The Use of the Surface Roughness Value to Quantify the Extent of Supercritical CO 2 Involved Geochemical Reaction at a CO 2 Sequestration Site

Changes in the physical properties of the supercritical CO2 (scCO2) reservoir rock is one of the most important factors in controlling the storage safety at a scCO2 sequestration site. According to recent studies, it is probable that geochemical reactions influence changes in the rock properties after a CO2 injection in the subsurface, but quantitative data that reveal the interrelationship of the factors involved and the parameters needed to evaluate the extent of scCO2-rock-groundwater reactions have not yet been presented. In this study, the potential for employing the surface roughness value (SRRMS) to quantify the extent of the scCO2 involved reaction was evaluated by lab-scale experiments. For a total of 150 days of a simulation of the scCO2-sandstone-groundwater reaction at 100 bar and 50 ◦C, the trends in changes in the physical rock properties, pH change, and cation concentration change followed similar logarithmic patterns that were significantly correlated with the logarithmic increase in the SRRMS value. These findings suggest that changes in surface roughness can quantify the extent of the geochemical weathering process and can be used to evaluate leakage safety due to the progressive changes in rock properties at scCO2 storage sites.


Introduction
Various CO 2 capture and sequestration technologies have been developed to reduce CO 2 emissions and the International Energy Agency (IEA) has announced that CO 2 capture and sequestration (CCS) technology will cover 19% of the total CO 2 reductions by 2050 [1][2][3][4][5][6].Among them, geological sequestration will be deeply considered to reduce CO 2 emissions at low cost [7][8][9][10][11].Several CO 2 sequestration projects in the world have been successfully conducted [12][13][14][15], but some CO 2 injection sites were found to be impractical, even though their site characterization, such as geophysical exploration, geo-structural investigation, and drilling core analysis, was thoroughly conducted [16,17].For successful CO 2 sequestration in heterogeneous subsurface formations, the practicability of the CO 2 sequestration process based on storage capacity and leakage prevention should be guaranteed before the field scale CO 2 injection.The previous research over the last two decades revealed that the injection of supercritical CO 2 (scCO 2 ) into a reservoir rock initiates a geochemical reaction in the scCO 2 -rock-groundwater system, but it does not threaten the storage safety of CO 2 reservoir formations because of its long reaction time [18][19][20].However, in the past five years, several studies have cast doubts on whether the rate of the scCO 2 -rock-groundwater reaction is too slow to affect the change of CO 2 storage capacity and leakage safety after scCO 2 injection [21][22][23][24][25][26].
If there exists the possibility of a fast reaction, and thus the geochemical reaction is one of the more important parameters controlling CO 2 storage and leakage safety, then a quantitative understanding of the scCO 2 -rock-groundwater reaction occurring in the CO 2 storage site is essential for successful CO 2 sequestration in the subsurface [27][28][29].Over the last decade, meaningful information about what kinds of geochemical reactions dominate in CO 2 sequestration sites have been qualitatively acquired through laboratory scale experiments, but most of these have been limited to qualitative investigations such as compositional and/or mineralogical analyses [30][31][32][33][34][35][36].These results verified only the occurrence of a geochemical reaction at the CO 2 sequestration system, and they were limited in providing the extent of the reaction or discovering its influence on sequestration capacity, such as leakage or storage capacity, because it is difficult to reenact the geochemical reaction occurring at a CO 2 storage site and also to determine the interrelationship between the geochemical reaction and the other property changes of the reservoir rock [37][38][39].It is now more important than ever to search for the available CO 2 reservoir sites in the subsurface locations and to prevent the accidental leakage of CO 2 originating from a geochemical reaction.However, the development of a new quantitative parameter for evaluating the scCO 2 involved geochemical reaction in sequestration conditions has almost never been attempted in previous studies [40,41].
The present study focuses on the quantitative investigation of the scCO 2 involved reaction occurring at CO 2 storage sites and provides the potential use of changes in surface roughness of the CO 2 reservoir rock to quantify the extent of the scCO 2 -reservoir rock-groundwater reaction.The interrelationship among property changes originating from the geochemical reaction is also bound to be revealed by the laboratory scale experiments.This is a novel approach to developing a quantitative technique that uses the surface roughness to investigate the scCO 2 -rock-groundwater reaction and also to evaluate the long-term rock property changes originating from the geochemical reaction in scCO 2 storage conditions.The results of this study will provide ideas for the development of a method for safety estimation based on the extent of the geochemical reaction in a CO 2 sequestration site and of a future plan for strengthening the successful confrontation technology for CO 2 leakage.

