PUBLICATIONS. Water Resources Research

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1 PUBLICATIONS Water Resources Research RESEARCH ARTICLE Key Points: In situ saturation monitoring of vertical CO 2 transport in a stratified core Fluid velocity and CO 2 storage potential was assessed Effect of subscale heterogeneity on pressure and CO 2 saturation Correspondence to: K.-Y. Kim, kykim@kigam.re.kr Citation: Oh, J., K.-Y. Kim, W. S. Han, E. Park, and J.-C. Kim (2015), Migration behavior of supercritical and liquid CO 2 in a stratified system: Experiments and numerical simulations, Water Resour. Res., 51, , doi: / 2015WR Received 29 JAN 2015 Accepted 12 SEP 2015 Accepted article online 16 SEP 2015 Published online 3 OCT 2015 Migration behavior of supercritical and liquid CO 2 in a stratified system: Experiments and numerical simulations Junho Oh 1,2, Kue-Young Kim 1, Weon Shik Han 3, Eungyu Park 2, and Jeong-Chan Kim 1 1 Korea Institute of Geoscience and Mineral Resources, Daejeon, South Korea, 2 Department of Geology, Kyungpook National University, Daegu, South Korea, 3 Department of Geosciences, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin, USA Abstract Multiple scenarios of upward CO 2 migration driven by both injection-induced pressure and buoyancy force were investigated in a horizontally and vertically stratified core utilizing a core-flooding system with a 2-D X-ray scanner. Two reservoir-type scenarios were considered: (1) the terrestrial reservoir scenario (10 MPa and 508C), where CO 2 exists in a supercritical state and (2) the deep-sea sediment reservoir scenario (28 MPa and 258C), where CO 2 is stored in the liquid phase. The core-flooding experiments showed a 36% increase in migration rate in the vertical core setting compared with the horizontal setting, indicating the significance of the buoyancy force under the terrestrial reservoir scenario. Under both reservoir conditions, the injected CO 2 tended to find a preferential flow path (low capillary entry pressure and high-permeability (high-k) path) and bypass the unfavorable pathways, leaving low CO 2 saturation in the low-permeability (low-k) layers. No distinctive fingering was observed as the CO 2 moved upward, and the CO 2 movement was primarily controlled by media heterogeneity. The CO 2 saturation in the low-k layers exhibited a more sensitive response to injection rates, implying that the increase in CO 2 injection rates could be more effective in terms of storage capacity in the low-k layers in a stratified reservoir. Under the deep-sea sediment condition, the storage potential of liquid CO 2 was more than twice as high as that of supercritical CO 2 under the terrestrial reservoir scenario. In the end, multiphase transport simulations were conducted to assess the effects of heterogeneity on the spatial variation of pressure buildup, CO 2 saturation, and CO 2 flux. Finally, we showed that a high gravity number (N gr ) tended to be more influenced by the heterogeneity of the porous media. VC American Geophysical Union. All Rights Reserved. 1. Introduction A continued increase in the atmospheric concentration of carbon dioxide (CO 2 ) due to anthropogenic emissions is predicted to cause significant changes in the global climate [Cox et al., 2000]. CO 2 storage in geologic formations is one of the critical options for mitigating the impact of greenhouse gas emissions to the atmosphere [IPCC, 2005]. Hence, technologies related to CO 2 storage are receiving increasing attention, and research is being conducted to ensure safe and long-term secure storage of CO 2 in subsurface formations [Gale, 2004]. Many pilot- and demonstration-scale projects have been conducted to test and demonstrate monitoring and verification technologies in different subsurface geological environments [Litynski et al., 2008; Michael et al., 2010]. Underground storage reservoirs in terrestrial geologic formations are typically recommended to be at depths below m [Orr, 2009]; under these pressure and temperature conditions, CO 2 will be in a dense supercritical state. Although the supercritical CO 2 (scco 2 ) has gas-like properties (high diffusivity and low viscosity) allowing for effective injectivity, the density contrast between the scco 2 and the native formation of water causes the buoyant CO 2 to migrate upward until it is blocked by fine-grained, low-k formations that serve as permeability and capillary barriers limiting the vertical migration of the stored CO 2 [Lu et al., 2013; Plug and Bruining, 2007a]. However, when the CO 2 plume encounters imperfections (e.g., fracture zones, faults, or preexisting wells) in caprocks, part of the CO 2 could leak toward the land surface [Celia et al., 2004; Han et al., 2013; Shipton et al., 2004]. The leakage of CO 2 could result in unintended negative environmental and safety consequences [Keating et al., 2013a; Oldenburg et al., 2010; Siirila et al., 2012]. Alternatively, scientific communities consider storing CO 2 in deep-sea sediments [House et al., 2006; Koide et al., 1997; Schrag, 2009]. Beneath 2800 m of ocean, the pressure and temperature conditions compress OH ET AL. CO 2 MIGRATION BEHAVIOR IN A STRATIFIED SYSTEM 7937

2 Figure 1. A schematic illustration of CO 2 plume migration behavior for two cases. (a) Case 1: CO 2 injection through a partially penetrating well positioning at the bottom of the storage formation. (b) Case 2: CO 2 leakage from a storage aquifer to overlying formation through a leakage pathway in a caprock. CO 2 to a liquid phase. House et al. [2010] analyzed the long-term storage of buoyant liquid CO 2 injected into deep-sea sediments and compared the mobility of liquid CO 2 in marine sediments with that of scco 2 in geologically equivalent terrestrial reservoirs. They concluded that as long as the pressure and temperature conditions sufficiently yield liquid CO 2 with a density of approximately 90% of seawater, the stored liquid CO 2 is expected to be immobile in deep-sea formations. Recently, in conjunction with a controlled marine-based CO 2 release test (QICS: Quantifying and monitoring potential ecosystem Impacts of geological Carbon Storage), monitoring strategies based on environmental regulations effective on marine ecosystems have been reviewed [Blackford et al., 2014]. The suggested monitoring techniques must incorporate physical, chemical, acoustic, and biological observations based on a hierarchical approach, which uses a minimum number of techniques for cost-effectiveness. With emphases on lessons from various field-scale projects (including both onshore and offshore sites), laboratory experiments and numerical modeling studies are equivalently crucial to the understanding of twophase flow. Prior numerical studies evaluating CO 2 migration behavior for both terrestrial [Delshad et al., 2013] and deep-sea conditions [House et al., 2010] demonstrated the importance of integrating both field observations and computational verification/prediction. Moreover, a number of laboratory core-flooding experiments have revealed the dynamic behavior of scco 2 under reservoir conditions. These laboratory studies focused on permeability heterogeneity and multiphase flow [Chaouche et al., 1994; Krause et al., 2011; Pini and Benson, 2013a], measuring relative permeabilities [Akbarabadi and Piri, 2013; Bachu and Bennion, 2008; Krevor et al., 2012], interfacial tension [Aggelopolos et al., 2011; Chalbaud et al., 2009], and capillary pressures [Pentland et al., 2011; Pini and Benson, 2013b; Plug and Bruining, 2007b], as these are important factors controlling the behavior of two-phase fluids and residual CO 2 -trapping mechanisms in porous media. More recently, Oh et al. [2013] conducted experimental studies to assess the transport behavior of scco 2 and brine in a fractured rock. The upward migration of CO 2 has been studied in order to understand the leakage potentials through faults or abandoned wells [Keating et al., 2013b; Pruess, 2005b; Watson et al., 2014]. Often, CO 2 injection-induced pressure buildup is a driving force for abrupt leakage [Birkholzer et al., 2009; K. W. Chang et al., 2013; Kim et al., 2012]. Alternatively, without pressure buildup, pure buoyancy is also capable of driving a CO 2 plume to rise during a postinjection period [Bryant et al., 2008; Han et al., 2010b; Oldenburg et al., 2012]. In summary, many field and numerical studies indicate that the injection-induced pressure and the buoyancy force play a central role in vertical CO 2 flow. When CO 2 is injected into the targeted formation, the injection-induced pressure forces the CO 2 plume to horizontally expand. However, the upward movement of CO 2 is due to both injection-induced pressure and buoyancy force. Figure 1 delineates two exemplary cases where the vertical migration of a CO 2 plume OH ET AL. CO 2 MIGRATION BEHAVIOR IN A STRATIFIED SYSTEM 7938

