Interfacial Interactions between Reservoir Brine andCO2 at High Pressures and Elevated Temperatures

Daoyong Yang, Paitoon Tontiwachwuthikul, and Yongan Gu*

Petroleum Technology Research Centre (PTRC), Petroleum Systems Engineering,Faculty of Engineering, University of Regina, Regina, Saskatchewan S4S 0A2, Canada

Received August 18, 2004. Revised Manuscript Received October 26, 2004

In this paper, an experimental technique has been developed to study the interfacial interactionsof the reservoir brine-CO2 system at high pressures and elevated temperatures. Using theaxisymmetric drop shape analysis (ADSA) for the pendant drop case, this new technique makesit possible to determine the interfacial tension (IFT) and visualize the interfacial interactionsbetween the reservoir brine and CO2 under practical reservoir conditions. More specifically, thedynamic IFT between the reservoir brine and CO2 is measured as a function of pressure at twodifferent temperatures. It is found that the dynamic IFT gradually reduces to a constant value,which is termed as the equilibrium IFT. The equilibrium IFT decreases as the pressure increases,whereas the same parameter increases as the temperature increases. The major interfacialinteractions observed in this study include interface disappearance, the swelling effect, theshrinking effect, and wettability alteration. At an elevated temperature (T ) 58 C), the pendantbrine drop cannot be observed in the CO2 phase at a pressure of P g 12.238 MPa. It is also foundthat CO2 solubility in the brine phase, which is measured using the PVT system, approaches itsmaximum value under the same conditions. Further increases in pressure do not noticeablyincrease the CO2 solubility in the brine phase. The brine swelling effect is observed at all thepressures and temperatures tested, whereas the brine shrinking effect occurs only at highpressures and a lower temperature (T ) 27 C). In addition, the wettability of the reservoir brine-CO2 system changes from the hydrophilic case to the hydrophobic case when the CO2 changesfrom the gas phase to the liquid phase.

Introduction

Geological sequestration of anthropologic CO2 is apotential technology for greatly mitigating greenhousegas emissions. In this way, CO2 is either sequestratedin a depleted oil/gas reservoir or disposed from station-ary sources into a saline aquifer. Successful sequestra-tion of CO2 is largely controlled by the interfacialinteractions among brine, CO2, and the reservoir miner-als.1 The major interfacial interactions include theinterfacial tension (IFT), wettability, capillarity, andinterface mass transfer. It has long been recognized thatthese interactions govern both the distribution of fluidsand their flow behavior in porous media.2,3 As a firststep, it is of fundamental and practical importance tothoroughly study the interfacial interactions betweenthe reservoir brine and CO2 at high pressures andelevated temperatures.

Recently, a field-scale project for CO2 enhanced oilrecovery (EOR) and CO2 sequestration has been under-

taken in the Weyburn oil field1 and a large-scale CO2sequestration project is underway in the Sleipner salineaquifer.4 The International Energy Agency (IEA) Green-house Gas (GHG) Weyburn CO2 Monitoring and StorageProject is recognized as the worlds largest joint fieldtest for sequestrating CO2 in a depleted oil reservoir,although it is currently still at the EOR stage.5 AfterCO2 is injected into a depleted reservoir or saline aquiferunder the supercritical condition, most of the injectedCO2 remains in the supercritical state, whereas theremainder dissolves in the crude oil and/or brine phaseand interacts with the native minerals. The IFT phe-nomenon under high pressures controls the migrationof CO2 and brine in the porous media. Although thereare numerous dynamic and/or equilibrium IFT data forthe water-CO2 systems at different pressures andtemperatures,6-9 no IFT data are made available for thereservoir brine-CO2 system under practical reservoirconditions.

An updated literature search shows that few attemptsare made to relate the interfacial properties of the

* Author to whom correspondence should be addressed. Telephone:1-306-585-4630. Fax: 1-306-585-4855. E-mail: Peter.Gu@Uregina.Ca.

(1) Emberley, S.; Hutcheon, I.; Shevalier, M.; Durocher, K.; Gunter,W. D.; Perkins, E. H. Energy 2004, 29 (9-10), 1393-1401.

(2) Arendt, B.; Dittmar, D.; Eggers, R. Int. J. Heat Mass Transfer2004, 47 (17-18), 3649-3657.