Sandstone Cores and Groundwater Samples
Currently, South Korea is ranked seventh in the world in terms of CO 2 emissions, having produced a total of 739 megatons in 2011 alone [42], and the Korean government currently plans to inject more than 100 megatons of CO 2 into the subsurface by 2020 [43].From a previous study, the Sindong lithostratigraphic group of the Gyeongsang basin was considered as an available CO 2 storage site on the Korean peninsula (CO 2 storage capacity of 1011 megatons) [44].In 2014, the Gyeongsang basin was announced as one of the candidate sites for a scCO 2 storage test in Korea, and continuous drill cores (4.2 cm of average diameter) were collected at two drilling sites (average 1200 m in depth).From the property analysis of these cores, three coarse sandstone formations (850, 1000, and 1050 m in depth) with medium porosity (porosity of 7.0%-9.8%)and moderate thickness (total of 100 m) were found, and their sandstone cores (S1, S2, and S3) were used for this study.A total of nine consolidated cylindrical sandstone cores (each 4.2 cm in diameter and 6-7 cm in length) without cracks or fractures were used to measure the physical and geochemical property changes of the cores during the scCO 2 -water-core reaction.For each sandstone core, several thin slabs (1 × 1 × 0.2 cm each) were additionally prepared to observe the surface roughness change and the cation concentration change for the whole reaction time.The geological map around the drilling site, the sandstone core, and the slab used for the experiment are shown in Figure 1.The mineral composition of each core was identified by modal analysis and more than 500 locations on each thin section surface were observed using the point counter attached in a polarizing microscope.
Appl.Sci.2017, 7, x 3 of 18 was identified by modal analysis and more than 500 locations on each thin section surface were observed using the point counter attached in a polarizing microscope.
Figure 1.Geological map around the drilling site ( ) and photographs of sandstone cores and a slab for the experiment (modified from [22]).
The results of the modal analysis for the three cores (S1, S2, and S3) reveal that the sandstone surface is mostly composed of quartz, perthite, plagioclase, calcite, muscovite, biotite, and chlorite, and their average proportions were 47.4%, 18.5%, 10.5%, 5.4%, 1.7%, 1.5%, and 1.4%, respectively (Table 1).The surface roughness changes for these seven minerals were specifically targeted during the experiment.was identified by modal analysis and more than 500 locations on each thin section surface were observed using the point counter attached in a polarizing microscope.
Figure 1.Geological map around the drilling site ( ) and photographs of sandstone cores and a slab for the experiment (modified from [22]).
) and photographs of sandstone cores and a slab for the experiment (modified from [22]).
The results of the modal analysis for the three cores (S1, S2, and S3) reveal that the sandstone surface is mostly composed of quartz, perthite, plagioclase, calcite, muscovite, biotite, and chlorite, and their average proportions were 47.4%, 18.5%, 10.5%, 5.4%, 1.7%, 1.5%, and 1.4%, respectively (Table 1).The surface roughness changes for these seven minerals were specifically targeted during the experiment.Groundwater was sampled from a monitoring well near the drilling site (>400 m in depth) and analytical results for the groundwater quality are shown in Table 2.The concentrations of Na + and Cl − were higher than those of fresh water, suggesting the groundwater was mixed with sea water below the drilling site.The identification of the scCO 2 -rock-pore water geochemical reaction at scCO 2 sequestration conditions has been studied by various analytical processes.These approaches include qualitative processes mostly depending on visual inspection or compositional analysis using SEM/EDS (Scanning Electron Microscope/Energy Dispersive X-ray Spectrometer; VEGA II LSU, Tescan, Brno, Czech Republic), XRD (X-ray Diffractometer; X Pert-MPD, Philips, Eindhoven, The Netherlands), and XRF (X-ray Fluorescence Spectrometer; XRF-1800, Shimadzu, Kyoto, Japan) [45,46].However, quantitative information acquired from this study was very limited, such as in mineral composition transfer and structural change [47,48].To supplement these limitations, the surface roughness change of the rock surface during the scCO 2 -sandstone-groundwater reaction was investigated, and the feasibility of its use as a parameter to evaluate the geochemical weathering process was verified by comparing the physical property change of the sandstone with the cation concentration change in groundwater during the geochemical reaction.
To observe the surface roughness change of the slab surface over the reaction time, one surface of each slab prepared from the sandstone core was polished with diamond-coated paper and the other slab surface was attached to a glass slide plate with epoxy resin.After being washed with methanol, each thin slab was placed at the bottom of a Teflon beaker, which was fixed inside a high pressurized stainless steel cell (with a capacity of 150 mL), and 100 mL of groundwater was poured into the Teflon beaker.The inside of the cell was pressurized up to 100 bar using a high-pressure syringe pump (Isco-D260, TELEDYNE ISCO, Lincoln, NE, USA) and the empty space in the cell was filled with scCO 2 by a gas flow regulator attached to a high-pressure syringe pump.Each cell was placed in an oven to maintain a constant temperature of 50 • C, which led to a scCO 2 -rock-groundwater reaction in the cell at scCO 2 sequestration conditions.The scCO 2 -sandstone slab-groundwater reaction in the cell was executed at 100 bar and 50 • C for 150 days and the apparatus of the slab experiment is shown in Figure 2. To observe the surface roughness change of the slab surface over the reaction time, one surface of each slab prepared from the sandstone core was polished with diamond-coated paper and the other slab surface was attached to a glass slide plate with epoxy resin.After being washed with methanol, each thin slab was placed at the bottom of a Teflon beaker, which was fixed inside a high pressurized stainless steel cell (with a capacity of 150 mL), and 100 mL of groundwater was poured into the Teflon beaker.The inside of the cell was pressurized up to 100 bar using a high-pressure syringe pump (Isco-D260, TELEDYNE ISCO, Lincoln, NE, USA) and the empty space in the cell was filled