3 prevails from these two factors. Figure 1a is a case where CO 2 is injected into the lower part of the storage formation. During the injection period, CO 2 migrates both to the horizontal and vertical directions due to the injection pressure and buoyancy force. The area of interest is shown as the red-box, where the laboratory experiments tend to replicate true vertical CO 2 migration by utilizing vertical core-flooding experiments. Alternative examples of the study of interest are shown in Figure 1b, where the CO 2 plume is leaked from fractures or faults in a caprock to the overlying aquifer. In both circumstances, characteristics of multiphase CO 2 migrations must be identified through the vertical core-flooding experiments where the CO 2 plume migrates opposite to the gravitational direction. In this study, we are especially interested in a stratified system that consists of a number of alternating high and low-k layers. This type of reservoir system is observed in the Utsira formation in Norway, where the aquifer sandstone is divided by nearly horizontal discontinuous thin mudstone layers [Lindeberg et al., 2001]. In this circumstance, the two-phase migration of CO 2 and brine is controlled by not only the fluid properties but also the media properties such as the k-contrast and capillary entry pressure between the high-k layer and its alternating low-k layer. The increase in the pressure of the nonwetting phase (CO 2 )at the layer interface may exceed the capillary entry pressure, forcing CO 2 to penetrate into the low-k layer. This accumulation-penetration-breakthrough phenomenon in the stratified formation can significantly retard the upward migration of CO 2 [Hayek et al., 2009; Zhou et al., 2010]. Many previous CO 2 core-flooding tests were conducted at horizontally positioned cores [e.g., Krevor et al., 2012; Oh et al., 2013; Shi et al., 2011] where gravitational force is perpendicular to the displacing direction of CO 2. The experimental setup where downward gravitation force is parallel to the upward direction of CO 2 will represent an accurate condition of buoyancy-driven CO 2 displacement (Figure 1). To elucidate the buoyancy effect for CO 2 /brine flow, the core was set up in both vertical and horizontal directions, and the migration behaviors of the two cases were compared. Additionally, two reservoir-type scenarios were considered: (1) the terrestrial reservoir scenario (10 MPa and 508C) where CO 2 exists in a supercritical state; (2) the deepsea sediment reservoir scenario (28 MPa and 258C), where CO 2 is stored in the liquid phase. Although the degree of rock consolidation under the deep-sea reservoir scenario can be different from that under the terrestrial reservoir scenario, the standard Berea sandstone core was chosen in both settings to evaluate the difference in CO 2 migration entirely due to the thermophysical conditions in the storage formations. Experimental investigations and numerical simulations were interactively conducted to assess the migration characteristics of scco 2 and liquid CO 2 under different reservoir conditions. Finally, we discussed the effect of bedding orientation on CO 2 migration in core scales. 2. Materials and Methods 2.1. Experimental Setup Core-flooding experiments were performed with an apparatus that comprises a fluid-injection system, a core-holder system, a confining and back-pressure control system, and an X-ray scanning system (Figure 2). The fluid-injection system is composed of two sets of high-pressure syringe pumps used for CO 2 and brine injection. The core-holder system was constructed from graphite composite with titanium endplates. The embedded multiple spiral grooves on the inlet face end-plug forces the injected fluid to be evenly distributed onto the face of the core. An integrated heating system with temperature-monitoring thermocouples allowed temperature to be controlled during the tests. The confining-pressure controller generated and maintained a constant confining pressure while the back-pressure regulator controlled flow to maintain pore pressure. Line-pressure measurements were performed at both upstream and downstream ends of the core with pressure transducers. One distinctive feature of the system is that the container of the core-holder and the X-ray scanning system can be rotated from 08 to 908 which enables the core holder to be set up in both the horizontal and vertical directions (Figure 2). The X-ray scanning system consists of an X-ray tube and a detector. When a beam of X-rays passes through a material, a portion of the beam is absorbed or scattered out of the beam. The attenuation of the intensity depends on the electron density of the material, the energy of radiation, and the bulk density of the sample material [Wildenschild et al., 2002]. The detector board has two arrays of 128 channels each, corresponding to 256 detectors with a detector pitch of 0.4 mm. The array of the X-ray detector is perpendicular to the axis of the core and it scans parallel to the axis of the core. The data output OH ET AL. CO 2 MIGRATION BEHAVIOR IN A STRATIFIED SYSTEM 7939

