Geochemical modelling of water-rock interaction

CO2 geological storage is one of the most promising technologies for reducing atmospheric emissions of greenhouse gas. In this work we present and discuss a new approach geochemical modelling for evaluating the effects of short-medium term CO2 disposal in deep geologic formations that has been tested in the Weyburn test site (Saskatchewan, Canada), where since September 2000 5000 t/day of supercritical CO2 are injected. The geochemical modeling has been performed by using the code PRHEEQC (V2.11) software package, via thermodynamic corrections to the code default database. First, we reconstructed the in-situ reservoir (62°C and 0.1 MPa) chemical composition, including pH, by the chemical equilibrium among the various phases, and we evaluated the boundary conditions (e.g. PCO2 , PH2S), which are necessary for the implementation of reaction path modeling. This is the starting point to assess the geochemical impact of CO2 into the oil reservoir and, as main target, to quantify water-gas-rock reactions. Furthermore, we identified possible compositions of the initially reservoir liquid phases by assuming the equilibrium conditions for the mineral assemblage with respect to a Na-Cl water (Cl/Na=1.2). Then we computed the kinetic evolution of the CO2-rich Weyburn brines interacting with the host-rock minerals, performed over 100 years after injection. Results of reaction path modeling suggest that, in this period, CO2 can be neutralized by solubility (as CO2 (aq)) and mineral trapping through Dawsonite precipitation. In order to validate our geochemical model we have simulated the geochemical impact of three years of CO2 injection (September 2000-2003) by kinetically controlled reactions and we have compared the computed and measured data. The calculated chemical composition after the CO2 injection is consistent with the analytical data of samples collected in 2003 with an error within 5 % for most analytical species, with the exception of the Ca and Mg contents (error > 90%), likely due to the complexation effect of carboxilic acid

An approach to the geochemical modelling of water-rock interaction in CO2 storage geological reservoirs: the Weyburn Project (Canada) case study

Geological storage is one of the most promising technologies for reducing anthropogenic atmospheric emissions of CO2. Among the several CO2 storage techniques, sequestration in deep-seated saline aquifers implies four processes: a) supercritical fluid into geologic structure (physical trapping), b) dissolved CO2(aq) due to very long flow path (hydrodynamic trapping), c) dissolved CO2(aq) (solubility trapping), and d) secondary carbonates (mineral trapping). The appealing concept that CO2 can permanently be retained underground has prompted several experimental studies in Europe and North America sponsored by IEA GHG R&D, EU and numerous international industrials and governments, the most important project being the International Energy Agency Weyburn CO2 Monitoring & Storage, an EnCana’s CO2 injection EOR project at Weyburn (Saskatchewan, Canada). Owing to the possible risks associated to this technique, numerical modelling procedures of geochemical processes are necessary to investigate the short- to long-term consequences of CO2 storage. Assumptions and gap-acceptance are made to reconstruct the reservoir conditions (pressure, pH, chemistry, and mineral assemblage), although most strategic geochemical parameters of deep fluids are computed by a posteriori procedure due to the sampling collection at the wellhead, i.e. using depressurised aliquots. In this work a new approach to geochemical model capable of to reconstruct the reservoir chemical composition (T, P, boundary conditions and pH) is proposed using surface analytical data to simulate the short-medium term reservoir evolution during and after the CO2 injection. The PRHEEQC (V2.11) Software Package via thermodynamic corrections to the code default database has been used to obtain a more realistic modelling. The main modifications brought about the Software Package are: i) addition of new solid phases, ii) use of P>0.1 Mpa, iii) variation of the CO2 supercritical fugacity and solubility under reservoir conditions, iv) addition of kinetic rate equations of several minerals and v) calculation of reaction surface area. The Weyburn Project was selected as case study to test our model. The Weyburn oil-pull is recovered from the Midale Beds (1300-1500 m deep) that consist of two units of Mississippian shallow marine carbonate-evaporites: i) the dolomitic “Marly” and ii) the underlying calcitic “Vuggy”, sealed by an anhydrite cap-rock. About 3 billions mc of supercritical CO2 have been injected into the “Phase A1” injection area. The INGV and the University of Calgary (Canada), have carried out a geochemical monitoring program (ca. thrice yearly- from pre-injection trip: “Baseline” trip, August 2000, to September 2004). The merged experimental data are the base of the present geochemical modeling. On the basis of the available data, i.e. a) bulk mineralogy of the Marly and Vuggy reservoirs; b) mean gas-cap composition at the wellheads and c) selected pre- and post-CO2 injection water samples, the in-situ (62 °C and 0.1 MPa) reservoir chemical composition (including pH and the boundary conditions as PCO2, PH2S) has been re-built by the chemical equilibrium among the various phases, minimizing the effects of the past 30-years of water flooding in the oil field. The kinetic evolution of the CO2-rich Weyburn brines interacting with the host-rock minerals performed over 100 years after injection have also been computed. The reaction path modeling suggests that CO2 can mainly be neutralized by solubility and mineral trapping via Dawsonite precipitation. To validate our model the geochemical impact of three years of CO2 injection (September 2000-2003) has been simulated by kinetically controlled reactions. The calculated chemical composition after the CO2 injection is consistent with the analytical data of samples collected in 2003 with a <5 % error for most analytical species, with the exception of Ca and Mg (error >90%), likely due to the complexation effect of carboxilic acid

