Table of Contents

Supporting Information: Alterations of fractures in carbonate rocks by CO2-acidified brines Hang Deng1, Jeffrey P. Fitts1, Dustin Crandall2, Dustin McIntyre2, Catherine A. Peters1,* 1

Department of Civil and Environmental Engineering, Princeton University, Princeton NJ 2 National Energy Technology Laboratory, Morgantown, WV * corresponding author [email protected]

Environmental Science & Technology, July 2015

Table of Contents Indiana Limestone Cores ............................................................................................................................................. S1 Details of the high-pressure core-flow experiments ................................................................................................... S1 xCT scanning ................................................................................................................................................................ S2 Initial fracture aperture statistics ................................................................................................................................ S3 1D reactive transport model details ............................................................................................................................ S4 Kinetic parameter estimation ...................................................................................................................................... S4 Effluent chemistry - pH and saturation indices ........................................................................................................... S6 References ................................................................................................................................................................... S6

List of Figures SI-Figure 1 Photo of a fractured core taken before the experiment with the fracture surfaces of the two core halves facing the camera. .............................................................................................................................................. S1 SI-Figure 2 Residual graphs for (a) linear fitting, and (b) non-linear fitting. ................................................................ S5 SI-Figure 3. Measured inlet and outlet pH (a) and calculated saturation index (b) over time for the low PCO2 experiment (blue), and the high PCO2 experiment (red). Downward arrows indicate when xCT scans were performed. The steps in the influent pH measurements were caused by refill of CO2. The pump used for bubbling CO2 into the brine is a small total volume. Several batches were made throughout a single experiment. ........................................................................................................................................................ S6

List of Tables SI-Table 1 Estimated average aperture change [m] during xCT scan. ......................................................................... S3 SI-Table 2 Statistical characterizations of the initial fractures. ................................................................................... S3 SI-Table 3 Chemical reactions modeled, and the equilibrium parameters (the equilibrium parameters are after PHREEQC database). ........................................................................................................................................... S4 SI-Table 4 Calcite dissolution kinetic rate parameters from the literature, and determined in this study for high pressure conditions. ........................................................................................................................................... S5

i

Indiana Limestone Cores The Indiana Limestone specimen was purchased from Kocurek Industries (product ID B-101a). The rock was cored (1’’D X 2’’L), and in each core, a single clean fracture was created using the modified Brazilian method. The two core halves were put together with offset of several millimeters, trimmed, stabilized by coating the exterior of the core with epoxy, and turned down on a lathe. This ensured that the core maintained a tight seal with the confining rubber sleeve once placed in a TEMCO carbon fiber core holder (Core Laboratories L.P.). To initialize a core before the flow experiment, it was subjected to three confining pressure cycles, up to 27 bar of effective confining pressure, to achieve a linear elastic fracture1.

SI-Figure 1 Photo of a fractured core taken before the experiment with the fracture surfaces of the two core halves facing the camera.

Details of the high-pressure core-flow experiments The flow system for these experiments was constructed in the climate-controlled xCT scanner room at NETL. Here we provide details about the system presented in Fig. 1 in the manuscript. The flow system pumps were dual-syringe pumps (Teledyne ISCO). Prepared brine was placed into the high-pressure mixing vessel, where it was equilibrated with CO2 at the desired pressure. The chemicals used were reagent grade, and the CO2 was high purity grade. For all experiments, the flow conditions were the same. The upstream pump adjusted the injection pressure so that the CO2-acidified brine was injected at a constant rate of 0.5 ml/min, and the downstream receiving pump maintained the back-pressure at 100 bar. A separate pump maintained the confining pressure at 120 bar. Pressure transducers were installed upstream and downstream of the core, but ultimately these data were not used because the pressure differences were too small to be measurable. The flow times for the experiments ranged from 2 to 3 days.

