Mitigation of asphaltics deposition during CO
2
¯ood by
enhancing CO
2
solvency with chemical modi®ers
R.J. Hwang *, J. Ortiz
Chevron Research Technology Company, Richmond, CA 94802, USA
Abstract
CO2injected in the reservoir of McElroy Field, TX, for a CO2¯ood was in the supercritical state. Supercritical CO2
¯uid is capable of extracting light and intermediate hydrocarbons from rocks but is unable to extract heavy hydro-carbons and asphaltics. Therefore, plugging of asphaltics in reservoir rocks and a consequent reduction in injectivity and recovery may result when CO2 only is used in enhanced oil recovery. By adding common solvents as chemical
modi®ers, the ¯ooding ¯uid shows marked improvement in solvency for heavy components of crudes due to its increased density and polarity. Numerous supercritical CO2 ¯uid extractions of dolomite rock from the Grayburg
Formation containing known amount of spiked McElroy crude oil have been carried out to evaluate extraction e-ciencies of CO2and CO2with chemical modi®ers at various temperatures and pressures. All experiments show that
extraction eciency increases with increasing CO2 pressure but decreases with increasing temperature. Addition of
chemical modi®ers to CO2also shows improved extraction eciency and reduced asphaltic deposits. Under the
pres-sure and temperature similar to McElroy reservoir conditions; chemically modi®ed CO2yielded almost 3 times as much
oil extracts as those in extractions with CO2only. It also reduced the asphaltics content in extracted rocks to nearly one
half; indicating its potential for mitigating asphaltics plugging of formation rocks# 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Mitigation; Organic deposition; Carbon dioxide; Asphaltics; Miscible ¯ood; Supercritical ¯uid; Chemical modi®ers; Extraction eciency; Enhanced oil recovery; Reservoir; Injectivity
1. Introduction
Oil recovery processes involving reservoir injection of gases, such as light hydrocarbons and CO2 (miscible
¯ood), often induce organic deposits that plug rock pores and thus reduce rock permeability (Shelton and Yarborough, 1977; Stalkup, 1983; Tuttle, 1983; Mon-ger, 1985; Mazzocchi et al., 1998). The organic deposits, mostly asphaltic components of crude oils, can sig-ni®cantly reduce oil recovery and hence operation pro®ts. Many ®elds have been removed from candidacy for gas ¯oods because of serious asphaltene problems experienced in some ®elds. Little can be done to remediate reservoir formation damage caused by asphaltic deposits. Although production stimulation
methods (such as acidization) are eective in remediat-ing certain kinds of formation damage, they focus on inorganic scales in the near well-bore region (Thomas and Crowe, 1981; Da Motta et al., 1992; Davies et al., 1992; Wehunt et al., 1993). Prevention of organic deposition in miscible gas ¯oods would not only max-imize recovery, but also would save tremendous costs associated with formation damage remediation.
The extent of organic deposition depends largely on crude oil composition, solvation power of injection gas, and reservoir temperature and pressure (Monger and Fu, 1987). Therefore, characterizations of oils and of the interactions between oils and injection gas are essential in designing recovery processes to minimize gas-induced formation of organic deposits.
Under supercritical conditions, ¯uids such as CO2
and light hydrocarbons are powerful solvents capable of extracting oils from rocks (Monin et al., 1988; Hwang et al., 1996). Their solvation power can be further
0146-6380/00/$ - see front matter#2000 Elsevier Science Ltd. All rights reserved. P I I : S 0 1 4 6 - 6 3 8 0 ( 0 0 ) 0 0 0 8 2 - 6
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enhanced by adding organic solvents known as chemical modi®ers (Klink et al., 1994; Yang et al., 1995). Thus supercritical ¯uids with modi®ers have the potential to dissolve some asphaltic components and to minimize asphaltene precipitation and formation damage. This study of supercritical ¯uid extraction (SFE) has sought to develop more ecient oil extraction processes by enhancing oil solubility in CO2and minimizing
asphal-tene precipitation.
The SFE concept is important to certain enhanced oil recovery processes, since the performance of these pro-cesses depends on the extraction of oil components by a solvent that is typically in a supercritical state (Deo et al., 1992). For instance, CO2has been injected at
pres-sures of 75±82 atm (1100±1200 psi) in the McElroy
Field (Permian Basin, TX) for a pilot CO2 ¯ood
(Hwang and Ortiz, 1998). These injection pressures are slightly greater than the reservoir pressure 75 atm
(1100 psi) and ensure that the CO2is in a supercritical
state at reservoir temperatures 32C (
90F), since the
critical temperature and pressure of CO2are 31C and
73 atm, respectively.
