4th World Conference on
Applied Sciences, Engineering & Technology
24-26 October 2015, Kumamoto University, Japan
Experimental study and numerical simulation of methane solubility in
crude oil for gas injection enhanced oil recovery
MOHAMMAD REZA GHULAMI, KYURO SASAKI, YUICHI SUGAI
Department of Earth Resources Engineering, Graduate School of Engineering, Kyushu University, Japan
Email: [email protected], [email protected], [email protected]
Abstract: Gas injection enhanced oil recovery has long been used in petroleum industry to increase oil recovery
factor. Carbon dioxide, nitrogen, flue gas and lean hydrocarbon gases were all being utilized as displacing
agents. The oil swelling is an indicator of amount of gas dissolved into oil phase. Swelled oil has lower viscosity
and flows more easily, then it improves recovery factor.
In current study, a crude oil sample was used for experimental investigation. Solubility of methane gas in oil and
swelling factor of the oil were measured by a Pressure-Volume-Temperature apparatus and a High-Pressure-Cell
equipment, respectively.
Furthermore, a reservoir simulation software, CMG-WINPROP, was used to estimate the PVT properties of gasoil mixture and compare it with experimental data. It was used to calculate gas solubility and oil swelling factor,
and also tune the equation of state with experimental data.
Experimental result shows that the solubility of methane in oil phase and swelling factor of oil increase
proportionally with pressure. Moreover, reservoir fluid phase behaviour was predicted using a tuned equation of
state against experimental data.
Keywords: Oil Swelling, Gas Solubility, Gas Injection EOR, Phase Behavior
Introduction:
Enhanced oil recovery (EOR) involves the application
of external forces, and substances to manipulate
chemical and physical interactions in hydrocarbon
reservoirs in a manner that promotes favourable
recovery conditions [1].
During the life cycle of an oil field there are different
stages of recovery. Utilization of natural energy of the
reservoir (primary recovery) and water or gas injection
for the purpose of pressure maintenance (secondary
recovery) are common in most of conventional oil
fields. EOR (tertiary recovery) techniques are
implemented to recover trapped oil in the reservoir
after primary and secondary recovery stages. But, they
can also be applied as the primary and/or secondary
recovery methods where oil production rate is not
favourable.
There are three main EOR techniques; thermal,
chemical and gas injection enhanced oil recoveries.
Out of 652 EOR projects around the world from 1959
to 2010, fifty percent were thermal, chemical EOR
shared ten percent, while forty percent of the projects
were gas injection [1].
Injecting gas into an oil reservoir can improve the
recovery through maintaining the reservoir pressure,
displacing oil, or vaporising the intermediate and
heavy fractions of the oil [2].
Crude oil swelling due to solvent dissolution is a wellknown phenomenon. Relative permeability of oil
could be increased due to an increase in the volume of
the oil. Furthermore, oil viscosity reduction is also
associated with swelling effect [3-5]. The residual oil
left in the reservoir after gas flooding is roughly
inversely proportional to the swelling factor.
Present study tends to investigate crude oil swelling
effect due to solubility of lean hydrocarbon gas. For
this purpose, experimental study on solubility of
methane gas and oil swelling effect was conducted
using an intermediate gravity crude oil. PengRobinson Equation of State (PREOS) [6] was used for
calculation of vapour-liquid equilibria and verification
of experimental data. Subsequently, CMG-WINPROP
was used to tune the equation of state that can be used
for prediction of phase behaviour of gas-oil system.
Materials and Procedures:
1. Materials:
Experimental measurements were carried out on a
crude oil sample with physical properties as outlined
in Table 1.
Table 1: crude oil sample physical properties
Sample
ρ @ 20 C
(g/cm3)
API (-)
Viscosity @
20 C (cp)
A-1
0.87
31
9.2
Methane (CH4) was used as displacing gas. It was
supplied by Itochu Industry gas Ltd (Japan) and had a
purity of 99.99%. Decane was used as a reference
material. It was supplied by Junsei Chemical Co. Ltd
(Japan) and had the purity of min. 95.0%.
2. Experimental Apparatus and Procedures:
Solubility of injection gas in crude oil was measured
using an equipment known as PVT (pressure-volumetemperature). Schematic the PVT cell is shown in Fig.
1. Gas was injected into the cell at desired pressure.
Cell volume was kept constant using a mechanical
piston. A pressure drop could be observed due to gas
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MOHAMMAD REZA GHULAMI, KYURO SASAKI, YUICHI SUGAI
3. Phase Behaviour Calculations:
In order to estimate the properties of both vapour and
liquid phases at each equilibrium pressure, two-phase
flash calculation was carried out using PREOS.
