energies-12-04308.pdf

energies

Article

Huff-n-Puff Experimental Studies of CO2 with
Heavy Oil

Evgeny Shilov 1,*,† , Alexey Cheremisin 1 , Kirill Maksakov 2 and Sergey Kharlanov 3

1 Center for Hydrocarbon Recovery, Skolkovo Institute of Science and Technology, Moscow 121205, Russia;
[email protected]

2 Department of Technology Development for High Viscosity and Heavy Oils, LUKOIL-Engineering LCC,
Moscow 109028, Russia; [email protected]

3 Management Department of Scientific and Technical Projects, RITEK JSC, Moscow 115035, Russia;
[email protected]

* Correspondence: [email protected]
† Current address: Skolkovo Innovation Center, Bolshoy Bulvar 30-1, Moscow 121205, Russia.

Received: 30 September 2019; Accepted: 6 November 2019; Published: 12 November 2019 �����������������

Abstract: This work is devoted to CO2 Huff-n-Puff studies on heavy oil. Oil recovery for heavy oil
reservoirs is sufficiently small in comparison with conventional reservoirs, and, due to the physical
limitation of oil flow through porous media, a strong need for better understanding of tertiary
recovery mechanisms of heavy oil exists. Notwithstanding that the idea of Huff-n-Puff gas injection
technology for enhanced oil recovery has existed for dozens of years, there is still no any precise
methodology for evaluating the applicability and efficiency of this technology in heavy oil reservoirs.
Oil recovery factor is a question of vital importance for heavy oil reservoirs. In this work, we repeated
Huff-n-Puff tests more than three times at five distinct pressure points to evaluate the applicability
and efficiency of CO2 Huff-n-Puff injection to the heavy oil reservoirs. Additionally, the most critical
factor that affects oil recovery in gas injection operation is the condition of miscibility. Experimental
data allowed to distinguish the mixing zone of the light fractions of studied heavy oil samples.
The experimental results showed that the pressure increase in the Huff-n-Puff injection process does
not affect the oil recovery when the injection pressure stays between miscibility pressure of light
components of oil and minimum miscibility pressure. It was detected that permeability decreases
after Huff-n-Puff CO2 tests.

Keywords: enhanced oil recovery; CO2 injection; heavy oil; Huff-n-Puff; miscibility evaluation;
permeability reduction

1. Introduction

The continuous increase in energy consumption [1] enhances the total demand for every type of
hydrocarbon resource [2,3]. Despite the very challenging production of oil from heavy oil reservoirs,
heavy oil is becoming a very prospective and valuable source of energy these days. Combined with
the facts that (1) oil production from conventional reservoirs is decreasing [4], (2) tremendous heavy
reserves are more than 3 times bigger than conventional oil reserves [5], and (3) tertiary oil recovery
techniques are advancing each year [6,7], we predict that more and more studies about heavy oil will
be published in the upcoming years.

This work is devoted to the experimental studies of CO2 applicability, efficiency, and miscibility
with heavy oil by Huff-n-Puff tests. Oil recovery in heavy oil reservoirs is sufficiently small in
comparison with oil recovery from conventional reservoirs due to the physical limitation of the oil flow
through the porous media. As heavy oil reservoirs do not effectively respond to secondary recovery

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methods, the tertiary enhanced oil recovery techniques are the only practical and possible options.
Therefore, a need for better understanding of tertiary recovery mechanisms for heavy oil reservoirs
exists nowadays. Notwithstanding that the idea of gas injection technology for enhanced oil recovery
operations, either in flooding or the Huff-n-Puff regime, was developed long time ago [8], and the
first experimental studies of CO2 applicability in heavy oil reservoirs were conducted in the 1980s
and 1990s [9,10], there is no precise methodology for the evaluation of applicability and efficiency of
Huff-n-Puff gas injection technology in heavy oil reservoirs [11].

