Experimental and Numerical Studies of CO2 Injection Into Water ...

Available online at www.sciencedirect.com

Energy Procedia 37 (2013) 5511 – 5519

GHGT-11

Experimental and numerical studies of CO2 injection into water-saturated porous medium: capillary to viscous to fracture fingering phenomenon Amina Islam1 *, Sylvie Chevalier1, Mohamed Sassi1 1

Masdar Institute of Science and Technology, P.O.Box 54224, Abu Dhabi, UAE

Abstract

Injection of anthropogenic carbon dioxide (CO2) into deep saline geological formations is a possible long-term solution of removing CO2 from the atmosphere. The early injection of CO2 into saline aquifer is known as drainage and can cause unstable fingers to extend into the saturated rock pores. In this work, the crossover from capillary invasion to viscous fingering to fracturing is studied. Experimentally, a vertical Hele-Shaw cell filled with micro-beads was set up to study different injection rates of gas invasion into saturated porous media. The visualized invasion patterns showed that the crossover from capillary to viscous to fracture can occur in the same vertical cell but at different heights due to gas bursts occurring at different overburden and pore pressures. Numerically, 2D simulations of the invasion using STOMP (Subsurface Transport Over Multiple Phases) overpredicted the width of fingers obtained experimentally due to the averaging process of the model. © 2013 2013 The © The Authors. Authors.Published PublishedbybyElsevier ElsevierLtd. Ltd. Selection and/or peer-review under responsibility of of GHGT Selection and/or peer-review under responsibility GHGT Keywords: Hele-Shaw cell; two-phase flow; Carbon dioxide capture and storage (CCS); visualization; porous media

1. Introduction Drainage is one of the phenomena involved in CO2 geological storage. It happens during the early stage of CO2 injection into saline aquifers, and it describes the immiscible displacement of the wetting phase (brine) by the inviscid non-wetting phase (CO2). Carbon Capture and Storage (CCS) is seen as a possible method of mitigating atmospheric CO2 [1] and lowering its contribution to global warming. Deep saline aquifers have a high potential global capacity for long-term CO2 storage. Thus understanding the

* Corresponding author. Tel.: +971-55-8924288 E-mail address: [email protected].

1876-6102 © 2013 The Authors. Published by Elsevier Ltd.

Selection and/or peer-review under responsibility of GHGT doi:10.1016/j.egypro.2013.06.471

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Amina Islam et al. / Energy Procedia 37 (2013) 5511 – 5519