with scCO2 by a gas flow regulator attached to a high-pressure syringe pump.Each cell was placed in an oven to maintain a constant temperature of 50 °C, which led to a scCO2-rockgroundwater reaction in the cell at scCO2 sequestration conditions.The scCO2-sandstone slabgroundwater reaction in the cell was executed at 100 bar and 50 °C for 150 days and the apparatus of the slab experiment is shown in Figure 2. According to previous studies related to geochemical reactions, the weathering process of the reservoir rock after scCO2 injection was closely related to the surface roughness change of the rock because dissolution and precipitation occur simultaneously and lead to surface roughness change until the geochemical system turns back to a status of equilibrium [22,49].To observe the surface roughness change of sandstone slabs during the geochemical reaction, seven minerals, quartz, perthite, plagioclase, muscovite, calcite, biotite, and chlorite for each slab surface were determined According to previous studies related to geochemical reactions, the weathering process of the reservoir rock after scCO 2 injection was closely related to the surface roughness change of the rock because dissolution and precipitation occur simultaneously and lead to surface roughness change until the geochemical system turns back to a status of equilibrium [22,49].To observe the surface roughness change of sandstone slabs during the geochemical reaction, seven minerals, quartz, perthite, plagioclase, muscovite, calcite, biotite, and chlorite for each slab surface were determined using a reflecting microscope (Nikon eclipse LV100N, Nikon, Fukasawa, Japan), and three locations for each mineral were randomly selected to acquire 3-dimensional surface images.Each slab was taken out from the cell and dried in an oven at certain reaction time intervals.Then, a total of 262,144 points (512 × 512) around each selected location (the certain surface domain of 50 × 50 µm) of the slab were scanned by an SPM (Scanning Probe Microscope; Icon-PT-PLUS, Bruker, Bremen, Germany).The SPM included a cantilever tip connected to a probe with the laser beam, which was used to measure the height difference at each point and at the standard level (mean height), in nanometers, finally acquiring a 3-dimensional surface image profile for the certain surface domain of a slab (Figure 3).The information acquired from SPM analysis is useful not only for the surface image but also for more quantitative parameters to evaluate the roughness degree of the surface [50,51].Several surface roughness parameters have been developed, and in this study, a surface roughness value (SR RMS ) was used to reduce all of the information in the 3-dimensional surface image to a single number representing the degree of the surface roughness [52,53].For this study, the SR RMS for the specific surface domain on each slab was calculated from the RMS (root mean square), for which the data can be acquired by measuring the height difference between the standard level (Z 0 is the mean height from all points in a surface domain) and the height of each point in a domain [52].
Appl.Sci.2017, 7, x 6 of 18 be acquired by measuring the height difference between the standard level (Z0 is the mean height from all points in a surface domain) and the height of each point in a domain [52].
The following equation was used to calculate the SRRMS (unit: µm) for a specific surface domain: where n is the total point number (262,144) in the surface domain, and Δzi is the vertical height difference from the mean height (Zo) to the ith point in the domain [54].The degree to which the initial surface roughness differed from the reacted surface roughness at a certain reaction time was determined by comparing their SRRMS values.The interrelationship between the degree of the surface roughness and the extent of the geochemical reaction was investigated and the potential application of SRRMS to quantify the weathering process originating from the scCO2-sandstone-groundwater reaction was also evaluated by the experiment.The SRRMS values acquired from all minerals′ SRRMS values were plotted at different reaction times, and the trend of SRRMS change was compared with the trend of the physical property change of sandstones and the cation concentration change in groundwater, which verified that the surface roughness change The following equation was used to calculate the SR RMS (unit: µm) for a specific surface domain: where n is the total point number (262,144) in the surface domain, and ∆z i is the vertical height difference from the mean height (Z o ) to the ith point in the domain [54].
The degree to which the initial surface roughness differed from the reacted surface roughness at a certain reaction time was determined by comparing their SR RMS values.The interrelationship between the degree of the surface roughness and the extent of the geochemical reaction was investigated and the potential application of SR RMS to quantify the weathering process originating from the scCO 2 -sandstone-groundwater reaction was also evaluated by the experiment.The SR RMS values acquired from all minerals SR RMS values were plotted at different reaction times, and the trend of SR RMS change was compared with the trend of the physical property change of sandstones and the cation concentration change in groundwater, which verified that the surface roughness change reflects the extent of the geochemical weathering process and the rock property change.To verify that the scCO 2 -sandstone-groundwater reaction occurs actively, SEM/EDS analyses for the slab surface were performed and the mineralogical transfer and compositional change on the slab surface were investigated.Three milliliters of groundwater were sampled from the cell at different reaction times (maximum of 150 days) for the chemical analysis, and the same amount of groundwater was added into the cell.The pH and the concentrations of six main cations (Al 3+ , Ca 2+ , Fe 2+ , K + , Na + , and Si 4+ ) in the groundwater sample were analyzed via ICP/AES (Inductively Coupled Plasma/Atomic Emission Spectrometer; Optima 7300V, Perkin Elmer, Waltham, MA, USA) and using a pH meter, and they were plotted at different reaction times (0-150 reaction days).The relationship among pH change, cation concentration change, physical property change, and SR RMS change was investigated to verify the feasibility of SR RMS usage as a parameter to quantify the extent of the geochemical reaction.

