Scholarly article on topic 'Immiscible Displacements and Capillary Trapping in CO2 Storage'

Immiscible Displacements and Capillary Trapping in CO2 Storage Academic research paper on "Chemical engineering"

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{"Capillary trapping" / "Residual saturation" / "Trapping capacity" / "Porous plate" / "Contact angle"}

Abstract of research paper on Chemical engineering, author of scientific article — C.H. Pentland, R. El-Maghraby, A. Georgiadis, S. Iglauer, M.J. Blunt

Abstract We measure the residual non-wetting phase saturation of super-critical carbon dioxide in a Berea sandstone core. We use the porous plate method while a stirred reactor ensures equilibrium between the carbon dioxide and brine. We also measure carbon dioxide-brine contact angles on the porous plate to understand wetting behavior in the experiment. The application of the work is for carbon dioxide storage in aquifers, where capillary trapping is a rapid and effective mechanism to render the injected fluid immobile. The experiment was performed at temperature and pressure representative of potential subsurface storage formations. The measured residual saturation is 37% which is lower than the measured residual for an oil-brine system on a similar core (48%), but higher than measured by other authors for super-critical CO2 in Berea sandstone. We suggest that super-critical CO2 is still non-wetting in sandstones with considerable trapping and discuss the implications for CO2 storage in aquifers.

Academic research paper on topic "Immiscible Displacements and Capillary Trapping in CO2 Storage"

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Energy Procedía 4 (2011) 4969-4976


Immiscible Displacements and Capillary Trapping in CO2 Storage

C. H. Pentlanda*, R. El-Maghrabya, A. Georgiadisb, S. Iglauera, M. J. Blunta

a Department of Earth Science and Engineering, Imperial College London, SW7 2AZ, United Kingdom b Department of Chemical Engineering and Chemical Technology, Imperial College London, SW7 2AZ, United Kingdom


We measure the residual non-wetting phase saturation of super-critical carbon dioxide in a Berea sandstone core. We use the porous plate method while a stirred reactor ensures equilibrium between the carbon dioxide and brine. We also measure carbon dioxide-brine contact angles on the porous plate to understand wetting behavior in the experiment. The application of the work is for carbon dioxide storage in aquifers, where capillary trapping is a rapid and effective mechanism to render the injected fluid immobile. The experiment was performed at temperature and pressure representative of potential subsurface storage formations. The measured residual saturation is 37% which is lower than the measured residual for an oil-brine system on a similar core (48%), but higher than measured by other authors for super-critical CO2 in Berea sandstone. We suggest that super-critical CO2 is still non-wetting in sandstones with considerable trapping and discuss the implications for CO2 storage in aquifers.

© 2011 Published by Elsevier Ltd.

Keywords: Capillary trapping; Residual saturation; Trapping capacity; Porous plate; Contact angle

1. Introduction

Saline aquifers have been identified as suitable storage formations for injected carbon dioxide (CO2) due to their wide geographical spread and large storage capacities [1-3]. When CO2 is injected into a saline aquifer its movement is controlled by a number of parameters including: the solubility of CO2 in the formation brine; the phase densities and viscosities; the petrophysical properties of the aquifer; and the injection rate and duration. Mass transfer between CO2 and brine will occur at the leading edge of the injected CO2 plume as it comes into contact with fresh formation brine. Behind the leading edge the CO2 and brine in the pore space are mutually saturated and immiscible displacements dominate. As the CO2 plume rises upwards in the formation - due to the density contrast with brine - there will be secondary imbibition at the trailing edge of the plume [4,5]. The re-imbibition of brine into the pore space will trap CO2 as a residual discontinuous phase held in place by local capillary forces - this process is called capillary trapping.

Capillary trapping has been extensively studied due to its importance for hydrocarbon recovery and contaminant remediation. It is proposed that capillary trapping can also play an important role in CO2 storage - as it is a fast and secure method of storing CO2 in porous media. In this context it is desirable to

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maximize the trapped non-wetting phase (CO2) within the porous media. One way of achieving this is to engineer the process, i.e. by the injection of additional brine into the formation after CO2 injection [6,7].

Capillary trapping can be characterized by measuring the form of the initial versus residual non-wetting phase saturation curve - the trapping curve. This has been extensively measured for gas-water and oil-water sand/sandstone systems [8-19]. A number of studies have proposed empirical equations to generate the trapping curve if the end point of the curve is known - that is the maximum residual saturation (5™ax) for a given maximum initial saturation (Sjmax). The advantage being that only one measurement is required instead of several, saving considerable time. Recent research [19] shows that the Land, [18] and Spiteri et al., [20] equations provide the best match to measurements on consolidated sandstone samples. The Aissaoui, [21] equation provides the best match to unconsolidated sand pack data [16].