Leaking And Non-leaking Systems: Study Of Natural CO2 Accumulations For Geological Sequestration

The potential risks of geological CO2 storage must be understood and geologists are required to predict how CO2 may behave once stored underground. As natural geological accumulations of carbon dioxide occur in many basins in Italy and volcanic and seismically active areas allow CO2 rich fluids to migrate to the near surface, many of these areas have been investigated in order to study long-term geochemical processes that may occur following geological storage of anthropogenic CO2. A study representing an example of "leaking" system is the Solfatara crater (Campi Flegrei, Southern Italy) characterised by the presence of both CO2 rich-waters and fumarole. Soil gas flux measurements show that the entire area discharges between 1200 and 1500 tons of CO2 a day. Most part of analysed waters is the effect of a mixing between a shallow meteoric water and a deep thermal Na-Cl end-member and/or seawater, resulting in sodiumchloride waters. A high dissolved CO2 content (max value 566.28 cc/l) is also present. Furthermore, the Campi Flegrei frequently undergo bradyseism related to the elastic response of the shallow crust to increasing pressure within a shallow magma chamber. The study of this phenomenon could be useful to detect ground deformation linked to geomechanical changes in a geological CO2 reservoir. In contrast, an example of "non-leaking" system is the Pisticci oil and gas Field (Southern Italy) where a great variety of hydrocarbons traps are formed by horst and tilted blocks in the Mesozoic carbonate substratum covered by an almost continuous sequence of Lower Pliocene marls and Middle Pliocene-Pleistocene marly blue clays. Soil gas surveys were performed after a MD 4.5 earthquake and two years later to test the permanence of the gas distribution pattern. CO2 distribution in soil gas seems not to be affected by changes in stress, as suggested by the average values of both surveys. The principal aim of our research has been to evaluate and mitigate risks for local populations as the studied areas are densely populated. To date, the obtained results suggest that gas uprising is generally well localised around restricted areas, often controlled by local tectonics (faults and/or fractures). This implies that, in the frame of geological CO2 sequestration, it is necessary to carefully assess the presence of pathways (fault and/or fractures) that might allow the migration of CO2 out of the reservoir