4 Figure 2. A schematic diagram of the core-flooding experimental setup. The system consists of a core holder, two sets of high-pressure syringe pumps, a thermal controller, a backpressure regulator, and a X-ray scanner. The core holder is rotatable, enabling it to be positioned at both horizontal and vertical directions. is in 16 bit format ranging between 0 and 65,535. The scanning system generated two-dimensional (2-D) images for measuring CO 2 saturation along the core. The obtained CO 2 saturation map may include noise mainly due to the relatively small density contrast between the two fluid phases (CO 2 and brine) compared with the contrast between any one of the fluids and the rock [Pini et al., 2012]. One of the suggested strategies for circumventing low density contrasts between the two fluids is to add dopants (e.g., iodinated medium) to one or the other fluid phase [Withjack, 1988]. In the experiments, NaI was used as a dopant to enhance the X-ray attenuation so that the CO 2 saturation could be distinguished from the brine in the scanned image. The brine was synthesized with 150 g/l total solids (50 g/l NaCl and 100 g/l NaI). The X-ray source (voltage and current) was set to 150 kv and 1.2 ma. The CO 2 saturation during the core-flooding experiments was calculated using the equation S CO2 5 I exp2i brinesat I CO2 2I brinesat (1) where I CO2 and I brinesat are the values of gray-scale intensity obtained from the background scans of the core saturated with CO 2 and brine, respectively, and I exp is the value obtained for the CO 2 -flooding experiments. The scan rate was 0.3 mm/s and the size of the pixel was 0.4 mm mm Preparation of the Core Sample A Berea sandstone block with silt stratification was prepared for the experiments. The sandstone block was drilled to obtain a core, which is perpendicular to the bedding planes. The dimensions of the core were 200 mm length and 38 mm diameter (Figure 3). To characterize the drilled core with bedding planes, the grain-size distribution was analyzed by means of laser diffraction spectroscopy (model: Malvern Mastersizer 2000). For the analysis, fragments were separated from the coarse and fine-grained layers of the Berea sandstone block. The particle-size distribution showed that one subsample representing coarse materials was composed of 21.0% silt, 76.9%, sand and 2.1% clay, and the other subsample representing fine materials was composed of 92.5% silt and 7.5% clay (Figure 3). Thus, we hereafter designate the coarse material as sand and the fine material as silt. The drilled core was fired at a temperature of 7008C to stabilize clay minerals, thereby reducing clay swelling and the migration of fine grains during the CO 2 -flooding experiments [Krevor et al., 2012; Pini et al., 2012]. The core was wrapped with a heat shrinkable Teflon tube followed by aluminum foil and another Teflon OH ET AL. CO 2 MIGRATION BEHAVIOR IN A STRATIFIED SYSTEM 7940

5 Figure 3. A photo of the drilled Berea sandstone core interlayered with thin beds of silt and the particle-size distribution of the fine and coarse matrix in the core. tube before mounting in the core holder. The core has a porosity of 21% and a permeability of m 2. The pore volume (PV) of the core sample was 47.6 ml Experimental Procedure The experiments were conducted under two different sets of CO 2 storage conditions representing a typical terrestrial reservoir scenario and a deepsea sediment reservoir scenario. For the terrestrial reservoir scenario, where CO 2 exists in a supercritical phase, the downstream pressure was maintained at 10 MPa and the temperature was set at 508C to replicate a hypothetical 1 km subsurface environment. On the other hand, the downstream pressure was maintained at 28 MPa and the temperature was set at 258C to reproduce the deep-sea sediment conditions typically found 800 m below the seafloor and 2000 m water depth. In this circumstance, CO 2 is expected to be stored in the liquid phase. In the first experiment, a capillary pressure-saturation relationship was estimated using mercury injection capillary pressure (MICP). To perform MICP tests, two small subsamples (0.6 cm 3 ) were taken from the sandstone block each representing coarse (sand) and fine (silt)-grained layers. The subsamples were dried in a vacuum oven at 708C for 24 h prior to the tests. The MICP and corresponding mercury saturation measured for the mercury/air system were transformed to capillary pressure and the corresponding CO 2 saturation in a CO 2 /brine system using the following relationship: P c;co2 5 r CO 2 cosh CO2 (2) P c;hg r Hg cosh Hg where P c (Pa) is the capillary pressure, r (mn m 21 ) is the interfacial tension (IFT) between two fluids, and h (8) is the contact angle measured in the wetting phase. The IFT of r Hg mn m 21 was applied in the above equation, whereas r CO and 32.6 mn m 21 were used in the conversion for the terrestrial and deep-sea sediment conditions, respectively [Li et al., 2012]. The contact angles of the two systems were assumed to be equal [Pentland et al., 2011]. In the second experiment, vertical core-flooding tests were conducted to evaluate the vertical migration of the CO 2 plume under the terrestrial reservoir scenario as introduced in Figure 1. The CO 2 -flooding tests in the brine-saturated core were performed at five different injection rates of CO 2 (q , 0.1, 0.5, 1.0, and 5.0 ml/min). After each drainage test, an imbibition test was conducted with the injection rate of brine (q ml/min). After each CO 2 -flooding test with a different rate (q), the core was flushed with brine at a Table 1. Dimensionless Parameters for the Terrestrial and Deep-Sea Conditions a Reservoir Fluids (CO 2 /Brine) N B N M N Bo q (ml/min) N c N gr q brine, and Ngr : gravity number N gr 5 Terrestrial condition P (MPa) T (8C) q (kg/m 3 ) 384.3/ l ( Pa s) 2.84/ r (mn/m) Deep-sea condition P (MPa) T (8C) q (kg/m 3 ) 957.6/ l ( Pa s) 10.7/ r (mn/m) a N B : Buoyancy number N B 5 q brine2q co2, N M : mobility number N M 5 l brine ð, N Bo : Bond number N Bo 5 q brine2q co2 Þgr 2, N c : capillary number N c 5 vl brine r kv LDqg Hu l : l co2 r OH ET AL. CO 2 MIGRATION BEHAVIOR IN A STRATIFIED SYSTEM 7941

6 Figure 4. P c curves as a function of water saturation. Experimental data are from MICP results converted to the CO 2 /water system at terrestrial condition (10 MPa and 508C) and deep-sea sediments condition (28 MPa and 258C) while solid lines represent the P c curves fitted from van Genuchten model. high injection rate (q 5 15 ml/min) until the X-ray scan results showed negligible CO 2 saturation in the core. In the third experiment, horizontal CO 2 -flooding tests were conducted under the same conditions as the vertical CO 2 -flooding experiment for comparison. Finally, vertical flooding tests were conducted under the deep-sea sediment conditions. All of the experiments were conducted under a capillarydominated displacement regime. The capillary number, N c 5 tl/r (where t is the Darcy velocity), was within the range of and under terrestrial and deep-sea reservoir scenario, respectively. A detailed list of flow conditions and fluid properties at different reservoir conditions is provided in Table 1. The dimensionless parameters, buoyancy number (N B ), mobility number (N M,) and Bond number (N Bo ) are much smaller under the deep-sea reservoir scenario than under the terrestrial reservoir scenario. 3. Results 3.1. Capillary Pressure The experimentally measured capillary pressure (P c ) data are shown as a function of water saturation (S w ) with fitted curves from the van Genuchten model [van Genuchten, 1980] (Figure 4). The blue and red colors denote the P c data obtained from coarse (sand) and fine (silt) materials in the Berea sandstone core, respectively. The P c from the silt layer is more than 2 orders of magnitude greater than that from the sand layer. The elevated P c in the silt is mainly due to smaller sizes of pore throats, which determines the primary drainage entry of the nonwetting phase (CO 2 ). Additionally, the P c of both sand and silt for the deep-sea reservoir scenario are consistently lower than that for the terrestrial reservoir scenario, although its difference was minor. As seen in equation (2), the P c is proportional to IFT between the CO 2 and the brine. The smaller IFT in CO 2 /brine under the deep-sea reservoir scenario (32.6 mn/m) relative to that under the terrestrial reservoir scenario (38.5 mn/m) is attributed to both elevated pressure (10! 28 MPa) and reduced temperature (50! 258C) (Table 1); the IFT decreases with increasing pressure when the salinity and the temperature are kept constant [Chalbaud et al., 2009; Li et al., 2012]. Although the IFT variation as a function of temperature is more complicated, previous research revealed that with decreasing temperature the CO 2 solubility in water increases and results in a decrease in the IFT [Bachu and Bennion, 2008; Hebach et al., 2002]. Overall, it is plausible that heterogeneity would be a prevailing factor affecting P c variation, more so than changes in the thermodynamic conditions between the terrestrial and deep-sea reservoir scenario. Finally, P c data were fitted with the van Genuchten model [van Genuchten, 1980]: n o ð12kþ P c 52P 0 ðs Þ 21=k 21 (3) S 5ðS w 2S wir Þ= ð12s wir Þ (4) where P 0 is the strength coefficient, k is a parameter depending on pore geometry, S w is water saturation, and S wir is irreducible water saturation. The data were best fitted for drainage capillary pressure with a OH ET AL. CO 2 MIGRATION BEHAVIOR IN A STRATIFIED SYSTEM 7942