Supercritical CO2is being used in many laboratories
for a wide range of analytical applications (Favati et al., 1991). It has physical properties (such as density and solvating power) similar to those of liquid solvents. However, supercritical CO2 does not have the polarity
needed for extracting complex mixtures of analytes with a very wide range of molecular weights and physical properties such as those found in crude oil.
The low polarity of supercritical CO2causes the
pre-cipitation of asphaltenes and perhaps other heavy organic components, such as nitrogen-, sulfur-, and/or oxygen-containing (NSO or resins) compounds and heavy parans (Hawthrone et al., 1994). Increasing amounts of these precipitates can reduce the porosity of the reservoir. It has been found that oils obtained from the pilot CO2area in the McElroy Field after initiation
of the ¯ood contained 50% less asphaltenes and
dis-played slightly higher API gravity than oils collected prior to the ¯ood (Hwang and Ortiz, 1998). Further-more, the McElroy producing oils showed a signi®cant reduction in the heavy paran content after the start of CO2 injection, suggesting occurrence of deposition of
heavy hydrocarbons (i.e. >C25) as well (Hwang and
Ortiz, 1998). These deposits were thought to have con-tributed to the loss of injectivity in the CO2¯ood pilot
area of the ®eld.
Investigation of the interaction between chemically modi®ed CO2 and crude oil in rock using the
super-critical ¯uid extraction (SFE) technique is the focus of this study. Speci®cally, the study is to determine if che-mically modi®ed CO2 can improve the extraction
e-ciency of a composite McElroy ®eld crude oil from dolomite reservoir rocks and to examine the chemical make-up of extraction residues. By analyzing the
extraction residues, one can better understand the interactions between CO2and the crude oil in reservoirs
and between the crude oil and reservoir rocks. A series of experiments has been done to test the eectiveness of supercritical CO2 and chemically modi®ed CO2 as
extraction solvents.
2. Background
Gaseous CO2 has a density of 0.00198 g/ml at 0C
and one atm pressure. Under these conditions it is not very eective as a solvent for liquids and solids; how-ever, as pressure is increased Ð which causes an increase in density Ð the extractive power of CO2improves. The
solubility of organic compounds in CO2increases with
higher density and temperature. Subcritical liquid CO2
at 20C and its equilibrium pressure of 56 atm is a useful
solvent because its density is relatively high Ð 0.77 g/ml (Sims, 1982). As temperature is increased toward 31C,
with an accompanying pressure increase, there is less and less dierence between densities of the saturated gas and liquid. At 31C, the critical temperature, there is no
dierence in any of the physical properties, and in fact two phases no longer exist. Above 31C (88F) and 73
atm (1071 psi, critical pressure), there exists only one phase, a supercritical ¯uid CO2. The phase diagram in
Fig. 1 illustrates this point.
A supercritical ¯uid exhibits unique physical and chemical properties; it acts chemically like a liquid and physically like a gas. The viscosity of the ¯uid resembles that of a gas that gives the ¯uid the ability to penetrate a matrix very rapidly, yet the ¯uid retains much of the solvating power of a liquid. The solvating power of a supercritical ¯uid is proportional to its density: the higher the density, the more substrate the ¯uid can extract from the matrix (Pipkin, 1990). Factors that aect ¯uid density (including pressure and temperature)
can have a signi®cant in¯uence on the extraction e-ciency of a supercritical ¯uid.
Injection of chemically modi®ed CO2 to oil ®elds
could potentially improve injectivity of a water-alter-nate-gas (WAG) injection process, and oil recovery through minimizing asphaltic and wax deposition. Common modi®ers are methanol, methylene chloride, water and hexane (Levy et al., 1993). Little work has been done to study the use of modi®ed CO2systems for
extracting highly polar compounds such as asphaltics from rocks. In contrast, signi®cant amount of the work has been carried out on the use of chemically modi®ed CO2in extracting highly polar herbicides and pesticides
from sediments (Langenfeld et al., 1994).
3. Experimental
Numerous supercritical CO2 ¯uid extractions of
dolomite rock (from the Grayburg formation) spiked with known amounts of McElroy crude oil have been completed for evaluating extraction eciencies of CO2
and chemically modi®ed CO2 at various temperatures
and pressures. The McElroy reservoir is known for its low pressure75 atm and low temperature 32C
(Har-ris and Walker, 1990). To mimic oil extraction processes occurred in the reservoir during CO2 ¯ood, SFE
laboratory experiments were conducted at pressure and temperature conditions similar to those of the McElroy reservoir.
Supercritical ¯uid extraction was performed in a Suprex model SFE 50 supercritical extractor, equipped with a piston pump, an extraction cell of 1 ml (Keystone Scienti®c), and a 1-m long capillary restrictor (50 mm i.d.) as a transfer line. High purity CO2was used in the
experiments, which was pressurized by a piston pump to above its critical pressure. The extraction vessel that was capable of withstanding pressure up to 1020 atm
(15,000 psi) was situated in a temperature-controlled oven to allow extraction to be conducted at various temperatures. The schematic of the SFE system is illu-strated in Fig. 2. It automatically measures and displays
density of supercritical ¯uid under experimental condi-tions during extraction.
