Directory UMM :Data Elmu:jurnal:O:Organic Geochemistry:Vol31.Issue12.Dec2000:
Organic Geochemistry 31 (2000) 1451±1462
www.elsevier.nl/locate/orggeochem
Mitigation of asphaltics deposition during CO2 ¯ood by
enhancing CO2 solvency with chemical modi®ers
R.J. Hwang *, J. Ortiz
Chevron Research Technology Company, Richmond, CA 94802, USA
Abstract
CO2 injected 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 hydrocarbons 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 eciencies of CO2 and CO2 with 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 CO2 also shows improved extraction eciency and reduced asphaltic deposits. Under the pressure and temperature similar to McElroy reservoir conditions; chemically modi®ed CO2 yielded almost 3 times as much
oil extracts as those in extractions with CO2 only. 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; Monger, 1985; Mazzocchi et al., 1998). The organic deposits,
mostly asphaltic components of crude oils, can signi®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
* Corresponding author.
methods (such as acidization) are eective in remediating 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 maximize 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.
PII: S0146-6380(00)00082-6
1452
R.J. Hwang, J. Ortiz / Organic Geochemistry 31 (2000) 1451±1462
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 CO2 and minimizing asphaltene precipitation.
The SFE concept is important to certain enhanced oil
recovery processes, since the performance of these processes depends on the extraction of oil components by a
solvent that is typically in a supercritical state (Deo et
al., 1992). For instance, CO2 has been injected at pressures 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 CO2 is in a supercritical
state at reservoir temperatures 32 C (90 F), since the
critical temperature and pressure of CO2 are 31 C and
73 atm, respectively.
Supercritical CO2 is 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 CO2 causes the precipitation 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 CO2 area in the McElroy Field after initiation
of the ¯ood contained 50% less asphaltenes and displayed slightly higher API gravity than oils collected
prior to the ¯ood (Hwang and Ortiz, 1998). Furthermore, 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 contributed 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 supercritical ¯uid extraction (SFE) technique is the focus of
this study. Speci®cally, the study is to determine if chemically modi®ed CO2 can improve the extraction eciency 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 CO2 and 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 0 C
and one atm pressure. Under these conditions it is not
very eective as a solvent for liquids and solids; however, as pressure is increased Ð which causes an increase
in density Ð the extractive power of CO2 improves. The
solubility of organic compounds in CO2 increases with
higher density and temperature. Subcritical liquid CO2
at 20 C 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 31 C,
with an accompanying pressure increase, there is less
and less dierence between densities of the saturated gas
and liquid. At 31 C, the critical temperature, there is no
dierence in any of the physical properties, and in fact
two phases no longer exist. Above 31 C (88 F) 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)
Fig. 1. Phase diagram of CO2.
R.J. Hwang, J. Ortiz / Organic Geochemistry 31 (2000) 1451±1462
can have a signi®cant in¯uence on the extraction eciency of a supercritical ¯uid.
Injection of chemically modi®ed CO2 to oil ®elds
could potentially improve injectivity of a water-alternate-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 CO2 systems 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
CO2 in 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 pressure 75 atm and low temperature 32 C (Harris 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 CO2 was 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 illustrated in Fig. 2. It automatically measures and displays
Fig. 2. Block diagram of supercritical ¯uid extraction system.
1453
density of supercritical ¯uid under experimental conditions during extraction.
Oil extraction was taking place when supercritical
CO2 ¯uid ¯owed through the extraction cell at predetermined 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-pressure 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-extracted 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 CO2 were 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 CO2 to attain the desired concentration of the
modi®er. Mechanically rolling the cylinder in a horizontal position aids mixing of the components. SFE
experiments were carried out with CO2 plus modi®ers at
pressures of 80, 100 and 120 atmospheres. Five premixed 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 compression and expansion of the pump prior to extraction.
LAH is a hydrocracking product derived from reformating heavy resid in oil re®neries. It contains predominantly 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 immediately 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
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R.J. Hwang, J. Ortiz / Organic Geochemistry 31 (2000) 1451±1462
Table 1
Extraction yields of oil from dolomite using supercritical ¯uid CO2 with chemical modi®ers at various pressuresa
Solventb
CO2 only
CO2/MeOH
CO2/MeOH/toluene
CO2/MeOH/toluene
CO2/IPA/toluene
CO2/toluene
CO2/hexane
CO2/IPA
CO2/MeOH/toluene
CO2/MeOH/toluene
CO2/LAH
a
b
c
Solvent composition
CO2: modi®er
100/0
90/10
90/8/2
90/2/8
90/3/7
90/10
90/10
90/10
97.5/0.5/2
95/1/4
90/10
Yield (%)
80 atm
S. D.c
100 atm
S. D.c
120 atm
S. D.c
22.4
77.3
81.7
88.4
88.4
94.3
84.4
86.5
57.8
70.7
87.8
1.3
0.9
2.1
1.3
2.6
2.3
1.4
0.4
2.8
2.2
0.2
47.3
80.1
84.0
88.7
88.5
95.0
86.0
83.9
71.2
74.2
88.4
1.9
1.3
0.7
1.7
1.2
1.4
1.3
0.8
2.8
1.8
1.6
53.5
82.2
86.0
89.2
89.4
94.9
86.5
85.1
75.1
77.4
90.1
2.6
1.9
0.9
1.4
1.7
0.8
1.6
1.5
1.2
2.5
1.5
All SFE extractions were carried out at 35 C (95 F); atm, atmospheric pressure (76 cm Hg).