transport of CO2 into saline aquifers is important as they potentially could widely be used as storage reservoirs. During drainage, three different flow regimes can occur: capillary invasion, viscous fingering or fracturing. Their conditions of occurrence relate to the competition between capillary forces, viscous forces, gravity and rock matrix deformation [2]. When capillary forces overcome viscous forces, the nonwetting fluid injected progresses through the largest pores and capillary fingering is observed [3]. This regime provides the best efficiency for storage in porous media as invasion occurs in fat clusters. The prevailing of viscous forces over capillary forces results in preferential flow where long, thin viscous fingers start branching out [2]. The viscous fingering regime can lead to fractional invasion of the porous media. Fracture can occur within the rock matrix due to the increase of the pressure within the pore spaces. This last phenomenon limits the invasion within the porous media but also questions the integrity of the storage due to possible leakage. Historically, drainage in porous media has been studied through Hele-Shaw cell experiments for the concern of petroleum engineering. None of the experiments studies gas invasion of water-saturated unconsolidated porous media in a vertical Hele-Shaw cell with visualization of the three flow regimes, as it is done in this work. Interesting descriptions and results of viscous fingering studies in Hele-Shaw cell can be found in [4] [5] [6] [7] and [8]. Some works refer to configurations partially similar to the configuration under study in this work. For example, Chen and Wilkinson [9] studied experiments for horizontal, radial oil invasion into glycerin in etched-glass networks. Smirnoff et al. [10] led microgravity experiments of linearly injected dyed-water into a horizontal Hele-Shall cell saturated by a water-glycerine solution. Johnsen et al. [11] analyzed the pattern formation during air injection in a horizontal circular Hele-Shaw cell. Lovoll et al. [2] considered air injection in water saturated random porous medium under the effect of gravity but analyzed only the transition between capillary and viscous regimes. Fauria and Rempel [12] led experiments of gas invasion into water-saturated porous media. But the experimental set-up did not allow visualization of fingerings and injection was not done directly in the porous medium. Crandall et al. [13] studied air injection in a horizontal water saturated flowcell. The system consisted however of a consolidated porous medium and no fracturing regime could be studied. Jankov et al. [14] considered effects of pressure oscillations on drainage using a horizontal cell with an injection of air in a glycerolwater solution. Recently, Holtzmann and Juanes [15] analytically and numerically studied the crossover from –capillary or viscous- fingering to fracturing in deformable, disordered media at the pore scale. They also did experimentations into a horizontal cylindrical cell filled with glass beads [16]. Depending on the studies, previous focus has been put on morphological visualisation of the invading structure and their classification, on description of the invasion dynamic (bursting phenomenon) or on the determination of transition parameters between flow regimes. The present work focuses on the transition from capillary invasion to viscous fingering to fracturing in vertical Hele-shaw cells, thus taking into account gravitational effects. 2. Experimental Details The experimental set up for the drainage experiment is shown in Fig. 1. Hele-Shaw cells were made of standard glass plates that are 300 mm by 200 mm in size. Pencil leads separated the plates and were located along the three borders to be sealed by silicon. The precise diameter of pencil leads ensured the uniformity of the gap between the plates. Two diameters of pencil leads were used, 1 and 2mm. The cell was sealed by silicon on three sides and the top was left open to enable the air to escape. Before completely sealing the bottom, a G20 hypodermic needle was placed vertically between the two plates at the centre of the bottom side. The needle had an internal diameter of 0.686 mm allowing the gas injection.


The water-filled cell was tightly packed with glass microbeads and shaken to ensure uniform packing. Glass microbeads had diameters that were close in range, the first set ranging from 0.32-0.43 mm and the second set ranging from 0.57-0.70 mm. For the two sets, the porosity values were found around 33%, however intrinsic permeabilities were estimated around 2.8x10-12 and 8x10-12 m2 respectively. Due to the similar physical properties between air and CO2 at atmospheric pressure and temperature, experiments were conducted using air. Air was injected at a constant flow rate through a pump syringe (NE-4000) from the single entry point at the center bottom of the Hele-Shaw cell. The injected air moves from the needle tip through the vertical cell. The injection rate was varied and the resulting invasion patterns were visualized. The back lighting unit was an LED Light Table with uniform lighting to illuminate the Hele-Shaw cell. Snapshots were captured at regular intervals of 0.1s using a CCD camera (Sony XCL 5005CR CCD Camera) that was directly connected to the computer through the PIXCI Imaging board [15]. For the drainage image acquisition, an advanced image processing software called XCAPTM software [16] was used.

Fig. 1. Experimental set up to visualize drainage

3. Experimental Results and Discussion In this section, we present images of four experimental runs among the most representative ones. The images show the invasion pattern as they proceed with time. The invasion pattern for each experimental run showed characteristics of either two or three of the different regimes: capillary invasion, viscous fingering or fracturing. The classification of the displacement pattern was based on the visual appearance. However we are currently working on the fractal characterization of the different features.

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3.1 Case 1: 0.57 – 0.70 mm beads with injection rate = 2 mL/min Fig. 2 shows the invasion pattern for a Hele-Shaw cell filled with glass microbeads with diameters between 0.57-0.70 mm. Air was injected at 2 mL/min and the invasion pattern shows characteristics of all three regimes; capillary invasion, viscous fingering and fracturing. Uniform invasion characteristic of capillary invasion can be seen at the lower left side of the cell, while the viscous fingering could be seen in the middle of the cell. The invasion is quite slow as it takes the air 118s to cross the Hele-Shaw cell and escape right after causing fracture towards the top of the cell.