Huff-n-puff CO2 injection is an enhanced oil recovery technique that is widely used for
conventional oil reservoirs [12] and is known as cyclic injection of CO2. The Huff-n-Puff injection is
presented by three divided stages: (1) the injection stage, (2) the soaking stage, and (3) the production
stage [9]; all three stages will be experimentally modeled. Oil recovery factor is a question of vital
importance that petroleum companies have about hard to recover resources. The most critical factor
that affects oil recovery in any gas injection operation is the condition of miscibility [13]. The injected
gas can be either in the miscible or immiscible regime. Commonly, gas injection over minimum
miscibility pressure (MMP) is considered as the most favorable regime with the highest possible
oil recovery [14]. Miscibility depends on the number of parameters like oil composition, swelling
coefficient, diffusion, solubility, and mainly on pressure, which is above MMP [15]. MMP is an
indication of pressure, temperature, or gas composition when gas and oil are completely miscible in
any proportions [16].

Regarding the light oil of conventional reservoirs, the determination of MMP is not very
challenging: MMP of CO2 and oil can be predicted by empirical correlations, numerical simulations,
and rapid experimental techniques. Most of the empirical correlations were made with the help of
experimental data obtained from experiments performed with light oil. Therefore, they are hardly
applicable to heavy oil [17]. For the proper numerical simulation, the composition of the oil is needed.
However, the composition of heavy oil is very hard to precisely determine, and it is sufficiently different
in its behavior from field to field. Due to the high compositional differences of heavy oil, it tends to
have two distinct miscibility pressures at which we can reach miscibility, with light components of oil
first, and only then with heavy components of oil. In the most cases related to heavy oil reservoirs,
researches assume that heavy oil is not miscible because MMP in heavy oil reservoir is usually much
higher than the reservoir pressure [18–20]. However, it also coincides with another problem—MMP
determination for heavy oil is very challenging. Traditionally, MMP for the light oil can be determined
by such conventional methods as the slim tube, Vanishing Interfacial Tension (VIT), and Rising Bubble
Apparatus (RBA) tests. However, these techniques fail when dealing with high viscosity oil. The
industrially accepted method, the slim tube test, is not physically affordable for high viscosity oil [21].

To evaluate heavy oil recovery by Huff-n-Puff CO2 injection and evaluate the miscibility conditions
of CO2 and heavy oil, we implement the Huff-n-Puff gas injection tests. The studied pressure range was
restricted to the field injection limitations. Obtaining oil recovery factors after five Huff-n-Puff cycles of
five different pressure points expanded our knowledge on the Huff-n-Puff process and its efficiency for
heavy oil reserves. Forty Huff-n-Puff gas injection tests were conducted in total. Experiments coincide
with other studies where heavy oil is mainly extracted by the first three cycles of CO2 Huff-n-Puff
injection [11,22]. Then, the received data allowed to distinguish the mixing zone for the light fractions
of studied oil samples with CO2, proving that it is applicable for possible miscibility evaluation.
Experiments shown no difference for oil recovery when injection pressure is between miscibility
pressure of light components and MMP. Additionally, the change of oil composition after Huff-n-Puff
tests was analyzed. Permeability reduction was observed by running core flood experiments with
different solvents.

2. Materials

In this study, wellhead oil samples from heavy oilfields #1 and #2 were provided by
LUKOIL-Engineering; the oil samples were free from gas. Reservoir properties are presented in Table 1.

Energies 2019, 12, 4308 3 of 15

As some part of the experimental procedure is meant to be conducted under atmospheric pressure,
there was no need to make recombined oil samples. Berea Sandstone core plugs were used for these
Huff-n-Puff tests. The main properties of dead oil samples were measured by PVT 3000-L (Chandler
Engineering, Tulsa, OK, USA) and are presented in Table 2. The compositions of the oil samples
were measured by gas chromatography/mass spectrometry and presented in Table 3. The initial
water content of the oilfield #1 and oilfield #2 oil samples was over 20%. Therefore, the samples
were dewatered by (1) thermostabilization at 90 ◦C for 72 h and (2) the centrifuge, PrO-ASTM BC
(Centurion Scientific Ltd., Chichester, UK), at 75 ◦C for 80 min. Every core sample was drilled from
one homogeneous block of Berea sandstone. Core diameters, lengths, and helium permeability are
presented in Table 4. The volume difference of used samples does not exceed 20%, the porosity is
~20%, and permeability ranges from 154 to 175 mD. The injection gas in these Huff-n-Puff tests is CO2
with purity greater than 99.99%.

Table 1. Reservoir properties.

Reservoir Oilfield #1 Oilfield #2

Temperature ( ◦C) 27.4 21.6
Pressure (MPa) 13.8 10.9

Table 2. Measured properties of dead oil samples from oilfield #1 and oilfield #2.