MeOH, methanol; IPA, isoprpyl alcohol; LAH, light aromtic hydrocarbon mixture.
Standard deviation; seven extractions were done for each experiment.
methanol/toluene mixture to remove the remaining
highly polar organic compounds based on the procedures 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 CO2 plus modi®er (LAH). Details of core
¯ood experiments have been described previously
(Hwang and Ortiz, 1998).
4.1. Supercritical ¯uid extraction of crude oil with CO2
only
can be expected. However, the pressure of CO2 used in a
CO2 ¯ood in an oil ®eld is limited by its reservoir pressure 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 CO2 density, 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 CO2 only, 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.
4.1.1. Pressure eect
Crude oil is a complex mixture of tens of thousands of
saturated and aromatic hydrocarbons and non-hydrocarbons (asphaltics). These crude oil components dier
signi®cantly in their physico-chemical properties, such
as molecular weight, density, and polarity. Their susceptibility to extraction by supercritical CO2 ¯uid (and
hence their extraction yields) vary widely. Fig. 3 shows
that both the density and extraction eciency of CO2 at
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 eciency of crude oil.
With regard to oil recovery using CO2 in the ®eld, the
higher the pressure (density), the better the recovery that
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 complex parameter for controlling extraction. At the low
temperature range (30±100 C) commonly seen for petroleum 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 100 C than those at 45 C when
pressure is less than 270 atm (4000 psi). Given the low
pressure 75 atm (1100 psi) and low temperature
32 C (90 F) of the McElroy reservoir, increases in
CO2 polarity and density by adding chemical modi®ers
would improve the eciency of oil extraction during a
CO2 ¯ood.
4. Results and discussion
R.J. Hwang, J. Ortiz / Organic Geochemistry 31 (2000) 1451±1462
Fig. 3. CO2 extraction eciency of McElroy crude oil (at
100 C) as function of its density.
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, isopropyl alcohol (IPA), hexane, toluene, methanol/
toluene mixtures, and light aromatic hydrocarbons
(LAH) mixtures.
1455
increasing carbon number. Little or no hydrocarbons
heavier than C22 were extracted. Abundant hydrocarbons heavier than C12 remain in the extracted rock
(Fig. 6a), providing evidence that the extraction using
pure CO2 is mainly eective for gasoline range hydrocarbons. 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 ranges 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 intermediate (diesel) and heavy hydrocarbons, which is
expected to lead to greater oil recovery when applied to
®eld production.
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 modi®ed CO2 displays enhanced solvating power and greater
extraction eciency of many organic compounds (Levy
et al., 1993). This added extraction power is demonstrated by comparing supercritical ¯uid extractions of
McElroy oil from dolomite rock using CO2 with and
without chemical modi®ers. Under conditions of 31 C
and 80 atm, pure CO2 eectively extracts hydrocarbons
up to C12, as shown in Fig. 6b. For hydrocarbons heavier than C12, the extraction yields decrease rapidly with
4.2.2. Extraction yields
4.2.2.1. Effect of various modifiers. At all pressures studied, the SFE yields with chemically modi®ed CO2 are all
signi®cantly higher than with CO2 only (Table 1), indicating 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 eciency of 94.3% compared to 22.4% with CO2 only at
80 atm and 35 C. The increase in CO2 extraction eciency 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 eciency of CO2 considerably 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. 4. Variation of supercritical ¯uid density with (w) and
without (wo) addition of a chemical modi®er (methanol) as a
function of pressure (constant temperature of 45 C).
Fig. 5. Small temperature eect on eciency of supercritical CO2
¯uid extraction of McElroy oil from rock at various pressures.
1456
R.J. Hwang, J. Ortiz / Organic Geochemistry 31 (2000) 1451±1462
Fig. 6. Supercritical CO2 ¯uid extractions (SFE) of clean Grayburg dolomite rock mixed with McElroy produced oil. Gas chromatograms of (a) SFE extraction residue, (b) SFE extract, and (c) original McElroy crude.