To date the trapping curve has not been measured for CO2-brine fluid systems. Research has primarily focused on relative permeability [22-24], the influence of sub core scale heterogeneity [24-26], mineral-CO2 interactions [27-29], and the measurement of residual saturations [30-32]. This paper complements the data on residual saturation - specifically smax - with a new CO2-brine coreflood experiment performed at flow rates representative of storage conditions.

If smax and porosity are known then the capillary trapping capacity (Ctrap) of a given rock can be determined as:

Ctrap = &rmaX (1)

The influence of porosity on Ctrap has been studied previously for oil-water systems [17].

System wettability is a key control on the ability to trap CO2 as a residual immobile phase as it determines fluid distribution on the pore scale. If CO2 were to wet the rock surface the amount of trapping would be very limited - however there is little evidence from measurements that this is the case. Water-CO2 contact angles have been measured on various substrates [33-35]. The results for untreated silicate minerals show that water will be the wetting phase in these systems (defined as 9 < 90° [36] where 9 is measured through the denser brine phase).

An immiscible displacement coreflood experiment was performed to measure smax on a Berea sandstone core-plug. The results are analyzed in terms of Ctrap and compared with the Berea CO2-brine data of Suekane et al., [30]. The system wettability is inferred from coreflood results and the solubility of CO2 in brine is measured. In addition contact angle measurements are performed on a silicate mineral substrate.

2. Experimental Conditions and Equipment

Experimental conditions representative of saline aquifers (T = 343 K; Pore pressure = 9 MPa) were considered. The cylindrical Berea core-plug used was 0.0384 m in diameter and 0.0754 m in length. The wetting phase in the experiment was 5wt.%-sodium chloride, 1wt.%-potassium chloride synthetic brine. Table 1 summarizes the petrophysical properties of the Berea core-plug.

The importance of immiscible displacements to CO2-brine capillary trapping has been highlighted. In order to perform such laboratory displacement experiments special care must be taken in the equilibration of the CO2 and brine phases. If the phases are not mutually saturated then there will be mass transfer across the phase interface leading to saturation changes in the pore space which are not controlled by capillary forces. In order to achieve equilibrium between the CO2 and brine at the experimental conditions a coreflood experiment was designed and built around an equilibration reactor (1200 mL C276 autoclave with gas entrainment stirrer - Parr Instruments Co., IL, USA).

Table 1: Petrophysical properties of the Berea core-plug used in the CO2-water coreflood experiments.

Porosity Brine Permeability (m2) Formation Factor

0.222 4.4x10-13 14

The coreflood displacement method used to achieve initial and residual CO2 saturations is of paramount importance. Steady and unsteady state CO2-brine displacement methods have been reported elsewhere in the literature [24-26,30-32], however these methods are unable to achieve a homogenous saturation profile along the length of the core-plug due to the influence of capillary end effects. In addition a study of the literature indicates that high initial CO2 saturations (> 80%) are rarely achieved with these methods. A modified porous plate method was used in this study. A hydrophilic ceramic disk (15bar, Aluminium Silicate, Weatherford Laboratories, Stavanger, Norway) was placed immediately downstream of the core-plug. The porous plate retains the non-wetting CO2 phase within the core-plug due to the capillary entry pressure constraints of the porous plate's small diameter pores. During CO2 injection equilibrium corresponds to a uniform pressure distribution along the core-plug length in each of the phases. The difference in the equilibrium pressure of the two phases corresponds to the capillary pressure (Pc). At equilibrium the phase saturations are assumed to be uniform along the length of the core-plug. No additional CO2 enters the core-plug and no brine is produced. The porous plate method has been used previously to measure CO2-water capillary pressure curves for unconsolidated sand [37]. We modified the porous plate to shorten the duration of waterflooding. A hole was drilled through the centre of the porous plate through which a 3.175*10-3 m (1/8") tube could pass. This tube was used during initial saturation and waterflooding to avoid large pressure drops across the porous plate. The tube was connected to a valve which was closed during primary drainage to prevent CO2 production. A fluoroelastomer (Viton®) o-ring provided a seal between tube and porous plate to prevent bypassing.

The core-plug was housed in a custom Hassler type core holder. All flowlines and valves were C276 alloy metal to mitigate the risk of corrosion. The equilibration reactor was housed in an oil bath and the core holder in an air bath - both maintained at the experimental temperature. Experimental pressure and displacement rates were controlled by high precision syringe pumps (Teledyne ISCO 500D & 1000D, Lincoln, NE, USA). Super-critical CO2 (scCO2) was generated inside a custom 1000 mL piston accumulator (Phoenix Instruments, Splendora, TX, USA) controlled by a syringe pump. The coreflood effluent was received in an identical piston accumulator, again controlled by a syringe pump. This effluent accumulator controlled the system back pressure. Figure 1 shows the arrangement of the experimental equipment.