7 Figure 5. Snapshots of the CO 2 saturation at eight different injection PVs during CO 2 flooding under terrestrial conditions (q ml/min). power exponent (k) of 0.4 and P 0 of Pa for the sand layer. In the case of the silt layer, the van Genuchten model was best matched with power exponent (k) of and P 0 of 200 kpa Vertical CO 2 Migration Behavior Terrestrial Reservoir Scenario (10 MPa and 508C) The terrestrial reservoir scenario considered in this study is at 1 km depth. Under this condition, the density and viscosity of CO 2 are q g/cm 3 and l Pa s, respectively [Span and Wagner, 1996] and CO 2 exists in the supercritical state (Table 1). The CO 2 density is at the lower end of what is expected in terrestrial CO 2 storage scenarios. Figure 5 shows snapshots of the CO 2 saturation at different pore volumes (PVs) during drainage CO 2 flooding with an injection rate of q ml/min. Six PVs of CO 2 were injected into the core to ensure that the steady state condition was achieved. The observed pressure difference between the core inlet and outlet (DP in-out ) was stabilized at 6 kpa. The CO 2 reached at the core end after the injection of 0.15 PVs. No significant change in CO 2 saturation was typically observed after more than three PVs of CO 2 was injected. High CO 2 saturation ( ) was observed at sand layers whereas low CO 2 saturation (0 0.3) was observed at silt layers indicating that sand layers are likely to store more CO 2 than silt layers. In detail, both pressure gradient (DP in-out 5 6 kpa) and buoyancy forces drove the advancement of the CO 2 front immediately after the CO 2 injection at the core bottom. Upward movement of CO 2 was greatly influenced by dips and thicknesses of intervening silt layers. CO 2 tended to migrate through high-k pathways preserving low capillary entry pressure bypassing the unfavorable pathways. For example, there is a dark blue area representing almost zero CO 2 saturation at approximately z 5 40 mm (A in Figure 5) at 0.03 PV. Enhancement of PV shrinks the area of the zero CO 2 saturation zone, but this area is still distinct even after six PVs. The presence of alowco 2 saturation zone indicates that the CO 2 movement is blocked by a silt layer, which is typical for a layer with a relatively high capillary entry pressure (Figure 4). Similarly, zones of low CO 2 saturation where CO 2 movement is hampered by the silt layers were observed at z 5 70, 140, and 170 mm (B, C, and D in Figure 5). Overall, the experiments showed no distinctive fingering as CO 2 moved upward, but the results plausibly indicated that CO 2 movement is controlled by stratified heterogeneity in the core. This result corresponded to the finding of Bryant et al. [2008] where CO 2 rising under the buoyancy force was governed by the primary updip direction of the bedding plane. The small-scale heterogeneities such as anisotropy, configuration of the intervening low-k layers, and the associated capillary pressure, were important factors. Figure 6a shows the CO 2 saturation map at the steady state condition (six PVs) with the injection rate of q ml/min. Figure 6b shows the vertical profile of horizontally averaged CO 2 saturations at different PVs. Because the bedding in the core is horizontally stratified, horizontally averaged CO 2 saturation is OH ET AL. CO 2 MIGRATION BEHAVIOR IN A STRATIFIED SYSTEM 7943

8 Figure 6. (a) CO 2 saturation map after six PVs of injection (q ml/min). (b) Horizontally averaged CO 2 saturation profiles at different PVs (q ml/min). (c) Horizontally averaged CO 2 saturation profiles at the steady state condition (six PVs) with different injection rates. considered to represent the result without the loss of important information. Consecutive snapshots of PVs from 0.01 to 0.15 showed the evolution of the advancing CO 2 front (Figure 6b). After the CO 2 front reached the core end at 0.15 PV, additional injection developed the CO 2 saturation profiles with similar fluctuating patterns from inlet to outlet. Overall, CO 2 saturation was decreased from inlet to outlet, but its profiles fluctuated depending on the presence of the silt beds. To assess the effect of injection rates, horizontally averaged CO 2 saturation was plotted at five different injection rates (q , 0.1, 0.5, 1.0, and 5.0 ml/min) after reaching steady state (six PVs) conditions (Figure 6c). The observed pressure difference between the core inlet and outlet (DP in-out ) was negligible for the case of q ml/min, implying that the buoyancy force is the primary driving force. With an increasing injection rate, DP in-out gradually increased from 6, 25, 70 to 120 kpa. When the injection rate was 0.01 ml/ min, the CO 2 saturation was and in the silt and sand layers, respectively. As the injection rate was increased to 0.5 ml/min or greater, the maximum CO 2 saturation reached up to 0.7 in the sand layers adjacent to the core inlet. Overall, the vertical profiles of average CO 2 saturation were small with lower injection rates and became larger with higher injection rates, but similar trends were maintained among all injection rates. This result suggested that small-scale bedding consistently impacted CO 2 plume migration at different injection rates and injected amounts. To evaluate the effect of bedding orientation on CO 2 saturation profiles, two cores were prepared; one core was drilled parallel to the bedding plane (150 mm length), and the other core was drilled perpendicular to the bedding plane (200 mm length). Although the average porosity of the two cores was equal to 0.2, the permeability of the core that was drilled parallel to the bedding was m 2, which was nearly twice that of the core that was drilled perpendicular to the bedding ( m 2 ). Figure 7 shows the stationary CO 2 saturation distribution from the horizontal flooding experiments. The core drilled parallel to the bedding showed a relatively smooth decline of CO 2 saturation along the distance varying from the inlet (0.4) to the outlet (0.2). In contrast, CO 2 profiles perpendicular to the bedding showed substantial fluctuation of CO 2 saturation varying from 0.3 to 0.7. Even the lowest CO 2 saturation obtained from the flow test perpendicular to the bedding was greater than that from the flow test parallel to the bedding. This implies that the connectivity of sand layers is important to the CO 2 saturation distribution, as similarly discussed in solute transport studies [Knudby et al., 2006; Renard and Allard, 2013]. In the core drilled perpendicular to the bedding, the sand layers were interceded by the silt layers; each sand layer was almost completely OH ET AL. CO 2 MIGRATION BEHAVIOR IN A STRATIFIED SYSTEM 7944