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 CO2 is a very important factor in determining
economics of the operation.
Clearly, chemical modi®ers enable CO2 to attain high
extraction eciency that would otherwise require extremely high pressure to attain if CO2 only 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 CO2 pressure 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 CO2 alone and their eect would be equivalent to
reducing minimum miscibility pressure (MMP), the
threshold pressure for obtaining the miscible phase
between crude oil and CO2 in petroleum reservoirs. Thus,
the modi®ers appear to have potential not only to
increase eectiveness of CO2 sweeping but also to make
the CO2 ¯ood a viable approach for enhancing oil recovery 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
CO2 extraction eciency by adding chemical modi®ers
is greatly reduced, although the improvement is still
signi®cant. For example, the increase in extraction eciency 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 eciency of supercritical CO2 ¯uid is sensitive to the
amount of modi®ers present in the ¯uid. A positive
correlation has been observed between extraction eciency and concentration of a chemical modi®er. For
example, for toluene at 80 atm, the extraction eciency
increases drastically with increasing concentration of the
R.J. Hwang, J. Ortiz / Organic Geochemistry 31 (2000) 1451±1462
1457
Fig. 7. Supercritical (CO2+methanol) ¯uid extractions of clean Grayburg dolomite rock mixed with McElroy produced. Gas chromatograms of (a) SFE extraction residue, (b) SFE extract, and (c) original McElroy crude.
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 CO2 is
the optimum concentration because its extraction eciency 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 CO2 volume is reduced to one third (Fig.
11, Table 2). Reduction of CO2 volume was achieved by
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 35 C.
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. 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
variation of pressure.
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R.J. Hwang, J. Ortiz / Organic Geochemistry 31 (2000) 1451±1462
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
(%)
CO2 only
100/0
2.0
3.0
6.0
14.0
18.7
22.9
CO2/MeOH
90/10
0.7
1.4
2.1
69.9
69.0
75.5
CO2/MeOH/toluene
90/8/2
0.7
1.4
2.1
71.2
77.3
79.3
CO2/MeOH/toluene
90/2/8
1.0
2.0
3.0
82.6
86.5
89.6
CO2/IPA/toluene
90/3/7
1.0
2.0
3.0
80.1
87.2
86.6
CO2/toluene
90/10
0.7
1.4
2.1
86.4
93.7
93.5
CO2/hexane
90/10
0.7
1.4
2.1
82.1
84.1
83.0
CO2/IPA
90/10
1.0
2.0
3.0
78.1
82.9
82.2
CO2/LHA
90/10
1.0
2.0
3.0
80.1
88.9
91.4
a
SFE extractions were carried out at 80 atm, 35 C.
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 CO2 has 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.
Fig. 10. Eect of modi®er concentration on extraction eciency of CO2 at various pressures.
Fig. 11. Variation of extraction eciency with volume of
suerpcritical CO2 ¯uid used for oil extraction from dolomite
reservoir rocks.
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R.J. Hwang, J. Ortiz / Organic Geochemistry 31 (2000) 1451±1462
Table 3
Bulk composition of residual oils in extracted rocks from CO2 supercritical ¯uid extractiona
SFE solvent
Solvent composition
CO2/modi®ers
Pressure
(atm)b
Saturates
(%)
Aromatics
(%)
NSO
(%)
Asphaltenes
(%)
CO2 only
CO2 only
CO2 only
100/0
100/0
100/0
80
100
120
16.5
5.3
4.7
9.3
6.1
5.6
10.8
10.7
11.2
1.1
0.7
0.8
CO2/MeOH
CO2/MeOH
CO2/MeOH
90/10
90/10
90/10
80
100
120
5.5
4.6
4.0
5.2
4.4
3.9
9.5
9.1
8.9
0.7
0.7
0.7
CO2/MeOH/toluene
CO2/MeOH/toluene
CO2/MeOH/toluene
90/8/2
90/8/2
90/8/2
80
100
120
4.4
3.8
3.3
3.6
3.2
2.4
8.7
8.7
8.3
1.1
0.8
1.0
CO2/MeOH/toluene
CO2/MeOH/toluene
CO2/MeOH/toluene
90/2/8
90/2/8
90/2/8
80
100
120
2.4
2.5
3.3
1.8
2.3
1.8
7.2
6.7
6.4
0.9
0.8
0.7
CO2/IPA/toluene
CO2/IPA/toluene
CO2/IPA/toluene
90/3/7
90/3/7
90/3/7
80
100
120
±
2.5
1.6
±
2.6
2.3
6.6
7.6
6.8
0.8
0.7
0.7
CO2/toluene
CO2/toluene
CO2/toluene
90/10
90/10
90/10
80
100
120
1.6
1.3
1.6
0.6
0.6
0.6
3.4
3.0
3.9
0.8
0.8
0.8
CO2/LHA
CO2/LHA
CO2/LHA
90/10
90/10
90/10
80
100
120
0.8
1.1
0.5
1.1
1.6
0.7
6.7
7.2
5.3
0.7
0.6
0.6
33.1
22.2
13.0
1.0
Original crude oil
a
The composition was determined based on the original amount of crude oil spiked in the rocks prior to the supercritical CO2 ¯uid
extraction and was not normalized to the amount of recovery.