Measurement of Physical Property Changes for the Sandstone Cores
Several previous studies confirmed that physical properties, such as the porosity, density, and seismic wave velocity of CO 2 reservoir rock can change after CO 2 injection into the reservoir formation [23,32,38,55].However, it has not been clearly proved what the main reason is for the property change of the reservoir rock and how the change proceeds so rapidly.It is important to quantify the extent of the rock property change because it can affect both leakage safety and CO 2 storage capacity of the reservoir.Changes in porosity, density, and Primary/Secondary (P/S) seismic velocity for nine sandstone cores were measured at different scCO 2 -core-groundwater reaction times, and the results were compared with those of the SR RMS results.To quantify the physical property changes of three sandstones (S1, S2, and S3) due to the geochemical reaction, each core (three cores for each sandstone) was placed in a large stainless steel chamber (1.5 L of capacity), and 1 L of groundwater was added at 100 bar and 50 • C.Then, scCO 2 was injected into the chamber by a high-pressure syringe pump to fill the empty space of the chamber.The inside pressure of the chamber was maintained at 100 bar by a back-pressure controller and the chamber was coated with a heating jacket at 50 • C to maintain the CO 2 as a super critical fluid during the whole reaction time.After each core was taken from the chamber and dried in an oven, the physical properties of the core were measured at different reaction times (total 150 reaction days).The control experiment without scCO 2 in the chamber (zero air instead) was also repeated to separate the effects of scCO 2 injection on rock property changes.Dry density, porosity, and seismic velocity of the cores were measured by the "ASTM/C97" test, "ASTM/C97M-09" test, and "ASTM/D2845-08" test referred from the American Society of Testing Materials.A more detailed procedure for the core experiment can be found in [54].The property values for the three sandstones vs. reaction time were plotted and their changes were also compared with those of SR RMS and the cation concentration in groundwater.The schematic of the core experiment to measure the physical property changes of sandstone is shown in Figure 4.
jacket at 50 °C to maintain the CO2 as a super critical fluid during the whole reaction time.After each core was taken from the chamber and dried in an oven, the physical properties of the core were measured at different reaction times (total 150 reaction days).The control experiment without scCO2 in the chamber (zero air instead) was also repeated to separate the effects of scCO2 injection on rock property changes.Dry density, porosity, and seismic velocity of the cores were measured by the "ASTM/C97" test, "ASTM/C97M-09" test, and "ASTM/D2845-08" test referred from the American Society of Testing Materials.A more detailed procedure for the core experiment can be found in [54].The property values for the three sandstones vs. reaction time were plotted and their changes were also compared with those of SRRMS and the cation concentration in groundwater.The schematic of the core experiment to measure the physical property changes of sandstone is shown in Figure 4.