Dead Oil Sample Oilfield #1 Oilfield #2

Dead oil density at 20.0 ◦C (kg m3) 1006.70 931.03
Dead oil viscosity at 25.0 ◦C (mPa s) 1116.00 421.8
Dead oil molecular weight (g mol−1) 346.74 499.51
Dead oil compressibility (1/MPa) 4.45 × 10−4 6.29 × 10−4
Water content after dewatering (%) 0.78 0.33
Paraffin crystallization temperature at 0.2 MPa ( ◦C) 30.4 33.1

Table 3. Results of compisitional analysis of the dead oil samples from oilfield #1 and oilfield #2.

Component
Oilfield #1 Oilfield #2

Component
Oilfield #1 Oilfield #2

wt.% mol.% wt.% mol.% wt.% mol.% wt.% mol.%

H 0 0.0003 0 0 C17 0.0784 0.1651 0.3508 0.5132
H2S 0.0012 0.0170 0 0 C18 0.0993 0.1975 0.3914 0.5407
CO2 0.0135 0.1532 0.0005 0.0040 C19 0.0726 0.1378 0.2503 0.3299
N2 0.2919 5.2056 0.0188 0.2330 C20 0.0636 0.1156 0.2486 0.3134
C1 0.0207 0.6442 0.0139 0.2996 C21 0.0577 0.0991 0.2079 0.2477
C2 0.0266 0.4413 0.0460 0.5304 C22 0.0431 0.0706 0.1984 0.2256
C3 0.0310 0.3511 0.4608 3.6235 C23 0.0399 0.0627 0.1891 0.2062
iC4 0.0128 0.1104 0.0442 0.2639 C24 0.0358 0.0541 0.1687 0.1768
nC4 0.0349 0.2997 0.7141 4.2601 C25 0.0322 0.0466 0.1425 0.1432
iC5 0.0281 0.1948 0.2672 1.2843 C26 0.0314 0.0436 0.1360 0.1313
nC5 0.0266 0.1844 0.5736 2.7567 C27 0.0251 0.0335 0.1050 0.0973
C6 0.1033 0.6143 1.5685 6.4746 C28 0.0219 0.0282 0.0856 0.0765
C7 0.1189 0.6185 1.5188 5.4856 C29 0.0188 0.0234 0.0640 0.0552
C8 0.1040 0.4857 0.8171 2.6479 C30 0.0162 0.0195 0.0493 0.0411
C9 0.0453 0.1870 0.2532 0.7256 C31 0.0144 0.0168 0.0459 0.0370
C10 0.0465 0.1733 0.2283 0.5907 C32 0.0138 0.0156 0.0352 0.0275
C11 0.0484 0.1644 0.2064 0.4869 C33 0.0122 0.0134 0.0147 0.0111
C12 0.0495 0.1535 0.2478 0.5336 C34 0.0087 0.0092 0.0108 0.0079
C13 0.0467 0.1333 0.2829 0.5606 C35 0.0066 0.0068 0.0081 0.0058
C14 0.0666 0.1751 0.2864 0.5227 C36+ 98.0613 88.229 89.2027 64.6417
C15 0.0661 0.1603 0.2775 0.4671 Total 100 100 100 100
C16 0.0643 0.1447 0.2689 0.4201

Energies 2019, 12, 4308 4 of 15

Table 4. Summary of physical properties of the cores applied in this study.

Sample No.
Length Diameter Volume Surface Area/Volume Porosity Permeability

cm cm cm3 m2/cm3 % mD

1 4.819 2.940 32.72 1.64 20.14 153.5
2 4.760 2.941 32.34 1.65 20.21 157.6
3 4.764 2.941 32.36 1.65 20.16 153.2
4 4.725 2.938 32.03 1.65 20.16 156.4
5 4.698 2.943 31.96 1.65 20.15 156.5
6 4.503 2.941 30.58 1.66 20.35 168.3
7 4.375 2.940 29.71 1.67 20.40 173.7
8 4.371 2.940 29.68 1.67 20.42 174.9
9 4.282 2.941 29.08 1.68 20.42 171.4
10 3.892 2.937 26.36 1.71 20.63 174.8