9 Figure 7. Comparison of stationary CO 2 saturation profiles between two different cores: one drilled parallel to the bedding and the other drilled perpendicular to the bedding. disconnected by overlying and underlying silt layers. However, in the horizontal flooding experiment with the core drilled parallel to the bedding, CO 2 was able to flow through the connected sand bodies. It is clear from the CO 2 saturation profiles that the orientation of silt beds plays a role in CO 2 storage capacity Deep-Sea Reservoir Scenario (28 MPa and 258C) In deep-sea sediments, CO 2 exists in a liquid phase, with its density and viscosity corresponding to q g/ cm 3 and l Pa s, respectively (Table 1) [Span and Wagner, 1996]. Relative to the terrestrial reservoir scenario, both CO 2 density and viscosity were significantly enhanced, suggesting that the mobility of CO 2 would be retarded. Figure 8 shows snapshots of the CO 2 saturation at different PVs with an injection rate of q ml/min. Again, six PVs of CO 2 were injected into the core to ensure that the steady state condition was achieved. DP in-out was stabilized at 6 kpa, similar to the experiment under the terrestrial reservoir scenario. The CO 2 front reached the core end after 0.5 PVs of CO 2, which is more than three times of the PV relative to the flooding experiment under the terrestrial reservoir scenario (0.15 PVs). Excluding the bottom part of the core, which is under the influence of the injection pressure, the overall CO 2 saturation in the sand layers was , whereas it was in the silt layers. Note is that the liquid CO 2 intruded into the silt layers more deeply than scco 2 under the same experimental conditions. For instance, the early stage of CO 2 injection at 0.18 PV showed that liquid CO 2 bypassed the silt layer at z 5 40 mm (A in Figure 8) as it did under the terrestrial reservoir scenario. Although the silt layer remained unsaturated with respect to CO 2 even after the steady state condition was achieved under the terrestrial reservoir scenario, the CO 2 at the deep-sea reservoir scenario intruded into the silt layer (z 5 40 mm) only after 0.5 PV. Indeed, the CO 2 saturation rose up to 0.3 after six PVs of CO 2 injection. Similar results were also observed at z 5 70, 140, and 170 mm (B, C, and D in Figure Figure 8. Snapshots of the CO 2 saturation at eight different injection pore volumes during CO 2 flooding under deep-sea conditions (q ml/min). OH ET AL. CO 2 MIGRATION BEHAVIOR IN A STRATIFIED SYSTEM 7945

10 Figure 9. (a) CO 2 saturation map after six PVs of injection (q ml/min). (b) Horizontally averaged CO 2 saturation profile at different PVs (q ml/min). (c) Horizontally averaged CO 2 saturation profile at steady state condition (six PVs) with different injection rates. The difference of CO 2 saturation between the cases of q and 5.0 ml/min are presented as DSco 2 (ds refers to deep-sea condition whereas tr refers to terrestrial condition). 8). It is presumed that the lower capillary entry pressure and the greater capillary number (N c ) under the deep-sea reservoir scenario caused the CO 2 saturation difference between the two reservoir conditions. The horizontally averaged CO 2 saturation at different PVs and the effect of CO 2 injection rates (q 5 0.1, 0.5, 1.0, and 5.0 ml/min) on CO 2 saturation profiles are presented in Figure 9. The CO 2 front advanced at a nearly constant rate with respect to PVs, and CO 2 breakthrough occurred at the core outlet after 0.5 PV. Similar to the terrestrial reservoir scenario, the increase in injection rate built up the CO 2 saturation along the core. To assess the degree of CO 2 saturation change due to the injection rates, the difference of CO 2 saturation (DSco 2 ) between the cases of q and 5.0 ml/min was plotted (Figure 9c). The relative change of CO 2 saturation under the deep-sea reservoir scenario was ranged from 2.3% (at z 5 52 mm) to 123% (at z mm) as the CO 2 injection rate increased from q to 5.0 ml/min. The average increase in CO 2 saturation was 31% under the deep-sea reservoir scenario. The DSco 2 is inversely correlated with Sco 2 revealing that the silt layers are more sensitive to injection rate than the sand layers. In the case of the terrestrial reservoir scenario (DSco 2 tr, gray solid line), the increased CO 2 saturation ranged from 4.5% (at z 5 52 mm) to 69% at (z mm), with an average increase of 27%. These results imply that an increased CO 2 injection rate will be more effective in terms of storage capacity in the low-k layers in a stratified reservoir, whereas the effect of CO 2 injection rates will be minor in high-k layers Terrestrial Versus Deep-Sea Reservoir Scenarios CO 2 Saturation Horizontally averaged CO 2 saturation profiles between the scco 2 (under the terrestrial reservoir scenario) and liquid CO 2 (under the deep-sea reservoir scenario) were compared at four different injection rates in a vertical core (Figure 10a). At a small injection rate (q ml/min), the scco 2 saturation fluctuated between 0.3 and 0.5 in the lower half of the core (closer to the inlet), whereas it varied between 0.2 and 0.4 in the upper half (closer to the outlet). Throughout the core, liquid CO 2 saturation was lower than scco 2, but the fluctuation patterns were similar to each other, suggesting that the small-scale heterogeneity equally governs the distribution of both scco 2 and liquid CO 2. For example, in silty (low-k) layers,theliquidco 2 saturation was similar or even OH ET AL. CO 2 MIGRATION BEHAVIOR IN A STRATIFIED SYSTEM 7946

