Mineralogy and geochemical trapping of CO2 in an Italian carbonatic deep saline aquifer: preliminary results

摘要

CO2 Capture & Storage (CCS) is presently one of the most promising technologiesfor reducing anthropogenic emissions of CO2 . Among the several potential geologi-cal CO2 storage sites, e.g. depleted oil and gas field, unexploitable coal beds, salineaquifers, the latter are estimated to have the highest potential capacity (350-1000 GtCO2 ) and, being relatively common worldwide, a higher probability to be locatedclose to major CO2 anthropogenic sources. In these sites CO2 can safely be retainedat depth for long times, as follows: a) physical trapping into geologic structures; b) hy-drodynamic trapping where CO2(aq) slowly migrates in an aquifer, c) solubility trap-ping after the dissolution of CO2(aq) and d) mineral trapping as secondary carbon-ates precipitate. Despite the potential advantages of CO2 geo-sequestration, risks ofCO2 leakage from the reservoir have to be carefully evaluated by both monitoringtechniques and numerical modeling used in “CO2 analogues”, although seepage fromsaline aquifers is unlikely to be occurring. The fate of CO2 once injected into a salineaquifer can be predicted by means of numerical modelling procedures of geochemicalprocesses, these theoretical calculations being one of the few approaches for inves-tigating the short-long-term consequences of CO2 storage. This study is focused onsome Italian deep-seated (>800 m) saline aquifers by assessing solubility and min-eral trapping potentiality as strategic need for some feasibility studies that are aboutto be started in Italy. Preliminary results obtained by numerical simulations of a geo-chemical modeling applied to an off-shore Italian carbonatic saline aquifer potentialsuitable to geological CO2 storage are here presented and discussed. Deep well data,still covered by industrial confidentiality, show that the saline aquifer, includes sixLate Triassic-Early Jurassic carbonatic formations at the depth of 2500-3700 m b.s.l.These formations, belonging to Tuscan Nappe, consist of porous limestones (mainlycalcite) and marly limestones sealed, on the top, by an effective and thick cap-rock(around 2500 m) of clay flysch belonging to the Liguride Units. The evaluation of thepotential geochemical impact of CO2 storage and the quantification of water-gas-rockreactions (solubility and mineral trapping) of injection reservoir have been performedby the PRHEEQC (V2.11) Software Package via corrections to the code default ther-modynamic database to obtain a more realistic modelling. The main modifications tothe Software Package are, as follows: i) addition of new solid phases, ii) variationof the CO2 supercritical fugacity and solubility under reservoir conditions, iii) addi-tion of kinetic rate equations of several minerals and iv) calculation of reaction sur-face area. Available site-specific data include only basic physical parameters such astemperature, pressure, and salinity of the formation waters. Rocks sampling of eachconsidered formation in the contiguous in-shore zones was carried out. Mineralogywas determined by X-Ray diffraction analysis and Scanning Electronic Microscopyon thin sections. As chemical composition of the aquifer pore water is unknown, thishas been inferred by batch modeling assuming thermodynamic equilibrium betweenminerals and a NaCl equivalent brine at reservoir conditions (up to 135 ̊C and 251atm). Kinetic modelling was carried out for isothermal conditions (135 ̊C), under aCO2 injection constant pressure of 251 atm, between: a) bulk mineralogy of the sixformations constituting the aquifer, and b) pre-CO2 injection water. The kinetic evolu-tion of the CO2 -rich brines interacting with the host-rock minerals performed over 100years after injection suggests that solubility trapping is prevailing in this early stageof CO2 injection. Further and detailed multidisciplinary studies on rock properties,geochemical and micro seismic monitoring and 3D reservoir simulation are necessaryto better characterize the potential storage site and asses the CO2 storage capacity.