3. Experimental Setup

The assemblies used in saturation operations and Huff-n-Puff tests are presented in Figures 1 and
2, respectively. The assemblies are presented by two piston vessels, a climate chamber, vacuum pump,
and the pump. The pump, Quizix QX (Chandler Engineering, Tulsa, OK, USA), is a piston pump with
two cylinders that works in the regime of continuous delivery and receiving; it pushes the piston of
the cylinder filled by oil or gas and replaces the liquid into high-pressure stainless steel container 1,
which is filled by the vertically placed rock samples. The samples are placed on the piston inside the
annulus of container 1, and they are free from any sleeves. The piston is adjusted on the desired level
by water on the bottom side of container 1. The pump allowed to inject the gas at the pressure up to
24 MPa. The climate chamber, MK 240 (Binder GmbH, Tuttlingen, Germany), maintained the reservoir
temperature with fluctuations up to 0.5 ◦C.

Version October 28, 2019 submitted to Energies 4 of 16

Table 4. Summary of physical properties of the cores applied in this study.

Sample no. Length Diameter Volume Surface Area/Volume Porosity Permeabilitycm cm cm3 m2∕cm3 % mD
1 4.819 2.940 32.72 1.64 20.14 153.5
2 4.760 2.941 32.34 1.65 20.21 157.6
3 4.764 2.941 32.36 1.65 20.16 153.2
4 4.725 2.938 32.03 1.65 20.16 156.4
5 4.698 2.943 31.96 1.65 20.15 156.5
6 4.503 2.941 30.58 1.66 20.35 168.3
7 4.375 2.940 29.71 1.67 20.40 173.7
8 4.371 2.940 29.68 1.67 20.42 174.9
9 4.282 2.941 29.08 1.68 20.42 171.4
10 3.892 2.937 26.36 1.71 20.63 174.8

Climate
Chamber

Co
nta

ine
r1

Co
nta

ine
r2

Pump

Vacuum
PumpOil

WaterWater

Figure 1. Schematic of experimental setup used for saturation.

3. Experimental setup81
The assemblies used in saturation operations and Huff-n-Puff tests are presented on fig. 1 and fig. 282

respectively. The assemblies are presented by two piston vessels, climate chamber, vacuum pump, and the pump.83
The pump, Quizix QX (Chandler Engineering), is a piston pump with two cylinders that can work in the regime84
of continuous delivery and receiving; it pushes the piston of the cylinder filled by oil or gas and replaces the85
liquid into high-pressure stainless steel container 1 filled by the rock samples placed vertically. The samples are86
placed on the piston inside the annulus of container 1; and they are free from any sleeves. The piston is adjusted87
on the desired level by water on the bottom side of container 1. The pump allowed to inject the gas at the pressure88
up to 24 MPa. Climate chamber, MK 240 (Binder), allowed to keep the reservoir temperature with fluctuation up89
to 0.5 ◦C.90
4. Experimental procedures91

The experimental procedure [23] was adopted in this work to the experiments with heavy oil. In the original92
procedure, the Huff-n-Puff injection of gas happens at each pressure regimes with every shale core sample.93
Despite the real rock samples had moderately high porosity-permeability, it was decided to work with highly94
homogeneous samples of Berea sandstone to speed up the original procedure of CO2 Huff-n-Puff injection at95
different pressure regimes. The experimental procedure is presented by the step of core saturation (1) and by the96
part of Huff-n-Puff gas injection tests (2).97

Figure 1. Schematic of experimental setup used for saturation.

Energies 2019, 12, 4308 5 of 15Version October 28, 2019 submitted to Energies 5 of 16

Climate
Chamber

Co
nta

ine
r1

Co
nta

ine
r2

CO2

Pump

WaterWater

Figure 2. Schematic of experimental setup for the CO2 Huff-n-Puff test.