(3) Yang, D.; Gu, Y. Interfacial Phenomena of the Oil-Fluid-RockSystems in Carbon Dioxide Flooding Reservoirs. In Recent Develop-ments in Colloids and Interface Research; Pandalai, S. G., Ed.;Transworld Research Network: Kerala, India, 2003; Vol. 1, pp 115-127.

(4) Arts, R.; Eiken, O.; Chadwick, A.; Zweigel, P.; van der Meer, L.Energy 2004, 29 (9-10), 1383-1392.

(5) Moritis, G. Oil Gas J. 2004, 102 (14), 45-52.(6) Jho, C.; Nealon, D.; Schobola, S.; King, A. D., Jr. J. Colloid

Interface Sci. 1978, 65 (1), 141-154.(7) Chun, B. S.; Wilkinson, G. T. Ind. Eng. Chem. Res. 1995, 34 (12),

4371-4377.(8) Hebach, A.; Oberhof, A.; Dahmen, N.; Kogel, A.; Ederer, H.;

Dinjus, E. J. Chem. Eng. Data 2002, 47 (6), 1540-1546.(9) Tewes, F.; Boury, F. J. Phys. Chem. B 2004, 108 (7), 2405-2412.

216 Energy & Fuels 2005, 19, 216-223

10.1021/ef049792z CCC: $30.25 2005 American Chemical SocietyPublished on Web 12/13/2004

binary systems to their miscibility and solubility underreservoir conditions. In practice, miscibility is consid-ered to occur if there is no interface between the twophases involved (i.e., zero IFT) or if the two phases canbe mixed together at any ratio.10 On the other hand,solubility of CO2 in the reservoir brine represents howmuch CO2 dissolves in the reservoir fluids at theequilibrium state, depending on the pressure and tem-perature. To date, the miscibility between reservoirbrine and CO2 has not been reported in the literature.Although the solubility data of the water-CO2 systemscovering large pressure and temperature ranges can befound elsewhere,11-16 few data are available for thereservoir brine-CO2 systems at high pressures andelevated temperatures.16,17 Generally, it has been as-sumed that CO2 solubility in the brine phase is lowbecause of its high salinity.11 Furthermore, the complexrelationships among the equilibrium IFT, miscibility,and solubility are not explored.

In this paper, an experimental technique is developedto study the interfacial interactions of the reservoirbrine-CO2 system under practical reservoir conditions.Based on the axisymmetric drop shape analysis (ADSA)for the pendant drop case, this new technique is usedto measure the IFTs and visualize the interfacialinteractions between the reservoir brine and CO2 athigh pressures and elevated temperatures. More specif-ically, the dynamic and equilibrium IFTs between thereservoir brine and CO2 are measured at differentpressures and two temperatures. Several importantphysical phenomena are observed after the fresh brinephase is made in contact with CO2. The measured IFTdata and the visualized interfacial interactions underreservoir conditions should facilitate design of the CO2EOR and CO2 sequestration processes. Certain relation-ships also are found to exist among the equilibriumIFT, miscibility, and solubility for the reservoir brine-CO2 system at high pressures and elevated tempera-tures.

Experimental Section

Materials. The brine sample was collected from the Instowfield in Saskatchewan, Canada. Its density is 1.003 g/cm3 at15 C, and its pH value is 8.66 at 22 C. This brine samplemainly contained 1500 mg/L of sodium, 2050 mg/L of chloride,and 928 mg/L of bicarbonate, and the total dissolved solids(TDS) content was 4270 mg/L at 110 C. The densities of CO2at different pressures and temperatures are calculated froma standard property table.18 The purity of CO2 and nitrogen

(each from Praxair, USA) was 99.99% and 99.998%, respec-tively. For CO2, its critical pressure is 7.38 MPa and its criticaltemperature is 30.95 C.18

