Asphaltene Deposition During CO2Flooding

R.K. Srivastava,* SPE, S.S. Huang, SPE, and Mingzhe Dong, SPE, Saskatchewan Research Councilt

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mofnd itSummaryIn this article we present results of dynamic and static preciption tests to investigate the likelihood of asphaltene deposiproblems in southeast Saskatchewans Weyburn reservoir. Twere conducted at the reservoir temperature and pressure ctions. The effect on asphaltene flocculation/precipitation ofoperating pressure, CO2 concentration, gas contaminants in CO2,and presence of formation brine was investigated for three difent oil samples using static pressure/volume/temperature~PVT!tests. The extent of asphaltene deposition was also assethrough coreflood experiments and through an x-ray compuaided tomograph~CAT!-scanning visualization experiment.

Static tests indicated the most important factor on whichasphaltene precipitation depended was the CO2 concentration. Foroils belonging to the same pool, the increase in asphaltenecipitation with solvent concentration was proportional to the itial asphaltene contents of the oil. Coreflood experiments shoa considerable increase in asphaltene deposition in the core mfollowing CO2 injection. Pore topography of the core matrplayed an important role in the extent of CO2-induced asphaltenedeposition. X-ray CAT-scanning tests depicted localized areaasphaltene deposition along the length of the core, with significdeposition suspected to be occurring near the inlet of the cor

IntroductionAfter initial waterflooding, many light and medium oil reservoiare subjected to miscible or near-miscible CO2 or hydrocarbonflooding for enhanced oil recovery. In the US, 60 active misciCO2 projects were in operation in 1996, whereas, in Canada,drocarbon miscible floods are more common and number aro40 active projects.1 In Saskatchewan, Canada, most of the lightreservoirs have reached their economic limit of productionwaterflooding2 and are suitable candidates for miscible/nemiscible CO2 flooding.

3 The injected CO2, when it contacts thereservoir oil, can cause changes in the fluid behavior and equrium conditions which favor precipitation of organic solidmainly asphaltenes.4 Asphaltene precipitation can change the wtability of the reservoir matrix and consequently affect the floperformance.5 It can also cause formation damage and wellbplugging, requiring expensive treatment and cleanprocedures.6-10 Asphaltene deposition problems are not limitedmiscible floods,11 they are also encountered during natural deption, gas-lift operations, caustic flooding, and matrix acidizing

Asphaltenes are the polar, polyaromatic, and high molecweight hydrocarbon fraction of crude oil that are generally chacterized as insoluble inn-hexane or inn-pentane. They are believed to exist either dissolved in oil or as a finely dispersed cloidal suspension in oil stabilized by resins adsorbed on thsurface. The asphaltene/resin ratio and high/low molecular wecomponent ratio determine which crude oil can precipitatephaltenes. Application of chemical, mechanical, or electriforces can alter these ratios and destabilize resins and asphalThe fine particles of destabilized asphaltenes coalesce and cflocculation. Flocculated asphaltenes may contain sizable amoof entrapped oil10 which inhibits deposition. Asphaltene precip

*Now a consultant.

Copyright 1999 Society of Petroleum Engineers

This paper (SPE 59092) was revised for publication from paper SPE 37468, first presentedat the 1997 SPE Production Operations Symposium held in Oklahoma City, Oklahoma,911 March. Original manuscript received for review 9 March 1997. Revised manuscriptreceived 20 July 1999. Paper peer approved 2 August 1999.SPE Prod. & Facilities14 ~4!, November 1999ita-ionestsndi-he

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tation is considered to occur when the flocculated asphalteseparate from the oil phase. However, precipitated asphalt~which are hard to observe visually because of their similar dcolor! can return to solution if the asphaltene/resin ratio of tprecipitated phase is the same as that of the original oil. In CO2 orhydrocarbon flooding, the asphaltene-to-resin ratio of crude oaltered, causing asphaltene precipitation and thereby its deption. In static PVT tests, asphaltene flocculation is believedoccur, whereas in coreflood tests asphaltene precipitatdeposition in the core matrix may occur.

Asphaltene precipitation is not clearly understood at presThe role of resins in stabilizing asphaltenes is well recognizBut the exact mechanism of how the asphaltenes are stabilizethe presence of resins is not well established for light oils.recent literature11,12 it was suggested that asphaltenes may agggate with resins to form relatively small molecules with a molecular weight around 2,000 g/gmol. At low dilution ratios nearthe onset of precipitation, asphaltenes seem to precipitateliquid component. At high ratios, the separation into pure asphenes and resins is believed to occur.

One of the major problems that confronts reservoir/productengineers considering a miscible CO2 flood for a field is the needto assess the likelihood of asphaltene precipitation and conseqoil recovery and monetary losses. To do this, experimental stuor modeling techniques are initiated to determine when ahow much asphaltene will be precipitated. Severmethods13-17 were reported for measuring the onset of asphaltprecipitation and also the extent of this precipitation. These minclude measurement of the electrical conductivity14 andviscosity,15 and spectrophotometry13,16 and gravimetry.17 In thisinvestigation, we have used a spectrophotometric technique11 formeasuring the asphaltene content of crude oil samples. The meling approaches~none was attempted! rely on the utilization ofFlory-Huggins polymer solution theory,17,18 application ofequation-of-state calculations,19 use of thermodynamic colloidamodels,20 and, more recently, thermodynamic micellizatiomodels21 for prediction of asphaltene precipitation. However,techniques require experimental data for model validation.

There are many factors that affect the asphaltene precipitaprocess.4 These may include the nature of the rock matrix, tasphaltene and resin contents of the reservoir oil, the amounformation brine and its composition, the nature of injection gthe presence of contaminants in the injection gas, and temperaand pressure conditions. The present investigation focuses on2injection in the Weyburn reservoir and examines the effectasphaltene flocculation/precipitation of operating pressure, C2concentration, presence of formation brine, and contaminantCO2 such as methane and nitrogen. Weyburn is a light oil resvoir ~2835 API gravity! located in southeast Saskatchewan. Itcharacterized by a higher permeability Vuggy zone at the botand a Marly zone at the top. The injected CO2 is more likely tocontact and mobilize the reservoir oil in the Marly zone whicould be the more susceptible area for asphaltene depositionhave therefore also investigated, by dynamic coreflood expments, the extent of asphaltene deposition likely in the Weybreservoir that represents zones of increasing permeability. Parthe core matrix that are more susceptible to asphaltene deposand flocculation were examined by a suitably designed x-CAT-scanning experiment.