Oil extraction was taking place when supercritical CO2 ¯uid ¯owed through the extraction cell at
pre-determined pressure and temperature. Depressurization of the ¯uid through a restrictor led to separation of extractables from the supercritical ¯uid. The restrictor was passed through a needle and into a 5 ml conical vial with a septum cap that contained either hexane or a 50/50 mixture of methanol and toluene. The extractables were collected in the solvent trap before being subjected to gas chromatographic analysis. Group type separation of the original oil and all the residues were performed by a combination of solid phase extraction and high-pres-sure liquid chromatography (Hwang, 1990).
The oil used in the experiments was a composite of oils produced from various wells in the McElroy ®eld. The bulk composition of the oil is listed in Table 1. Prior to SFE extraction, a ®xed amount of the oil (50
mg) was mixed with pre-weighed (500 mg) ground
dolomite rock (125±495 um particle sizes) obtained
from those McElroy cores free of oil stains. To ensure its cleanness, the ground dolomite rock was pre-extrac-ted with methylene chloride followed by a mixture of methanol/toluene. Each SFE extraction was conducted at a ®xed length of time; 5.0 min static extraction (zero ¯ow rate) followed by 45 min dynamic extraction with a ¯ow rate of 20 ml/min.
CO2 and chemically modi®ed CO2were supplied by
Scott Specialty Gases. According to Scott, the blending of supercritical ¯uid CO2 with chemical modi®ers is
typically accomplished by ®rst adding a predetermined amount of modi®er to an evacuated cylinder followed by adding CO2to attain the desired concentration of the
modi®er. Mechanically rolling the cylinder in a hor-izontal position aids mixing of the components. SFE experiments were carried out with CO2plus modi®ers at
pressures of 80, 100 and 120 atmospheres. Five pre-mixed tanks from Scott used in the study included: CO2/
methanol (90/10), CO2/methanol/toluene (90/8/2), CO2/
methanol/toluene (90/2/8), CO2/isopropanol/toluene
(90/3/7), and CO2/toluene (90/10). For CO2 with light
aromatic hydrocarbons (LAH) mixtures, mixing was completed by adding pre-determined amount of CO2
and LAH in the SFE pump followed by repeated com-pression and expansion of the pump prior to extraction. LAH is a hydrocracking product derived from refor-mating heavy resid in oil re®neries. It contains pre-dominantly toluene, xylenes, and alkylbenzenes.
The extraction eciency was calculated based on weight losses of rock samples through extraction. For all experiments, the oil residues were extracted immedi-ately from the rocks after supercritical ¯uid extraction with methylene chloride and analyzed by GC without solvent evaporation to yield hydrocarbon pro®les of the residues. The rocks were further extracted with a 50/50
methanol/toluene mixture to remove the remaining highly polar organic compounds based on the proce-dures published previously (Baskin et al., 1995). The extracts were combined for determining residue weight after solvent evaporation. Seven SFE extractions for each modi®ed CO2 solvent system were performed
allowing calculation of standard deviation of extraction yield (Table 1).
Core ¯ood experiments were carried out with CO2
only and with CO2plus modi®er (LAH). Details of core
¯ood experiments have been described previously (Hwang and Ortiz, 1998).
4. Results and discussion
4.1. Supercritical ¯uid extraction of crude oil with CO2 only
4.1.1. Pressure eect
Crude oil is a complex mixture of tens of thousands of saturated and aromatic hydrocarbons and non-hydro-carbons (asphaltics). These crude oil components dier signi®cantly in their physico-chemical properties, such as molecular weight, density, and polarity. Their sus-ceptibility to extraction by supercritical CO2¯uid (and
hence their extraction yields) vary widely. Fig. 3 shows that both the density and extraction eciency of CO2at
a given temperature increase with increasing pressure. The pressure eect on the density matches with that on the extraction eciency of the CO2¯uid. Obviously, the
density of CO2 plays a key role in its extraction
e-ciency of crude oil.
With regard to oil recovery using CO2in the ®eld, the
higher the pressure (density), the better the recovery that
can be expected. However, the pressure of CO2used in a
CO2¯ood in an oil ®eld is limited by its reservoir
pres-sure and can not be increased to the optimum level for hydrocarbon recovery. Chemical modi®cation of CO2
by adding modi®ers prior to extraction provides an alternative method for increasing the CO2density, since
the modi®ers increase the density to a small degree at pressures less than 135 atm (2000 psi), as depicted in Fig. 4. Compared to CO2only, chemically modi®ed CO2
is not only slightly denser, but also is more polar in chemical nature. These added characteristics enhance the capacity of CO2 in extracting heavy hydrocarbons
and asphaltics from rocks and therefore increasing oil recovery. As shown later in discussion, the presence of small amounts of the modi®ers in CO2 leads to large
increases in its oil extraction yields. The results indicate that the density of modi®ed CO2 is of secondary
importance to solvent polarity in enhancing its solvency.
