C02 Workshop

Introduction Carbon dioxide capture and storage (CCS) is continuously discussed in the scientific community as a transient tool for reduction of anthropogenic CO2 emissions, thus mitigating adverse effects of global warming and climate change (e.g. Metz et al., 2005; Wildenborg et al., 2013). In the last decades, many studies addressed the geophysical and geochemical impacts of supercritical CO2 fluids on suitable storage reservoirs, such as depleted gas fields, unmineable coal beds and deep saline aquifers. In recent years, not only the evaluation of pure CO2 injection scenarios, but especially the impacts of impurities, which are contained in the separated CO2 flue gas streams from power and industrial sources, drew growing scientific interest (Bacon et al., 2009; Jung et al., 2013; Knauss et al., 2005; Pearce et al., 2014; Wilke et al., 2012; Xu et al., 2007; Zhang et al., 2011). In this context settles the EU FP7 funded project CO2QUEST (impact of the quality of CO2 on storage and transport).The here presented work is part of the work package WP3 “Storage Reservoir Integrity”. One of the most important flue gas impurities is sulfur dioxide (SO2), which develops, beside some changes of physical properties like density, especially a strong chemical impact on the reservoir system. It is important to note that this high relevance is based on the high acidity of sulfuric acid, H2SO4, which is the product of oxidation or disproportionation reactions (Waldmann et al., 2014):

SO 2(aq) + H 2 O  H 2SO3  HSO3- + H + (1) − hydrolysis 2SO 2(aq) + 2H 2 O + O 2(aq)  2H 2SO 4

(2) − oxidation

4SO 2(aq) + 4H 2 O  H 2S + 3H 2SO 4

(3) − disproportionation

Once these acid sulfur species are produced in the aqueous phase they will dissociate and thereby lower pH, which in turn increases mineral dissolution rates. Due to enhanced mineral dissolution metal cations are released in solution and, depending on the transport and the concentrations of those metal species, precipitation of secondary minerals may evolve. Hence, considerable changes of the mineral assemblage are induced by dissolution of SO2 into the formation water, which attain different spatial extents of mineral reactions depending on fluid flow. The main focus of this work is studying the influence of the spatial dissolution pattern of SO2 on the geochemical reservoir system. In order to predict reasonable CO2 injection scenarios in a saline aquifer, reactive transport simulations using the THC code (thermal – hydrological – chemical) TOUGHREACT were applied. As the degree of quantitative changes in this highly coupled system strongly depend on many initial parameters, such as, e.g., porosity, permeability and mineral composition, the presented work in this study focuses on the comparison and sensitivity of results and inferences on selected main parameters as described below. The goal is to determine critical sets and ranges of parameters, which lead to changing or even impairing conclusions and descriptions of the reservoir system in terms of geochemistry, feasibility and safety of CO2 injection operations, compared to the chosen reference scenario. Simulation methods The applied numerical code TOUGHREACT V3.0-OMP (Xu et al., 2014) is capable of simulating subsurface fluid and heat flow, including the transport and reactions of solutes in the aqueous and, since the recent release of the current version, also gaseous phase. The chemistry and flow module of the simulation are solved sequentially, thereby incorporating feedback between both modules (e.g. chemically induced changes of the porosity). The simulations were carried out on a 500 cell one dimensional radial grid with 30 km extent. Its cell size increases exponentially for proper description of short term processes occurring close to the wellbore and pressure dissipation at larger distances. The petrophysical properties, sediment mineralogy and composition of the formation water are based on data from the Heletz test site in Israel (Shtivelman et al., 2011). The reservoir rock at Heletz is a sandstone that is mainly composed of quartz (70 wt. %), feldspars (16 wt. %), clay minerals (8 wt. %) and carbonates (4 wt. %). The The Third Sustainable Earth Science Conference & Exhibition Use of the Deep Sub-surface to serve the Energy Transition 13-15 October 2015, Celle, Germany