Contact angle measurements were performed on a separate apparatus which consisted of a view cell (Sitec, model 740.2093-5, Zurich, Switzerland) connected to a custom designed high pressure apparatus (Eurotechnica GmbH, model PD-E700 LL, Bargtecheide, Germany) with operating pressure and temperature range up to 70 MPa and 473 K respectively. A detailed description of the apparatus and operating procedures is given elsewhere [38].

3. Experimental Procedure

Contact Angle Measurement. Before performing saturation measurements the contact angle of the CO2-water-porous plate system was investigated. This was done to develop confidence in the ability of the porous plate to prevent CO2 production during primary drainage and to investigate wettability in silicate mineral systems. A segment of porous plate was placed inside a view-cell which was filled with de-ionized water and heated to 354.5 K. The water was pressurized and CO2 introduced onto the surface of the porous plate via a capillary tube from a screw injector. The porous plate was placed at an angle of 21° from the

horizontal to investigate advancing and receding contact angles (raindrop effect - Bear, [36]). Contact angles were measured for pressures between 26.6 MPa and 48.9 MPa.

Figure 1: Schematic diagram of the experimental setup for measuring capillary trapping in scCO2-brine-sandstone systems.

Phase Equilibration. The equilibration reactor was half filled with de-aired brine and half filled with scCO2. The top of the autoclave was connected to a reservoir of scCO2 maintained at constant pressure by a syringe pump. The reactor's internal gas entrainment stirrer was then rotated at 200 r.p.m. to mix the phases. Brine was sampled from a bottom drain valve and the solubility of CO2 in brine measured. Equilibrium was reached when no more CO2 entered the reactor from the reservoir with continued rotation of the stirrer shaft. The equilibrated brine was stored in the reactor at constant pressure and temperature until required in the experiment.

Initial Saturation. The core-plug pore space was initially vacuumed for 15 minutes with the experimental confining pressure applied (11.72 MPa). Fresh (unsaturated with CO2) de-aired brine was then injected into the core-plug from right to left (Figure 1). The fresh brine was initially introduced at ambient conditions then pressurized to 9 MPa. Five pore volumes (PVs) of fresh brine were injected through the core-plug. Flooding was then switched to the equilibrated brine for a further five PVs of injection. The flowlines connected to both valves 4 and 5 were used during injection. Valve 4 was connected to the flowline terminating at the outside face of the porous plate. Valve 5 was connected to the flowline that passed through the centre of the porous plate terminating at the face of the core-plug. Brine production was from the flowline connected to valve 2.

Primary Drainage. During primary drainage CO2 was injected from left to right in the core-plug. CO2 was introduced via the valve 2 flowline. At the outlet face the valve 5 flowline was closed to prevent bypassing of the porous plate. As CO2 entered the core-plug the displaced brine was produced through the porous plate and out of the core holder via the valve 4 flowline. The injection of CO2 was controlled by operating the inlet and outlet syringe pumps in constant pressure mode. The difference in pump pressures was 0.28

MPa at equilibrium - corresponding to the applied Pc. Equilibrium was reached when the production of brine and injection of CO2 ceased. The brine produced was considered equal in volume to the CO2 injected. The initial CO2 saturation was calculated based on this volume measurement. The duration of primary drainage was 20 hours.

Waterflooding. The core-plug was waterflooded to residual CO2 saturation with equilibrated brine. Waterflooding took place from right to left in the same manner as initial saturation. Five PVs of equilibrated brine were injected at a capillary number of 4.2 x 10-7.

Residual Saturation Measurement. The residual CO2 saturation was measured by isothermal expansion of the core-plug pore space, as per Suekane et al,. [30]. The pore space was initially isolated from the experimental system before connection to an empty syringe pump (via valve 3). The pump was maintained at the experimental temperature and the internal volume increased while the pressure was monitored. Knowing the pore volume, dead volume, volume of expansion, pressure, CO2 density and CO2 solubility in brine the CO2 saturation was calculated.

4. Results

The contact angle measurements are summarized in Table 2. In addition Figure 2 shows the CO2-water-porous plate contact angle at the maximum pressure (48.9 MPa). The contact angles on the two sides of the drop were found to be almost identical (within 1-2 degrees). Smaller contact angles at lower pressures were possibly an effect of small contact force (CO2 drop just reached the substrate); whereas at higher pressures (39.5 - 48.9 MPa) good contact was achieved and thus contact angles remained stable at 40°. Another explanation for the observed change in contact angle with pressure is the increase of CO2 solubility in brine and subsequent shift in the reactions CO2 + H2O O H2CO3 O H+ + HCO3- O 2 H+ + CO32- to the right side. The drop in the pH value and an increase in HCO3- and CO32- concentrations may have changed the surface chemistry of the substrate and the electric double-layer on it, influencing the spreading of CO2. This effect requires further study.