b
atm, atmospheric pressure (76 cm Hg); extraction temperature 35 C.
4.2.3. Extraction residues
All CO2 extractions 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-
Fig. 12. Reduction in asphaltics (NSO+asphaltenes) content
of residual oils as function of chemical modi®er. Asphaltics
content of the oil spiked in the rock matrix was 14%.
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 modi®ed CO2 led 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 temperature 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, 35 C), extraction
with CO2 plus 10% modi®ers yielded residual oils with
the asphaltics content ranging from 4 to 10%, which
compares favorably to 11.9% when CO2 only was used
in extraction (Fig. 12). This represents a reduction ranging 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
1460
R.J. Hwang, J. Ortiz / Organic Geochemistry 31 (2000) 1451±1462
Fig. 13. Hydrocarbon content of residual oils in extracted
rocks from supercritical ¯uid extraction with chemically modi®ed CO2 are much lower than that with CO2 only.
Fig. 15. 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 modi®er to extract
asphaltics and heavy hydrocarbons.
dual oils shift to high carbon numbers as the concentration of toluene in CO2 increases.
4.3. Core ¯ood
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 CO2 indicating
CO2 solvency for heavy hydrocarbons is improved as the
amount of toluene is increased.
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 extraction, but also reduce asphaltics deposition induced by
CO2 injection 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 extractions with various amounts of toluene as the modi®er
(Fig. 14). Distribution maxima of hydrocarbons of resi-
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. CO2 miscible displacements with and without
modi®ers were conducted in a 1.5 m long stacked McElroy core under reservoir conditions. The CO2 core ¯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 CO2 injection during the CO2 core ¯ood with
and without a modi®er. Observations were made previously in both laboratory core ¯oods and ®eld production that API gravity of produced oils increased with
increasing CO2 injection (Hwang and Ortiz, 1998). The
increase in API gravity was attributed to the results of
precipitation of asphaltics and heavy hydrocarbons
induced by CO2 injection. 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
CO2 only core ¯ood indicate the ability of the modi®er to
extract asphaltics and heavy hydrocarbons and to mitigate 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
R.J. Hwang, J. Ortiz / Organic Geochemistry 31 (2000) 1451±1462
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 distribution from an injector to a producer in the ®eld. Fig.
16 shows the asphaltics content of the extracts obtained
after both CO2 only ¯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 suggesting that majority of CO2 induced asphaltene precipitation in the ®eld occurs near injectors. In
comparison, the CO2/modi®er ¯ood yielded the asphaltics 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 CO2 only into oil reservoirs is likely to reduce
asphaltics deposition and pore plugging for increased
recovery.
1461
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 produced 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 contributed to the low injectivity problem.
The results of the current study suggest that supercritical CO2 ¯uid with chemical modi®ers can dissolve
some asphaltic components, minimize asphaltene precipitation, 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 CO2 only. 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 chemical modi®ers to CO2 is likely to increase economics
viability for the CO2 ¯ood for enhanced oil recovery.
5. Conclusions
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
with modi®er in CO2.
The results from the study suggest that supercritical
CO2 ¯uid with chemical modi®ers can dissolve some
asphaltic components, minimize asphaltene precipitation, and increase recovery of heavy hydrocarbons
(>C25) during a CO2 ¯ood. Chemical modi®ers appear
to have the potential to substantially improve the economics of CO2 miscible ¯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 CO2 only 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 CO2 would probably not be higher than when CO2 only is used. Because
of its enhanced extraction eciency, chemically modi®ed CO2 does not require as much volume as CO2 only
to achieve the same extraction yield, a fact which would
result in substantial savings in CO2 costs. Furthermore,
chemically modi®ed CO2 can 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
1462
R.J. Hwang, J. Ortiz / Organic Geochemistry 31 (2000) 1451±1462
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 reservoir rocks and hence potential improvement in injectivity
of wells during a CO2 ¯ood. Clearly, the potential economic 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 CO2 miscibility 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 CO2 only.