4.1.2. Temperature eect
Temperature and density of supercritical CO2 ¯uid
vary inversely and nonlinearly (Gere and Derrico, 1994). Therefore, temperature is an important but com-plex parameter for controlling extraction. At the low temperature range (30±100C) commonly seen for
pet-roleum reservoirs, the eect of temperature on CO2
extraction eciency is small. Fig. 5 illustrates that supercritical CO2¯uid extraction of McElroy oil gives
slightly better yields at 100C than those at 45C when
pressure is less than 270 atm (4000 psi). Given the low pressure 75 atm (1100 psi) and low temperature
32C (
90F) of the McElroy reservoir, increases in
CO2polarity and density by adding chemical modi®ers
would improve the eciency of oil extraction during a CO2¯ood.
Table 1
Extraction yields of oil from dolomite using supercritical ¯uid CO2with chemical modi®ers at various pressuresa
Solventb Solvent composition
CO2: modi®er
Yield (%)
80 atm S. D.c 100 atm S. D.c 120 atm S. D.c
CO2only 100/0 22.4 1.3 47.3 1.9 53.5 2.6
CO2/MeOH 90/10 77.3 0.9 80.1 1.3 82.2 1.9
CO2/MeOH/toluene 90/8/2 81.7 2.1 84.0 0.7 86.0 0.9
CO2/MeOH/toluene 90/2/8 88.4 1.3 88.7 1.7 89.2 1.4
CO2/IPA/toluene 90/3/7 88.4 2.6 88.5 1.2 89.4 1.7
CO2/toluene 90/10 94.3 2.3 95.0 1.4 94.9 0.8
CO2/hexane 90/10 84.4 1.4 86.0 1.3 86.5 1.6
CO2/IPA 90/10 86.5 0.4 83.9 0.8 85.1 1.5
CO2/MeOH/toluene 97.5/0.5/2 57.8 2.8 71.2 2.8 75.1 1.2
CO2/MeOH/toluene 95/1/4 70.7 2.2 74.2 1.8 77.4 2.5
CO2/LAH 90/10 87.8 0.2 88.4 1.6 90.1 1.5
a All SFE extractions were carried out at 35C (95F); atm, atmospheric pressure (76 cm Hg).
4.2. Supercritical ¯uid extraction of oil with chemically modi®ed CO2
Chemicals commonly used in laboratories as solvents were added in small amounts to CO2 to enhance its
extraction power of oils from rocks. These chemical modi®ers evaluated in this study include methanol, iso-propyl alcohol (IPA), hexane, toluene, methanol/ toluene mixtures, and light aromatic hydrocarbons (LAH) mixtures.
4.2.1. Composition of extracts
As mentioned earlier, addition of chemicals to CO2
generally results in increases in density and polarity of the ¯uid. Compared to pure CO2, the chemically
mod-i®ed CO2displays enhanced solvating power and greater
extraction eciency of many organic compounds (Levy et al., 1993). This added extraction power is demon-strated by comparing supercritical ¯uid extractions of McElroy oil from dolomite rock using CO2 with and
without chemical modi®ers. Under conditions of 31C
and 80 atm, pure CO2eectively extracts hydrocarbons
up to C12, as shown in Fig. 6b. For hydrocarbons
hea-vier than C12, the extraction yields decrease rapidly with
increasing carbon number. Little or no hydrocarbons heavier than C22 were extracted. Abundant
hydro-carbons heavier than C12 remain in the extracted rock
(Fig. 6a), providing evidence that the extraction using pure CO2 is mainly eective for gasoline range
hydro-carbons. Fig. 6c shows the hydrocarbon pro®le of the original crude used in the SFE experiments.
In contrast, CO2 with 10% methanol is capable of
extracting hydrocarbons up to C30 (Fig. 7b) including
quantitative extraction of both gasoline and diesel ran-ges hydrocarbons (up to C22). The hydrocarbon pro®le
of the extract is highly similar to that of the crude oil (Fig. 7c). There were only very small amounts of heavy hydrocarbons (>C25) remaining in the SFE extracted
rock (Fig. 7a). These results illustrate that CO2 with a
small amount of chemical modi®er can extract a wider range of hydrocarbons and greater amounts of inter-mediate (diesel) and heavy hydrocarbons, which is expected to lead to greater oil recovery when applied to ®eld production.