Measurement of the Surface Roughness Value for the Sandstone Slabs
Three-dimensional SPM surface images of the sandstone slab before and after the reaction confirmed that the serious surface roughness change occurred rapidly by the scCO2-sandstone-

Measurement of the Surface Roughness Value for the Sandstone Slabs
Three-dimensional SPM surface images of the sandstone slab before and after the reaction confirmed that the serious surface roughness change occurred rapidly by the scCO 2 -sandstone-groundwater reaction (Figure 5).As seen by comparison with the initial surface, the polished surface of three major minerals (quartz, plagioclase, and biotite) transformed into a bumpy surface at 120 reaction days, suggesting that the weathering processes such as dissolution and precipitation actively occurred on the mineral surfaces of the S1 sandstone, directly increasing the surface roughness.SPM analysis of the calcite surfaces for the sandstone slabs was impossible because the calcites were perforated or seriously damaged by dissolution within 30 reaction days.Results of the SPM image analyses confirmed that the scCO 2 -groundwater-sandstone reaction directly resulted in a surface roughness change of the sandstones.
The SR RMS values for three different locations of three minerals for the three sandstone slabs were used to estimate the extent of the surface weathering process of the sandstone at different reaction times.A total of nine SR RMS values for each sandstone slab at a certain reaction time and their average value for each sandstone (S1, S2, and S3) are shown in Table 3 and all the SR RMS values are plotted in Figure 6.In the case of the S1 slab, the average SR RMS of quartz increased from 0.573 to 1.293 µm within 30 reaction days, and the rate of the change became gradually stable after 60 days (Table 3).The average SR RMS of plagioclase and biotite also increased from 0.344 and 0.772 µm to 1.905 and 1.485 µm, respectively, within 30 days, and their SR RMS values gently increased after 60 days.The trends of the SR RMS change for the S2 and the S3 slabs were very similar to that for the S1 slab.The results suggest that the surface roughness increased actively at the early reaction stage, but the change in roughness stabilized after a few months of reaction.
reaction days, suggesting that the weathering processes such as dissolution and precipitation actively occurred on the mineral surfaces of the S1 sandstone, directly increasing the surface roughness.SPM analysis of the calcite surfaces for the sandstone slabs was impossible because the calcites were perforated or seriously damaged by dissolution within 30 reaction days.Results of the SPM image analyses confirmed that the scCO2-groundwater-sandstone reaction directly resulted in a surface roughness change of the sandstones.The SRRMS values for three different locations of three minerals for the three sandstone slabs were used to estimate the extent of the surface weathering process of the sandstone at different reaction   Figure 6 shows the average SR RMS for the three sandstones at different reaction days, comparing SR RMS with the control slab (S1 slab reacted with only groundwater).The increase in the average SR RMS for the S1 slab with only groundwater was negligible for 120 reaction days (only a 2.8% increase).However, the average SR RMS for the S1 slab with scCO 2 and groundwater increased by 2.5 times during 120 reaction days and 4.4 and 3.5 times for the S2 and the S3 slab, respectively.For all three sandstones with the scCO 2 and groundwater reaction, the average SR RMS increased precipitously for 30 days of the reaction.After 60 days of the reaction, the SR RMS increase became less severe, suggesting that no more serious increase in surface roughness occurred.For quantitative analysis of the surface roughness change for the three sandstones (S1, S2, and S3), the curve fitting results for their SR RMS values using the logarithmic Equation (2) are delineated in Figure 6: where t is the reaction time (day); a is the rate constant of the surface roughness change; and b is the intercept constant [54].The r-square values for all fitted curves are higher than 0.83 (average of 0.891 in Figure 6), indicating that the SR RMS change follows a well-defined logarithmic pattern.
values using the logarithmic Equation 2 are delineated in Figure 6: where t is the reaction time (day); a is the rate constant of the surface roughness change; and b is the intercept constant [54].The r-square values for all fitted curves are higher than 0.83 (average of 0.891 in Figure 6), indicating that the SRRMS change follows a well-defined logarithmic pattern.The pH of the groundwater decreased to 5.3 due to the dissolved scCO2, which led to the dissolution of the sandstone slab in groundwater, but it became stable after 60 reaction days, suggesting that the amount of hydrogen ions in the groundwater increased due to the dissolution of CO2 at an early stage, but the pH levels remained stable due to the buffering of dissolved ions with the increase of the reaction time (within 90-120 days).The trend of the pH decrease in the groundwater vs. the reaction time was also very similar to that of the SRRMS increase of the slabs (the logarithmic decrease (or increase) pattern in Figure 6), indicating that the geochemical weathering process of the sandstone reservoir was very active in the early CO2 injection time but stabilized within a few months.If the trend of the scCO2 involved geochemical reaction is similar to that of the SRRMS, the surface roughness change could be used to quantify the extent of the scCO2-rock-groundwater reaction.
In this study, the trend of the dissolved ion concentration change in the groundwater during the scCO2-involved reaction was also compared to the trend of the SRRMS and pH change.The concentration of five cations in the groundwater during the reaction is shown in Figure 7.The pH of the groundwater decreased to 5.3 due to the dissolved scCO 2 , which led to the dissolution of the sandstone slab in groundwater, but it became stable after 60 reaction days, suggesting that the amount of hydrogen ions in the groundwater increased due to the dissolution of CO 2 at an early stage, but the pH levels remained stable due to the buffering of dissolved ions with the increase of the reaction time (within 90-120 days).The trend of the pH decrease in the groundwater vs. the reaction time was also very similar to that of the SR RMS increase of the slabs (the logarithmic decrease (or increase) pattern in Figure 6), indicating that the geochemical weathering process of the sandstone reservoir was very active in the early CO 2 injection time but stabilized within a few months.If the trend of the scCO 2 involved geochemical reaction is similar to that of the SR RMS , the surface roughness change could be used to quantify the extent of the scCO 2 -rock-groundwater reaction.