Apparatus. In this study, the ADSA technique for thependant drop case is used to measure the dynamic andequilibrium IFTs of the reservoir brine-CO2 system at differ-ent pressures and temperatures. The ADSA technique isprobably the most advanced and accurate method for measur-ing the IFT in a large range of pressures and temperatures.19,20In principle, the ADSA technique determines the IFT from thependant drop shape analysis. In experiment, a pendant brinedrop in CO2 phase is formed at the tip of a syringe needle. Itsdigital image then is acquired, using a digital image acquisi-tion system. Through application of computer digital imageanalysis and processing techniques, an accurate interfacialprofile of the pendant brine drop is obtained. Finally, the IFTof the pendant brine drop surrounded by CO2 phase isdetermined by solving the Laplace equation of capillarity andfinding the best fit of the numerically calculated interfacialprofile to the physically observed drop profile. In comparisonwith the other existing methods, the ADSA technique for thependant drop case is accurate for the IFT measurement ((0.05dyne/cm), fully automatic, and completely free of the operatorssubjectivity. At present, this technique becomes a standardmethod for measuring the IFT.

A schematic diagram of the ADSA system for the pendantbrine drop case used in this study is shown in Figure 1. Themajor component of this system is a high-pressure cell withtwo see-through windows (IFT-10, Temco, USA). The maxi-mum operating pressure and temperature of this pressure cellare 69 MPa and 177 C, respectively. The temperature duringthe test is maintained by wrapping the pressure cell with twoheating tapes (HT955041, Electrothermal, USA), which areconnected to a stepless temperature controller (CN45515,Thermolyne, USA). There are a total of six ports around thepressure cell. In this study, the top port is used to introduce apendant brine drop in the presence of the CO2 phase, and thebottom port serves as a draining outlet. Among the other fourports on the side, one is for pressure measurement, using adigital pressure gauge (DTG-6000, 3D Instruments, USA),one is for temperature measurement using a thermocouple(JMQSS-125U-6, Omega, USA) and a temperature display(MDSS 41-T-A, Omega, USA), one is for installation of arupture valve (P-7019, Oseco, USA), and the last port is forthe injection of CO2.

In the ADSA system, the see-through windowed high-pres-sure cell is placed between a light source and a MZ6 microscopecamera (Leica, Germany). A pendant brine drop is formed inthe CO2 phase at the tip of a stainless-steel needle, which isinstalled at the top of the pressure cell. The entire experimen-tal setup is mounted on a vibration-free table (RS 4000,Newport, USA). A Dell desktop computer is used to acquirethe digital image of the pendant brine drop and perform thesubsequent drop image analysis, digitization, and computation.

Figure 2 shows a block diagram of the experimental setupused to measure the IFT and visualize the interfacial interac-tions of the reservoir brine-CO2 system at high pressures and

(10) Green, D. W.; Willhite, G. P. Enhanced Oil Recovery; SPETextbook Series, Vol. 6; Society of Petroleum Engineers: Richardson,TX, 1998.

(11) Klins, M. A. Carbon Dioxide Flooding: Basic Mechanisms andProject Design; International Human Resources Development Corpora-tion: Boston, MA, 1984.

(12) Mori, Y. H.; Mochizuki, T. Energy Convers. Manage. 1998, 39(7) 567-578.

(13) Teng, H.; Yamasaki, A. J. Chem. Eng. Data 1998, 43 (1), 2-5.(14) Dhima, A.; de Hemptinne, J.; Jose, J. Ind. Eng. Chem. Res.

1999, 38 (8), 3144-3161.(15) Diamond, L. W.; Akinfiev, N. N. Fluid Phase Equilib. 2003,

208 (1-2), 265-290.(16) Enick, R. M.; Klara, S. M. Chem. Eng. Commun. 1990, 90, 23-

33.(17) Li, Z.; Dong, M.; Li, S.; Dai, L. J. Chem. Eng. Data 2004, 49

(4), 1026-1031.(18) Jarrell, P. M.; Fox, C. E.; Stein, M. H.; Webb, S. L. Practical

Aspects of CO2 Flooding; SPE Monograph Series, Vol. 22; Society ofPetroleum Engineers: Richardson, TX, 2002.

(19) Rotenberg, Y.; Boruvka, L.; Neumann, A. W. J. Colloid InterfaceSci. 1983, 93 (1), 169-183.

(20) Cheng, P.; Li, D.; Boruvka, L.; Rotenberg, Y.; Neumann, A. W.Colloids Surf. 1990, 43 (2), 151-167.

Figure 1. Schematic of the axisymmetric drop shape analysis(ADSA) system for the pendant brine drop case.