4.2.2. Extraction yields
4.2.2.1. Effect of various modifiers.At all pressures stud-ied, the SFE yields with chemically modi®ed CO2are all
signi®cantly higher than with CO2only (Table 1),
indi-cating higher extraction eciency for the chemically modi®ed CO2. As shown in Fig. 8, toluene is the most
eective modi®er among the chemicals studied. The CO2/toluene (90/10) mixture yielded an extraction
e-ciency of 94.3% compared to 22.4% with CO2only at
80 atm and 35C. The increase in CO
2 extraction
e-ciency by adding 10% toluene is over four times as much as that of CO2 only. Although less spectacular,
10% methanol in CO2 also improves extraction
e-ciency of CO2considerably from 22.4 to 77.3%. Binary
modi®ers, such as methanol/toluene and IPA/toluene mixtures, also show improvements that are intermediate between those of toluene and methanol. Increasing the toluene content in the mixture modi®ers led to an increase in the extraction eciency.
Fig. 5. Small temperature eect on eciency of supercritical CO2
¯uid extraction of McElroy oil from rock at various pressures. Fig. 3. CO2 extraction eciency of McElroy crude oil (at
100C) as function of its density.
A light aromatic hydrocarbon mixture (LAH) as the modi®er is an interesting alternative to the solvents mentioned above. Once added to CO2, LAH provides
nearly as high extraction eciency as toluene does (Fig. 8). This high eciency can probably be attributed to its richness in light aromatic compounds such as toluene, xylenes, and other alkyl benzenes. Using LAH as a modi®er is advantageous due to its lower cost than other chemical modi®ers that are high purity solvents. Given the enormous amount of the chemical needed in the tertiary recovery process, the cost of chemicals added to CO2is a very important factor in determining
economics of the operation.
Clearly, chemical modi®ers enable CO2to attain high
extraction eciency that would otherwise require extre-mely high pressure to attain if CO2only were used for
oil extraction from the rock in reservoirs. The results here have a signi®cant implication for the CO2¯ood. As
mentioned earlier, an increase in CO2pressure favors oil
extraction and hence enhances oil recovery. However, it is impractical to maintain high pressure for the CO2
¯ood in low-pressure reservoirs such as the one in the McElroy ®eld. Chemical modi®ers would make the pressure requirement for the CO2 ¯ood less stringent
than CO2alone and their eect would be equivalent to
reducing minimum miscibility pressure (MMP), the threshold pressure for obtaining the miscible phase
between crude oil and CO2in petroleum reservoirs. Thus,
the modi®ers appear to have potential not only to increase eectiveness of CO2sweeping but also to make
the CO2¯ood a viable approach for enhancing oil
recov-ery in previously unsuitable, low-pressure reservoirs.
4.2.2.2. Effect of pressure.Unlike pure CO2, chemically
modi®ed CO2 mixtures are ecient extractants at the
low-pressure range (80±120 atm) and their extraction eciencies are not very sensitive to pressure changes (Fig. 9). Increases in extraction eciency with increasing pressure are relatively small for chemically modi®ed CO2. At higher pressures, the degree of improvement in
CO2extraction eciency by adding chemical modi®ers
is greatly reduced, although the improvement is still signi®cant. For example, the increase in extraction e-ciency is less than two-fold (94.9% for CO2/toluene vs.
53.5% for CO2 only) at 120 atm as compared to over
four-fold at 80 atm (Fig. 9).
4.2.2.3. Effect of modifier concentration.Extraction e-ciency of supercritical CO2 ¯uid is sensitive to the
amount of modi®ers present in the ¯uid. A positive correlation has been observed between extraction e-ciency and concentration of a chemical modi®er. For example, for toluene at 80 atm, the extraction eciency increases drastically with increasing concentration of the
Fig. 6. Supercritical CO2¯uid extractions (SFE) of clean Grayburg dolomite rock mixed with McElroy produced oil. Gas
modi®er, as shown in Fig. 10. These increases, however, are not as drastic at higher pressures as those at lower pressures. It appears that 8±10% of modi®er in CO2is
the optimum concentration because its extraction e-ciency is around 90%.
4.2.2.4. Effect of CO2 volume. Volume of CO2 has an
enormous eect on the extraction eciency when CO2
only is used in extraction. The extraction yield drops to one half when CO2volume is reduced to one third (Fig.
11, Table 2). Reduction of CO2volume was achieved by
decreasing extraction time at constant ¯ow rate. In contrast, extraction eciencies of CO2 with chemical
modi®ers are not very sensitive to volume of CO2(Fig.
