction of thermo-physical properties of an off-shore deep well (situated in the medium Tyrrhenian Sea, only 5 miles from the coast, in the frame of a distensive and relatively high heat flux regime as a whole,with good outcrops, on-shore, of its stratigraphy includes six Late Triassic-Early Jurassic carbonatic formations at the depth of 2500-3700 m b.s.l). We used the well-log coupled with temperature profile and new mineralogical analyses of the outcrops geological formations, being the original core data lacking. This kind of procedure is new as a whole, and it is useful to create background petro-physical data, for reservoir engineering numerical simulations both of mass-transport and geochemical as well as geo-mechanical, in order to asses its general properties, without re-opening the well itself for industrial use, such as CO2 geological storage. The profile of thermal capacity and conductivity, as well as porosity and permeability resulted very well constrained and detailed for further numerical simulation uses. Porosity is a very important parameter for reservoir engineering, mainly for numerical simulations including geochemical modelling, being strongly necessary for CO2 geological storage feasibility studies, because it allows to compute: i) the reservoir storage capacity for each trapping mechanisms (some algorithms are discussed in the presentation) and ii) the water/rock ratio (one of the input parameter requested by the geochemical software codes). A common problem, working with closed wells with, available the well-log report only, is to obtain data on the thermo-physical properties of the rock. Usually the available well-log report the temperature profile measured during drilling, the mud-loss and some other information on water and gas phase presence. In this work we present a procedure that allow to estimate porosity and permeability of the rock formation from the well-log data joint with a rough mineralogical analyses of the corresponding geological formations outcrop with the use of a boundary condition such as shallow heat flow measurements; a similar approach were presented from some authors that dealt with similar problems e.g. Singh V.K., (2007). The analyses of the rock samples proceed by using i) petro-graphical analyses; ii) calcimetry with Dietrich-Fruhling apparatus in order to analyse the carbonate content of each sample; iii) XRD Rietveld analyses in order to quantify the major mineralogy of each sample and to apply the dolomite correction to the results of calcimetry determination. Rietveld quantification procedure were performed by using Maud v 2.2.; iv) SEM analyses have been accomplished later in details. Successively, hints about the subsequent geochemical modelling approach are presented. Chemical composition of the aquifer pore water has been has been inferred by batch modeling assuming thermodynamic equilibrium between minerals and a NaCl equivalent brine at reservoir conditions (up to 70 °C and 200 bar). Numerical simulations has been carried out by the PRHEEQC (V2.11) Software Package via corrections to the code default thermodynamic to obtain a more realistic modeling.

Stabilize and reduce the atmospheric concentration of anthropogenic greenhouse gases is one of the principal goal that have to be accomplished in short time, in order to reduce the climate changes and the global warming, following the World Energy Outlook 2007 program by IEA. The most promising remedy, proposed for large CO2 sources like thermoelectric power plants, refineries and cement industries, is to separate the flue gas capturing the CO2 and to store it into deep sub-surface geological reservoirs, such as deep saline aquifers, depleted oil and gas fields and unminable coal beds. Among these options, deep saline aquifers are considered the reservoirs with the larger storage potentiality, as a consequence of a wide availability with respect to deep coal seems, depleted oil fields and gas reservoirs. The identification of a possible storage site necessarily passes through the demonstration that CO2 can be injected in extremely safe conditions into geological deep formations, with impermeable caprock above the aquifer/s, which physic-chemical-mineralogical conditions are useful to a better mineral and solubility trapping as well as the hydrodynamic or physical/ structural ones. In order to support the identification of potential storage reservoirs in Italy, INGV jointly with CESI RICERCA S.p.A. accomplished a detailed reworking of available geological, geophysical, geochemical and seismological data, in order to support the existing European GESTCO as well as the CO2GeoCapacity projects. Aim of this work is to establish some site selection criteria to demonstrate the possibility of the geological storage of CO2 in Italy, even if it is located in an active geodynamical domain. This research started from the study of 7575 wells drilled on Italian territory during the last 50 years for gas/oil and geothermal exploration. Among this data-set as a whole, only 1700 wells (deeper than 800 m) have been selected. Only 1290 of these wells have a public-available composite log and fit with the