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

Modeling composition of Ca-Fe-Mg carbonates in a natural CO2 reservoir

Understanding the physical-chemical features of liquid, gas and solid phases in natural analogue reservoirs of Carbon Capture and Sequestration (CCS) site is fundamental as they can provide key data for building up both conceptual and numerical modeling of reaction path for gas-water-rock interaction in high pCO2 systems. The aim of this work is improve the knowledge about these processes, by employing appropriate methods for compositional data on a case study, focusing on the solid (minerals) phases. In the early eighties, the PSS1 well (Eastern Tuscany, Central Italy), drilled down to almost 5,000 m for oil exploration by ENI (Italian National Agency of Hydrocarbons), intercepted a high pressure (≈700 bar) CO2 reservoir at the temperature of 120 °C. The reservoir rocks in the fertile horizon, located at about 3,800 m, consist of altered volcanic deposits interbedded with gypsumdolomite- bearing evaporites (“Burano Formation”). Surveys for determining the actual paragenesis of volcanic rocks, carried out on the drill core samples, corresponded to the top of CO2 reservoir (3,864-3,871 m depths from surface on the PSS1 bore-well log). Quartz, Ca-Fe-Mg carbonates, clay minerals (illite and chlorite series) and Fe-Ti oxides were found as principal mineralogical phases. Electron Microprobe Analysis on the carbonates has allowed to recognize the presence of ankerite and calcites. Compositional data, related to atomic % content of Ca, Fe, and Mg in carbonates minerals, were transformed by using Isometric Log-Ratio balances, whilst the variability affecting the data pattern was investigated in simple binary diagrams. The stoichiometric substitution processes governing the presence of Ca, Fe and Mg in carbonates were modeled by using regression techniques in the new space defined by ilrs coordinates. Results have evidenced the different role of Fe and Mg in substituting or not Ca in both carbonate minerals of these CO2-bearing reservoir rocks

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

Fluid geochemistry and seismic activityin the period 1998-2002 at Turrialba Volcano (Costa Rica)

Turrialba Volcano, located in Central-Southern Costa Rica, has been characterized, since the last period of eruptive activity in 1884-1886, by a weak and discontinuous fumarolic activity in the western area of its summit. During the 1998-2002 period, fumaroles discharging from central and West craters were collected for chemical analyses of major and trace gas compounds, 13C/12C in CO2 and 18O/16O and D/H (in one fumarolic condensate), isotopic ratios. Geophysical measurements (seismic activity and ground deformation), monitored in the same period, were compared to geochemical data to define the status of the volcanic system. Chemical and isotopic characteristics of fumaroles of Turrialba Volcano seem to be related to interaction processes between a magmatic source and a shallower hydrothermal aquifer. Since February 1997, seismicity at Turrialba Volcano gradually increased, while since August 2001 new fumaroles start to discharge from a new fracture system located in the area between central and West craters. Since September 2001, strong compositional changes of gas discharges have been recorded at central crater. These occurrences are possibly due to variations in the permeability of the conduit system feeding the fumaroles. Heat pulse episodes from a magmatic source have possibly caused the increase of vapour pressure at depth and, consequently, favoured the uprising of the magmatic fluids toward the surface. The observed evolution of chemical and physical parameters suggests that to forecast a possible renewal of the volcanic activity in the near future a full program of both geochemical and geophysical surveillance must be provided at Turrialba Volcano