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. diPrimio, P. Taylor, and A. Wihelms.
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www.elsevier.nl/locate/orggeochem
Mitigation of asphaltics deposition during CO2 ¯ood by
enhancing CO2 solvency with chemical modi®ers
R.J. Hwang *, J. Ortiz
Chevron Research Technology Company, Richmond, CA 94802, USA
Abstract
CO2 injected 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 hydrocarbons 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 eciencies of CO2 and CO2 with 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 CO2 also shows improved extraction eciency and reduced asphaltic deposits. Under the pressure and temperature similar to McElroy reservoir conditions; chemically modi®ed CO2 yielded almost 3 times as much
oil extracts as those in extractions with CO2 only. 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; Monger, 1985; Mazzocchi et al., 1998). The organic deposits,
mostly asphaltic components of crude oils, can signi®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
* Corresponding author.
methods (such as acidization) are eective in remediating 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 maximize 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.
PII: S0146-6380(00)00082-6
1452
R.J. Hwang, J. Ortiz / Organic Geochemistry 31 (2000) 1451±1462
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 CO2 and minimizing asphaltene precipitation.
The SFE concept is important to certain enhanced oil
recovery processes, since the performance of these processes depends on the extraction of oil components by a
solvent that is typically in a supercritical state (Deo et
al., 1992). For instance, CO2 has been injected at pressures 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 CO2 is in a supercritical
state at reservoir temperatures 32 C (90 F), since the
critical temperature and pressure of CO2 are 31 C and
73 atm, respectively.
Supercritical CO2 is 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 CO2 causes the precipitation 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 CO2 area in the McElroy Field after initiation
of the ¯ood contained 50% less asphaltenes and displayed slightly higher API gravity than oils collected
prior to the ¯ood (Hwang and Ortiz, 1998). Furthermore, 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 contributed 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 supercritical ¯uid extraction (SFE) technique is the focus of
this study. Speci®cally, the study is to determine if chemically modi®ed CO2 can improve the extraction eciency 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 CO2 and 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 0 C
and one atm pressure. Under these conditions it is not
very eective as a solvent for liquids and solids; however, as pressure is increased Ð which causes an increase
in density Ð the extractive power of CO2 improves. The
solubility of organic compounds in CO2 increases with
higher density and temperature. Subcritical liquid CO2
at 20 C 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 31 C,
with an accompanying pressure increase, there is less
and less dierence between densities of the saturated gas
and liquid. At 31 C, the critical temperature, there is no
dierence in any of the physical properties, and in fact
two phases no longer exist. Above 31 C (88 F) 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)
Fig. 1. Phase diagram of CO2.
R.J. Hwang, J. Ortiz / Organic Geochemistry 31 (2000) 1451±1462
can have a signi®cant in¯uence on the extraction eciency of a supercritical ¯uid.
Injection of chemically modi®ed CO2 to oil ®elds
could potentially improve injectivity of a water-alternate-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 CO2 systems 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
CO2 in 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 pressure 75 atm and low temperature 32 C (Harris 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 CO2 was 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 illustrated in Fig. 2. It automatically measures and displays
Fig. 2. Block diagram of supercritical ¯uid extraction system.
1453
density of supercritical ¯uid under experimental conditions during extraction.
Oil extraction was taking place when supercritical
CO2 ¯uid ¯owed through the extraction cell at predetermined 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-pressure 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-extracted 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 CO2 were 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 CO2 to attain the desired concentration of the
modi®er. Mechanically rolling the cylinder in a horizontal position aids mixing of the components. SFE
experiments were carried out with CO2 plus modi®ers at
pressures of 80, 100 and 120 atmospheres. Five premixed 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 compression and expansion of the pump prior to extraction.
LAH is a hydrocracking product derived from reformating heavy resid in oil re®neries. It contains predominantly 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 immediately 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
1454
R.J. Hwang, J. Ortiz / Organic Geochemistry 31 (2000) 1451±1462
Table 1
Extraction yields of oil from dolomite using supercritical ¯uid CO2 with chemical modi®ers at various pressuresa
Solventb
CO2 only
CO2/MeOH
CO2/MeOH/toluene
CO2/MeOH/toluene
CO2/IPA/toluene
CO2/toluene
CO2/hexane
CO2/IPA
CO2/MeOH/toluene
CO2/MeOH/toluene
CO2/LAH
a
b
c
Solvent composition
CO2: modi®er
100/0
90/10
90/8/2
90/2/8
90/3/7
90/10
90/10
90/10
97.5/0.5/2
95/1/4
90/10
Yield (%)
80 atm
S. D.c
100 atm
S. D.c
120 atm
S. D.c
22.4
77.3
81.7
88.4
88.4
94.3
84.4
86.5
57.8
70.7
87.8
1.3
0.9
2.1
1.3
2.6
2.3
1.4
0.4
2.8
2.2
0.2
47.3
80.1
84.0
88.7
88.5
95.0
86.0
83.9
71.2
74.2
88.4
1.9
1.3
0.7
1.7
1.2
1.4
1.3
0.8
2.8
1.8
1.6
53.5
82.2
86.0
89.2
89.4
94.9
86.5
85.1
75.1
77.4
90.1
2.6
1.9
0.9
1.4
1.7
0.8
1.6
1.5
1.2
2.5
1.5
All SFE extractions were carried out at 35 C (95 F); atm, atmospheric pressure (76 cm Hg).