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

CO2 Capture & Storage (CCS) is presently one of the most promising technologies for reducing anthropogenic emissions of CO2 . Among the several potential geologi- cal CO2 storage sites, e.g. depleted oil and gas field, unexploitable coal beds, saline aquifers, the latter are estimated to have the highest potential capacity (350-1000 Gt CO2 ) and, being relatively common worldwide, a higher probability to be located close to major CO2 anthropogenic sources. In these sites CO2 can safely be retained at 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 of CO2 leakage from the reservoir have to be carefully evaluated by both monitoring techniques and numerical modeling used in “CO2 analogues”, although seepage from saline aquifers is unlikely to be occurring. The fate of CO2 once injected into a saline aquifer can be predicted by means of numerical modelling procedures of geochemical processes, these theoretical calculations being one of the few approaches for inves- tigating the short-long-term consequences of CO2 storage. This study is focused on some Italian deep-seated (>800 m) saline aquifers by assessing solubility and min- eral trapping potentiality as strategic need for some feasibility studies that are about to be started in Italy. Preliminary results obtained by numerical simulations of a geo- chemical modeling applied to an off-shore Italian carbonatic saline aquifer potential suitable to geological CO2 storage are here presented and discussed. Deep well data, still covered by industrial confidentiality, show that the saline aquifer, includes six Late 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 (mainly calcite) 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 the potential geochemical impact of CO2 storage and the quantification of water-gas-rock reactions (solubility and mineral trapping) of injection reservoir have been performed by the PRHEEQC (V2.11) Software Package via corrections to the code default ther- modynamic database to obtain a more realistic modelling. The main modifications to the Software Package are, as follows: i) addition of new solid phases, ii) variation of 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 as temperature, pressure, and salinity of the formation waters. Rocks sampling of each considered formation in the contiguous in-shore zones was carried out. Mineralogy was determined by X-Ray diffraction analysis and Scanning Electronic Microscopy on thin sections. As chemical composition of the aquifer pore water is unknown, this has been inferred by batch modeling assuming thermodynamic equilibrium between minerals and a NaCl equivalent brine at reservoir conditions (up to 135 ̊C and 251 atm). Kinetic modelling was carried out for isothermal conditions (135 ̊C), under a CO2 injection constant pressure of 251 atm, between: a) bulk mineralogy of the six formations 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 100 years after injection suggests that solubility trapping is prevailing in this early stage of CO2 injection. Further and detailed multidisciplinary studies on rock properties, geochemical and micro seismic monitoring and 3D reservoir simulation are necessary to better characterize the potential storage site and asses the CO2 storage capacity

Overview of the geochemical modeling on CO2 capture & storage in Italian feasibility studies

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

Development of an Italian catalogue of potential CO2storage sites: an approach from deep wells data

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

BARRIER EFFECT IN CO2 CAPTURE AND STORAGE FEASIBILITY STUDY

CO2 Capture & Storage (CCS) in saline aquifer is one of the most promising technologies for reducing anthropogenic emission of CO2. Feasibility studies for CO2 geo-sequestration in Italy have increased in the last few years. Before planning a CCS plant an appropriate precision and accuracy in the prediction of the reservoir evolution during injection, in terms of both geochemical calculation and fluid flow properties, is demanded. In this work a geochemical model will be presented for an offshore well in the Tyrrhenian Sea where the injection of 1.5 million ton/year of CO2 is planned. The dimension of the trapping structure requires to study an area of about 100 km2 and 4 km deep. Consequently, three different simulations were performed by means of TOUGHREACT code with Equation Of State module ECO2N. The first simulation is a stratigraphic column with a size of 110*110*4,000 meters and a metric resolution in the injection/cap-rock area (total of 8,470 elements), performed in order to asses the geochemical evolution of the cap-rock and to ensure the sealing of the system. The second simulation is at large scale in order to assess the CO2 path from the injection towards the spill point (total of about 154,000 elements). During this simulation, the effect of the full coupling of chemistry with fluid flow and a relevant effect in the expected CO2 diffusion velocity was recognized. Owing to the effect of chemical reaction and coupling terms (porosity/permeability variation with mineral dissolution/precipitation), the diffusion velocity results to be 20% slower than in a pure fluid flow simulation. In order to give a better picture of this 'barrier' effect, where the diffusion of the CO2-rich acidic water into the carbonate reservoir originates a complex precipitation/dissolution area, a small volume simulation with a 0.1 m grid was elapsed. This effect may potentially i) have a big impact on CO2 sequestration due to the reduction of available storage volume reached by the CO2 plume in 20 years and/or the enhanced injection pressure and ii) outline the relevance of a full geochemical simulation in an accurate prediction of the reservoir properties

The Tor Caldara CO2 Diffuse Degassing Structure (DDS): 222Rn/220Rn output before and after the August, 22, 2005 Anzio Earthquake (Mw=4.6).