formation water is a NaCl-dominated brine with little amounts of dissolved Ca2+ and Mg2+. The prevailing reservoir conditions are 14.7 MPa, 66 °C and 5.5 wt. % salinity. The reservoir sandstone has a porosity of 20 %, the permeability is 100 mD. Note that this study does not intend to investigate site-specific scenarios for this particular storage reservoir, but rather uses this real data for developing more generalised scenarios as realistic as possible. A selection of parameters is furthermore varied over plausible value ranges in order to evaluate to what extent a reservoir with a similar setting might respond to the injection of an impure CO2 stream in the same way or not. Results and parameter sensitivity As expected, the simulations of the described standard scenario showed that the impact on the chemical system and the minerals mainly relies on: • The relative speed of CO2 fluid phase transport in comparison to the timescales of chemical processes involving the aqueous phase (mineral dissolution / precipitation as well as aqueous chemical conversion, e.g. SO2 oxidation). • The amount and type of carbonate minerals (i.e. Ca-rich calcite vs. Ca-bearing ankerite), which are available for pH buffering and thus strongly regulate further mineral dissolution rates. Next, the most prominent model parameters that influence one of these two sensitivity aspects were varied: • Permeability (between 30 and 700 mD, default 100 mD) • Porosity (between 5 and 30 %, default 20 %) • Injection rate (as low as 0.9 kg/s, default 9.0 kg/s) • SO2(g) concentration (between 0.1 and 3.0 %, default 1.0 %) • Carbonate fraction (as low as 0.37 %, default 3.7 %) • Type of carbonate (calcite Ca2+ to CO32- ratio 1:1 and ankerite Ca2+ to CO32- ratio 1:2) Figure 1 exemplarily shows porosity profiles of the default scenario with ankerite being the primary carbonate phase, and the varied scenario with calcite as primary carbonate phase. The grey line depicts the injection of pure CO2 as a reference case. The dashed dark grey line corresponds to the injection of CO2 including 1 % of SO2, whereby its aqueous speciation reactions are computed by solving equilibrium equations, which is basically equivalent to an infinitely fast chemical conversion. The black line shows the same injection scenario, but the chemical conversion after physical dissolution of SO2 is kinetically delayed, thus mimicking slow chemical processes relative to fluid phase transport. In the two SO2 simulations in the upper panel of figure 1 with ankerite as primary carbonate mineral it is apparent that the porosity is increased compared to the profile of pure CO2 injection. Depending on the speed of SO2 chemistry, this increase is either continuously elevated (equilibrium speciation computation, intense acid formation) or smoothly developing (kinetically delayed speciation). In any case, the porosity profiles are mainly based on the transformation of ankerite to anhydrite. However, in case of primary calcite (lower panel of figure 1) more Ca2+ is released due to the acid buffering, leading to an increased amount of precipitated anhydrite compared to the primary ankerite case. As anhydrite occupies a larger molar volume than calcite, the transformation of calcite to anhydrite leads to a net decrease of pore volume and porosity, respectively. Especially these porosity profiles are of high importance for predicting the feasibility and reliability of CO2 injection operations in terms of pore clogging and injectivity. Other parameters are varied in the same way. They were qualitatively and quantitatively evaluated in our work in order to analyse critical parameter ranges or combinations, which may complicate CO2 injection at specific storage sites similar to Heletz.

The Third Sustainable Earth Science Conference & Exhibition Use of the Deep Sub-surface to serve the Energy Transition 13-15 October 2015, Celle, Germany

Figure 1 Porosity profiles of two different scenarios. The upper panel shows the simulation with ankerite as the primary carbonate mineral. In the lower panel the primary carbonate mineral is calcite with a different Ca2+ to CO32- ratio (1:1 instead of 1:2 in ankerite).

Conclusions In order to analyse the effect of co-injected SO2 into saline aquifers, reactive transport simulations using TOUGHREACT V3.0-OMP were performed. Based on a default scenario relying on real data from the CCS test site in Heletz, Israel, different sets of initial parameters were varied to test for the sensitivity of the geochemical system on specific conditions under the disturbance of impure CO2 containing SO2. It was found that the relative speed of CO2 fluid phase migration compared to the rate of chemical reactions in the aqueous phase (especially sulfuric acid formation) are of crucial concern for the influence on mineral reactions and subsequent changes of porosity. Qualitative and quantitative impacts of selected initial reservoir parameters are discussed in detail in our work. In summary, based on our modelling results the total impact of SO2 on the reservoir system is rather limited as long as enough carbonate minerals are locally available for pH buffering and consequently preventing high reactivity hot spots. Acknowledgements The authors thank Sebastian Fischer for the thorough review of the work presented here. The continuous support and supervision of the project by Franz May is gratefully acknowledged. The research leading to the results described in this presentation has received funding from the European Union 7th Framework Program FP7-ENERGY-2012-1-2STAGE under grant agreement number 309102. The presentation reflects only the authors’ view and the European Union is not liable for any use that may be made of the information contained therein. The Third Sustainable Earth Science Conference & Exhibition Use of the Deep Sub-surface to serve the Energy Transition 13-15 October 2015, Celle, Germany

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The Third Sustainable Earth Science Conference & Exhibition Use of the Deep Sub-surface to serve the Energy Transition 13-15 October 2015, Celle, Germany