Available online at www.sciencedirect.com ScienceDirect Procedia Earth and Planetary Science 17 (2017) 480 – 483 15th Water-Rock Interaction International Symposium, WRI-15 Reactive transport simulations of impure CO2 injection into saline aquifers using different modelling approaches provided by TOUGHREACT V3.0-OMP J.L Wolfa,1, S Fischerb, H Rüttersa, D Rebschera a Federal Institute for Geosciences and Natural Resources (BGR), Stilleweg 2, 30655 Hannover, Germany Federal Institute for Geosciences and Natural Resources (BGR), Wilhelmstr 25-30, 13593 Berlin, Germany b Abstract In carbon dioxide capture and storage operations (CCS), impurities in CO2 flue gas are an important aspect concerning the geochemical reactions between the injected gas, the rock minerals in a storage reservoir and the brine Numerical reactive transport simulations are capable to evaluate the main mineral reactions and their spatial and temporal occurrences in the reservoir under the impact of co-injected impurities For the inclusion of the impurities in such simulations, different modeling approaches are evaluated (additional brine injection, trace gas transport, or a combination of the two) The impact of the chosen approach on qualitative and quantitative results of the simulations is discussed in this study using the recently released program TOUGHREACT V3.0-OMP © 2017 2017The TheAuthors Authors Published by Elsevier Published by Elsevier B.V.B.V This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the organizing committee of WRI-15 Peer-review under responsibility of the organizing committee of WRI-15 Keywords: reactive transport; CO2 storage; impurities; numerical simulation; modelling; TOUGHREACT V3.0-OMP Nomenclature CCS Sg ) I Carbon Dioxide Capture and Storage Gas saturation [ ] Porosity [ ] Mineral volume fraction [ ] * Corresponding author E-mail address: JanLennard.Wolf@bgr.de 1878-5220 © 2017 The Authors Published by Elsevier B.V This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the organizing committee of WRI-15 doi:10.1016/j.proeps.2016.12.121 J.L Wolf et al / Procedia Earth and Planetary Science 17 (2017) 480 – 483 Introduction Injection of CO2 into deep saline aquifers is discussed for years as a technique for mitigating adverse climate changes due to anthropogenic CO2 emissions1 The aspect of impurities within a CO2 stream is important for the whole carbon dioxide capture and storage chain (CCS): In general, a higher allowance for the amount of impurities lowers the costs on the CO2 capture side2, but might increase technical and financial efforts for its transportation and storage3 Here, one has to distinguish between (a) inert gases such as Ar, N 2, or O2, mainly changing physical characteristics, e.g effect viscosity and storage capacity and (b) geochemical active impurities such as SO x, NOx, CO, H2, or H2S The latter enhance geochemical interactions between the CO2 stream with rock and brine in the subsurface as pH or redox conditions are modified Redox reactivity and amount of impurities within a CO2 stream depend on the source (e.g fossil powered power plant, cement production, refinery, steel plant) as well as on the implemented capture method (pre-combustion, post-combustion or oxyfuel)4 A realistic scenario for a whole CCS chain integrates a set of CO2 sources to a network of transport pipelines This so-called CCS-cluster is connected to one single storage reservoir Depending on operational modes and capture techniques of all industrial sources involved, the resulting CO2 stream varies temporarily in composition As a matter of cause, variable compositions of the CO2 stream effect transport via pipelines and subsequent storage in the subsurface The complex interdependencies, potential impacts, and subsequent requirements are addressed in the project CLUSTER, funded by the German Federal Ministry of Economic Affairs and Energy Reactive transport modelling approaches Numerical reactive transport simulations have proven to provide a viable tool for the evaluation of geochemical impacts of impurities on reservoir rocks5,6 A widely tested and successfully applied program for reactive transport simulations is TOUGHREACT7 The suite of code is developed at Lawrence Berkeley National Laboratory, California and allows the full coupling of thermal, hydrological, and chemical processes In addition, the latest release (2014) of version V3.0-OMP offers the possibility of simulating the gas phase transport of trace gases (such as NO, NO2, SO2, H2S, O2, H2, CH4), while assuming flow properties of a pure CO2 plume Simulating injection of impure CO2 into deep saline aquifers including geochemical interactions of CO2 and impurities with the rock minerals6,8,9, different modelling approaches are described in the literature Here, the most common approach is to compute impurity species to be in equilibrium with the brine of the aquifer at the in-situ conditions, i.e pressure, temperature, salinity, and fugacity of impurities In these simulations, this artificial brine is injected in addition to pure CO2 Some studies apply the method