Reservoir Brine-CO2 Interfacial Interactions Energy & Fuels, Vol. 19, No. 1, 2005 217

elevated temperatures. This setup consists of the ADSAsystem, a control panel, and a fluids handling system. Mountedon the vibration-free table, the control panel has eight needlevalves that are used to control the flow rate and direction. Inthe fluids handling system, three cylinders are used, to cleanthe pressure cell and the entire tubing system with acetone(EM Science, USA), pressurize the pressure cell with CO2, andintroduce the pendant brine drop, respectively.

In addition, a mercury-free PVT system (PVT-0150-100-200-316-155, DBR, Canada) is used to conduct the solubilitymeasurements of CO2 in the reservoir brine. Such measuredCO2 solubility in the brine phase is related to the physicalphenomena observed in the pendant brine drop experiments.The maximum operating pressure and temperature of the PVTsystem are 69 MPa and 200 C, respectively. The majorcomponent of this PVT system is a visual PVT cell, where fluidsamples are encapsulated inside a glass tube and isolated fromthe hydraulic oil by a moveable isolation piston. The cellpressure can be readily adjusted by changing the pressure inthe hydraulic system to move the isolation piston down-ward or upward. The visual PVT cell, together with a video-based digital cathetometer, allows direct and accurate mea-surement of the phase volumes. It is also equipped with amagnetically coupled mixer, which can effectively mix thefluid-liquid system. The PVT cell and two sample cylindersare kept in a constant-temperature air bath: one cylinder isfilled with CO2, and the other is filled with the reservoirbrine. The constant temperature inside the air bath ismaintained using a microprocessor-based controller, in con-junction with a resistance temperature device (RTD) sensor.An automatic positive displacement pump (PMP-500-1-10-MB-316-M4-CO, DBR, Canada) is used to generate high fluidpressures and accurately displace prespecified amounts of fluidsamples from the sample cylinders to the PVT cell. A computer-based data acquisition system is used to automatically recordthe cell pressure, cell temperature, and pump displacementvolume.

Experimental Procedure

IFT Measurement. Prior to the first experiment, theentire system is tested for leakage with deionized water,then cleaned with acetone, flushed with nitrogen, andfinally evacuated. The pressure cell is then pressurizedwith CO2 to a prespecified pressure, using a manualpositive displacement pump (PMP-500-1-10-HB, DBR,

Canada). After the CO2 is injected, usually 30-60 minare needed for the pressure and temperature inside thepressure cell to reach their stable values. Note that thebrine and CO2 are not pre-equilibrated in the experi-ments, because the purpose of this study is to determineboth the dynamic and equilibrium IFTs when CO2dissolves into a fresh brine phase. This is conducted tomodel the process during which CO2 is placed in contactwith the brine phase for the first time after CO2 isinjected into a saline aquifer or an oil reservoir for CO2sequestration and/or CO2 EOR.

The general experimental procedure for the IFT mea-surements and visual observations is briefly describedas follows. A pendant brine drop is introduced from thebrine cylinder, whose pressure is maintained 0.1-0.5MPa higher than that of CO2 phase inside the pressurecell. The pendant brine drop is formed at the tip of thestainless-steel needle, using a specially designed high-pressure syringe delivery system. After the pendantbrine drop is formed in CO2 phase, its digital image iswell-focused, acquired sequentially, and stored auto-matically in the computer memory. For each digitalbrine drop image, a standard grid image is used tocalibrate the drop image and correct possible opticaldistortion. The ADSA program for the pendant drop casethen is executed to determine the dynamic and equi-librium IFTs of the pendant brine drop, as well as theentire drop profile. The output data also include theradius of the curvature at the apex point, and thevolume and surface area of the pendant brine drop. Onlythe local gravitational acceleration and the densitydifference between the brine phase and CO2 are re-quired as the input data for this program.

The IFT measurements are conducted for at leastthree different pendant brine drops, to ensure satisfac-tory repeatability at the specified pressure and temper-ature. After each test, the pressure cell and the entiretubing system are cleaned with acetone, then flushedwith nitrogen and finally vacuumed. In this study, ninepressures are tested, in the range of 0.1-30.0 MPa, andtwo temperatures (27 and 58 C) are selected to covermost practical cases of interest.

Figure 2. Block diagram of the experimental setup used to study the interfacial interactions of the reservoir brine-CO2 systemat high pressures and elevated temperatures.