4.1. Saturation Step98

In order to saturate the samples, the collection of samples was extracted by kerosen and then dried in the99
oven under 120 ◦C during 24 hours. The weight (Wd) of dried core samples was measured. Then the samples100
were placed in the container 1 (fig. A2a) and vacuumed in the container during 6 hours. The saturation process101
was conducted at the temperature two times bigger than the temperature of paraffin crystallization (table 2). At102
the next stage Quizix QX pump pushed the piston of container 2 and oil filled container 1 with samples with the103
speed of 5 ml/min and restored the reservoir pressure. The pressure of container 2 was maintained for at least104
24 hours (section A). Then the core samples were taken out, cleaned by cleaning paper and the weight of each105
sample was measured and registered as Ws. Then the samples were stored in desiccator to keep the oil in the106
samples. The core samples were saturated by 100% with oil without connate water.107
4.2. Huff-n-Puff gas injection procedure108

Part of Huff-n-Puff gas injection tests is presented on fig. 2 and should simulate the Huff-n-Puff injection of109
CO2 in the oil field. Initially, the air in the container 1 is blown off by multiple reinjections of CO2 from external110
gas cylinder under 0.25 MPa. Then the lines between the containers are vacuumed by a vacuum pump. Then the111
CO2 in injected from container 2 to container 1 during 30 minutes, and then the pressure should be raised to the112
experimental pressure by piston pump if it is needed. The soaking time is equal to 6 hours. After the soaking113
period, the pressure should be released with depletion rate of 0.5 MPa/min to the atmospheric pressure. Then the114
production period is equal to 6 hours. So in the end, the the samples are taken out from the vessel (fig. A2b) and115
cleaned (fig. A3). Then weight of the core samples is measured (Wi). The next injection cycle starts immediately116
after weight measuring. The same procedure is repeated for all subsequent cycles after the first cycle of injection.117
The weight measurements are done to estimate oil recovery using the following eq. (1). But the eq. (1) does not118
estimate the density of original oil is similar to the remaining oils at different stages.119

Oil Recovery in cycle i =
Wi − Wd
Ws − Wd

× 100% (1)
In the original procedure developed for the shale samples, each sample should go through all the injection120

pressure regimes. However, in our case, homogeneous Berea sandstone samples were used. Despite high121
similarities in sizes, porosity and permeability among the samples, oil recovery of each sample should be122
normalized according to their surface area and volume using eq. (2). The first injection cycle yielded the highest123
incremental oil recovery from 20.70% to 27.67% for different injection pressure regimes. The porosity has not124
been used in this normalizing factor since it is taken into account in the oil recovery values itself.125

Oil Recoverynormalized =
Oil Recoverycycle i

Surface Area
V olume

×
Mean Surface Area

Mean Volume (2)

Figure 2. Schematic of experimental setup for the CO2 Huff-n-Puff test.

4. Experimental Procedures

The experimental procedure used in this work [23] was adapted to the experiments with heavy
oil. In the original procedure, the Huff-n-Puff injection of gas occurs at each pressure regime with
every shale core sample. Despite the real rock samples having moderately high porosity–permeability,
we decided to use highly homogeneous samples of Berea sandstone to speed up the original procedure
of CO2 Huff-n-Puff injection at different pressure regimes. The experimental procedure presented has
two steps: (1) core saturation and (2) the Huff-n-Puff gas injection tests.

4.1. Saturation Step

To saturate the samples, the collection of samples was extracted by kerosene and then dried
in the oven under 120 ◦C during 24 h. The weight (Wd) of the dried core samples was measured.
Then, the samples were placed in the container 1 (Appendix A) and vacuumed in the container for
6 h. The saturation process was conducted at a temperature two times greater than the temperature
of paraffin crystallization (Table 2). At the next stage, the Quizix QX pump pushed the pistons of
container 2 and oil-filled container 1 with samples at a speed of 5 mL/min and restored the reservoir
pressure. The pressure of container 2 was maintained for at least 24 h (Appendix B). Then, the core
samples were taken out, cleaned by cleaning paper, and the weight of each sample was measured and
registered as Ws. Then, the samples were stored in desiccator to keep the oil in the samples between
the experiments. The core samples were saturated to 100% with oil without connate water.

4.2. Huff-n-Puff Gas Injection Procedure

Part of the Huff-n-Puff gas injection tests is presented in Figure 2 and should simulate the
Huff-n-Puff injection of CO2 in the oil field. Initially, the air in container 1 is blown off by multiple
reinjections of CO2 from the external gas cylinder under 0.25 MPa. Then, the lines between the
containers are vacuumed by a vacuum pump. Then, the CO2 is injected from container 2 to container
1 for 30 min, and then the pressure should be raised to the experimental pressure by piston pump
if it is needed. The soaking time is 6 h. After the soaking period, the pressure should be released
with depletion rate of 0.5 MPa/min to the atmospheric pressure. Then, the production period is 6 h.
Therefore, finally, the samples are taken out from the vessel (Figure A1b) and cleaned (Figure A2).
Then, the core samples are weighed (Wi). The next injection cycle starts immediately after weighing.