of direct gas phase transport of trace gases, using predominantly nonavailable code developments such as TOUGH2/TMGAS10,11, a modification of the TMVOS EOS module12, or STOMP-COMP13 Through the release of TOUGHREACT V3.0, this modelling approach becomes accessible to the scientific community Comparing the two approaches listed above, the one based on trace gas transport seems to be more realistic as no artificial mass injection of additional brine is needed to simulate the geochemical reactions of CO2 impurities with reservoir rock minerals14,15 This is especially valid in the direct vicinity of the injection well bore, since some of the impurities tend to preferentially dissolve into the brine 12,13,16 However, the usage of trace gas transport modelling leads to the accumulation of ionic species in certain grid cells under the impact of impurity dissolution, which causes computational restrictions in TOUGHREACT V3.0 14 To be more specific, TOUGHREACT skips geochemical computations in grid cells with an ionic strength above a fixed threshold, while in reality minerals would still continue to react This computational intricacy needs to be taken into account while analysing simulation results, especially geochemical changes of the mineral composition14 Methods and results In order to enhance the understanding of the impact of the chosen modelling approach for calculating reactions of CO2 impurities with the reservoir minerals, comparable simulations of a generic CO injection scenario were performed Therefore both, the common approach of additional brine injection as well as the trace gas transport approach, were used In addition, a third approach was developed as a hybrid of the former two: brine which 481 482 J.L Wolf et al / Procedia Earth and Planetary Science 17 (2017) 480 – 483 contains impurities is injected into grid cells when needed, i.e cells receive the right amount of impurities depending on the motion of the CO2 plume Hereby the gas phase transport of impurity species can be mimicked by “moving the location” of impurity dissolution while still keeping ionic strength below the critical threshold For all simulations, the mineralogy of rock material is based on the analysis of German Bunter Sandstone developed in the COORAL project17,18 Special emphasis is given on the modelling of temporal variable CO flue gas compositions Example results using the trace gas transport approach are presented in Fig They show the acidic dissolution of the initial calcite and dolomite phases in the vicinity of the injection under the impact of dissolved SO after ten years of continuous injection at a rate of 284 kt/a in a 1D radial simulation The dissolution of Ca2+ bearing carbonate minerals is coupled to the precipitation of anhydrite, which results in a net porosity decrease from initially 20 % to 17 % As a result, this may lower the permeability locally, and hence, the injectivity The gas saturation profile, representing the development of the dry-out zone, is added as a black line in the same figure for comparison Fig Spatial profile of gas saturation Sg, porosity ) , and volume fractions I of calcite, dolomite, and anhydrite after ten years of CO2 coinjection including % SO2 Within the radius of SO2 impact (approximately 80 m, indicated by the dashed vertical line), the initial calcite and dolomite phases are almost completely converted to anhydrite, which dominates the overall changes of porosity (blue line) Conclusion In general, the comparison between the three different modelling approaches presented here lead mainly to similar qualitative results in regard to the conversion of Ca2+ bearing carbonate phases to anhydrite However, the quantitative impact, i.e the spatial extent of impurity affected minerals and the intensity of mineral dissolution and precipitation reactions, depends highly on the chosen model The advantage of using the trace gas transport lies apparently in the more realistic computation of the CO plume motion and temporal evolution of gas saturation, while the additional brine injection approach allows for a more precise computation of mineral reactions on a long term scale Hence, depending on the scope of an intended study on impurity reactions, a modelling approach should be chosen accordingly, respecting each particular modelling restrictions for interpretation of results J.L Wolf et al / Procedia Earth and Planetary Science 17 (2017) 480 – 483 Acknowledgements The work is funded by the German Federal Ministry of Economic Affairs and Energy within the CLUSTER project under grant agreement 03ET7031A References Gale, J CCS 20 Years On and the Challenges 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Reservoir Transport Porous Med., 201 1 90: p 183- 200 12 Zhang, W., T Xu, and Y Li, Modeling of fate and transport of coinjection of H 2S with CO2 in deep saline formations J Geophys Res., 201 1 116:... Benefits and restrictions of 2D reactive transport simulations of CO2 and SO2 co -injection into a saline aquifer using TOUGHREACT V3. 0- OMP Int J Greenh Gas Control, 201 6 accepted 15 Rebscher, D., J.L