11 Figure 10. (a) Comparison of horizontally averaged CO 2 saturation profiles between the supercritical CO 2 (under terrestrial condition) and liquid CO 2 (under deep-sea condition) at different injection rates. (b) CO 2 saturation contrast (S contrast 5 Sco 2 ds 2 Sco 2 tr) between the deep-sea and terrestrial conditions. The red color in the binary map indicates the positive S contrast, while the blue color shows the negative S contrast. greater than the scco 2 saturation. At high injection rate (q ml/min), the liquid CO 2 saturation showed a dramatic decrease in fluctuation and appeared to be constant at approximately 0.4. To assess the CO 2 saturation difference between the scco 2 and liquid CO 2 in detail, a contrast saturation map (S contrast 5 Sco 2 ds Sco 2 tr) between the deep-sea and terrestrial reservoir scenarios was constructed (Figure 10b). A CO 2 saturation map under the terrestrial reservoir scenario was presented for reference. At a small injection rate (q ml/min), areas exhibiting negative S contrast and positive S contrast corresponded to the sand and silt layers, respectively. These results imply that more CO 2 intruded into the silt layers under the deep-sea reservoir scenario. At high injection rate (q ml/min), however, the positive S contrast value (corresponding to the silt layers) decreased. This is because the high viscous force accelerated the intrusion of scco 2 into the silt layers and resulted in a more evenly distributed CO 2 saturation in the core. Such a result indicates that under the high-pressure gradient (or injection rate) condition, the distributions of both scco 2 and liquid CO 2 will be similar to each other regardless of the subscale heterogeneity and the capillary pressure. A binary map of S contrast further highlighted the locations where Sco 2 ds is greater than Sco 2 tr. Similarly, S contrast decreased with greater injection rates CO 2 Storage Capacity The assessment of storage capacity is one of the central issues in geologic CO 2 sequestration [Bradshaw et al., 2007], which is mainly controlled by heterogeneous porosity, permeability, and thermodynamic conditions in a storage formation. Using the identical core, the stored CO 2 mass was estimated under both terrestrial and deep-sea reservoir scenarios. The stored CO 2 mass (M) can be calculated based on the following: ð M5 uvqs g (5) where u is porosity, V is the volume of a voxel element in the core determined from the scanning apparatus, q is the fluid density, and S g is the CO 2 saturation. Because the scanner used was a 2-D system, the core OH ET AL. CO 2 MIGRATION BEHAVIOR IN A STRATIFIED SYSTEM 7947

12 Table 2. CO 2 Mass Stored at Two Different P, T Conditions a Injection Rate q ml/min q ml/min q ml/min q ml/min P, T conditions M cond1 (g/cm) M cond2 (g/cm) M cond1 (g/cm) M cond2 (g/cm) M cond1 (g/cm) M cond2 (g/cm) M cond1 (g/cm) M cond2 (g/cm) 6 PV a M cond1 : the CO 2 mass stored under terrestrial condition (P 5 10 MPa and T 5 508C); M cond2 : the CO 2 mass stored under deep-sea condition (P 5 28 MPa and T 5 258C). image was projected onto the 2-D array of the X-ray detector, resulting in an averaged CO 2 saturation value. Due to the limitations of using the volume (V) of the voxel element, the area (A) of the pixel was used to calculate the CO 2 mass, which is why the mass dimension is expressed as mass per unit length [M L 21 ]. The size of each pixel from the scanner was 0.4 mm mm and the CO 2 density was q g/cm 3 under the terrestrial reservoir scenario (P 5 10 MPa and T 5 508C) and q g/cm 3 under the deep-sea reservoir scenario (P 5 28 MPa and T 5 258C) [Span and Wagner, 1996]. Table 2 shows the storage potentials in the core calculated at six PVs for four injection rates for each of the two storage conditions. At q ml/min, the CO 2 mass stored under the terrestrial reservoir scenario was 1.37 g/cm, and it increased with a large injection rate; at q ml/min, the stored CO 2 mass was 1.71 g/cm (25% increase). With both the terrestrial and deep-sea reservoir scenario, the CO 2 saturation values fall in a similar range (Figures 5 and 8), but the stored mass was distinctively inconsistent due to the discrepancy in CO 2 density. For instance, at q ml/min, the CO 2 mass under the deep-sea reservoir scenario was calculated as 3.06 g/cm, which was more than twice as much as that under the terrestrial reservoir scenario (1.37 g/cm) Dimensionless Rate of CO 2 Migration To assess the upward CO 2 migration rate, a dimensionless migration rate of the CO 2 front (v D ) was defined as the ratio of the fraction of the core length to the number of pore volumes (v D 5 D f =L PV, where D f is the traveling distance of the CO 2 front [L], L is the core length [L], and PV is the amount of CO 2 injected in terms of pore volume). Figure 11 presents dimensionless CO 2 migration rates for three different cases. The vertical core setting under the terrestrial reservoir scenario (circle) showed the fastest CO 2 migration rate of 7.5, whereas the horizontal core setting (rectangle) showed a rate of 5.5 (36% increase), implying that the buoyancy force is significant in the vertical core experiment. The rate of advance of the CO 2 front under the deep-sea reservoir scenario (triangle) in the vertical setting was the slowest (v D 5 2.7), which is one third of the rate under the terrestrial reservoir scenario with a vertical core. Figure 11. Dimensionless CO 2 migration rates for three different cases: vertical and horizontal settings at the terrestrial condition and vertical setting at the deep-sea sediment condition Brine Imbibition Residual CO 2 trapping occurs when brine displaces the CO 2 as discontinuous droplets at the trailing edge of a rising CO 2 plume, or when engineered through brine injection [Han et al., 2010a; Ide et al., 2007; Juanes et al., 2006]. This type of trapping mechanism takes place in a region where the connate brine has become saturated with CO 2, and the CO 2 is saturated with brine so that immiscible displacement occurs. Spiteri et al. [2008] predicted that the residual CO 2 saturation can be almost 0.5 in the Berea sandstone after the pore-scale modeling. Krevor et al. [2012] conducted core-flooding tests with CO 2 and water, which were saturated with each other, and showed residual CO 2 saturation values ranging between 0.1 and Recent analyses in a compilation of experimental data OH ET AL. CO 2 MIGRATION BEHAVIOR IN A STRATIFIED SYSTEM 7948