Because the effect of water on the hydrocarbon phase
behaviour can be neglected in most cases [7], vapourliquid equilibria calculations were conducted by
assuming only two phases, liquid and vapour.
For a liquid-gas system with n components, the
necessary condition for equilibrium is
(1)
.
Figure 1: PVT analysing cell
diffusion into the oil phase. When the system reaches
an equilibrium state, in which pressure remains almost
constant, it was assumed that the oil phase is saturated
at current pressure and no more component exchange
will occur between phases. At this point, the volume
was changed in order to enter the second stage of
vapour-liquid contact. This procedure was repeated for
a number of steps.
That is, the fugacity of each component i should be
equal in both liquid and vapour phases.
Let’s assume one mole of mixture is flashed at
pressure P and temperature T into
moles of liquid
and
moles of vapour. Total material balance for the
system is,
(2)
Material balance for each component is given by
(3)
where , , and are mole fractions of component i
in the mixture, liquid and vapour phases, respectively.
where N shows the total number of components in the
system. By considering equilibrium ratio as
(5)
Figure 2: High pressure cell
Swelling factor of crude oil was measured using a high
pressure cell (HPC) apparatus as shown in Fig. 2.
Oil sample was inserted into the glass tube. The glass
tube is attached to a bolt at one end and is open at the
other. The glass tube was placed inside of the high
pressure cell by fastening the bolt from the bottom of
the apparatus. Then the displacing gas was injected at
desired pressure from the top. Pressure change in the
cell was recorded using a GE DPI 104 digital pressure
gauge. Subsequently, oil volume change was optically
measured through the observation window on the side
of the equipment. This procedure was repeated for a
number of steps. At each equilibrium pressure,
saturation pressure and volume of the oil was
measured and thereby swelling factor of the oil was
calculated.
Table 2 gives the upper bound of operating conditions
of both high pressure apparatus and PVT cell.
Table 2: Properties of experimental apparatus
Parameter
HPC
PVT
P, MPa
7.5
70
T, oC
150
200
V, ml
50
360
Substituting equilibrium ratio into Eq. (3), and solving
for and using Eq. (2) results in,
Calculation procedure of phase properties of
equilibrated phases is shown in Fig. 3.
First estimation of equilibrium ratio (
was made
using the Wilson’s equation [8] expressed as Eq. (8).
where
and
are critical pressure and temperature
of component i, respectively, P and T experimental
pressure and temperature, and
is the acentric factor
of component i.
Then using Eq. (6) and (7) mole fraction of each
component in liquid and vapour phases were
calculated.
To describe the gas-liquid mixture Peng-Robinson
equation of state was used. The general formulation of
PREOS is given by Eq. (9).
Proceedings of the 4th World Conference on Applied Sciences, Engineering and Technology
24-26 October 2015, Kumamoto University, Japan, ISBN 13: 978-81-930222-1-4, pp 209-212
Experimental study and numerical simulation of methane solubility in
crude oil for gas injection enhanced oil recovery
reservoir fluids, it is common to encounter such
defects by tuning the EOS against experimental data.
Where for mixtures,
(12)
(14)
(15)
and
PREOS in terms of the compressibility factor Z can be
formulated as follows,
Figure 3: Calculation procedure
Fugacity coefficient is defined as the ratio of fugacity
to pressure,
Where
(22)
(18)
(19)
Eq. (17) can be solved to find compressibility factor of
liquid and vapour phases at equilibrium condition.
Consequently, fugacity coefficient of component i in
each phase can be computed using Eq. (20).
4. Results and Discussion:
Due to availability of experimental data on solubility
of methane in pure hydrocarbons, in this study decane
was used as a reference fluid. Solubility of methane in
decane and crude oil were measured at various
temperatures and different initial conditions.
Figure 4. shows the measured values of gas mole
fraction in liquid phase for methane and decane
mixture at various temperatures.
Where
(21)
Rearranging Eq. (23) the fugacity of component i in
each phase could be calculated. When fugacity of
component i in vapour and liquid phases converges, it
is concluded that calculated phase properties are
representative of properties of equilibrated phases.
And if fugacity does not converge,
should be
refined and calculation be repeated until fugacity
difference is smaller than an error margin value.
By following this procedure molar fraction, molar
volume, and number of moles of component i in liquid
and vapour phases could be obtained.
Figure 4: Mole fraction of dissolved methane in ndecane at 310.9 K: (□) this work, (○) Srivastan et al
(■) WINPROP, at 344.3 K: (Δ) this work
Real reservoir fluids are composed of thousands of
compounds. An equation of state represents the
reservoir fluid using a limited number of components,
generally, a combination of pure fluids. Because of
deficiencies of EOS in predicting phase behaviour of
It was observed that the result of this work and
WINPROP estimation of solubility are in close
agreement with available literature data [9]. So, the
same procedure was used for calculation of methane
solubility in the crude oil.