Weyburn Reservoir. The Weyburn pool is located about 130 k~81 miles! southeast of the city of Regina in the provinceSaskatchewan, Canada. The field was discovered in 1954 a1064-668X/99/14~4!/235/11/$3.5010.15 235

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bytivecovers approximately 180 km2 ~70 sq miles!. It was completelydelineated by vertical drilling and by 1991 consisted of 627 pducing wells and 162 water injection wells. A horizontal infidrilling program was implemented in 1991 to improve productiwhich was about 25.8% of the original oil in place.

The Midale beds of the Weyburn reservoir that representMississippian Charles Formation were deposited on a shallowbonate shelf in the Williston Basin. The field is uniformly subdvided into an upper Marly and a lower Vuggy zone. The Mazone is essentially a chalky dolostone with occasional limestinterbeds. The porosity of the Marly dolostone ranges from 1to 26%. The permeability varies from 1 to 100 md.

The Vuggy zone is a heterogeneous, subtidal limestonecontains two distinct rock typesIntershoal and Shoal. The Inshoal Vuggy covers a larger area, whereas Shoals or high-gsize Vuggy zones have a limited areal extent but tend to domiperformance because of their high permeability. The porosithroughout the Vuggy zone range from 3% to 18% and the pmeabilities from 0.01 to 500 md. Shoal permeabilities are genally an order of magnitude higher than Intershoal ones. Fogiven porosity, Shoal Vuggy rocks tend to have the highest pmeability, while Marly rocks have the least, with IntershoVuggy lying somewhere in between. The Vuggy zone is mfractured than the Marly and it controls the magnitude and dirtion of the permeability anisotropy. More details on the Weybureservoir and its geology can be found elsewhere.22,23

Experiment

Crude Oil Characterization and Reservoir FluidReconstitution. The fluid properties of the Weyburn reservovary widely. From 1955 to 1961, the oil densities ranged from 8kg/m3 ~34API! to 904.2 kg/m3 ~25API!, the saturation pressurfrom 2.2 to 6.7 MPa~319 to 972 psi!, and the instantaneous gaoil ratios from 17 to 32 m3/m3 ~95 to 180 scf/bbl!. The oil densi-ties are higher in the southern portion and lower in the northportion of the reservoir. The variations are believed to be relato the geologic and depositional environment of the reservoir

For this work, separator oil and gas samples were collecfrom three different well locations in the Weyburn reservoir coering three distinct areas of the reservoir. These well locatiwere 14-17-6-13 W2M~oil well 1!, 3-11-7-13 W2M~oil well 2!,and Hz 12-18-6-13 W2M~oil well 3!. The respective reservoitemperatures for these oils were 59, 61, and 63C~138, 142, and145F!. The crude oils were characterized by measuring the dsity and viscosity as a function of temperature and as a functiopressure at the respective reservoir temperature.

The Weyburn reservoir fluid W1 was reconstituted by recobining the separator oil and gas samples to a gas-oil ratio~GOR!of 19 m3/m3 ~107 scf/bbl! at 59C~138F!, fluid W2 to a GOR of23 m3/m3 ~129 scf/bbl!, and fluid W3 to a GOR of 32 m3/m3 ~180scf/bbl!. The PVT properties of these reservoir fluids were msured. More details of the experimental apparatus and measment procedure can be found in our previous publications.24,25

Static Asphaltene Flocculation Studies.Asphaltene precipita-tion/flocculation tests were carried out in a PVT apparatus. TPVT cell, maintained at the desired reservoir temperature,charged with the reconstituted reservoir fluid and pure or impCO2 to attain a desired concentration of CO2 at the desired pressure, usually 16 MPa~2,321 psi!. The mixture was equilibratedand the GOR of the mixture was measured. A significant portof the reservoir fluid-CO2 mixture was transferred to another piton cylinder through a 0.5mm in-line filter ~seeFig. 1!. The pistoncylinder was slowly depressurized and the free gas was relethrough a condenser to minimize the loss of light ends. The piscylinder was then slowly cooled to the ambient conditions. Tasphaltene content of the filtered and flashed crude oil was dmined by spectrophotometry using a Hitachi dual-beam specphotometer. The amount of asphaltene flocculated/precipitwas obtained by the difference in the asphaltene content offlashed crude oil and the original Weyburn oil.236 Srivastava, Huang, and Dong: Asphaltene Deposition During C2 Fo-lln

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For the system containing brine, the PVT cell was first chargwith the pure/impure CO2 and formation brine, and the contentwere mixed. The desired amount of Weyburn reservoir fluid wthen charged into the PVT cell, and the procedure outlined abwas repeated.

Dynamic Asphaltene Precipitation Tests.The asphaltene deposition in the core matrix was measured using a single Marly pand stacked Vuggy composite cores. The composite cores geally comprised four to five good plugs, of 2.5 cm~1 in.! diameter,from three Weyburn Wells. The sequence of the stacking ofcore plugs was determined using the method described in RefThe core was mounted in a triaxially loaded core holder. The ovtemperature was raised to 59C~138F! and the operating pressure was set at 16 MPa~2,321 psi!.

The core was saturated with Weyburn dead oil W1 at irredible water saturation at a flow rate of 2 cm3/hr ~0.31 in.2/hr!. Afteroil breakthrough, the oil samples produced~approximately 1 geach! were collected and the asphaltene content was determine~a1 g oil sample is the minimum amount required for an accurmeasurement! by spectrophotometry. The oil injection was continued until the oil produced had the same asphaltene contenthe injected oil. The amount of asphaltene adsorption was calated. A record of the brine and oil production provided thamount of initial oil saturation in the core.

A secondary CO2 injection was started on the core at the initioil saturation to determine the additional amount of asphaltedeposition induced by CO2. Once again, the oil samples produceapproximately 1 g each, were collected and the asphaltene conwas measured. CO2 injection was stopped when the gas-oil ratreached 10,000 m3/m3 ~56,000 scf/bbl!. The residual oil saturationin the core was determined from the amount of oil produced. Tcore was subjected to a four-stage blowdown and cooled.connate water measured and the residual oil saturation data, awith the asphaltene content of the oil produced, were used to cout a materials balance and to determine the amount of asphene deposition in the core matrix during CO2 injection.