218 Energy & Fuels, Vol. 19, No. 1, 2005 Yang et al.

Solubility Measurement. Prior to each solubilitymeasurement, the PVT cell and the fluids handlingsystem in the PVT system are thoroughly cleaned withacetone and then nitrogen three times and finallyevacuated, to remove any traces of the cleaning re-agents. The temperature of the air bath is set to thedesired value 24 h before the measurement, to allow thesample cylinders and the PVT cell to reach the presettemperature.

To measure the solubility of CO2 in the brine sample,the PVT cell is filled with CO2 first. After the cell isallowed to stabilize for 4 h, the initial CO2 pressure (P0),CO2 temperature (T0), and CO2 phase volume (V0) arerecorded. Next, a total brine sample volume of Vbrine )35 cm3 is injected into the PVT cell at the injection rateof 2000 cm3/h, using the automatic positive displace-ment pump. The magnetic mixer then is turned on, tovigorously mix the brine sample with CO2. Finally, afterthe brine-CO2 mixture reaches the equilibrium state,as indicated by constant cell pressure and temperature,the mixer is stopped and the equilibrium CO2 pressure(Pe), CO2 temperature (Te), and CO2 phase volume (Ve)are obtained. Note that, during the solubility measure-ments, the equilibrium CO2 temperature (Te) is almostsame as the initial CO2 temperature (T0), because thesample cylinders and the entire PVT cell are placedinside the constant-temperature air bath. By applyingthe conservation of mass and the equation of state fora real gas, the solubility of CO2 in the brine sample ()can be determined from the following equation:

where R is the universal gas constant, MCO2 is themolecular weight of CO2, Fbrine is the density of the freshbrine sample with no dissolution of CO2, and Z0 and Zeare the compressibility factors (Z-factors) of CO2 at theinitial state (P0, T0) and at the equilibrium state (Pe,Te), respectively.

Results and Discussion

Swelling Effect and Shrinking Effect. During theexperiments, the joint effect of brine swelling andshrinking is manifested by a change in the volume ofthe pendant brine drop. The pendant brine drop swellsafter it is formed in the presence of CO2 inside thepressure cell. Figure 3a shows that, at P ) 0.121 MPaand T ) 27 C, the pendant brine drop continues to swelland its volume continues to increase from 7.954 mm3at t ) 0 s to 8.858 mm3 at t ) 9022 s. This is mainlyascribed to CO2 dissolution in the brine phase. Finally,the brine drop detaches from the needle tip due to theforce imbalance between the downward gravity and theupward surface force. When pressure is increased to P) 8.506 MPa, as shown in Figure 3b, or higher at T )27 C, the brine drop initially swells, because of the CO2dissolution in the brine phase, and then shrinks,because of the mass transfer from the pendant brinedrop to the CO2 phase. A similar phenomenon has alsobeen reported by Kogel et al.21 for the water-CO2system.

The observed swelling and shrinking effects areexplained as follows. In the brine-CO2 binary system,

(21) Kogel, A.; Dahman, N.; Ederer, H. J. Supercrit. Fluids 2003,29 (3), 237-249.

Figure 3. Sequential digital images of the pendant brine drop in CO2 with the swelling effect and shrinking effect: (a) P ) 0.121MPa at T ) 27 C, (b) P ) 8.506 MPa at T ) 27 C, and (c) P ) 0.130 MPa at T ) 58 C.

)MCO2

RFbrineVbrine(P0V0Z0T0 - PeVeZeTe) 100

Reservoir Brine-CO2 Interfacial Interactions Energy & Fuels, Vol. 19, No. 1, 2005 219

CO2 contacts the brine-CO2 interface first, then trans-fers across the interface and finally diffuses into thependant brine drop. It has been found that the solubilityof CO2 in water is 4.0 mol %, whereas that of waterin CO2 is 85 s at P ) 8.506 MPa and T ) 27C. At equilibrium, there exists the mutual solubilityof CO2 and brine.