The solubility of CO2 in brine was measured as 0.0136 mole fraction ± 0.0020 mole fraction (32.4 kg m-3 ± 4.8 kg m-3) for the experimental conditions investigated. This is in the range of solubilities measured on similar systems [39-43]. The solubility was measured by the same isothermal expansion procedure used in pore space saturation measurement. The coreflood experiment gave a maximum residual CO2 saturation of 37% (± 2%) for a corresponding initial CO2 saturation of 90% (± 5%). Applying equation 1 to these results yielded a Ctrap of 8.2%. These results are summarized and compared to those of Suekane et al., [30] in Table 3.

Table 2: Contact angle measurements at 354.5 K between the de-ionized water and scCO2 phase interface on a porous plate (aluminium silicate). Contact angles are reported for the denser brine phase._

Pressure (MPa) 9 (degrees)

26.6 10 ± 5

29.7 32 ± 2

32.0 30 ± 2

39.5 39 ± 2

47.0 40 ± 2

48.9 40 ± 3

Figure 2: A drop of scCO2 generated from a capillary tube is brought into contact with a porous plate substrate angled at 21°. The bulk phase is de-ionized water (P = 48.9 MPa).

The CO2-brine 5™ax can be compared with the oil-brine 5™ax of 48% reported previously [19] - table 4. Both experiments were performed at the same conditions of temperature and pressure and on core-plugs from the same batch of Berea sandstone.

Table 3: Summary of coreflood results from this study and a comparison with literature capillary trapping data.

Study Sample $ T P (K) (MPa) Si Sr ^trap

Suekane et al., 2008 Berea This study Berea 0.198 308 - 323 7.6 - 10.0 0.222 343 9.0 - 0.25 - 0.28 0.90 0.37 0.050 - 0.055 0.082

Table 4: Summary of oil-brine and scCO2-brine residual saturation coreflood data.

Study Sample ^ Phases IFT Si (mN m-1) Sr

Pentland et al., 2010 Berea This study Berea 0.221 n-decane - brine 0.222 CO2 - brine 48.3a 0.88 32.5b 0.90 0.48 0.37

a Zeppieri et al., 2001; linearly extrapolated to 343K.

b Chalbaud et al, 2009; experimental data for 345.1 K, 9.56 MPa and 0.87 m NaCl salinity. 5. Conclusions

Laboratory measurements of contact angle and capillary trapping were presented for a CO2-brine system. Capillary trapping was measured by performing immiscible displacement sequences in a water-wet Berea sandstone using the porous plate method. Contact angles were measured on an aluminium silicate substrate which was found to be water-wet. For Berea sandstone the maximum residual CO2 saturation was shown to be higher than literature measurements indicating that up to 8.2% of the gross rock volume can contain CO2 trapped on the pore scale by capillary forces. This is over 50% higher than previous measurements [30]. Lower residual saturations in previous studies may have been a result of the experimental procedure (unsteady state versus porous plate) where lower initial CO2 saturations would be expected.

The residual saturation measured in this study indicates that CO2 was the non-wetting phase in the system. If the CO2 had completely wet the sandstone then lower residual saturations would have been expected as the CO2 drained from the sample through wetting layers during waterflooding. The contact angles measured in this study add weight to this theory - CO2 was clearly shown to be the non-wetting phase in the presence of water on an aluminium silicate substrate.

The CO2-brine residual saturation measured in this study was compared to a literature measurement of oil-brine residual saturation for the same rock type [19]. The lower CO2-brine residual saturation could be attributed to the lower CO2-brine interfacial tension and associated changes in contact angle. For a given waterflood capillary pressure more CO2 than oil would be produced from the same rock sample. However, the reduction in residual saturation compared to analogue oil-brine systems is modest and implies that super-critical CO2 still behaves as a strongly non-wetting phase in clean sandstones.

Future work will focus on investigating system wettability further and measuring the shape of the trapping curve for CO2-brine systems and different rock types.


Ctrap = capillary trapping capacity Pc = applied capillary pressure, Pa

5jmax = maximum initial (post-primary drainage) saturation ^max = maximum residual (post-secondary imbibition) saturation Si = initial (post-primary drainage) non-wetting phase saturation Sr = residual (post-imbibition) non-wetting phase saturation 9 = macroscopic intrinsic contact angle, ° porosity


We would like to thank Shell under the Grand Challenge on Clean Fossil Fuels for supporting this work. The authors gratefully acknowledge Alexander Toth and Graham Nash for providing technical laboratory support and advice.


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