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

Densely populated settings: the challenge of siting geological facilities for deep geothermics, CO2 and natural gas storage, and radioactive waste disposal Underground Coexistence and Synergies for a Sound Energy Mix in the Post-Kyoto Era

The abstracts herein – collected for the 34th Course of the International School of Geophysics, held in Erice, Italy (“Ettore Majorana” Foundation and Centre for Scientific Culture, 25-30 September, 2010) – focus on geophysical, geological and geochemical methods applied to the planning of the soundest energy mix in densely populated countries, where the coexistence of different technologies requires unique underground facilities and resources. In the framework of IEA and EU programmes, where the concepts of “smart grids” and “smart cities” are prevailing, we rather propose the concept of “smart region” planning the use of both underground and surface areas in a new social-energetic paradigm of “zero kilometer” life. The coexistence of geological storage of CO2 and natural gas, geothermics and, possibly, nuclear waste temporary storage (near surface or geological) is today necessary owing to the progressive decrease of space and resources. In this context, the following technologies turn out to be very important: renewables (geothermal energy), nuclear power, clean coal technologies via CO2 Capture and Storage (CCS), Enhanced Oil Recovery (EOR), Enhanced Coal Bed Methane (ECBM), non-conventional gas exploitation, and seasonal storage of natural gas (also for strategic reserves). These technologies have been recently emphasized in Italy by the Ministry of Economic Development and by the Ministry of the Environment and Territory, as well as by research institutions such as INGV and CNR. Key topics addressed during the Course were: • Geological storage and disposal: assessment of available volume and structures. • Subsurface geological resources: management of potential conflicts among various technologies. • Geological site characterization and risk assessment for policy makers and regulators: the role of the energy industry. • New high tech frontiers for geothermal power production. • New concepts in nuclear waste disposal. • Numerical simulation software for geothermal exploration, geological storage and nuclear waste disposal. • Sharing subsurface data coming from oil & gas and geothermal exploration. • High resolution characterization of shallow aquifers and reservoirs: multi-strata exploitation by different energy technologies. • Case histories and natural analogues: “learning by doing” and “acceptable risk” concepts. The 34th Course of the International School of Geophysics is dedicated to students and young contract researchers starting their carreers in a period of energetic-environmental global crisis. Although their scientific contribution is of high quality, they are usually underpaid in public research institutions with respect to volatile staff of some international organizations who, making use of the results of governmentfunded research, make final decisions on low-carbon energy technologies

Modellizzazione delle variazioni composizionali delle specie dell’azoto (NH4 +, NO2 -, NO3 -) nelle acque di falda del Comune di Arezzo (Toscana)♦

Gli elementi chimici disciolti nelle acque continentali provengono dall’alterazione della crosta terrestre. L’acqua erode e dissolve i minerali delle rocce attraverso l’alterazione chimica avvalendosi del contributo dei gas presenti in atmosfera o nel sottosuolo. Il nitrato, una delle sostanze responsabili delle più gravi forme di inquinamento delle acque nei paesi in via di sviluppo, è un nutriente essenziale per la crescita delle piante e rappresenta un anello fondamentale del ciclo biogeochimico dell'azoto, in quanto viene prodotto dai batteri a partire dall'azoto atmosferico. In quantità eccessive il nitrato può essere dannoso per gli uomini e per gli animali. Elevati livelli di nitrato nell'acqua sono causati in larga misura dall'uso di fertilizzanti ricchi di nitrato e dal letame. In questo contesto, le condizioni redox delle acque naturali, che controllano la speciazione dei composti dell’azoto, sono altamente variabili perché controllate prevalentemente dall’attività biologica. In particolare, il bilancio fra i due processi dell’attività biologica, la fotosintesi e la respirazione (o decomposizione della sostanza organica), determina la presenza nel sistema di condizioni ossidanti o riducenti. I composti dell’azoto possono quindi essere considerati utili indicatori dello stato di salute di un acquifero superficiale. In questo lavoro sono analizzati i dati relativi ai tenori delle specie dell’azoto NH4 +, NO2 - e NO3 - relativi ad acque di falda campionate nell’area aretina nel corso della realizzazione dell’Atlante Geochimico delle Acque di Falda e di Scorrimento Superficiale del Comune di Arezzo. I dati sono analizzati proponendo nuove metodologie grafiche e numeriche per visualizzare lo stato del territorio nei confronti della pressione antropica come rilevata dal comportamento spaziale e temporale delle specie suddette