Results and DiscussionWeyburn Crude Oil and Reservoir Fluid Characterization.The density and viscosity of the Weyburn crude oil samples wmeasured and they are reported inTable 1. Crude oil W1 had anAPI gravity of about 29 whereas oils W2 and W3 had gravities36 and 31API. The asphaltene contents of the crude oils WW2, and W3 were 4.8, 4.0, and 4.9 wt %, respectively. Fromlimited number of oil samples collected, it was difficult to ascetain if the asphaltene content of the Weyburn crude oil was crelated to the API gravity.

The Weyburn reservoir fluids or live oils were reconstitutedrecombining appropriate oil and gas samples at their respec

Fig. 1Modified light oil PVT apparatus for asphaltene floccu-lation studies.Olooding SPE Prod. & Facilities, Vol. 14, No. 4, November 1999

Srivastava, HuanTABLE 1 CHEMICAL AND PHYSICAL PROPERTIES OF WEYBURN DEAD OILS

Temperature(C)

Oil W1* Oil W2** Oil W3

Density(kg/m3)

Viscosity(mPas)

Density(kg/m3)

Viscosity(mPas)

Density(kg/m3)

Viscosity(mPas)

15 878.9 854.9 869.2 11.7620 875.9 12.8 842.4 4.60 864.4 9.4059 846.1 4.2 61 813.1 2.35 63 839.4 3.15

Pressure(MPa)

Density(at 59C)

Viscosity(at 59C)

Density(at 61C)

Viscosity(at 61C)

Density(at 63C)

Viscosity(at 63C)

0.1 846.1 4.2 813.1 2.35 839.4 3.153.54 849.2 816.4 2.49 842.4 3.266.99 852.4 819.6 2.62 845.2 3.37

10.44 858.0 822.9 2.76 848.4 3.4917.33 860.9 829.3 3.04 854.7 3.71

Basic sediment and water,vol %

0.1 0.2 0.5

Molecular weight, g/gmol 230 203 215(Component) (wt %) (wt %) (wt %)

Saturates 48.5 55.3 48.4Aromatics 33.5 31.1 33.5Resins 13.2 9.6 13.2Asphaltenes 4.8 4.0 4.9

*Collected from Weyburn well 14-17-6-13 W2M.** Collected from Weyburn well 3-11-7-13 W2M.Collected from Weyburn well Hz 12-18-6-13 W2M.Reservoir temperature for the oil samples.-

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hea-entsci-ultsin ar actreservoir temperatures.Table 2 presents the measured PVT dafor the reservoir fluids. The saturation pressure varied fromMPa ~421 psi! for fluid W1 to 3.5 MPa~363 psi! for fluid W2 and4.9 MPa ~711 psi! for fluid W3. The relatively heavier components (C6

1) increased in the reservoir fluid as the bubblepopressure of the fluids increased~Table 3!.

Asphaltene Flocculation.Asphaltene flocculation tests were coducted for three Weyburn reservoir fluids to determine the efof the following parameters: operating pressure, CO2 concentra-tion at 16 MPa~2,321 psi!, the presence of formation brine, animpure CO2 ~I-CO2! containing 2.7 mol % N2 and2.9 mol % CH4. ~The potential source of CO2 for southeastSaskatchewan is power plant flue gas. Thus N2 and CH4 fromrecycled gas are the most likely contaminants for CO2. The com-position selected for the studies is based on a separate study wg, and Dong: Asphaltene Deposition During CO2 Floodingta2.9

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the multiple-contact minimum miscibility pressure for Weybureservoir fluid W1 was investigated.24,27!

It is worth pointing out that the static experiments conductedthis work are only predictive for the asphaltene precipitation incase of the first-contact miscibility. In the miscible CO2 injectionprocess, CO2 and oil are not first-contact miscible but achievmiscibility by dissolution of CO2 into the oil and by evaporationor extraction of hydrocarbon components from the oil into tCO2 phase.

28 As a result of vapor-liquid separation, the precipittion of asphaltenes will be increased because the light componare stripped away from the crude which in a first-contact misbility stabilizes the asphaltenes in the crude. However, the resobtained in this work can be used to tune the parametersthermodynamic model like that described in Refs. 7 or 17 foprediction of asphaltene precipitation in the multiple-contaprocess.TABLE 2 PHYSICAL PROPERTIES OF WEYBURN RESERVOIR FLUIDS

Properties Units Fluid W1* Fluid W2** Fluid W3

Reservoir temperature C 59 61 63Saturation pressure MPa 2.89 3.47 4.92Viscosity mPas@psat 3.01 1.45 1.76Density kg/m3@psat 797.2 806.4Formation volume factor m3/m3 1.087 1.102 1.124Swelling factor m3/m3 1.074 1.060 1.085Gas-oil ratio m3/m3 19.3 23.4 32.0

*Collected from Weyburn well 14-17-6-13 W2M.** Collected from Weyburn well 3-11-7-13 W2M.Collected from Weyburn well Hz 12-18-6-13 W2M.Formation volume factor5volume of reservoir fluid at psat and Tres/volume of reservoir oil at 1 atm and 15C.Swelling factor5volume of reservoir fluid at psat and Tres/volume of reservoir oil at 1 atm and Tres .SPE Prod. & Facilities, Vol. 14, No. 4, November 1999 237

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Weyburn Reservoir Fluid W1 and CO2 Mixtures. The amountof asphaltene flocculation was measured for Weyburn reserfluid W1 at 59C ~138F!. Three different oil samples~W1A,W1B, and W1C! were collected from the same well. They showslight variations in the oil properties.13 The asphaltene content fothese oil samples varied between 4.8 and 5.3 wt %. Becausthis variation, the asphaltene flocculation data were normalizethe initial asphaltene content of the oil.

Fig. 2 presents the normalized asphaltene flocculation datafunction of CO2 concentration.Table 4 shows a part of the data~see Ref. 13 for complete data!. The data show that the onset oasphaltene flocculation is nearly the same for the three data~oils W1A, W1B, and W1C!. After the onset, the asphaltene floculation follows a nearly linear increase with increasing CO2 con-centration. The data not following the linear trend appear to brelatively high CO2 concentrations which possibly represents ttwo-phase region. These studies suggest that the asphaltene

TABLE 3 COMPOSITION OF WEYBURN RESERVOIRFLUIDS

Component Fluid W1 Fluid W2 Fluid W3

N2 0.96 1.59 2.07CO2 0.58 0.23 0.74H2S 0.30 - 0.12Methane 4.49 4.54 7.49Ethane 2.99 2.07 4.22Propane 4.75 4.41 7.85i-Butane 0.81 1.23 1.58Butane 1.92 2.59 4.97i-Pentane 1.27 4.53 2.01Pentane 2.19 4.96 2.58C69 25.73 27.34 21.56C1017 26.98 27.86 22.02C1827 13.28 11.78 10.27C28

1 13.75 6.87 12.52100.00 100.00 100.00238 Srivastava, Huang, and Dong: Asphaltene Deposition During C2 Fvoir

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culation in Weyburn reservoir fluid/CO2 mixtures was insensitiveto the operating pressure@in the pressure range of 10 to 20 MP~1450 to 2900 psi! investigated#.