In this study, the trend of the dissolved ion concentration change in the groundwater during the scCO 2 -involved reaction was also compared to the trend of the SR RMS and pH change.The concentration of five cations in the groundwater during the reaction is shown in Figure 7.The average K + and Si 4+ concentrations in the groundwater increased from 23.3 and 12.4 mg/L to 30.0 and 26.0 mg/L after 30 reaction days, respectively, and were then stabilized by gradually decreasing after 90 days.The results suggest that the concentration of K + and Si 4+ mainly increased by the dissolution of silicate minerals such as K-feldspar and then decreased by the precipitation of secondary minerals, including K + ions (this is also supported by SEM/EDS analyses in Figure 8 and Figure 9).The average concentration of Ca 2+ and Na + increased from 547.1 and 633.2 mg/L to 663.8 and 692.0 mg/L after 30 days, respectively.However, after 90 days, they gradually decreased to 579.4 and 627.0 mg/L.The results showed that most minerals in the sandstone slab had a tendency to dissolve in the groundwater during the early reaction days (about 30-60 days), but the dissolution became stable after 90 days of reaction, resulting from the secondary precipitations overcoming the continuous dissolution during the The average K + and Si 4+ concentrations in the groundwater increased from 23.3 and 12.4 mg/L to 30.0 and 26.0 mg/L after 30 reaction days, respectively, and were then stabilized by gradually decreasing after 90 days.The results suggest that the concentration of K + and Si 4+ mainly increased by the dissolution of silicate minerals such as K-feldspar and then decreased by the precipitation of secondary minerals, including K + ions (this is also supported by SEM/EDS analyses in Figures 8  and 9).The average concentration of Ca 2+ and Na + increased from 547.1 and 633.2 mg/L to 663.8 and 692.0 mg/L after 30 days, respectively.However, after 90 days, they gradually decreased to 579.4 and 627.0 mg/L.The results showed that most minerals in the sandstone slab had a tendency to dissolve in the groundwater during the early reaction days (about 30-60 days), but the dissolution became stable after 90 days of reaction, resulting from the secondary precipitations overcoming the continuous dissolution during the geochemical reaction in the cell.The trend of cation concentration change in the groundwater after 30 days was also similar to those of the SR RMS and pH change during the scCO 2 -sandstone-groundwater reaction, showing an interesting possibility for the use of SR RMS change of the rock surface to estimate the process of the geochemical reaction (Figure 7).From these comparisons of trends, it was concluded that the dissolution process of the CO 2 reservoir sandstones was dominant at the early CO 2 injection time (less than 60 days), but precipitation also occurred sequentially to decrease the concentrations of dissolved ions and to stabilize the unbalanced geochemical system within a few months after CO 2 injection.
To identify the weathering process of the reservoir rock by the scCO 2 -reservoir rock-groundwater reaction, the surfaces of the S1-1 sandstone slab were analyzed using SEM/EDS before the reaction and after 120 reaction days and their SEM images are shown in Figure 8.The average K + and Si 4+ concentrations in the groundwater increased from 23.3 and 12.4 mg/L to 30.0 and mg/L after 30 reaction days, respectively, and were then stabilized by gradually decreasing after 90 days.The results suggest that the concentration of K + and Si 4+ mainly increased by the dissolution of silicate minerals such as K-feldspar and then decreased by the precipitation of secondary minerals, including K + ions (this is also supported by SEM/EDS analyses in Figure 8 and Figure 9).The average concentration of Ca 2+ and Na + increased from 547.1 and 633.2 mg/L to 663.8 and 692.0 mg/L after 30 days, respectively.However, after 90 days, they gradually decreased to 579.4 and 627.0 mg/L.The results showed that most minerals in the sandstone slab had a tendency to dissolve in the groundwater during the early reaction days (about 30-60 days), but the dissolution became stable after 90 days of reaction, resulting from the secondary precipitations overcoming the continuous dissolution during the geochemical reaction in the cell.The trend of cation concentration change in the groundwater after 30 days was also similar to those of the SRRMS and pH change during the scCO2-sandstone-groundwater reaction, showing an interesting possibility for the use of SRRMS change of the rock surface to estimate the process of the geochemical reaction (Figure 7).From these comparisons of trends, it was concluded that the dissolution process of the CO2 reservoir sandstones was dominant at the early CO2 injection time (less than 60 days), but precipitation also occurred sequentially to decrease the concentrations of dissolved ions and to stabilize the unbalanced geochemical system within a few months after CO2 injection.
To identify the weathering process of the reservoir rock by the scCO2-reservoir rock-groundwater reaction, the surfaces of the S1-1 sandstone slab were analyzed using SEM/EDS before the reaction and after 120 reaction days and their SEM images are shown in Figure 8.The SEM images show that the slab surface was heavily deformed by the geochemical reaction and the weathering process actively occurred along the existing fractures or mineral boundaries.The crack that existed at the mineral boundaries before the reaction was clearly expanded, and the calcites were perforated by the dissolution during the reaction, suggesting that these geochemical weathering processes can increase the porosity or the permeability of the reservoir rocks (this is discussed in detail in Section 3.2).In contrast, some pre-existing pores on the mineral surface were filled with secondary precipitates after the reaction, and additional precipitation on mineral surfaces such as quartz and biotite also occurred (Figure 8).The typical precipitates created on the S2-1 and S3-1 slab surfaces were analyzed using SEM/EDS after 120 reaction days, where they were determined to be amorphous silicate minerals as well as calcite, halite, and kaolinite, mostly consisting of Si 4+ , Na + , K + , Ca 2+ , Al 3+ , and Fe 2+ (Figure 9).These SEM/EDS analyses also indicated that both dissolution and precipitation occur on the sandstone surface due to the weathering process during the scCO 2 -rock-groundwater reaction, which directly caused the roughness change of the rock surface and which represented the extent of the geochemical reaction over the reaction time.If these experimental results confirm that the SR RMS change is directly proportional to the extent of the geochemical reaction, the trend of the SR RMS change can be used to estimate the progress of the reaction, which mostly results from dissolution and precipitation.If the active dissolution and precipitation of sandstones occur over a short period of reaction time, accompanying the SR RMS and physical property changes when CO 2 is injected into the sandstone reservoir, the activity of the geochemical reaction is likely to be an important parameter in determining the optimal CO 2 injection site, and further studies will be necessary to conclusively link the extent and influence with leakage safety of CO 2 sequestration.
Ca 2+ , Al 3+ , and Fe 2+ (Figure 9).These SEM/EDS analyses also indicated that both dissolution and precipitation occur on the sandstone surface due to the weathering process during the scCO2-rockgroundwater reaction, which directly caused the change of the rock surface and which represented the extent of the geochemical reaction over the reaction time.If these experimental results confirm that the SRRMS change is directly proportional to the extent of the geochemical reaction, the trend of the SRRMS change can be used to estimate the progress of the reaction, which mostly results from dissolution and precipitation.If the active dissolution and precipitation of sandstones occur over a short period of reaction time, accompanying the SRRMS and physical property changes when CO2 is injected into the sandstone reservoir, the activity of the geochemical reaction is likely to be an important parameter in determining the optimal CO2 injection site, and further studies will be necessary to conclusively link the extent and influence with leakage safety of CO2 sequestration.