Energies 2019, 12, 4308 6 of 15

The same procedure is repeated for all subsequent cycles after the first cycle of injection. The weight
measurements are done to estimate oil recovery using Equation (1). However, Equation (1) does not
estimate the density of the original oil as being similar to the remaining oils at different stages.

Oil Recovery in cycle i =
Wi − Wd
Ws − Wd

× 100% (1)

In the original procedure developed for the shale samples, each sample should go through all of
the injection pressure regimes. However, in our case, homogeneous Berea sandstone samples were
used. Despite the high similarity in size, porosity, and permeability among the samples, oil recovery
of each sample should be normalized according to their surface area and volume using Equation (2).
The first injection cycle yielded the highest incremental oil recovery from 20.70% to 27.67% for different
injection pressure regimes. The porosity has not been used in this normalizing factor as it is taken into
account in the oil recovery values itself.

Oil Recoverynormalized =
Oil Recoverycycle i

Surface Area
Volume

×
Mean Surface Area

Mean Volume (2)

5. Results and Discussion

5.1. Effect of Cycle Number

Each subsequent cycle of the Huff-n-Puff CO2 injection gives less incremental oil recovery than
the previous cycle. Experiments with oil from oilfield #1 was conducted earlier than experiment with
oil from oilfield #2. The biggest amount of oil from oilfield #1 is extracted during the first three cycles
of injection, and then just a small volume of oil can be produced. This is why we only use three cycles
of injection instead of six cycles in the experiments with oil from oilfield #2. Incremental oil recovery
for each injection cycle is shown in Tables 5 and 6 for both reservoirs. The decrease of subsequent
incremental oil recovery occurs because of the reduction of the contact area of CO2 and oil inside the
pore space, which caused the remaining oil to become heavier and more viscous (2) and the heavy oil
deposits to be destabilized, and they caused a decrease of permeability of the core samples (3). The first
two cycles can give an incremental recovery factor above 35%. After these cycles, the incremental oil
recovery does not usually exceed 3% in one cycle. Raw experimental data was previously published
by Maksakov and Cheremisin [24].

Table 5. Incremental oil recovery at each cycle of Huff-n-Puff CO2 injection for experiments with oil
from oilfield #1.

Sample No.
Pressure Cycle 1 Cycle 2 Cycle 3 Cycle 4 Cycle 5 Cycle 6

MPa % % % % % %

3 6 20.70 10.62 1.06 1.08 0.37 0.44
4 6 20.63 11.27 0.64 0.35 0.60 0.69
7 10 26.70 9.37 3.41 1.66 1.57 1.15
9 10 26.12 8.98 2.70 1.63 1.67 0.40
2 14 27.20 9.49 2.54 0.65 1.10 1.35
1 14 27.67 8.36 2.68 1.57 0.85 0.39
5 19 21.48 15.88 2.43 1.51 1.05 1.51
6 19 21.57 15.84 1.86 1.08 1.30 0.81
8 24 23.99 12.41 1.67 1.26 1.78 1.32
10 24 26.06 10.40 1.84 1.61 1.71 0.83

Energies 2019, 12, 4308 7 of 15

Table 6. Incremental oil recovery at each cycle of Huff-n-Puff CO2 injection for experiments with oil
from oilfield #2.

Sample No.
Pressure Cycle 1 Cycle 2 Cycle 3

MPa % % %

1 5 23.92 16.33 0.56
6 5 29.56 7.05 0.57
2 7.5 21.39 13.84 5.02
3 12 31.56 5.75 3.25
4 18 30.81 4.66 3.06
5 24 28.27 7.92 3.16

5.2. Cumulative Oil Recovery

The graphical representation of cumulative oil recovery for both oil reservoirs is presented in
Figure 3. Clearly, three cycles is not high enough for the experiments with oilfield #2, as shown
in Figure 3b, if the experiments continued cumulative oil recovery for different pressure regimes
would be equalized. According to the company’s prerequisite, the range of injection pressure of CO2
was predetermined by the limitation of the surface injection pumps and reservoir fracture pressure.
The cumulative heavy oil recovery can be over 40% after a number of Huff-n-Puff CO2 injection cycles.Version October 28, 2019 submitted to Energies 7 of 16