The most important parameter affecting the asphaltene floclation is the CO2 concentration. The effect of the presenceformation brine in the water-oil ratio of 3:7 seems to be neggible. The effect of small amounts of N2 and CH4 contaminationalso does not seem to affect the flocculation pattern.

Weyburn Reservoir Fluid W2 and I-CO2 Mixtures. Elevenmixtures of the Weyburn reservoir fluid W2 and impure CO2~I-CO2!, in the absence of brine, were investigated. The objecwas to determine the effect of the oil properties and the effecCO2 contaminants on asphaltene flocculation at a constant fioperating pressure. All the experiments were conducted at 16 M~2,321 psi! and 61C~142F!. Table 5 presents the asphaltenflocculation data for these mixtures with the I-CO2 concentrationranging from 29.4 to 85.0 mol %.

Fig. 2Normalized asphaltene flocculation in Weyburn reser-voir fluid W1 there are three different oil samples from thesame well with pure and impure CO 2 I-CO2 concentration at59C and 16 MPa.TABLE 4 EFFECT OF CO 2 CONCENTRATION ON ASPHALTENE FLOCCULATION FORWEYBURN RESERVOIR FLUID W1* AND CO2 MIXTURE, IN THE PRESENCE AND

ABSENCE OF BRINE, AT 16 MPa AND 59C

FluidCO2 Concentration

(mol %)

SaturationPressure

(MPa)Gas-Oil Ratio

(m3/m3)

AsphalteneFlocculated

(wt %)

Reservoir fluid1CO2 0.58 2.9 19.2 0.0016.4 4.6 43.9 0.0046.0 8.8 131.4 0.0053.5 10.2 172.7 0.9354.9 10.5 182.1 1.6865.3 12.8 277.7 2.6965.3 12.8 277.7 2.7475.0 .16.0 449.7 3.34

Reservoir fluid1CO21brine** 15.9 3.5 42.9 0.00

35.0 6.7 89.1 0.0046.6 8.7 134.3 0.3455.5 10.7 186.3 1.40

75 16.7 449.7 3.01

*Reservoir fluid from Weyburn well 14-17-6-13 W2M; asphaltene content 4.9 wt %.** Volume ratio of brine to oil53:7.Estimated.Amount of asphaltene flocculated was the difference between the asphaltene content of the oil determined before and after theexperiment.Olooding SPE Prod. & Facilities, Vol. 14, No. 4, November 1999

Srivastava, HuanTABLE 5 EFFECT OF IMPURITY ON ASPHALTENE FLOCCULATION FOR WEYBURNRESERVOIR FLUID W2* AND IMPURE CO2** MIXTURES, IN THE ABSENCE OF BRINE,

AT 16 MPa AND 61C

FluidCO2 Concentration

(mol %)

Saturation

Pressure(MPa)

Gas-Oil Ratio(m3/m3)

AsphalteneFlocculated

(wt %)

Reservoir fluid1impure CO2 29.4 6.3 70.3 0.0041.7 7.9 114.4 0.2544.9 8.4 131.1 0.5052.6 9.7 171.6 1.0254.6 10.0 186.1 1.7258.0 10.7 214.8 2.0763.8 11.9 281.3 2.8470.0 N/A 3.0475.0 N/A 3.2385.0 N/A 2.34

*Reservoir fluid W2 from well 3-11-7-13 W2M; asphaltene content 4.0 wt %.** Impure CO2 : 2.68 mol % N212.87 mol % CH4194.45 mol % CO2.Assuming pure CO2.The amount of asphaltene flocculated was the difference between the asphaltene content of the oil determined before and after theexperiment.h-

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d.e-oodThe results from the mixtures of reservoir fluid and I-CO2 with-out brine~Table 5 andFig. 3! indicate a smooth increase in thasphaltene flocculation with increasing I-CO2 concentration~greater than 41 mol %!. No asphaltene flocculation was obtaine@at 16 MPa~2,321 psi!# at concentrations of less than 41 mol %This agrees favorably with earlier results with Weyburn reservfluid W1 that the onset of asphaltene flocculation occurs at ab39 to 46 mol % CO2 concentration. These results show that teffect of contaminants in the CO2 stream is once again insignificant, as was observed with oil W1. Furthermore, a slight increin the operating temperature@from 59 to 61C~138 to 142F!# didnot change the asphaltene flocculation pattern.

Weyburn Reservoir Fluid W3 and CO2 Mixtures. Asphalteneflocculation tests were conducted for Weyburn reservoir fluid Win presence and absence of brine, at 63C~145F! and 16 MPa~2,321 psi! to determine the effect of brine on asphaltene floclation.

Without Brine. Seven mixtures of Weyburn reservoir fluid Wand CO2 were tested in the absence of brine. The onset pointasphaltene flocculation was found to be about 39 mol % CO2 con-centration for this oil~Table 6 and Fig. 4!. A linear increase inasphaltene flocculation was noted after the onset like it wasoils W1 and W2. The dotted line shows the approximate boundof the two-phase region at about 70 mol % CO2 concentration.

Fig. 3Asphaltene flocculation in Weyburn reservoir fluid W2as a function of I-CO 2 concentration at 61C and 16 MPa.g, and Dong: Asphaltene Deposition During CO2 Floodinge

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With Brine. Seven additional mixtures of fluid W3 and CO2were investigated in the presence of brine. In these experimethe brine was first saturated with CO2 at the operating pressurbefore the reservoir fluid was charged in the PVT cell. Fivethese mixtures used a brine-to-oil volume ratio of 3:7~70 vol %oil! like that used with fluid W1. However, two mixtures werinvestigated that had a brine-to-oil ratio of 3:1~25 vol % oil!. Thepurpose of these tests was to determine the effect of an increathe brine content of the mixture on asphaltene flocculation.