S1

Two sets of in-line high-pressure pH probes (Corr Instruments) were used. The upstream probe confirmed that the designed brine pH was achieved and maintained. The downstream probe was used to observe increases in pH to serve as a real-time indicator of calcite dissolution. Effluent samples were collected using a bleed valve, acidified by nitric acid and stored in sealed glass vials. They were analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES, Perkin Elmer Optima 4300 DV Optical Emission Spectrometer) for dissolved Ca concentrations. No filter was used for sampling. 2,3

Previous experimental studies have shown that the primary source for particle mobilization is preferential dissolution of carbonates and release of non-reactive particles such as clays. Given the homogeneous nature of Indiana Limestone, there was no preferential dissolution that releases particles.

There are two options for controlling temperature in flow experiments: thermal wrap and climate control. Thermal wrap would have allowed us to have higher temperature conditions, which are representative of the deep subsurface. This approach was not viable in our case because of the complexity of the tubing, valves and connections that couple the flow-through system and xCT imaging. Reliable temperature control of the system using thermal wrapping would have been extremely difficult. We were concerned that potential thermal gradients may actually cause artifacts and undermine the experiments. Therefore, we decided to conduct the experiments at the climate control conditions in the room at 25 °C. We are confident that we had a constant and spatially uniform temperature. The room temperature could not be elevated because of the sensitivity of the xCT scanner. The two experiments that are reported here are part of an original set of four experiments: two for each type of influent brine, representing an experimental effort totaling more than a six month period. Because of the unique challenges of xCT imaging of a high-pressure core-flow experiment, much of this time was devoted to method development. In the end, only two experiments, one for each type of brine, produced high quality xCT images. For the two reported experiments, reference points within the rock matrix that were unaffected by reactive flow were used to confirm that the fracture was mechanically stable throughout the experiments, and when combined with confirmation that the pumps maintained constant volumetric flux at the inlet boundary and constant pressure at the outlet provides reassurance that the observed fracture geometry evolution occurred under the intended reactive flow conditions.

xCT scanning Each scan took 1.5 hours. Scanning was done using a microfocus x-ray tube at 220 keV and 300 µA, and 16 bit grade option for the flat panel detector. The 2D radiographs collected at each scan were reconstructed using the proprietary software efX (North Star Imaging). During the scans, to eliminate pressure perturbations, the fractured cores were not drained and were in contact with the reactive fluid. In principle, the fracture surfaces were subject to reactions with the stagnant CO2–acidified brine during the scans. To investigate the extent to which the fractures might be altered during the 1.5 hours of xCT scanning, a batch system model was developed. The fracture was assumed to be a well-mixed system, and its volume was estimated from the xCT data. The initial fluid in

S2

the reactor had the same composition as the influent. The amount of calcite dissolution needed for the reactor to reach equilibrium was calculated. The average aperture change in the fracture resulting from calcite dissolution was calculated and reported in SI-Table 1 for all fracture geometries scanned. All the values are smaller than 1 µm, and are orders of magnitude smaller than the xCT resolutions. In other words, the possible change caused by reactions during the scan is negligible compared to the uncertainties of the xCT results, and the changes caused by the flow that are discernable on the xCT images. Because in reality the fracture during the scan was not a well-mixed reactor and the reaction in the fluid may not have reached equilibrium, our estimates are likely to be over-estimates. Therefore, even though the well-mixed reactor model is a simplification of reality, the conclusion that the reaction during scan is negligible is still valid. SI-Table 1 Estimated average aperture change [m] during xCT scan.

Scan #1 Scan #2 Scan #3 Scan #4 Scan #5 Scan #6

Low PCO2 High PCO2 experiment experiment 7.32E-08 2.48E-07 2.36E-07 2.88E-07 2.88E-07 5.12E-07 3.83E-07 6.15E-07 4.14E-07 6.49E-07 7.40E-07

Initial fracture aperture statistics The statistical characterizations of the initial fracture geometries of the cores in the two experiments are shown in SI-Table 2. These statistics were calculated from the geometries derived from the xCT scans of the cores under pressure at the beginning of the experiments. SI-Table 2 Statistical characterizations of the initial fractures.