Measurement of Physical Property Changes for the Sandstone Cores
Table 4 shows the porosity changes for the three sandstones (total nine cores) vs. the reaction time (total of 150 days).The initial average porosity of the S1 sandstone cores was 8.097%, and it increased to 8.642% after 150 reaction days (6.7% increase from the initial value).The average porosity of the S2 and S3 sandstones also increased to 8.2% and 9.9% compared with the initial values, respectively.Even though the porosity increase of the rock cores occurred at the early stage of the reaction because of crack extension and perforation dominated by the dissolution, this was moderated within a few months, showing a logarithmic increase vs. reaction time (Figure 10a).The average dry density of the sandstone cores decreased from 2.390 to 2.384 g/cm 3 for 120 reaction days (0.25% decrease from the initial value) (Table 5).The average seismic wave velocities of the sandstones also decreased in a logarithmic manner to 5.67% for P-wave and to 5.75% for S-wave from the initial values for 150 reaction days (Figure 10b and Table 5).The decrease in the dry density and seismic wave velocity for the sandstones was associated with an increase in porosity due to the

Measurement of Physical Property Changes for the Sandstone Cores
Table 4 shows the porosity changes for the three sandstones (total nine cores) vs. the reaction time (total of 150 days).The initial average porosity of the S1 sandstone cores was 8.097%, and it increased to 8.642% after 150 reaction days (6.7% increase from the initial value).The average porosity of the S2 and S3 sandstones also increased to 8.2% and 9.9% compared with the initial values, respectively.Even though the porosity increase of the rock cores occurred at the early stage of the reaction because of crack extension and perforation dominated by the dissolution, this was moderated within a few months, showing a logarithmic increase vs. reaction time (Figure 10a).The average dry density of the sandstone cores decreased from 2.390 to 2.384 g/cm 3 for 120 reaction days (0.25% decrease from the initial value) (Table 5).The average seismic wave velocities of the sandstones also decreased in a logarithmic manner to 5.67% for P-wave and to 5.75% for S-wave from the initial values for 150 reaction days (Figure 10b and Table 5).The decrease in the dry density and seismic wave velocity for the sandstones was associated with an increase in porosity due to the geochemical weathering process during the reaction, which considers their interrelation characteristics.For a more quantitative analysis of the physical property changes for the nine sandstone cores, the curve fitting results for their property values using the logarithmic Equation ( 2) are delineated in Figure 10.The r-squared values for all fitted curves are higher than 0.88 (average of 0.921), indicating that the rock property changes follow a well-defined logarithmic increase (or decrease) pattern, which were very similar to those of the geochemical parameter changes such as pH, cation concentration, and SR RMS over the reaction time.
sandstone cores, the curve fitting results for their property values using the logarithmic Equation ( 2) are delineated in Figure 10.The r-squared values for all fitted curves are higher than 0.88 (average of 0.921), indicating that the rock property changes follow a well-defined logarithmic increase (or decrease) pattern, which were very similar to those of the geochemical parameter changes such as pH, cation concentration, and SRRMS over the reaction time.These similarities are evidence demonstrating that the rock property change directly originated from the geochemical reaction with the injected scCO 2 , which further resulted in the rock surface roughness change and ion concentration change in the groundwater, and also that the surface roughness measurement provides a lot of quantitative information regarding the extent of the geochemical reaction related to the scCO 2 in the subsurface.The results showed that the physical properties of the sandstone changed rapidly within a few months due to the geochemical reaction with the injected CO 2 , but the rate decreased in a relatively short time (a few months), verifying that the physical property change of the CO 2 reservoir rocks occurred faster than expected from previous research [56][57][58].Because the effect of the geochemical reaction on the reservoir rock property change is moderated after a short time, attention to the geochemical reaction at the early CO 2 injection stage is very important in preventing leakage and in understanding CO 2 sequestration site characteristics from the CO 2 storage capacity point of view.