11). Their extraction yields decrease little even when volume of extractant is reduced signi®cantly. For instance, the yields drop only 7% for CO2 with 2%
methanol and 8% toluene as additives when volume of the extractant is reduced to one third. These results indicate that extractions with chemically modi®ed CO2
Fig. 7. Supercritical (CO2+methanol) ¯uid extractions of clean Grayburg dolomite rock mixed with McElroy produced. Gas
chro-matograms of (a) SFE extraction residue, (b) SFE extract, and (c) original McElroy crude.
Fig. 8. Positive eect of chemical modi®ers (10% in CO2) on
extraction eciency of oil from dolomite using supercritical CO2¯uid at 80 atm (atmospheric pressure) and 35C.
Fig. 9. Comparison of supercritical ¯uid extraction eciency of various chemical modi®ers in CO2 at dierent pressures;
with modi®ers, CO2 extraction eciency is not sensitive to
can save large volumes (amounts) of CO2 while still
giving much higher yields than the ones in which CO2
only is used. The ®ndings suggest that, when applied in oil ®elds, chemically modi®ed CO2has great potential in
reducing CO2 volumes required for the CO2 miscible
¯ood and increasing extraction yields or oil recovery. These eects are expected to have a positive impact on the economics of the CO2¯ood.
Table 2
Extraction yields of supercritical CO2¯uid with chemical modi®ers at various extractant volumesa
Solvent Solvent composition CO2/modi®er
Volume of solvent CO2/modi®er (ml)
Yield (%)
CO2only 100/0 2.0 14.0
3.0 18.7
6.0 22.9
CO2/MeOH 90/10 0.7 69.9
1.4 69.0
2.1 75.5
CO2/MeOH/toluene 90/8/2 0.7 71.2
1.4 77.3
2.1 79.3
CO2/MeOH/toluene 90/2/8 1.0 82.6
2.0 86.5
3.0 89.6
CO2/IPA/toluene 90/3/7 1.0 80.1
2.0 87.2
3.0 86.6
CO2/toluene 90/10 0.7 86.4
1.4 93.7
2.1 93.5
CO2/hexane 90/10 0.7 82.1
1.4 84.1
2.1 83.0
CO2/IPA 90/10 1.0 78.1
2.0 82.9
3.0 82.2
CO2/LHA 90/10 1.0 80.1
2.0 88.9
3.0 91.4
a SFE extractions were carried out at 80 atm, 35C.
Fig. 10. Eect of modi®er concentration on extraction e-ciency of CO2at various pressures.
Fig. 11. Variation of extraction eciency with volume of suerpcritical CO2 ¯uid used for oil extraction from dolomite
4.2.3. Extraction residues
All CO2extractions with chemical modi®ers used in
this study have yielded less asphaltics in extracted rocks than those from extractions with CO2 only (Table 3),
indicating the potential of modi®ers for mitigating CO2
-induced asphaltic plugging in formation rocks during the CO2¯ood. Addition of chemical modi®ers to CO2
has resulted in improved extraction eciencies not only for hydrocarbons, but also for asphaltics (resins plus asphaltenes). Apparently, increased solvency with mod-i®ed CO2led to reduction in the asphaltics content of
residual oils left in extracted rocks. The amounts of reduction vary, depending on type and concentration of chemical modi®er added to CO2, and pressure and
tem-perature associated with CO2 extraction. Table 3 lists
the asphaltics content of residual oils derived from CO2
extraction with various chemical modi®ers.
Under pressure and temperature conditions similar to those of McElroy reservoir (80 atm, 35C), extraction
with CO2plus 10% modi®ers yielded residual oils with
the asphaltics content ranging from4 to 10%, which
compares favorably to 11.9% when CO2only was used
in extraction (Fig. 12). This represents a reduction ran-ging from 65 to 14% with toluene as the most eective modi®er and methanol the least. This observation is consistent with the well-known fact that toluene is an excellent solvent for asphaltics but methanol is not as
Table 3
Bulk composition of residual oils in extracted rocks from CO2supercritical ¯uid extractiona
SFE solvent Solvent composition CO2/modi®ers
Pressure (atm)b
Saturates (%)
Aromatics (%)
NSO (%)
Asphaltenes (%)
CO2only 100/0 80 16.5 9.3 10.8 1.1
CO2only 100/0 100 5.3 6.1 10.7 0.7
CO2only 100/0 120 4.7 5.6 11.2 0.8
CO2/MeOH 90/10 80 5.5 5.2 9.5 0.7
CO2/MeOH 90/10 100 4.6 4.4 9.1 0.7
CO2/MeOH 90/10 120 4.0 3.9 8.9 0.7
CO2/MeOH/toluene 90/8/2 80 4.4 3.6 8.7 1.1
CO2/MeOH/toluene 90/8/2 100 3.8 3.2 8.7 0.8
CO2/MeOH/toluene 90/8/2 120 3.3 2.4 8.3 1.0
CO2/MeOH/toluene 90/2/8 80 2.4 1.8 7.2 0.9
CO2/MeOH/toluene 90/2/8 100 2.5 2.3 6.7 0.8
CO2/MeOH/toluene 90/2/8 120 3.3 1.8 6.4 0.7
CO2/IPA/toluene 90/3/7 80 ± ± 6.6 0.8
CO2/IPA/toluene 90/3/7 100 2.5 2.6 7.6 0.7
CO2/IPA/toluene 90/3/7 120 1.6 2.3 6.8 0.7
CO2/toluene 90/10 80 1.6 0.6 3.4 0.8
CO2/toluene 90/10 100 1.3 0.6 3.0 0.8
CO2/toluene 90/10 120 1.6 0.6 3.9 0.8
CO2/LHA 90/10 80 0.8 1.1 6.7 0.7
CO2/LHA 90/10 100 1.1 1.6 7.2 0.6
CO2/LHA 90/10 120 0.5 0.7 5.3 0.6
Original crude oil 33.1 22.2 13.0 1.0
a The composition was determined based on the original amount of crude oil spiked in the rocks prior to the supercritical CO 2¯uid
extraction and was not normalized to the amount of recovery.