1 2 3 4 5 6
20

25

30

35

40

45

Cycle Number

Cu
mu

lat
ive

Oi
lR

eco
ve
ry,

%

Sample 3 (6 MPa) Sample 4 (6 MPa)
Sample 7 (10 MPa) Sample 9 (10 MPa)
Sample 1 (14 MPa) Sample 2 (14 MPa)
Sample 5 (19 MPa) Sample 6 (19 MPa)
Sample 8 (24 MPa) Sample 10 (24 MPa)

(a) Oil from Oilfield #1

1 2 3
20

25

30

35

40

Cycle Number

Cu
mu

lat
ive

Oi
lR

eco
ve
ry,

%

Sample 1 (5 MPa) Sample 6 (5 MPa)
Sample 2 (7.5 MPa) Sample 3 (12 MPa)
Sample 4 (18 MPa) Sample 5 (24 MPa)

(b) Oil from Oilfield #2
Figure 3. Pressure effect on CO2 Huff-n-Puff performance.

equalised. According to the company’s prerequisite, the range of injection pressure of CO2 was predetermined by146
the limitation of the surface injection pumps and reservoir fracture pressure.147
5.3. Miscibility Studies148

In order to evaluate the miscibility of oil samples with CO2, vast Huff-n-Puff injection experiments were149
conducted at five different pressure regimes for both oil heavy samples. Each oil recovery was normalized150
according to the volume and surface area of used samples, and the mean value was taken if more than one samples151
were used in the Huff-n-Puff injection test. Normalized oil recovery for oil samples from oilfield #1 and oilfield152
#2 are presented on fig. 4 and fig. 5. For the normalized oil recovery studies (Oilfield #1) on figs. 4b to 4f two153
straight lines can be drawn. Since there is not a clear intersection of these lines, the miscibility pressure cannot154
be determined. However, taking into account the results of special PVT studies it is possible to conclude that155
the mixing zone of CO2 with light oil components of oil from oilfield #1, when CO2 becomes miscible with156
light components of oil, lies in the interval from 6 MPa to 10 MPa as it is shown on fig. 4f. The mixing zone of157
CO2 and light oil components of oil from oilfield #2 lies below 5 MPa as it is shown on fig. 5c. Therefore, the158
assumption is the following: when the miscibility pressure of CO2 with light components is achieved, the oil159
recovery remained the same until it reaches miscibility pressure with heavy components.160
5.4. Effect of CO2 injection on oil and rock properties161

The Huff-n-Puff injection of CO2 leads to the extraction of light components first and can accelerate heavy162
oil depositions. In order to prove the idea that light components of oil yield first, the extracted oil sample was163
collected after Huff-n-Puff CO2 injection tests and analyzed using GC/MS. Oil sample from oilfield #1 was164
collected after the 1st cycle of injection at 14 MPa, and oil sample from oilfield #2 was collected after the 1st165
cycle of injection at 5 MPa. Figure 6 presents the difference of composition of the original oil, extracted oil and166
the oil remained in the samples after yield. The composition of remained oil was calculated using compositions167
of initial and extracted oil, and the masses of oil before and after the experiments. This investigation of the change168
of oil composition after Huff-n-Puff injection of CO2 tests proved that the extracted oil from the samples is much169
lighter that the remained in the samples. However, it is an opened question about heavy oil deposits.170

Figure 3. Pressure effect on CO2 Huff-n-Puff performance.

5.3. Miscibility Studies

To evaluate the miscibility of oil samples with CO2, vast Huff-n-Puff injection experiments were
conducted at five different pressure regimes for both oil heavy samples. Each oil recovery was
normalized according to the volume and surface area of the used samples, and the mean value was
taken if more than one samples were used in the Huff-n-Puff injection test. Normalized oil recovery
for oil samples from oilfield #1 and oilfield #2 are presented in Figures 4 and 5. For the normalized
oil recovery studies (Oilfield #1) shown in Figure 4b–f, two straight lines can be drawn. As there is
not a clear intersection of these lines, the miscibility pressure cannot be determined. According to the
special PVT studies of oil samples from oilfield #1 and …