Fig. 2. Invasion Pattern for 0.57-0.70 mm beads at air injection rate = 2 mL/min

3.2 Case 2: 0.57 – 0.70 mm beads with injection rate = 4 mL/min This experimental run was done with microbeads of diameters between 0.57 and 0.7 mm and an air injection rate of 4 mL/min. As shown in Fig. 3, the invasion pattern presents again a transition from capillary to viscous fingering to fracture, with the fracture appearing right at the location of the walls. 3.3 Case 3: 0.32 – 0.43 mm beads with injection rate = 8 mL/min Microbeads of diameters between 0.32 and 0.43 mm were packed within the Hele-Shaw cell, and air was injected at a rate of 8 mL/min. For this experimental run also, the three regimes of invasion were observed in the same cell. As can be noted in Fig. 4, the capillary invasion occurs initially but then at a certain distance from the injection point, at 7s, a viscous finger breaks out. It transitions to fracture towards the surface of the porous media as the air encounters less resistance from the overlaying solid particles towards the top and it becomes easier for a fracture displacement to take place.


Fig. 3. Invasion pattern for 0.57-0.70 mm beads with injection rate = 4 mL/min

Fig. 4. Invasion pattern for 0.32-0.43 mm beads with injection rate = 8 mL/min

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3.4 Case 4: 0.57 – 0.70 mm beads with injection rate = 20 mL/min Air was injected at a rate of 20 mL/min in a Hele-Shaw cell packed with microbeads in the range between 0.57 mm and 0.70 mm. The invasion pattern can be seen in Fig. 5 and it shows viscous fingering as well as fracturing. No capillary invasion was observed. A single finger protrudes from the injection point vertically, branching out at 1.3s. A fracture can be seen to start at 2.1s and propagates across nearly the upper half of the cell due to the high injection rate of the air and due to a decrease of overburden pressure as the air moves upwards.

Fig. 5. Viscous fingering and fracturing at 20 mL/min for 0.57 – 0.7 mm beads

These four results show that a vertical orientation of the porous media would have the three regimes of invasion depending on the location in the cell. Main factors are the overburden pressure due to the beads and the hydrostatic pressure due to the water, which vary with the height within the cell. For instance, fracture was most likely to appear towards the top of the cell, because the overburden pressure due to the beads was lower, and could therefore be displaced easily during an invasion. 4. Numerical Simulation and Discussion Furthermore, 2D simulations of CO2 invasion are conducted using the simulator STOMP (Subsurface Transport Over Multiple Phases) developed by Pacific Northwest National Laboratory of the Department of Energy, USA. STOMP is a sequential numerical simulator for modeling multifluid flow and transport through geological porous media which allows the user to choose the solved governing equations and their related state equations. Solution of the discretized governing equations is based on the integral volume finite difference method and a backward Euler discretization for the time domain.


STOMP was used to simulate the experiments in section 3.4. STOMP Water-Air-Energy mode was selected. A grid consisting of 5,000 cells was made in uniform (X-Z) Cartesian coordinates. The simulation was initialized in room temperature (25oC) and pressure (1 atm). An outflow boundary condition was fixed at the top while the air is injected at the same rate as the experiments at the center of the bottom boundary. Zero flux boundary conditions were set at the sides of the cell as can be seen in Fig. 6. The porous medium was considered heterogeneous with permeability values in the range of the expected experimental permeability variation.

Fig. 6. Domain for the drainage numerical simulation

Fig. 7. Gas saturation contours that result from simulation

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A comparison was made between numerical simulation (Fig. 5) and experimental results (Fig. 7). The timescales of the invasions are in good agreement. However, the comparison showed that the finger width predicted numerically is much wider than the experimental one. Note that the amount of air injected and the conditions of injection were the same between experiments and simulations. Our simulations were not able to capture the invasion phenomena accurately. The pore scale phenomenon should be included in the simulation to obtain a better prediction of the invasion regimes. 5. Conclusion In this paper, drainage is studied. Drainage represents the initial stage of CO2 injection into brine solution. Experiments show the different flow regimes of gas invasion into a Hele-shaw cell with saturated porous media. Capillary invasion, viscous fingering and fracturing can all appear in the same cell, although at different heights. This emphasizes that the effect of gravity cannot be neglected. Additionally, a numerical simulator for multiphase flow, STOMP, was used in an attempt to simulate the phenomenon. The numerical simulations overpredicted the width of the fingers and did not capture the pore-scale phenomenon accurately.