MeOH, methanol; IPA, isoprpyl alcohol; LAH, light aromtic hydrocarbon mixture.
Standard deviation; seven extractions were done for each experiment.
methanol/toluene mixture to remove the remaining
highly polar organic compounds based on the procedures 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 CO2 plus modi®er (LAH). Details of core
¯ood experiments have been described previously
(Hwang and Ortiz, 1998).
4.1. Supercritical ¯uid extraction of crude oil with CO2
only
can be expected. However, the pressure of CO2 used in a
CO2 ¯ood in an oil ®eld is limited by its reservoir pressure 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 CO2 density, 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 CO2 only, 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.
4.1.1. Pressure eect
Crude oil is a complex mixture of tens of thousands of
saturated and aromatic hydrocarbons and non-hydrocarbons (asphaltics). These crude oil components dier
signi®cantly in their physico-chemical properties, such
as molecular weight, density, and polarity. Their susceptibility to extraction by supercritical CO2 ¯uid (and
hence their extraction yields) vary widely. Fig. 3 shows
that both the density and extraction eciency of CO2 at
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 eciency of crude oil.
With regard to oil recovery using CO2 in the ®eld, the
higher the pressure (density), the better the recovery that
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 complex parameter for controlling extraction. At the low
temperature range (30±100 C) commonly seen for petroleum 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 100 C than those at 45 C when
pressure is less than 270 atm (4000 psi). Given the low
pressure 75 atm (1100 psi) and low temperature
32 C (90 F) of the McElroy reservoir, increases in
CO2 polarity and density by adding chemical modi®ers
would improve the eciency of oil extraction during a
CO2 ¯ood.
4. Results and discussion
R.J. Hwang, J. Ortiz / Organic Geochemistry 31 (2000) 1451±1462
Fig. 3. CO2 extraction eciency of McElroy crude oil (at
100 C) as function of its density.
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, isopropyl alcohol (IPA), hexane, toluene, methanol/
toluene mixtures, and light aromatic hydrocarbons
(LAH) mixtures.
1455
increasing carbon number. Little or no hydrocarbons
heavier than C22 were extracted. Abundant hydrocarbons heavier than C12 remain in the extracted rock
(Fig. 6a), providing evidence that the extraction using
pure CO2 is mainly eective for gasoline range hydrocarbons. 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 ranges 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 intermediate (diesel) and heavy hydrocarbons, which is
expected to lead to greater oil recovery when applied to
®eld production.
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 modi®ed CO2 displays enhanced solvating power and greater
extraction eciency of many organic compounds (Levy
et al., 1993). This added extraction power is demonstrated by comparing supercritical ¯uid extractions of
McElroy oil from dolomite rock using CO2 with and
without chemical modi®ers. Under conditions of 31 C
and 80 atm, pure CO2 eectively extracts hydrocarbons
up to C12, as shown in Fig. 6b. For hydrocarbons heavier than C12, the extraction yields decrease rapidly with
4.2.2. Extraction yields
4.2.2.1. Effect of various modifiers. At all pressures studied, the SFE yields with chemically modi®ed CO2 are all
signi®cantly higher than with CO2 only (Table 1), indicating 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 eciency of 94.3% compared to 22.4% with CO2 only at
80 atm and 35 C. The increase in CO2 extraction eciency 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 eciency of CO2 considerably 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. 4. Variation of supercritical ¯uid density with (w) and
without (wo) addition of a chemical modi®er (methanol) as a
function of pressure (constant temperature of 45 C).
Fig. 5. Small temperature eect on eciency of supercritical CO2
¯uid extraction of McElroy oil from rock at various pressures.
1456
R.J. Hwang, J. Ortiz / Organic Geochemistry 31 (2000) 1451±1462
Fig. 6. Supercritical CO2 ¯uid extractions (SFE) of clean Grayburg dolomite rock mixed with McElroy produced oil. Gas chromatograms of (a) SFE extraction residue, (b) SFE extract, and (c) original McElroy crude.