Conclusions
During the last decade, one of the main issues for CO 2 storage sites has been how the scCO 2 involved geochemical reaction affects the storage capacity and the leakage safety after scCO 2 injection into the subsurface.Several studies confirmed that the geochemical reaction rate was much faster than expected.Such findings suggested the possibility that the geochemical reaction may generate further changes in the reservoir rock properties.For a specific CO 2 storage site, it becomes very important to evaluate whether or not the geochemical reaction is significant enough to threaten the leakage safety of the CO 2 sequestration site.Through a novel approach for determining the correlations among changes in SR RMS , physical properties, pH, and dissolved cation concentration in the groundwater during the scCO 2 -rock-groundwater reaction, this study successfully verified the potential of using SR RMS values as a new parameter to quantify the extent of a geochemical reaction.
For sandstone sites in the Gyeongsang basin of South Korea, the trends of several property changes originating from the scCO 2 involved reaction displayed similar logarithmic patterns, which confirmed that the scCO 2 -sandstone-groundwater reaction occurs actively in the early CO 2 injection stage (a few months), but it then becomes stable and further rock property changes are halted.As a result of this study, the CO 2 sequestration site in the Gyeongsang basin can be evaluated as an appropriate site from the viewpoint of geochemical safety if a careful observation over the geochemical reaction in the early CO 2 injection stage is performed.The use of SR RMS values to evaluate the extent of the geochemical reaction will be available for any other CO 2 site in the world to secure leakage safety for reservoirs or formations.

Figure 1 .
Figure 1.Geological map around the drilling site ( property change of the sandstone with the cation concentration change in groundwater during the geochemical reaction.

Figure 2 .
Figure 2. Apparatus used for the slab experiment to simulate the scCO2-sandstone-groundwater reaction.

Figure 2 .
Figure 2. Apparatus used for the slab experiment to simulate the scCO 2 -sandstone-groundwater reaction.

Figure 3 .
Figure 3. Schematic of the SPM (Scanning Probe Microscope) analysis to calculate SR RMS (surface roughness value).

Figure 4 .
Figure 4. Schematic of the sandstone core experiment for physical property changes.

Figure 4 .
Figure 4. Schematic of the sandstone core experiment for physical property changes.

Figure 5 .
Figure 5. Scanning Probe Microscope (SPM) images of three mineral surfaces for the S1 slab before (left) and after 120 reaction days (right).

Figure 5 .
Figure 5. Scanning Probe Microscope (SPM) images of three mineral surfaces for the S1 slab before (left) and after 120 reaction days (right).

Figure 6 .
Figure 6.The SRRMS and pH change by the scCO2-sandstone-groundwater reaction.

Figure 6 .
Figure 6.The SR RMS and pH change by the scCO 2 -sandstone-groundwater reaction.

Figure 7 .
Figure 7. Concentration change of five cations in the groundwater during the scCO2 involved reaction (trend lines calculated from the data after 30 days).

Figure 7 .
Figure 7. Concentration change of five cations in the groundwater during the scCO 2 involved reaction (trend lines calculated from the data after 30 days).

Figure 7 .
Figure 7. Concentration change of five cations in the groundwater during the scCO2 involved reaction (trend lines calculated from the data after 30 days).

Figure 10 .
Figure 10.Average porosity change (a) and average seismic wave velocity change (b) for the sandstone cores during the scCO2-sandstone-groundwater reaction.

Figure 10 .
Figure 10.Average porosity change (a) and average seismic wave velocity change (b) for the sandstone cores during the scCO 2 -sandstone-groundwater reaction.

Table 2 .
Temperature, pH, and chemical composition of groundwater used in the experiment.

Table 3 .
SR RMS (unit: µm) vs. reaction time for the three sandstone slabs.

Table 4 .
Porosity change for the three sandstone cores for 150 reaction days.

Table 5 .
Average density and seismic wave velocity changes for the three sandstones during the reaction.