References [1] "IPCC Special Report on Carbon Capture and Storage," Working Group 3, 2005. [Online]. Available: http://www.ipcc.ch/. [Accessed April 2011]. [2] G. Lovoll, Y. Meheust, K. J. Maloy, E. Aker and J. Schmittbuhl, "Competition of gravity, capillary and viscous forces during drainage in a two-dimensional porous medium, a pore study," Energy, vol. 30, pp. 861-872, 2005. [3] C. Zhang, C. Oostrom, T. Wiestsma, J. Grate and M. Warner, "Influence of Viscous and Capillary Forces on Immiscible Fluid Displacement: Pore-Scale Experimental Study in a Water-Wet Micromodel Demonstrating Viscous and Capillary Fingering," Energy & Fuels, vol. 25, pp. 3493-3505, 2011. [4] P. Saffman and G. Taylor, "The penetration of a fluid in a porous medium or Hele-Shaw cell containing a more viscous liquid," in Proceedings of the Royal Society of London, 1958. [5] N. Smirnov, V. Nikitin, A. Maximenko, M. Thiercelin and J. C. Legros, "Instability and mixing flux in frontal displacement of viscous fluids," Physics of Fluids, vol. 17, no. 084102, 2005. [6] G. M. Homsy, "Viscous ﬁngering in porous media," Annual Review of Fluid Mechanics, vol. 19, p. 271–311., 1987. [7] A. R. Kopf-Sill and G. Homsy, "Nonlinear unstable viscous fingers in Hele-Shaw flows, 1. Experiments," Physics of Fluids, vol. 31, no. 2, pp. 242-249, 1988. [8] C. Jiao and T. Maxworthy, "An experimental study of miscible displacement with gravity-override and viscosity-contrast in a Hele Shaw cell," Experiments in Fluids, vol. 44, pp. 781-794, 2008. [9] Wilkinson and Chen, "Pore scale viscous fingering in porous media," APS Physical Review Letters, vol. 55, pp. 1892-1895, 1985. [10] N. Smirnov, V. Nikitin, A. Maximenko and M. T. a. J. C. Legros, "Instability and mixing flux in frontal displacement of viscous fluids," Physics of Fluids, vol. 17, no. 084102, 2005. [11] O. Johnsen, R. Toussaint, K. J. Maloy and E. Flekkoy, "Pattern formation during air injection into granular materials conﬁned," Physical Review E, vol. 74, p. 011301, 2006. [12] K. E. Fauria and A. W. Rempel, "Gas invasion into water-saturated, unconsolidated porous media: Implications for gas hydrate reservoirs," Earth and Planetary Science Letters, vol. 312 (2011), p. 188–193, 2011. [13] D. Crandall, G. Ahmadi and M. F. a. D. H. Smith, "Distribution and occurrence of localized-bursts in two-phase flow through porous media," Physica A, vol. 388, pp. 574-584, 2009. [14] M. Jankov, G. Lovoll, H. Knudsen, K. Maloy, R. Planet, R. Toussaint and E. Flekkoy, "Effects of Pressure Oscillations on Drainage in an Elastic Porous Medium," Transport in Porous Media, vol. 84, no. 3, pp. 569-585, 2010. [15] R. Holtzman and R. Juanes, "Crossover from Fingering to Fracturing in Deformable Disordered Media," APS Physical Review E, vol. 82, no. 4, 2010. [16] R. Holtzman, M. Szulczewski and R. Juanes, "Capillary Fracturing in Granular Media," Physical Review Letters, vol. 108, no.


26, 2012. [17] Sony Inc., "Sony: XCL U1000c," [Online]. Available: http://pro.sony.com/bbsc/ssr/cat-industrialcameras/catcameralink/product-XCL5005CR/. [Accessed 2011]. [18] EPIX Inc., "PIXCI Imaging Boards," [Online]. Available: http://www.epixinc.com/products/xcap.htm. [Accessed 2011].

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