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 CO2 is a very important factor in determining
economics of the operation.
Clearly, chemical modi®ers enable CO2 to attain high
extraction eciency that would otherwise require extremely high pressure to attain if CO2 only 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 CO2 pressure 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 CO2 alone and their eect would be equivalent to
reducing minimum miscibility pressure (MMP), the
threshold pressure for obtaining the miscible phase
between crude oil and CO2 in petroleum reservoirs. Thus,
the modi®ers appear to have potential not only to
increase eectiveness of CO2 sweeping but also to make
the CO2 ¯ood a viable approach for enhancing oil recovery 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
CO2 extraction eciency by adding chemical modi®ers
is greatly reduced, although the improvement is still
signi®cant. For example, the increase in extraction eciency 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 eciency of supercritical CO2 ¯uid is sensitive to the
amount of modi®ers present in the ¯uid. A positive
correlation has been observed between extraction eciency and concentration of a chemical modi®er. For
example, for toluene at 80 atm, the extraction eciency
increases drastically with increasing concentration of the
R.J. Hwang, J. Ortiz / Organic Geochemistry 31 (2000) 1451±1462
1457
Fig. 7. Supercritical (CO2+methanol) ¯uid extractions of clean Grayburg dolomite rock mixed with McElroy produced. Gas chromatograms of (a) SFE extraction residue, (b) SFE extract, and (c) original McElroy crude.
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 CO2 is
the optimum concentration because its extraction eciency 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 CO2 volume is reduced to one third (Fig.
11, Table 2). Reduction of CO2 volume was achieved by
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 35 C.
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. 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
variation of pressure.
1458
R.J. Hwang, J. Ortiz / Organic Geochemistry 31 (2000) 1451±1462
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
(%)
CO2 only
100/0
2.0
3.0
6.0
14.0
18.7
22.9
CO2/MeOH
90/10
0.7
1.4
2.1
69.9
69.0
75.5
CO2/MeOH/toluene
90/8/2
0.7
1.4
2.1
71.2
77.3
79.3
CO2/MeOH/toluene
90/2/8
1.0
2.0
3.0
82.6
86.5
89.6
CO2/IPA/toluene
90/3/7
1.0
2.0
3.0
80.1
87.2
86.6
CO2/toluene
90/10
0.7
1.4
2.1
86.4
93.7
93.5
CO2/hexane
90/10
0.7
1.4
2.1
82.1
84.1
83.0
CO2/IPA
90/10
1.0
2.0
3.0
78.1
82.9
82.2
CO2/LHA
90/10
1.0
2.0
3.0
80.1
88.9
91.4
a
SFE extractions were carried out at 80 atm, 35 C.
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 CO2 has 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.
Fig. 10. Eect of modi®er concentration on extraction eciency of CO2 at various pressures.
Fig. 11. Variation of extraction eciency with volume of
suerpcritical CO2 ¯uid used for oil extraction from dolomite
reservoir rocks.
1459
R.J. Hwang, J. Ortiz / Organic Geochemistry 31 (2000) 1451±1462
Table 3
Bulk composition of residual oils in extracted rocks from CO2 supercritical ¯uid extractiona
SFE solvent
Solvent composition
CO2/modi®ers
Pressure
(atm)b
Saturates
(%)
Aromatics
(%)
NSO
(%)
Asphaltenes
(%)
CO2 only
CO2 only
CO2 only
100/0
100/0
100/0
80
100
120
16.5
5.3
4.7
9.3
6.1
5.6
10.8
10.7
11.2
1.1
0.7
0.8
CO2/MeOH
CO2/MeOH
CO2/MeOH
90/10
90/10
90/10
80
100
120
5.5
4.6
4.0
5.2
4.4
3.9
9.5
9.1
8.9
0.7
0.7
0.7
CO2/MeOH/toluene
CO2/MeOH/toluene
CO2/MeOH/toluene
90/8/2
90/8/2
90/8/2
80
100
120
4.4
3.8
3.3
3.6
3.2
2.4
8.7
8.7
8.3
1.1
0.8
1.0
CO2/MeOH/toluene
CO2/MeOH/toluene
CO2/MeOH/toluene
90/2/8
90/2/8
90/2/8
80
100
120
2.4
2.5
3.3
1.8
2.3
1.8
7.2
6.7
6.4
0.9
0.8
0.7
CO2/IPA/toluene
CO2/IPA/toluene
CO2/IPA/toluene
90/3/7
90/3/7
90/3/7
80
100
120
±
2.5
1.6
±
2.6
2.3
6.6
7.6
6.8
0.8
0.7
0.7
CO2/toluene
CO2/toluene
CO2/toluene
90/10
90/10
90/10
80
100
120
1.6
1.3
1.6
0.6
0.6
0.6
3.4
3.0
3.9
0.8
0.8
0.8
CO2/LHA
CO2/LHA
CO2/LHA
90/10
90/10
90/10
80
100
120
0.8
1.1
0.5
1.1
1.6
0.7
6.7
7.2
5.3
0.7
0.6
0.6
33.1
22.2
13.0
1.0
Original crude oil
a
The composition was determined based on the original amount of crude oil spiked in the rocks prior to the supercritical CO2 ¯uid
extraction and was not normalized to the amount of recovery.