Table 6 presents the flocculation data for these mixtures wthe CO2 concentration ranging from about 45 to 63 mol %. Fig.shows the plot on a normalized asphaltene content scale. Fshows that, for the same CO2 concentration, a small decreaseasphaltene flocculation is generally observed in the presencbrine. This indicates that the presence of brine tends to sliginhibit the asphaltene flocculation in the Weyburn reservoir fluThis behavior appears to be somewhat more pronounced witincrease in the amount of brine present in the system. HoweFig. 4 shows that the asphaltene flocculation data for the oil wand without brine still fall within the narrow band. This indicatethat the effect of brine on asphaltene flocculation is relativsmall.

Fig. 4 shows a narrow band where all the asphaltene flocction data for the three oils seem to fall. Therefore, it can be ccluded that differing oil properties of the samples collected frodifferent wells in the same pool have a negligible effect onphaltene flocculation.

Dynamic Coreflood Studies.Three coreflood experiments werconducted to assess the asphaltene precipitation during CO2 injec-tion. The tests used essentially the same experimental proceas above. However, each test used a different core matericleaned single Marly plug, a stacked composite Vuggy~IntershoalVuggy! core comprising four preserved plugs, and a stacked cposite high-grain-size Vuggy~Shoal Vuggy! core comprising fivecleaned plugs. In terms of permeability variation, the Marly plrepresented an absolute permeability of 0.5 md; the stackedserved Vuggy core had a harmonically averaged baseline oilmeability of 1.6 md~or a harmonically averaged air permeabilimeasured after the test of 4.3 md!; and the high-grain-size Vuggycore had a harmonically averaged air permeability of 62.5 mTables 7 and 8provide the permeability variations, stacking squences, and lengths of individual plugs used in the corefltests.SPE Prod. & Facilities, Vol. 14, No. 4, November 1999 239

240 SrivastavaTABLE 6 EFFECT OF CO 2 CONCENTRATION ON ASPHALTENE FLOCCULATIONFOR WEYBURN RESERVOIR FLUID W3* AND CO2 MIXTURE, IN THE PRESENCE

AND ABSENCE OF BRINE, AT 16 MPa AND 63C

FluidCO2 Concentration

(mol %)

SaturationPressure**

(MPa)Gas-Oil Ratio

(m3/m3)

AsphalteneFlocculated

(wt %)

Reservoir fluid1CO2 0.7 4.9 32.0 0.0036.6 9.2 120.0 0.0044.5 10.6 153.9 0.6752.4 12.3 207.6 2.1058.6 14.0 241.7 2.7065.2 16.1 304.4 3.4870 17.9 N/A 3.09

75 .20.2 N/A 3.16

Reservoir fluid1CO21brine** 45.0a 10.7 155.4 0.52

47.4b 11.2 166.1 0.4051.8a 12.2 193.0 1.1954.5a 12.8 214.9 1.9254.8b 12.9 215.1 1.7560.4a 14.5 270.0 2.7462.5a 15.2 299.4 3.30

*Reservoir fluid W3 from Weyburn well Hz 12-18-6-13 W2M; asphaltene content 4.85 wt %.** Estimated.The amount of asphaltene flocculated was the difference between the asphaltene content in the oil determined before and after theexperiment.

Two-phase fluid.Low value caused by incomplete mixing.aVolume ratio of brine to oil53:7.bVolume ratio of brine to oil53:1.o

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daryMarly Plug. In preparation for all the tests, the core plugcomposite core was first saturated with formation brine at 59~138F! and 16 MPa~2,321 psi!. Weyburn dead oil W1 from well14-17-6-13 W2M was injected into the core at a rate of 2 cm3/h~0.31 in.2/hr!. During the oil saturation stage, the asphaltene ctent of the oil produced was analyzed photometrically. Thephaltene content of the injected oil was measured at 4.75 wFig. 5 shows the variation in asphaltene content of the oil pduced with oil injection for the Marly plug. An initial sharp droin the asphaltene content~from the initial 4.75 to 4.5 wt %! wasobserved for the first sample. This clearly indicated the adsorpof asphaltenes by the clean core matrix. As the oil flood contin@until about 2.2 pore volume~PV! when the brine productionstopped#, the asphaltene content of the oil produced increaslowly but remained below 4.75 wt %. This showed that adtional adsorption of asphaltenes was still occurring, most likely

Fig. 4Normalized asphaltene flocculation in the Weyburn res-ervoir fluid W3/CO 2 concentration at 63C and 16 MPa., Huang, and Dong: Asphaltene Deposition During C2 FrC

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the new sites created by the outflow of brine. After the 2.2 PVoil injection, a continued high asphaltene content~.4.75 wt %! ofthe oil produced could possibly indicate a slight redissolutionthe adsorbed asphaltenes in the core matrix.

After completion of the oil saturation, the amount of asphaenes remaining in the Marly core matrix was calculated. Tcalculation was based on the amount of oil produced andasphaltene content of the oil produced. The asphaltene contethe oil residing in the core sample was determined to be 4wt %. This value is slightly higher than the 4.75 wt % asphaltein the original injected oil. The results, therefore, provide furthevidence of a mild adsorption of asphaltenes by the core maduring oil saturation.

For determining CO2-induced asphaltene precipitationflocculation, CO2 was injected at a rate of 2 cm

3/h ~1.28 ft/D! intothe oil-saturated core samples. This represented a secon

Fig. 5Asphaltene content of oil produced during Weyburndead oil W1 saturation of a Marly plug at 59C and 16 MPa.Olooding SPE Prod. & Facilities, Vol. 14, No. 4, November 1999

Srivastava, HuanTABLE 7 PROPERTIES * AND STACKING SEQUENCE OF PRESERVED VUGGY PLUGS **

Core Properties

Plug Identification Letter

M Q P N

Length (cm) 4.839 4.787 4.840 4.836Diameter (cm) 2.493 2.496 2.476 2.485Baseline oil permeabillity (md) 6.21 0.59 4.95 2.47Permeability to air (md) 19.8 3.37 2.0 12.8Porosity (fraction) 0.145 0.124 0.122 0.121

*Determined by Core Laboratories.** Vuggy plugs from PanCanadian Weyburn well 141/14-02-6-14 W2M.Indicates stacking sequence, i.e., plug M at the inlet and N at the outlet.Used for determining the stacking sequence.Determined after the core displacement test was completed.t

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ec-lyuntilCO2 injection into the core matrix which was felt to be adequafor the asphaltene precipitation studies.Table 9 provides a sum-mary of the coreflood run indicating the core properties, operaconditions, and results for various injection stages.Fig. 6 showsthe variation in the asphaltene content of the oil produced withof CO2 injected for the Marly plug. The asphaltene content of toil produced remained essentially unchanged until CO2 break-through which occurred at about 0.75 PV. Since the oil produhad not yet been in contact with the injected CO2, it showed verylittle change in the asphaltene content. The initially producedwas also partially the oil left in the production lines after thesaturation stage. The oil produced after the CO2 breakthroughshowed a sharp decrease in the asphaltene content of the oiduced. These reductions in asphaltene content demonstratetional asphaltene precipitation/flocculation in the core matrix ding CO2 injection.