Mean aperture (µm) Standard Deviation of aperture (µm) Relative Roughness (std/mean) Contact Ratio (%) Correlation length (longitudinal direction) (µm) Correlation length (transverse direction) (µm) Surface area (m2)

S3

Low PCO2 High PCO2 experiment experiment 187.3 244.0 196.1 160.4 1.05 0.66 16.3 7.32 98 45 966 885 0.0033 0.0035

1D reactive transport model details Here we present supporting information about the 1D reactive transport modeling. Only the fracture volume was considered part of the model system. Even though the rock matrix is porous, reactive flow within the rock matrix was not modeled. This was justified because the xCT images did not show measurable changes to the rock matrix porosity over the course of the experiments. In addition, the permeability difference between the fracture and rock matrix is consistent with literature reports of negligible mass transfer between them 4. The initial condition throughout was zero Ca and total carbon concentrations. At the inlet, continuous flux at a constant rate of 0.5 ml/min with solute concentrations documented in Table 1 (in the manuscript) was imposed. There was no concentration gradient for the components at the outlet, and there was no mass transfer at the lateral boundary. The transport and geochemical reaction equations were solved sequentially 5. The advection and diffusion were solved first, using an upwind scheme. The concentrations were updated and then used in calculating R. The amount of calcite dissolution was used to update b of the corresponding cell. SI-Table 3 lists the reactions that were modeled and their equilibrium constants. Activities were determined using activity coefficients calculated by the Davis equation. The reaction equilibrium equations and the charge balance were solved to speciate water and carbonic acid: [  ] + 2[  ] = [ ] + [ ] + 2[ ] SI-Table 3 Chemical reactions modeled, and the equilibrium parameters (the equilibrium parameters are after PHREEQC database). log10K 

  +   ⇔   +  Calcite dissolution mechanisms



  +   ∗ ⇔   + 2 

Carbonic acid dissociation Water dissociation

  ⇔   + 

-8.48

  ∗ ⇔ 2  + 

-6.35

 ⇔   + 

-10.33

  ⇔   + 

-14

Kinetic parameter estimation For this study, experimental data from Pokrovsky et al (2005, 2009) 6,7 were used to fit the kinetic parameters for calcite dissolution, according to eqn (2) in the manuscript. The data used are measurements at 25°C, at 425 rpm, and up to 50 atm PCO2. For this data set, the pH varies from 3.8 to 5.6. Because one of the independent variables - the activity of proton - varies by two orders of magnitude, and the measured reaction rate varies by one order of magnitude, a non-linear regression approach was S4

applied. Two objective functions were tested: one that fit the data in log space (SI-eqn(1)) and one that fit the data without transformation (SI-eqn(2)). min ∑

!"# $ %, + "# $ '(∗ , + "# )

SI-eqn(1)

!"# $ %, + "# $ '(∗ , + "# )

min ∑

SI-eqn(2)

To examine the alternative regression objective functions, the residuals were analyzed with regard to being independent, identically distributed and normal. As shown in SI-Figure 3, the residual analysis of linear fitting demonstrates evident curvilinear relation, while on the residual graph of non-linear fitting, the residual values are more randomly distributed. Therefore, it was determined that the parameters determined by fitting to rate data in log space were more defensible given adherence to assumptions of IIDN in statistical regression.

SI-Figure 2 Residual graphs for (a) linear fitting, and (b) non-linear fitting.

The resulting parameter estimates are shown in SI-Table 4, along with other parameters estimates reported in the literature. SI-Table 4 Calcite dissolution kinetic rate parameters from the literature, and determined in this study for high pressure conditions.

8

Chou et al. (1989) 9 Molins et al. (2014) using data published by Pokrovsky et al., (2005; 6,7 2009) . This study, using data published by Pokrovsky et 6,7 al., (2005; 2009) .