The swelling effect is still observed when the tem-perature is increased to T ) 58 C at P ) 0.130 MPa,as shown in Figure 3c, although the brine shrinkingeffect is expectedly stronger at a higher temperature.In the experiment, the pendant brine drop is observedto detach quickly (t ) 223 s or less) from the needle tipas the operating pressure increases. This is attributedto a higher solubility of CO2 at a higher pressure. Thebrine swelling is an important effect, which should betaken into account after CO2 is injected into a previouslywater-flooded oil reservoir or sequestrated in a depletedreservoir or saline aquifer.11,23,24 However, the brine

swelling effect is usually neglected in CO2 floodingprocesses, because increased salinity of the reservoirbrine results in significantly reduced CO2 solubility.11Therefore, accurate determination of the brine swellingeffect is needed, to estimate the maximum sequestrableamount of CO2 in the depleted reservoir or salineaquifer.

Wettability Alteration. It is observed that thependant brine drop is formed around the outer surfaceof the needle, as shown in Figure 4a at P e 4.013 MPaand T ) 27 C. At P g 8.506 MPa and T ) 27 C,however, the pendant brine drop is formed from theinner surface of the needle, as shown in Figure 4b. Notethat, at T ) 27 C, CO2 is in a gas state when thepressure is 4.013 MPa or lower, whereas it becomesliquid when pressure is 8.506 MPa or higher. Obviously,wettability between the reservoir brine and the needlesurface is altered when the CO2 changes from a gasphase to a liquid phase. The stainless-steel needlesurface is modified from a hydrophilic surface to ahydrophobic surface. This is because the needle surfaceis coated with an interfacial hydrate layer that is formedbetween the brine phase and CO2 at P g 8.506 MPaand T ) 27 C. It has been reported that the interfacialhydrate in a crystalline state is formed between waterand CO2 at P g 4.5 MPa and T e 10 C,9,22,25,26 whereasthe hydrate layer, probably in a quasi-crystalline state,is formed at P ) 6.0-30.0 MPa and T ) 15-40 C.9,22,26This means that, in the depleted oil/gas reservoirs or

(22) Teng, H.; Yamasaki, A. Int. J. Heat Mass Transfer 1998, 41(24), 4315-4325.

(23) Enick, R. M.; Klara, S. M. SPE Reservoir Eng. 1992, 7 (2), 253-258.

(24) Ipsen, K. H.; Jacobsen, F. L. Energy Convers. Manage. 1996,37 (6-8), 1161-1166.

(25) Ogasawara, K.; Yamasaki, A.; Teng, H. Energy Fuels 2001, 15(1), 147-150.

(26) Ohgaki, K.; Hirokawa, N.; Ueda, M. Chem. Eng. Sci. 1992, 47(8), 1819-1823.

Figure 4. Wettability alteration of the pendant brine drop in CO2 phase under different pressures at T ) 27 C: (a) CO2 in thegas phase and (b) CO2 in the liquid phase.

220 Energy & Fuels, Vol. 19, No. 1, 2005 Yang et al.

saline aquifers, the interfacial hydrate may prevent CO2from closely interacting with the reservoir rocks. Whenthe temperature is increased to T ) 58 C, no wettabilityalteration is observed in the pressure range tested inthis study.

Miscibility and Solubility. Figure 5 shows digitalimages of the pendant brine drops that formed atdifferent pressures, from P ) 0.130 MPa (CO2 in thegas state) to 7.410 MPa at T ) 58 C (CO2 in thesupercritical state). However, no pendant brine drop canbe formed and observed when P g 12.238 MPa. In thiscase, miscibility between the reservoir brine and CO2seems to be achieved, because no interface existsbetween the brine phase and the CO2, based on atraditional definition that miscibility is obtained whenno interface exists between the two phases that areinvolved.10 The interface disappearance phenomenonobserved in this study has not been documented in theliterature, although CO2 is claimed to be a water-miscible fluid in a CO2 sequestration process.27 In fact,the disappearance of such an observed interface mayresult from the two-way mass transfer, i.e., mutualsaturation of the brine phase and CO2. In the literature,it has been found that the mass transfer coefficient fromthe water drop to the CO2 phase is 2.173 10-3-2.404 10-3 cm/s, whereas the mass transfer coefficient fromthe CO2 phase to the water drop is 1.412 10-4-1.562 10-4 cm/s at P ) 23.0 MPa and T ) 62 C.21