After completion of the test, the amount of asphaltenes reming in the core matrix was calculated. Fig. 6 shows the cumulaasphaltene precipitation in the Marly plug during CO2 injection.At the end of the flood, nearly 0.058 g asphaltene was left incore. This translated into an asphaltene residual oil content ofwt % and amounted to an approximately 52% increase in thephaltene content over that of the injected oil, which was 4wt %. This increase was caused by the additional asphalflocculation/precipitation that occurred from secondary CO2 injec-tion.

A petrographic analysis was conducted by Core Laboratoon the core sample after the secondary CO2 injection. The purposeof the analysis was to identify and quantify the asphaltenes lethe core after the coreflood experiment. For this analysis, acm-long ~2-in-long! core plug was sectioned at 1 and 3 cm~0.39and 1.18 in., respectively! from the inlet end into three segmentAn approximate quantitative distribution of the asphaltenesg, and Dong: Asphaltene Deposition During CO2 Floodingte

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residual oil in the core matrix, determined by Core Laboratoriis as follows:

Component Inlet end1 cm fromthe inlet

3 cm fromthe inlet

bitumen/asphaltene~vol %! 2 3 2residual oil~vol %! 17 6 4

These results were obtained using a 300-point modal petrograanalysis of the thin sections.

The petrographic analysis results indicate that, for the inletsection, the core matrix contained 2% asphaltene/bitumen17% residual oil~other constituents include 11% monoquartz a70% dolomite!. A visual examination of the core after CO2 injec-tion revealed a considerably higher deposition of the suspeasphaltenes at the inlet end. It may be possible that the pegraphic analysis was not able to distinguish the oil and asphenes clearly and it should therefore be considered as only qtative in nature.

Preserved Composite Vuggy Core.The preserved Vuggy corewas saturated with dead oil W1 like in the test with the Maplug. Fig. 7 shows the variation in asphaltene content of theproduced during oil saturation for this core. The baseline oil pmeability was measured~Table 7! by Core Laboratories using acompatible oil. Since the core was already saturated with anthe initially produced oil asphaltene content does not appeadecrease but instead shows the asphaltene content of the resoil. It appears that the asphaltene content of the resident oilabout 5.4 wt %, whereas the injected oil had an asphaltene conof 5.0 wt %.

The variation in asphaltene content of oil produced with sondary CO2 injection was similar to that observed for the Marplug. The asphaltene content remained essentially unchangedTABLE 8 PROPERTIES * AND STACKING SEQUENCE OF CLEANED HIGH-GRAIN-SIZE VUGGY PLUGS **

Core Properties

Plug Identification Number

61A 41A 62A 49A 62B

Length (cm) 4.67 4.66 4.43 4.64 4.58Diameter (cm) 2.50 2.50 2.50 2.50 2.50Permeability to air (md) 19.8 267.5 95.2 240.0 95.2Porosity (fraction) 0.162 0.210 0.154 0.146 0.154Well location 3-33-5-13 W2M 2D-12-6-14 W2M 3-33-5-13 W2M 2D-12-6-12 W2M 3-33-5-13 W2M

*Determined by Core Laboratories.** Vuggy plugs from PanCanadian Weyburn wells.Indicates stacking sequence, i.e., plug 61A at the inlet and 62B at the outlet.Used for determining the stacking sequence.SPE Prod. & Facilities, Vol. 14, No. 4, November 1999 241

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lt-CO2 breakthrough at about 0.80 PV. Thereafter, the asphalcontent decreased sharply, indicating asphaltene deposition icore matrix during CO2 injection.

Upon completion of the test, the plugs were subjected to DStark analysis and core cleaning by Core Laboratories. Thtreatments determined the amount of residual oil and watermaining in the core. The porosity of each plug was also demined and it was used to calculate the PV of the core. A matals balance analysis on asphaltenes showed that the asphacontent of the residual oil was 9.1 wt %~Table 9!, an approxi-mately 80% increase over the initial asphaltene content ofinjected oil~5.0 wt %!. This analysis indicated that there was sustantially higher asphaltene deposition in the Vuggy core mathan in the Marly core following CO2 injection.

High-Grain-Size Vuggy Composite Core.The composite corewas saturated with Weyburn dead oil W1 at 59C~138F! and 16MPa ~2,321 psi! the same for the Marly plug and the Vuggcomposite core. The asphaltene content of the oil produced

Fig. 6Asphaltene content of oil produced and cumulative as-phaltene precipitation in Marly plug during CO 2 injection at59C and 16 MPa.242 Srivastava, Huang, and Dong: Asphaltene Deposition During C2 Fenethe

anesere-er-ri-

ltene

theb-trix

yith

oil injection for the composite core varied in a way similar to thobserved for the Marly plug.

To investigate CO2-induced asphaltene precipitationflocculation, CO2 was injected at a rate of 2 cm

3/hr ~3.66 ft/D;Table 9! into the oil-saturated core sample.Fig. 8 shows the varia-tion in the asphaltene content of the oil produced with PVs of C2injected. The asphaltene content of the oil produced remaiessentially unchanged until CO2 breakthrough at about 0.90 PVThereafter, it decreased sharply, demonstrating an increase iamount of asphaltene deposition in the core matrix during C2injection, like the case with the Marly plug and the Vuggy composite core. The asphaltene content of the oil produced at theof CO2 injection was nearly zero~0.03% by weight!.