Upper limit of CO2 pressure of the fitted data [bar] 1 10

pH range of the fitted data 4 to 10 4.5 to 5.4

50

3.8 to 5.6

k1 2 mol/m s (95% confidence interval)

k2 2 mol/m s (95% confidence interval)

0.89 0.08

5×10 -4 1.6×10

6.6×10 -5 1.25×10

0.083 (0.05 - 0.12)

1.1×10 -5 (9.8×10 -4 1.3×10 )

-4

1.5×10 -5 (1.0×10 -5 2.0×10 )

S5

-4

k3 2 mol/m s (95% confidence interval) -7

-5

Effluent chemistry - pH and saturation indices The low PCO2 brine in equilibrium with calcite would have resulted in an effluent pH of 5.4, while the equilibrium effluent pH of the high PCO2 brine would be 5.0. However, the pH measurements at the outlet are below the equilibrium values, ranging between 4.0 and 4.5. The measured pH and Ca concentrations were used to calculate saturation index, which is the logarithm of the '*% '(+ ⁄",ratio. The negative values suggest that the systems were under-saturated with respect to calcite.

SI-Figure 3. Measured inlet and outlet pH (a) and calculated saturation index (b) over time for the low PCO2 experiment (blue), and the high PCO2 experiment (red). Downward arrows indicate when xCT scans were performed. The steps in the influent pH measurements were caused by refill of CO2. The pump used for bubbling CO2 into the brine is a small total volume. Several batches were made throughout a single experiment.

References 1. Huerta, N. J.; Bryant, S. L.; Strazisar, B. R.; Kutchko, B. G.; Conrad, L. C. The influence of confining stress and chemical alteration on conductive pathways within wellbore cement. Energy Procedia; Greenhouse Gas Control Technologies 9 Proceedings of the 9th International Conference on Greenhouse Gas Control Technologies (GHGT-9), 16-20 November 2008, Washington DC, USA 2009, 1, 3571-3578. S6

2. Noiriel, C.; Made, B.; Gouze, P. Impact of coating development on the hydraulic and transport properties in argillaceous limestone fracture. Water Resour. Res. 2007, 43, W09406. 3. Ellis, B. R.; Fitts, J. P.; Bromhal, G. S.; McIntyre, D. L.; Tappero, R.; Peters, C. A. Dissolution-Driven Permeability Reduction of a Fractured Carbonate Caprock. Environ. Eng. Sci. 2013, 30, 187-193. 4. Gherardi, F.; Audigane, P. Modeling geochemical reactions in wellbore cement: assessing preinjection integrity in a site for CO2 geological storage. Greenhouse Gases-Science and Technology 2013, 3, 447-474. 5. Steefel, C. I.; MacQuarrie, K. T. B. Approaches to modeling of reactive transport in porous media. Reactive Transport in Porous Media 1996, 34, 83-129. 6. Pokrovsky, O. S.; Golubev, S. V.; Schott, J. Dissolution kinetics of calcite, dolomite and magnesite at 25 degrees C and 0 to 50 atm pCO2. Chem. Geol. 2005, 217, 239-255. 7. Pokrovsky, O. S.; Golubev, S. V.; Schott, J.; Castillo, A. Calcite, dolomite and magnesite dissolution kinetics in aqueous solutions at acid to circumneutral pH, 25 to 150 degrees C and 1 to 55 atm pCO2: New constraints on CO2 sequestration in sedimentary basins. Chem. Geol. 2009, 265, 20-32. 8. Chou, L.; Garrels, R. M.; Wollast, R. Comparative-Study of the Kinetics and Mechanisms of Dissolution of Carbonate Minerals. Chem. Geol. 1989, 78, 269-282. 9. Molins, S.; Trebotich, D.; Yang, L.; Ajo-Franklin, J. B.; Ligocki, T. J.; Shen, C.; Steefel, C. I. Pore-Scale Controls on Calcite Dissolution Rates from Flow-through Laboratory and Numerical Experiments. Environ. Sci. Technol. 2014, 48, 7453-7460.

S7