b atm, atmospheric pressure (76 cm Hg); extraction temperature 35C.
eective. The toluene/methanol and toluene/isopropyl alcohol mixtures are also promising as modi®ers for reducing the asphaltics residues, but to a lesser degree than toluene (Table 3). Due to its richness in toluene, xylenes, and other light aromatics, the light aromatic hydrocarbon mixture (LAH) is almost as eective as pure toluene in extracting asphaltenes from rocks when added to CO2 (Table 3). In a CO2 ¯ood, LAH as a
modi®er would not only increase the yield of oil extrac-tion, but also reduce asphaltics deposition induced by CO2injection into petroleum reservoirs.
Supercritical CO2 ¯uid extractions with chemical
modi®ers also yielded less hydrocarbons in the residual (non-extracted) oils than did CO2 only (Fig. 13),
another indication of enhanced solvation eects due to the presence of chemical modi®ers in CO2. This enhanced
solvency, particularly toward heavy hydrocarbons, is also illustrated by dierences in the hydrocarbon distributions of residual oils left behind in the rocks from CO2
extrac-tions with various amounts of toluene as the modi®er (Fig. 14). Distribution maxima of hydrocarbons of
resi-dual oils shift to high carbon numbers as the con-centration of toluene in CO2increases.
4.3. Core ¯ood
To further evaluate the eect of chemical modi®ers on CO2¯ood, laboratory core ¯ood experiments that better
simulate the oil recovery process used in the ®eld were carried out. CO2miscible displacements with and without
modi®ers were conducted in a 1.5 m long stacked McEl-roy core under reservoir conditions. The CO2core ¯oods,
based on the 1/1 WAG (water-alternating-gas) process, were preceded by water ¯ood (Hwang and Ortiz, 1998).
The eect of CO2/modi®er injection on oil chemistry
during the core ¯ood is not quite the same as that of the core ¯ood with CO2 only, which is illustrated by the
changes in API gravity of produced oils during the core ¯ood. Fig. 15 shows that API gravity of produced oils increased, at dierent rates, from mid-20 to low 40s with increasing CO2injection during the CO2core ¯ood with
and without a modi®er. Observations were made pre-viously in both laboratory core ¯oods and ®eld produc-tion that API gravity of produced oils increased with increasing CO2injection (Hwang and Ortiz, 1998). The
increase in API gravity was attributed to the results of precipitation of asphaltics and heavy hydrocarbons induced by CO2injection. In this study, the observation
of slow increases in API gravity of produced oils during CO2/modi®er WAG core ¯ood relative to those during
CO2only core ¯ood indicate the ability of the modi®er to
extract asphaltics and heavy hydrocarbons and to miti-gate asphaltics deposition for enhancing oil recovery.
Determination of asphaltics content of the residual oil and their distribution in the core after core ¯ood is also critical in assessing the eect of CO2/modi®er ¯ood on
composition of reservoired oils. At completion of the core ¯ood, the residual oil remaining in the core was extracted by ¯owing toluene through the core. Five
Fig. 13. Hydrocarbon content of residual oils in extracted rocks from supercritical ¯uid extraction with chemically mod-i®ed CO2are much lower than that with CO2only.
Fig. 14. Maxima of hydrocarbon distributions of residual oil from suerpcritical CO2¯uid extraction shift to higher carbon
number, with increasing amount of toluene in CO2indicating
CO2 solvency for heavy hydrocarbons is improved as the
amount of toluene is increased.