For this core, an extended waterflood~EWF! was carried outafter nearly 2.3 PV of CO2 injection when oil production ceasedThe oil sample collected during the EWF showed a sharp risthe asphaltene content, from 0.03 to 4.1 wt %. This indicatesflocculated asphaltenes during CO2 injection were picked up bythe flow of brine during the EWF. A negligible volume of oil~lessthan 0.12 in.3! was produced thereafter during the EWF andcould not be used for asphaltene analysis~the minimum volumerequired is approximately 0.6 in.3!. A summary of the test resultsis given in Table 9. The oil recovery by secondary CO2 injectionwas approximately 65% of the initial oil in place~IOIP! and thetotal recovery following the EWF was over 82% IOIP~Fig. 8!.

After the test was completed, the amount of asphaltenesmaining in the core matrix was calculated by a materials balanThe results indicated that the asphaltene content of the residuawas 11.5 wt % after the CO2 flood. This amounts to a 130% increase in the asphaltene content in the high-grain-size Vuggytrix by secondary CO2 flooding over that~5.0 wt %! of the in-jected Weyburn dead oil. The increase was approximately 8for the predominant Vuggy matrix, whereas it was only abo50% for the Marly matrix.

X-ray CAT-Scan Experiments.We tried to assess the CO2-induced asphaltene deposition/precipitation pattern along thelength. To do this, the stacked high-grain-size Vuggy core wx-ray scanned using CAT by Novacor Research & TechnoloCorporation before the coreflood experiment~clean core matrix!and afterwards~dirty core matrix containing essentially asphaTABLE 9 SUMMARY OF THE ASPHALTENE QUANTIFICATION STUDY CONDUCTED ONTHE MARLY, PRESERVED VUGGY COMPOSITE AND HIGH-GRAIN-SIZE VUGGY

COMPOSITE CORES AT 16 MPa AND 59C

Displacement Test Marly PlugPreserved

VuggyHigh-Grain-Size Vuggy

Core PropertiesPV (cm3) 5.79 11.91 18.6Porosity (% bulk volume) 25.8 12.8 16.5

Oil Saturation StageOil injection rate (m/d) 0.42 0.77 1.15Oil injection volume (PV) 20.3 8.8 15.9Initial oil-in-place (cm3) 4.81 7.97 15.8Asphaltene content of injected oil (wt %) 4.75 5.0 5.0Effective oil permeability (mm2) 0.44 0.61 19.7

Secondary CO2 FloodCO2 injection rate (m/d) 0.42 0.77 1.2CO2 injection volume (PV) 2.1 2.1 3.5Residual oil after CO2 injection (cm

3) 0.87 4.2 5.2Asphaltene content of residualoil left in core (wt %)

7.2 9.1 11.5

Extended WaterfloodBrine injection rate (m/d) 1.2Brine injection volume (PV) 2.5Residual oil after brine injection (cm3) 2.6Olooding SPE Prod. & Facilities, Vol. 14, No. 4, November 1999

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sity.theenes!. Both scans were conducted at the same locations at invals of 1 cm~0.39 in.! along the length of the core.

Post-coreflood treatments were carried out by flushing the cinitially with decane and later with methyl alcohol. Decane wused to remove the residual oil remaining in the core afterEWF and blowdown. In a bench test, decane was found to bsatisfactory solvent because it picked up hardly any asphaltfrom a filter paper during flow conditions. It was therefore prsumed that decane can flush out the residual oil from the cwithout unduly disturbing the precipitated/deposited asphalte~left in the core after CO2 injection!. Methyl alcohol was used toclean the core of decane and brine~left after the EWF!. Duringflushing with methyl alcohol, a high pressure drop was noacross the core with little production. The core was suspectebeing plugged. However, methyl alcohol and formation brwere evaporated from the core by keeping it in an oven at 9~194F! for over 24 hours. It was assumed that most of the decwas removed during the initial methyl alcohol flush~before thecore became plugged!.

Figs. 9 and 10depict the images generated from the CAT scfor the clean core and dirty core~containing asphaltenes!, respec-tively. The 23-cm-long~9.1-in.-long! core was scanned and imaged at 22 locations sequentially along the length, starting fthe inlet end. These images are presented in five rows in Figand 10, each row except the last row containing five images.first image in the top left corner represents a location appromately 0.5 cm~0.20 in.! from the inlet and each subsequent imafrom left to right portrays scan locations 1 cm~0.39 in.! apart. The

Fig. 7Asphaltene content of oil produced during Weyburndead oil W1 saturation of a preserved Vuggy composite core at59C and 16 MPa.

Fig. 8Oil recovery and asphaltene content of oil produced dur-ing the secondary CO 2 flood of a composite high-grain-sizeVuggy core at 59C and 16 MPa.Srivastava, Huang, and Dong: Asphaltene Deposition During CO2 Floodingter-

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last image located at the bottom right depicts the scan closethe outlet. A characteristic number called the CT number repsents the grain density of the matrix at a particular measuremlocation in the scanned images. The CT number in Figs. 9 andranges from 500 to 900. The highest density is representedwhite ~CT No. 900! and the lowest by black~CT No. 500!. Theshades of gray in the images represent an intermediate denThe CT number distribution in Figs. 9 and 10 thus representsdistribution of the matrix grain density.

Fig. 9X-ray CAT-scan images of a clean high-grain-size Vuggycore at 1 cm intervals along the length from top left to right.

Fig. 10X-ray CAT-scan images at 1 cm intervals along thelength for dirty core containing essentially asphaltenes fromtop left to right.SPE Prod. & Facilities, Vol. 14, No. 4, November 1999 243

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A comparison of the images in Figs. 9~clean core! and 10~dirty core containing asphaltenes! at individual scan locationsshows that the high-density~white! areas have generally increasein size in Fig. 10. This can be attributed to deposition/precipitatof asphaltenes in the core matrix. Scan locations 10 and 11@9.5and 10.5 cm~3.74 and 4.13 in., respectively! from the inlet# indi-cate an opposite trend, i.e., that high-density areas are moredominant in the clean core. The lowering of the grain densitythese scans following CO2 injection may indicate damage to thcore matrix. These scans also show the Vuggy nature of therepresented by isolated black areas~low-density zones!. Thus,x-ray CAT scanning can be used for visualizing the core maand asphaltene deposition pattern. However, it is difficult tocertain the exact location of precipitation/deposition sites ofphaltenes in the core matrix from these images.