Furthermore, CO2 solubility is measured using thePVT system and is related to the interface disappear-ance phenomenon observed in the pendant drop experi-ments in which the volume of the pendant brine dropis sufficiently smaller than that of CO2 phase. As shown

in Figure 6, the solubility of CO2 in the brine phaseincreases as the pressure increases, whereas it de-creases as the temperature increases. These two trendshave also been reported for the water-CO2 system.12-17The solubility measured in this study for the Instowreservoir brine-CO2 system is similar to that of theWeyburn reservoir brine-CO2 system.17 At T ) 27 C,solubility cannot be measured when the pressure ishigher than the saturation pressure of CO2. This isbecause, in this study, solubility is determined from theequation of state for a real gas, which is not valid whenCO2 becomes liquid. It is worthwhile to emphasize thatthe brine-CO2 interface disappearance phenomenon isnot observed in the solubility measurements in whichthe volume of the brine phase is approximately one-halfof the volume of the CO2 phase. Nevertheless, Figure 6

(27) Kaszuba, J. P.; Janecky, D. R.; Snow, M. G. Appl. Geochem.2003, 18 (7), 1065-1080.

Figure 6. Measured solubility data of the reservoir brine-CO2 system versus pressure at two different temperatures.

Figure 5. Digital images of the pendant brine drops in CO2 phase under different pressures at T ) 58 C: (a) CO2 in the gasstate and (b) CO2 in the supercritical state.

Reservoir Brine-CO2 Interfacial Interactions Energy & Fuels, Vol. 19, No. 1, 2005 221

shows that the solubility of CO2 in the brine phaseapproaches its maximum value at T ) 58 C and P )12.238 MPa, at which the interface disappearancephenomenon starts to occur in the pendant brine dropexperiments. Further increases in pressure do notappreciably increase CO2 solubility in the brine phase.Obviously, the maximum CO2 solubility in the brinephase is achieved as long as the pressure is higher thana threshold value, which corresponds to the pressureat which the brine-CO2 interface disappearance phe-nomenon starts to occur in the pendant brine dropexperiments.

Dynamic IFT. The measured dynamic IFTs of thereservoir brine-CO2 system versus time under differentpressures at T ) 27 C are shown in Figure 7. Note thatthe dynamic IFT reaches a constant value (i.e., theequilibrium IFT) after 10-100 s under different pres-sures. In the literature, it has been concluded that thedynamic IFT reduction is caused by both the adsorptionof CO2 molecules onto the pendant water drop surfaceand the reorientation of water molecules at the inter-face.9 In the experiments, at P ) 0.121 MPa, the IFT isslightly reduced in a period of 100 s and then remainsconstant for at least 2.5 h. However, the pendant brinedrops only stay at the tip of the syringe needle for 70and 36 s at P ) 1.027 and 4.013 MPa, respectively,probably because of a large force imbalance between thedownward gravity force and the upward surface force.When the pressure is further increased to 8.506 MPa,the dynamic IFT remains almost constant from the verybeginning. This means that the pendant brine drop andliquid CO2 can quickly reach the equilibrium state. Notethat the dynamic IFTs measured at P ) 8.506 MPa arehigher than those at P ) 4.013 MPa. A similar findinghas also been reported for the pure water-CO2 sys-tems.6-9 This is attributed to the phase change of CO2from gas to liquid and the formation of a second CO2-enriched phase7,8 and/or CO2 hydrates.9,22,26 The dy-namic IFT is further reduced when the pressure isincreased to P ) 12.907 MPa or higher. Also, thedynamic IFT reaches its constant value more quicklyat a higher pressure. The faster establishment of theequilibrium state may be attributed to a strongerconvective mixing at a higher pressure.2

Figure 8 shows the dynamic IFTs of the reservoirbrine-CO2 system versus time under different pres-sures at an elevated temperature of T ) 58 C. During

the experiments, it is observed that the pendant brinedrop stays much shorter under a higher pressure. Incomparison with that at T ) 27 C, as shown in Figure7, the dynamic IFT at T ) 58 C, as shown in Figure 8,is higher under the same pressure. This is because CO2solubility in the brine phase is lower at a highertemperature.11,16 No IFT data are available if P g 12.238MPa at T ) 58 C, because there is no interface formedbetween the brine phase and CO2 in the pendant dropexperiments, as described previously.