b
atm, atmospheric pressure (76 cm Hg); extraction temperature 35 C.
4.2.3. Extraction residues
All CO2 extractions 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-
Fig. 12. Reduction in asphaltics (NSO+asphaltenes) content
of residual oils as function of chemical modi®er. Asphaltics
content of the oil spiked in the rock matrix was 14%.
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 modi®ed CO2 led 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 temperature 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, 35 C), extraction
with CO2 plus 10% modi®ers yielded residual oils with
the asphaltics content ranging from 4 to 10%, which
compares favorably to 11.9% when CO2 only was used
in extraction (Fig. 12). This represents a reduction ranging 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
1460
R.J. Hwang, J. Ortiz / Organic Geochemistry 31 (2000) 1451±1462
Fig. 13. Hydrocarbon content of residual oils in extracted
rocks from supercritical ¯uid extraction with chemically modi®ed CO2 are much lower than that with CO2 only.
Fig. 15. 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 modi®er to extract
asphaltics and heavy hydrocarbons.
dual oils shift to high carbon numbers as the concentration of toluene in CO2 increases.
4.3. Core ¯ood
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 CO2 indicating
CO2 solvency for heavy hydrocarbons is improved as the
amount of toluene is increased.
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 extraction, but also reduce asphaltics deposition induced by
CO2 injection 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 extractions with various amounts of toluene as the modi®er
(Fig. 14). Distribution maxima of hydrocarbons of resi-
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. CO2 miscible displacements with and without
modi®ers were conducted in a 1.5 m long stacked McElroy core under reservoir conditions. The CO2 core ¯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 CO2 injection during the CO2 core ¯ood with
and without a modi®er. Observations were made previously in both laboratory core ¯oods and ®eld production that API gravity of produced oils increased with
increasing CO2 injection (Hwang and Ortiz, 1998). The
increase in API gravity was attributed to the results of
precipitation of asphaltics and heavy hydrocarbons
induced by CO2 injection. 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
CO2 only core ¯ood indicate the ability of the modi®er to
extract asphaltics and heavy hydrocarbons and to mitigate 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
R.J. Hwang, J. Ortiz / Organic Geochemistry 31 (2000) 1451±1462
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 distribution from an injector to a producer in the ®eld. Fig.
16 shows the asphaltics content of the extracts obtained
after both CO2 only ¯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 suggesting that majority of CO2 induced asphaltene precipitation in the ®eld occurs near injectors. In
comparison, the CO2/modi®er ¯ood yielded the asphaltics 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 CO2 only into oil reservoirs is likely to reduce
asphaltics deposition and pore plugging for increased
recovery.
1461
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 produced 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 contributed to the low injectivity problem.
The results of the current study suggest that supercritical CO2 ¯uid with chemical modi®ers can dissolve
some asphaltic components, minimize asphaltene precipitation, 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 CO2 only. 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 chemical modi®ers to CO2 is likely to increase economics
viability for the CO2 ¯ood for enhanced oil recovery.
5. Conclusions
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
with modi®er in CO2.
The results from the study suggest that supercritical
CO2 ¯uid with chemical modi®ers can dissolve some
asphaltic components, minimize asphaltene precipitation, and increase recovery of heavy hydrocarbons
(>C25) during a CO2 ¯ood. Chemical modi®ers appear
to have the potential to substantially improve the economics of CO2 miscible ¯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 CO2 only 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 CO2 would probably not be higher than when CO2 only is used. Because
of its enhanced extraction eciency, chemically modi®ed CO2 does not require as much volume as CO2 only
to achieve the same extraction yield, a fact which would
result in substantial savings in CO2 costs. Furthermore,
chemically modi®ed CO2 can 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
1462
R.J. Hwang, J. Ortiz / Organic Geochemistry 31 (2000) 1451±1462
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 reservoir rocks and hence potential improvement in injectivity
of wells during a CO2 ¯ood. Clearly, the potential economic 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 CO2 miscibility 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 CO2 only.
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. diPrimio, P. Taylor, and A. Wihelms.
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