13 Figure 12. Snapshots of the CO 2 saturation at eight different injection PVs during the brine flooding (q ml/min). showed that the residual CO 2 saturation ranged between and for various sandstones [Burnside and Naylor, 2014]. The cessation of CO 2 injection causes the pressure drop in the storage formation, and the counter-flowing brine displaces the CO 2 from the plume margin. Under this circumstance, two regions can be considered. The first region is where the mass transfer rate between CO 2 and brine has already reached the equilibrium state. Under this circumstance, immiscible displacement will cause no mutual dissolution between CO 2 and brine. The other region is where fresh formation brine or partially CO 2 -saturated brine displaces the CO 2, resulting in dissolution of CO 2 into the brine. Previously, a number of laboratory experiments were conducted with equilibrated fluid of both brine and CO 2 (no mass transfer) [e.g., Krevor et al., 2012; Shi et al., 2011], whereas only a few studies have conducted experiments with nonequilibrated fluids [e.g., C. Chang et al., 2013]. In this section, we aimed to reproduce the latter case of nonequilibrium imbibition where fresh brine displaces to CO 2 after the drainage tests. When brine enters the inlet of the core, it will initially dissolve a small amount of CO 2 at the inlet of the core and become saturated preventing it from dissolving more CO 2 during the rest of its residence time in the core [Akbarabadi and Piri, 2013]. Figure 12 shows the snapshots of the CO 2 saturation at different PVs during the imbibition test with a brine injection rate of q ml/min under the terrestrial reservoir scenario. At the initial stage (0.04 PV), the CO 2 saturation was decreased from 0.6 to 0.4 at the core bottom indicating that the brine displaced the CO 2 up to 30 mm from the inlet while leaving the residual CO 2 behind. After the 0.1 PVs of brine injection, the CO 2 saturation was decreased throughout the core, implying that the breakthrough of brine had already arrived at the core outlet. Between 0.1 and 0.5 PVs, the CO 2 saturation was not changed much except for the core bottom. Hence, it is presumed that the irreducible CO 2 saturation was reached throughout the core. Subsequently, with more than 0.5 PV of brine injection, the residually trapped CO 2 started to dissolve into the fresh brine entering from the core bottom, and the dissolution front advanced upward. The volume of dissolved CO 2 increased with additional injected PVs. The horizontally averaged CO 2 saturation profiles at different PVs under terrestrial and deep-sea reservoir scenarios are presented in Figure 13. Under the terrestrial reservoir scenario, the CO 2 was displaced by brine up to 30 and 100 mm from the core inlet at 0.04 and 0.07 PV of brine injection, respectively (Figure 13a). Between 0.1 and 0.5 PVs, CO 2 saturation profiles were stacked up on each other. More than 1 PV of brine injection resulted in reduction of the CO 2 saturation profiles relative to the profiles lower than 1 PV, indicating that the residually trapped CO 2 started to dissolve into brine between 0 and 60 mm. The five PVs of brine injection induced most of the residually trapped CO 2 to dissolve in brine. The CO 2 saturation profiles OH ET AL. CO 2 MIGRATION BEHAVIOR IN A STRATIFIED SYSTEM 7949

14 Figure 13. Horizontally averaged CO 2 saturation profiles at different PVs (q ml/min). The numbers in the graph indicate the PVs of brine: (a) terrestrial condition and (b) deep-sea condition. for the deep-sea reservoir scenario showed a similar trend, but more PVs of brine injection were required for dissolving the residually trapped CO 2 (Figure 13b). Many studies have reported measurements of capillary trapping in porous media, and several models predicted the trapped saturation of the nonwetting phase [e.g., Jerauld, 1997; Killough, 1976; Lenhard et al., 1989; Spiteri and Juanes, 2006]. Among these models, the Land model [Land, 1968] is widely used to describe the relationship between the trapped gas saturation and the initial gas saturation [e.g., Flett et al., 2007; Ide et al., 2007; Juanes et al., 2006; Kumar et al., 2006], and is given by: where C is a dimensionless constant known as the Land coefficient. S CO2;r 5 S CO 2;i 11C S CO2;i (6) As the CO 2 saturation exhibited relatively consistent values between 0.1 and 0.5 PVs, implying that the irreducible CO 2 saturation was reached throughout the core, we applied the equilibrium relationship (Land trapping model) for three PVs (0.10, 0.15, and 0.25 PVs). Figure 14 shows a trapping curve relating the initial CO 2 saturation prior to the imbibition test (Sco 2,i )toco 2 saturation (Sco 2,r ) at three different PVs (0.10, 0.15, and 0.25 PVs) of brine injection. The brine was injected at a rate of q ml/min for both reservoir conditions and each circle with the same color represents the horizontally averaged CO 2 saturation along the OH ET AL. CO 2 MIGRATION BEHAVIOR IN A STRATIFIED SYSTEM 7950

15 Figure 14. A trapping curve relating initial CO 2 saturation (Sco 2,i )toco 2 saturation (Sco 2,r ) at three different PVs (0.10, 0.15, and 0.25 PVs): (a) terrestrial condition and (b) deep-sea condition. The Sco 2,i refers to the initial CO 2 saturation prior to imbibition test while the Sco 2,r refers to the CO 2 saturation at different PVs. core length at specific PV. The trapping curve was fit to 0.10 PV, and the Land coefficient was 0.78 and 0.85 for the terrestrial and deep-sea reservoir scenarios, respectively, showing good agreement with the previous studies using Berea sandstone [Krevor et al., 2012]. Overall, the deep-sea reservoir scenario experiment predicted a slightly larger Land coefficient. Finally, with more than 0.5 PVs of brine injection, the amount of trapped CO 2 decreased near the core bottom due to CO 2 dissolution. Consequently, the Land coefficient was increased and eventually reached infinity, implying the reduction of residually trapped CO 2 (i.e., the increase of CO 2 dissolution in brine by a solubility trapping mechanism). 4. Effect of Beddings on CO 2 Migration 4.1. Numerical Simulations Numerical simulations were performed for comparison with a companion set of core-flooding experiments as well as for overcoming limitations in the laboratory measurements. The simulation was not aimed to accurately reproduce the CO 2 distribution at the core experiments, but rather attempted to understand the subscale heterogeneity effect on CO 2 transport under the terrestrial reservoir scenario. The simulations were conducted using the TOUGH2 [Pruess et al., 2012] and ECO2N module [Pruess, 2005a]. TOUGH2 is a numerical simulator for nonisothermal flows of multicomponent, multiphase fluids in one, two, and three-dimensional porous and fractured media. ECO2N is a fluid property module that was designed for applications to geologic CO 2 sequestration in saline aquifers. It includes a comprehensive description of thermophysical properties of H 2 O-NaCl-CO 2 mixtures for temperatures ranging between 10 and 1108C and pressures up to 60 MPa [Pruess, 2005a]. TOUGH2 assumes local equilibrium conditions for predicting phase partitioning. Additionally, the mass fraction of water in the CO 2 phase is calculated according to its solubility limit at a certain pressure, temperature and salinity [Spycher and Pruess, 2005]. The isothermal version of the simulator was implemented as no heat sources or sinks were involved in the core-flooding experiment. However, the temperature-dependent density and viscosity of CO 2 and brine were considered internally in the simulator. A two-dimensional (2-D) model representing the experimental core (200 mm 3 38 mm) was constructed. Although the 2-D model has limitations compared with a three-dimensional (3-D) model, especially when analyzing a heterogeneous system, the simplicity provides computational efficiency and intuitive understanding of the system. The z axis, uniformly discretized into 100 elements, was extended to 200 mm and the grid size was 2 mm. The 38 mm of the x axis was divided into 38 elements, each having a 1 mm grid OH ET AL. CO 2 MIGRATION BEHAVIOR IN A STRATIFIED SYSTEM 7951