Equilibrium IFT. A constant or equilibrium IFTalways exists at the end of each dynamic IFT measure-ment. The equilibrium IFTs of the reservoir brine-CO2system, versus pressure at two different temperatures,are plotted in Figure 9. This figure shows that theequilibrium IFT generally decreases as the pressureincreases, whereas the same parameter increases as thetemperature increases. These two patterns have alsobeen observed for the water-CO2 system.6-9 This isbecause CO2 solubility in the brine phase, as shown inFigure 6, is higher at a higher pressure and lower at ahigher temperature, respectively. Also, the pressureeffect on the IFT is comparable with the temperatureeffect for the reservoir brine-CO2 system. Note that,at T ) 27 C, the equilibrium IFT at P ) 8.506 MPa ishigher than that at P ) 4.013 MPa. This is mainlybecause of the phase change of CO2 from gas to liquidand the formation of a second CO2-enriched phase7,8

Figure 7. Measured dynamic IFTs of the reservoir brine-CO2 system versus time under different pressures at T )27 C.

Figure 8. Measured dynamic IFTs of the reservoir brine-CO2 system versus time under different pressures at T )58 C.

Figure 9. Measured equilibrium IFTs of the reservoir brine-CO2 system versus pressure at two different temperatures.

222 Energy & Fuels, Vol. 19, No. 1, 2005 Yang et al.

and/or CO2 hydrates.9,22,26 On the other hand, there isa condensation process of CO2 in the pressure range of7.410 MPa < P < 12.238 MPa at T ) 58 C. In this case,no clear digital image of the pendant brine drop can beobserved, and, thus, no IFT can be measured. Whenpressure is increased to P ) 12.238 MPa and higher atthis temperature, no interface exists between the brinephase and the CO2. This means that the equilibriumIFT between the brine phase and the CO2 becomeszero.10,28 In this case, the measured CO2 solubility inthe brine phase, which is obtained using the PVTsystem, approaches its maximum value.

Conclusions

In this paper, a new experimental technique has beendeveloped to measure the interfacial tension (IFT) andvisualize the interfacial interactions between the res-ervoir brine and CO2 under reservoir conditions. TheIFT measurements are conducted using the axisymmet-ric drop shape analysis (ADSA) technique for thependant drop case. The dynamic IFT reduces to aconstant value after 10-100 s at T ) 27 C and after10-60 s at T ) 58 C. The dynamic IFT reduction maybe explained by both the adsorption of CO2 moleculesand the reorientation of water molecules at the pendantbrine drop surface. It is also observed that the equilib-rium IFT decreases as the pressure increases, whereasthe same parameter increases as the temperatureincreases. This is attributed to higher CO2 solubility ata higher pressure and lower CO2 solubility at a higher

temperature, respectively. On the other hand, theswelling effect, the shrinking effect, wettability alter-ation, and interface disappearance are observed for thereservoir brine-CO2 system in the pendant brine dropexperiments. The swelling effect is caused by thedissolution of CO2 in the brine phase, whereas theshrinking effect is due to the permeation of brine intoCO2. This means that two-way mass transfer existsbetween the brine phase and CO2. Wettability alterationoccurs because of the interfacial hydrates formed be-tween the brine phase and CO2 at high pressures andT ) 27 C in the pendant drop experiments. No interfacebetween the reservoir brine and CO2 is observed at P g12.238 MPa and T ) 58 C. Under the same conditions,however, a clear interface is formed between the brinephase and CO2 and is observed using the PVT system,while the measured solubility of CO2 in the brine phaseapproaches its maximum value. Further increases inpressure do not lead to an appreciable increase of CO2solubility in the brine phase. In practice, the maximumCO2 solubility is achieved in a depleted reservoir orsaline aquifer as long as the operating pressure exceedsa threshold value.

Acknowledgment. The authors acknowledge a dis-covery grant from the Natural Sciences and EngineeringResearch Council (NSERC) of Canada and an innova-tion fund from the Petroleum Technology ResearchCentre (PTRC) at the University of Regina to Y.G. Theauthors also wish to thank Mr. Chaodong Yang, for hisassistance in the solubility measurements, and twoanonymous reviewers, for their helpful comments.

EF049792Z(28) Adamson, A. W. Physical Chemistry of Surfaces, 6th Edition;

Wiley: New York, 1996.

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