To obtain some quantitative information on asphalteprecipitation/deposition, the CT number distribution for a scannimage can be used to calculate an average CT number. Theage CT number provides an indication of the average densitthe matrix.Fig. 11 shows the variation in the average CT numbwith the core length for clean and dirty cores. A relatively hiaverage CT number for the dirty core observed close to the i@0.5 and 1.5 cm~0.2 and 0.59 in., respectively! locations# cansignify asphaltene precipitation and deposition. Since the errothe CT number measurement is estimated~by Novacor ResearchCorporation! to be about610 units, several measurement loctions such as 2.5, 5.0 to 9.0, or 20.5 cm~1, 2.0 to 3.5, or 8.1 in.,respectively! from the inlet represent essentially identical comatrix densities, or negligible asphaltene precipitation and desition, at these sites. It appears that most of the asphaltene dsition is close to the inlet~Fig. 11!. At 9.5 and 10.5 cm~3.74 and4.13 in., respectively! scan locations from the inlet, an abnormdrop is observed in the CT number values for the dirty core. Isuspected that it is caused by damage to the core matrix duCO2 injection, as mentioned earlier. However, this conclusmay require further investigation for confirmation.

ConclusionsThe following conclusions are drawn from the results of thstudy.

1. The most important factor on which the asphalteflocculation/precipitation depends is the CO2 or injection gas con-centration. The asphaltene flocculation determined from the sprecipitation tests appeared to be insensitive to the operating psure~when the fluid mixture was in single phase!.

2. The onset point for asphaltene flocculation for the Weybreservoir was in the range of 39 to 46 mol % CO2 concentration.The asphaltene flocculation increased linearly~in the single-phaseregion! with CO2 concentration after the onset.

Fig. 11Variation in the average CT number with core length forclean and dirty core containing essentially asphaltenes .244 Srivastava, Huang, and Dong: Asphaltene Deposition During C2 Fdon

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3. The effect of the presence of brine on asphaltene flocculaseemed to be negligible. However, an increase in the brine ccentration appeared to somewhat inhibit the asphaltene flocction.

4. The effect of contaminants~approximately 3 mol % N2 and 3mol % CH4! in CO2 on asphaltene flocculation was insignifican

5. The asphaltene flocculation data normalized to the iniasphaltene contents of the oils fell into a narrow band fordifferent oil samples collected from the Weyburn pool, indicatithe effect of oil properties on asphaltene flocculation to be almnegligible.

6. Laboratory core displacement tests showed that asphaprecipitation/adsorption depended on the pore topography ofcore matrix tested. The high-grain-size Vuggy matrix showedhighest asphaltene precipitation in the core during CO2 injection.

7. X-ray CAT scanning of the clean and dirty~containing de-posited asphaltenes! Vuggy core showed that the technique canused for visualizing the core matrix and the asphaltene depospattern.

Nomenclature

psat 5 saturation pressure, m/Lt2, psiTres 5 reservior temperatures, T, C

ACKNOWLEDGMENTSThe authors acknowledge the 11 oil companies, namely, AmCanada Petroleum, Gulf Canada Resources, Husky Oil, MaraOil, Mobil Oil Canada, Murphy Oil, Norcen Energy ResourcePanCanadian Petroleum, Talisman Energy, Shell Canada,Wascana Energy, and also the Alberta Department of EnergyCanada Center for Mineral and Energy Technology~CANMET!for their financial support of this work.

They wish to express their thanks to Dr. B. Verkoczy for hvaluable input on the asphaltene measurement technique by strophotometry; P. De Wit, B. Schnell, N. Shatilla, and K. Sterreberg for their contributions to the experimental work; Dr. A. Kanzas and Novacor Research Corporation for conducting x-ray Cscanning of the cores; and J. Dickinson and B. Tacik for helpthe preparation of the manuscript. Thanks also go to PanCanaPetroleum Limited and Wascana Energy Inc. for providing theand gas samples for this work.

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Hopes, Oil & Gas J. ~April 1996! 94, 39.2. Saskatchewan Energy and Mines: Reservoir Annual~1993!.3. Huang, S.S. and Dyer, S.B.: Miscible Displacement in the Weybu

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10. Leontaritis, K.J. and Mansoori, G.A.: Asphaltene Deposition:Survey of Field Experiences and Research Approaches,J. Pet. Sci.Eng. ~1988! 1, 229.Olooding SPE Prod. & Facilities, Vol. 14, No. 4, November 1999

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he

nd

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te

e-

sented at the 1993 43rd Canadian Chemical Engineering ConfereOttawa, Ontario, 36 October.

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SI Metric Conversion FactorsAPI 141.5/(131.51API) 5 g/cm3

bbl 3 1.589 873 E201 5 m3

cp 3 1.0* E203 5 Pasin. 3 2.54* E100 5 cm

in.3 3 1.638 706 E101 5 cm3

ft 3 3.048* E201 5 mF (F32)/1.8 5 C

in. 3 2.54* E200 5 cmlbm 3 4.535 924 E201 5 kgmd 3 9.869 233 E204 5 mm2

mile 3 1.609 344* E100 5 kmpsi 3 6.894 757 E200 5 kPa

sq mile 3 2.589 988 E100 5 km2

*Conversion factors are exact. SPEPF

Raj Srivastava is an independent consultant working as a leadtest engineer for MCI Worldcom in Bartlesville, Oklahoma. Hepreviously worked for BDM Petroleum Technologies as a seniorengineer and for the Saskatchewan Research Council (SRC)as a senior research engineer in gas/chemical flooding. Srivas-tava holds a BS degree from the Indian Inst. of Technologyand MS and PhD degrees from the U. of Waterloo, Ontario, allin chemical engineering. Sam Huang is a manager of gas/chemical enhanced oil recovery (EOR) with the petroleumbranch of the SRC in Regina, Saskatchewan, and an adjunctprofessor at the U. of Regina. e-mail: huang@src.sk.ca. He pre-viosly worked as a research scientist with Gulf Canada andwas involved in hydrocarbon and CO2 miscible EOR projects.Huang holds a PhD degree in physical chemistry from Mar-quette U., Wisconsin. Mingzhe Dong is a research engineer inthe petroleum branch of the SRC. e-mail: dong@src.sk.ca. Hisresearch interests include multiphase flow in porous media,phase behavior in immiscible and near-miscible gas-injectionprocesses, reservoir simulation, and surface phenomena inEOR. Dong holds a PhD degree in chemical engineering fromthe U. of Waterloo.SPE Prod. & Facilities, Vol. 14, No. 4, November 1999 245