Fig. 15. Slow increases in API gravity of produced oils during CO2/modi®er WAG core ¯ood relative to those during CO2
extracts were obtained as extraction was completed on ®ve dierent sections of the core. The asphaltics content of the extracts could mimic a pro®le of asphaltics dis-tribution from an injector to a producer in the ®eld. Fig. 16 shows the asphaltics content of the extracts obtained after both CO2only ¯ood and CO2/modi®er ¯ood. For
the CO2 only ¯ood, the asphaltics concentration near
the injector is about 1.8 times that in the original crude. It gradually decreases towards the producer end sug-gesting that majority of CO2 induced asphaltene
pre-cipitation in the ®eld occurs near injectors. In comparison, the CO2/modi®er ¯ood yielded the
asphal-tics concentration near the injector at about 1.4 times that in the original crude and lower asphaltics content in the residual oil across the core. The results indicate that the chemical modi®er enhances the ability of CO2¯uid
to extract asphaltics and other heavy components of crude oil from the rock. Injection of CO2+modi®er
instead of CO2only into oil reservoirs is likely to reduce
asphaltics deposition and pore plugging for increased recovery.
4.4. Implication for CO2¯ood in the McElroy ®eld
The CO2¯ood has been going on in the pilot area of
the McElroy ®eld since December 1992. Many injection wells have suered injectivity loss or reduction during WAG cycles (Hwang and Ortiz, 1998). It was discovered that the asphaltics and heavy parans content of pro-duced oils in the pilot area decreased signi®cantly after the start of the CO2 ¯ood indicating occurrence of
organic deposition in the reservoir (Hwang and Ortiz, 1998). Organic deposition was thought to have con-tributed to the low injectivity problem.
The results of the current study suggest that super-critical CO2¯uid with chemical modi®ers can dissolve
some asphaltic components, minimize asphaltene pre-cipitation, and increase recovery of heavy hydrocarbons (>C25) during a CO2¯ood. Chemical modi®ers appear
to have the potential to improve well injectivity during WAG of the CO2¯ood.
The cost of chemically modi®ed CO2 is certainly
higher than CO2only. However, the chemicals selected
as modi®ers are inexpensive industrial solvents and their concentrations used in enhancing solvency of CO2 are
small. The added cost for the solvents used are relatively small. For example, 10% toluene in CO2,we estimate
that the added cost is less than 5% the cost of CO2.
Therefore, with a small incremental in cost, adding chemi-cal modi®ers to CO2 is likely to increase economics
viability for the CO2¯ood for enhanced oil recovery.
5. Conclusions
The results from the study suggest that supercritical CO2 ¯uid with chemical modi®ers can dissolve some
asphaltic components, minimize asphaltene precipita-tion, and increase recovery of heavy hydrocarbons (>C25) during a CO2¯ood. Chemical modi®ers appear
to have the potential to substantially improve the eco-nomics of CO2miscible ¯oods.
CO2 only is incapable of extracting asphaltics and
heavy hydrocarbons from dolomite rock under McElroy reservoir conditions. Therefore, plugging of asphaltics and heavy hydrocarbons in reservoir rocks can occur when CO2only is used in enhanced oil recovery, which
may result in low injectivity and reduced recovery. Despite the cost of solvents, the overall cost of oil extraction using chemically modi®ed CO2would
prob-ably not be higher than when CO2only is used. Because
of its enhanced extraction eciency, chemically mod-i®ed CO2does not require as much volume as CO2only
to achieve the same extraction yield, a fact which would result in substantial savings in CO2costs. Furthermore,
chemically modi®ed CO2can extract three to four times
as much oil from oil-impregnated reservoir rock as can CO2 alone, which would result in an increase in oil
Fig. 16. Asphaltics content of residual oils along core after core ¯oods: (a) high concentration of asphaltics near injector after CO2 WAG, (b) reduction of aspahitics in residual oils
recovery when applied to the CO2 ¯ood. It can also
reduce asphaltics deposits in the rocks by one third to one half, leading to reduction of asphaltics plugging in reser-voir rocks and hence potential improvement in injectivity of wells during a CO2¯ood. Clearly, the potential
eco-nomic bene®t of using chemically modi®ed CO2 in
enhancing oil recovery appears to be very signi®cant. In addition, the ability of modi®ers to improve recovery of heavy ends of oil suggests that modi®ers may improve CO2miscibility and may lower minimum
miscibility pressure (MMP). These eects would make CO2¯oods a viable approach for enhancing oil recovery
in previously unsuitable reservoirs, since chemical modi®ers would make the pressure requirement for CO2
¯oods less stringent than for CO2only.
Acknowledgements
The authors would like to thank the management of Chevron for permission to publish this paper and to thank W.S. Fong and R. Ulrich for providing technical assistance in core ¯ood experiments. The manuscript was greatly improved by comments from Drs. R. diPri-mio, P. Taylor, and A. Wihelms.
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