basic prerequisites for CO2 storage potential, mostly as deep saline aquifer/s presence. Wells data have been organized into a geodatabase containing information about the nature and the thickness of geological formations, the presence of fresh, saline or brackish water, brine, gas and oil, the underground temperature, the permeability, porosity and geochemical characteristics of the caprock and the reservoirs lithologies. Available maps, seismic and geological profiles containing or closer to the analyzed wells have been catalogued too. In order to constrain the supercritical behaviour of the CO2 and to prevent the escape of gaseous CO2 to the surface, a first evaluation of the caprock presence and quality has been done on these selected wells. Using a numerical parameterization of the caprock lithologies, a “Caprock Quality Factor” (Fbp) has been defined, which clustered the wells into 5 different classes of caprock impermeability (ranging between the lowest 1 to highest 5). The analysis shows that more than 50% of the selected wells have an Fbp Factor between 4 and 5 (good and optimal quality of caprock), and are mostly located in foredeep basins of the Alps-Apenninic Chain. The geodatabase also includes: i) the seismogenetic sources (INGV DISS 3.0.4 Database of Individual Seismogenetic Sources), ii) an elaboration of seismic events catalogues (INGV CFTI, CPTI04, NT4.1), iii) the Diffuse Degassing Structures (DDS), as part of the INGV project V5 diffuse degassing in Italy geodatabase, considered as “CO2 analogue” field-tests, iv) the distribution of the thermal anomalies on the Italian Territory, linked to the presence of volcanic CO2 emissions, in order to consider the CO2 diffuse degassing risk assessment on the Italian territory Successively it has been created a geodatabase on the nature and quality of deep aquifers for the high-ranking wells sub-dataset (where the aquifers data are available), containing the following parameters: i) presence of one or more aquifers deeper than 800 meters; ii) thickness of the aquifer/s; iii) lithology of the reservoir/s; iv) available chemical analysis; v) distance from closer power plants or other anthropogenic CO2 sources.The final aim of these work is to help to find potential areas in Italy where CO2 storage feasibility studies can be done. In these cases it is necessary to implement the knowledge by: i) better evaluation of saline aquifer quality; ii) estimation of CO2 storage capacity by 3D-modeling of deep crustal structures; iii) fluid-dynamic and geochemical modelling of water-rock-CO2 interaction paths.

Soon after a 222Rn and 220Rn survey in soil gases, performed (June 2005) in the frame of the Diffuse Degassing in Italy risk assessment project, a moderate earthquake (Mw=4.6) occurred in the Anzio offshore, on August, 22, 2005, only 5 miles from the Tor Caldara Diffuse Degassing Structure (DDS onward). Having available the pre-earthquake 222Rn and 220Rn grid-map on around 50 soil-gas points and being 222Rn both a stress-pathfinder and a discriminative component of activated-faults, a mirrorlike survey was repeated on the same 50 sites, soon after the close earthquake. Later, during a quiescent-aseismic period (December, 2005), a CO2 flux survey was performed for the same 50 sites, adding detailed measurements (more than 100 sites) for the highest flux sectors. The aim of this survey was both to have an overall picture of the background CO2 flux and to calculate the total budget of CO2 flux throughout the DDS, to better interpret the 222Rn and 220Rn areal surveys before and after the seismic event. Herewith, we distinguish the contribution of organic, diffusive and advective CO2 flux. Hints of convection and strong degassing linked to the fracture field, inside the DDS, have been envisaged on selected points, where continuous monitoring stations could be strategic, for seismic, volcanic and NGH surveillance. Despite we found higher 222Rn values in soils after the earthquake, suggesting an enhanced local degassing probably linked to a stress signal throughout the DDS as a whole, the results highlight an unmodified shape and location of the 222Rn anomalies before and after the earthquake. This evidence excludes both that the activated seismogenic segment has affected in some ways both the DDS degassing patterns and that fracture field changed. A similar result could be expected if the activated fault was oriented along the DDS itself and reached the surface. This evidence is well correlated with the reconstructed focal mechanism of the earthquake, pertaining to the transfer structure of the Ardea Graben , located along a peripheral sector of the degassing Alban Hills volcano and intersecting the DDS Tor Caldara itself. The shape and location of 222Rn anomalies inside the DDS for both the surveys are strictly inversely correlated with the areal CO2 flux data. The geometry of the degassing pathways is probably linked to the barrier action (sealing power) of the clays cropping out in the study area. These clays are generated by the strong leaching of the outcropping sedimentary Pleistocene rocks due to the huge flux of volcanic gas -rich fluids.