16 Figure 15. (a) Two different numerical models with geostatistically generated permeability field; each represents flow parallel and perpendicular to the bedding. (b) The horizontally averaged pressure buildup (DP 5 P z P outlet ) between the pressure at height (z) and pressure at outlet and a comparison of stationary CO 2 saturation profiles between the two models. size. A grid convergence study was conducted and is provided in Appendix A. The core model was initially saturated with brine (15 wt % of NaCl). At the top-most grid blocks, the constant pressure boundary condition of 10 MPa was assigned for the terrestrial reservoir scenario. At the lateral boundaries, the no-flow Neumann boundary condition was imposed because the core was covered with a sleeve to prevent fluid flow in the experiment. In addition, the temperature was held at a constant 508C throughout the core. The source boundary was assigned at the bottom-most elements to simulate a constant CO 2 injection. Finally, Corey s model [Corey, 1954] was used for the relative permeability curves, with S lr and S gr For capillary pressure, the van Genuchten model [van Genuchten, 1980] was used with k 5 0.4, S lr 5 0.3, S gr , P max Pa, and 1/P Pa 21. To reproduce a stratified core, a two-dimensional, heterogeneous, spatially correlated field of permeability modification coefficients (PMX), f n, was generated using the sequential Gaussian simulation (SGSIM) method developed by GSLIB [Deutsch and Journel, 1992]. The PMXs, which are values multiplied to the intrinsic permeability for each grid block, were calculated as: f n 5k 0 n =k n (7) where k n is the intrinsic permeability initially assigned in the model and k 0 n is the generated permeability in the grid block n. The porosity was fixed at 0.2 and the mean permeability (k n ) was m 2, OH ET AL. CO 2 MIGRATION BEHAVIOR IN A STRATIFIED SYSTEM 7952

17 Figure 16. CO 2 saturation as a function of permeability modification coefficient (PMX) and capillary entry pressure (P 0 ). which are the values obtained from laboratory experiments. The spatial correlation of the PMXs follows a spherical semivariogram with a correlation length of 100 mm, a sill value of 1.0, and an anisotropy ratio of The k 0 n field includes some low-k layers extending to the horizontal extent in the core and others shorter than the total horizontal extent (Figure 15a). To assess the effect of the bedding orientation on the CO 2 saturation profile, two permeability fields representing flow parallel and perpendicular to the bedding were constructed. The capillary pressure curve for each grid block was scaled according to the Leverett scaling rule [Leverett, 1941]. It was assumed that the wetting properties (interfacial tension and contact angle) and the shape of the capillary pressure curve (the van- Genuchten parameters) do not spatially vary with subscale heterogeneity [Saadatpoor et al., 2010]. We also assume that the porosity does not vary spatially. Thus, the capillary pressure of the n th grid block is then: p p 0 cap;n 5 p cap;n = ffiffiffiffi f n (8) 4.2. Simulation Results In this section, numerical simulations were conducted with two representative experimental settings chosen from the criteria of gravity number (N gr ). N gr measures the relative importance of viscous and gravity forces and is defined as: N gr 5 k vldqg Hu l (9) where k v is the vertical permeability, L is the model width, Dq is the density difference between CO 2 and brine, H is the model height, u is the total average Darcy flow velocity, and l is the viscosity of CO 2. A large N gr indicates that gravity force dominates in the system, whereas a small N gr implies that the viscous force is important [Ide et al., 2007]. The core-flooding experiments reveal that N gr ranges from to 6.3 and from to for terrestrial and deep-sea reservoir scenarios, respectively (Table 1). Among these, two representative numerical simulations were chosen to represent a core-flooding experiment, with one having large N gr (0.5) and the other having small N gr (0.01). Figure 15a shows two geostatistically generated permeability maps under the terrestrial reservoir scenario. The horizontally averaged pressure buildup (DP 5 P z 2 P outlet ), which represents the discrepancy of the CO 2 pressure (P z ) and pressure at the outlet (P outlet ), is plotted (Figure 15b). For comparison, horizontally averaged CO 2 saturation profiles are also plotted. When CO 2 flow was parallel to the bedding, the DP smoothly decreased from 5 to 1 kpa and from 8 to 2 kpa for N gr and N gr , respectively. When the flow was perpendicular to the bedding, the DP still smoothly dropped from the inlet to the outlet, but it was elevated overall due to the difficulty in pressure propagation perpendicular through the low-k layers. Note that sudden drops in DP were observed wherever low-k layers completely sealed the pathways. For example, an abrupt drop of DP was observed at z 5 40, 130, and 175 mm, where the low-k layers completely segregated the flow connection. Similar to the experimental results in Figure 7, a smooth decline in CO 2 saturation appeared from OH ET AL. CO 2 MIGRATION BEHAVIOR IN A STRATIFIED SYSTEM 7953

18 Figure 17. (a) Permeability modification coefficient (PMX) and CO 2 saturation maps for two different N gr under the terrestrial condition. (b) The scaled z and x-directional CO 2 flux (Fz CO2 and Fx CO2 ) along lines A and B. the inlet to the outlet in the flow parallel to the bedding (Figure 15b); CO 2 saturation smoothly decreased from 0.12 to 0.03 and from 0.23 to 0.07 for N gr and N gr , respectively. However, when the flow was perpendicular to the bedding, the profiles of CO 2 saturation showed uneven fluctuations similar to the experimental results in Figure 7. The ranges of CO 2 fluctuation were from 0.07 to 0.36 and from 0.11 to 0.55 for N gr and N gr , respectively. The storage of CO 2 mass was predicted in both k fields. For flow parallel to the bedding, CO 2 mass was 0.11 g/cm 3 (N gr 5 0.5) and 0.23 g/cm 3 (N gr ). For flow perpendicular to the bedding, the CO 2 mass was consistently greater (0.26 g/cm 3 for N gr 5 0.5, and 0.40 g/cm 3 for N gr ). This implies that the presence of low-k layers perpendicular to flow plays a key role in improving CO 2 storage capacity. However, it is also noted that the injectivity could be deteriorated due to an increase in pressure. To assess the effect of capillary entry pressure and the permeability on CO 2 saturation, we examined the model that represents the flow perpendicular to the bedding. Figure 16 presents CO 2 saturation (Sco 2 ) as a function of the PMXs and capillary entry pressure (P 0 ). The Sco 2 is increased logarithmically with the PMXs, and it decreased linearly with P 0. The logarithmic regression of PMX and Sco 2 was Sco 2 50:113ln ðpmxþ10:18, with the coefficient of determination (r 2 ) equal to 0.8. The linear regression of Sco 2 and P 0 was Sco 2 523: P 0 10:38 with r 2 equal to 0.7. Figure 17a shows the PMX and CO 2 saturation maps for two different N gr under the terrestrial reservoir scenario. The Sco 2 varied from 0.04 to 0.47 for N gr 5 0.5, whereas higher values of Sco 2 ( ) were OH ET AL. CO 2 MIGRATION BEHAVIOR IN A STRATIFIED SYSTEM 7954