Proceedings of the 4th World Conference on Applied Sciences, Engineering and Technology
24-26 October 2015, Kumamoto University, Japan, ISBN 13: 978-81-930222-1-4, pp 209-212
MOHAMMAD REZA GHULAMI, KYURO SASAKI, YUICHI SUGAI
Figure 5. and 6. show swelling factor of oil and
methane gas solubility, respectively. WINPROP
estimation of solubility and swelling factor diverge
from experimental value. To address this issue,
regression procedure of Agarwal et al. [10] was used
in WINPROP to tune the EOS.
Different parameters were regressed to tune the EOS.
A combination of critical pressure (Pc) and acentric
factor (ω) represented the best possible match with
experimental result.
Tuned equation of state resulting from regression of Pc
and ω was then used to estimate methane solubility in
Figure 5: oil swelling factor at 296 K:
(Δ) winprop, (▲) experiment. Regression parameters:
(○) acentric factor (ω), (□) critical pressure (Pc), (▬)
Pc and ωcrude oil. Estimated properties of gas-oil
system were in close agreement with experimental
data after calibration procedure.
Figure 6: Mole fraction of dissolved methane at 296
K: (Δ) before tuning, (■) experiment, (▬) after tuning
Gas dissolution increases when oil is undersaturated or
the pressure is increased as a result of gas injection.
Viscosity of oil decreases due to dissolution of gas.
Dissolved gas can enhance mobility of oil. These
phenomena can increase the efficiency of a gas/oil
displacement process [11].
Conclusion:
In this research, experiments to measure gas solubility
and oil swelling factor were carried out. Calculated
values of phase properties by CMG-WINPROP were
matched to experimental data.
Followings are summarized:
a.
b.
c.
Dissolution of CH4 gas in crude oil and oil
swelling effect are almost proportional to
pressure, in pressures less than 10 MPa.
To acquire acceptable prediction of CH4 solubility
and oil swelling factor using an EOS, it was
necessary to tune the EOS against experimental
data.
Using critical pressure and acentric factor of the
crude oil as regression parameters for CMGWINPROP, produced reasonable calculation
results matching with experimental data.
References:
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Projects and Updated Screening Criteria”,
Journal of Petroleum Science and Engineering,
Volume 79, Issues 1-2, October 2011, pp 10-24
[2] Danesh A., (1998) “PVT and Phase Behaviour of
Petroleum Reservoir Fluids”, ISBN: 978-0-44482196-6
[3] Mulliken C. A.,
Sandier S. I., (1980) “The
Prediction of CO2 Solubility and Swelling
Factors for Enhanced Oil Recovery”, Ind. Eng.
Chem. Process Des. Dev., 1980, 19 (4), pp 709711
[4] Tsau J. S., Bui L. H., Willhite G. P., (2010)
“Swelling/Extraction Test of a Small Sample
Size for Phase Behavior Study”, SPE 129728
[5] Avaullee L., Neau E., Jaubert J. N., (1997)
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III. Reservoir Fluid Saturation Pressure. A
Complete PVT Property Estimation. Application
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87-104
[6] Peng D. Y., Robinson D. B., (1976) “A New
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Chem. Fundam., 15(1), pp 59-64
[7] Danesh A., (1998) “PVT and Phase Behaviour of
Petroleum Reservoir Fluids”, ISBN: 978-0-44482196-6
[8] Wilson G., (1968) “A Modified Redlich-Kwong
EOS, Application to General Physical Data
Calculations”, Paper No. 15C, presented at the
AIChE 65th National Meeting
[9] Srivastan S., Darwish N. A., Gasem K. A. M.,
Rabinson Jr. R. L., (1992) “Solubility of Methane
in Hexane, Decane, and Dodecane at
Temperatures from 311 to 423 K and Pressures to
10.4 MPa”, J. Chem. Eng., 37, pp 516-520
[10] Agarwal R. K., Li Y. K., Nghiem L., (1990) "A
Regression Technique with Dynamic Parameter
Selection for Phase-Behavior Matching", SPE
Reservoir Engineering, February 1990, pp 115120.
[11] Warner Jr. H. R., Holstein E. D., (2007)
“Immiscible Gas Injection In Oil Reservoirs, SPE
Petroleum Engineering Handbook V”, ISBN: 9781-55563-120-8, pp 1103-1147.
Proceedings of the 4th World Conference on Applied Sciences, Engineering and Technology
24-26 October 2015, Kumamoto University, Japan, ISBN 13: 978-81-930222-1-4, pp 209-212