Carbon Dioxide Minimum Miscibility Pressure with Nanopore Confinement in Tight Oil Reservoirs

Abstract

CO2-injection is one of the capable processes in EOR from low-permeable reservoirs and MMP determination is a key factor in estimating the displacement efficiency of the CO2 in the EOR processes. The laboratory procedures for MMP determination recognized in the oil industry are slim-tube, rising-bubble, or vanishing interfacial tension (VIT). However, the presence of nanopores in tight formations influences phase equilibrium, causing reduction in MMP. Instead, the existing MMP correlations need to be modified for tight reservoirs that might result in reliable MMP. Therefore, MMP measurement is performed using WinProp to validate correlations for tight oil samples. This study presents MMP determination experimentally using VIT for tight oil samples in both recombined-oil and dead-oil conditions. Subsequently, results obtained from VIT are compared with slim-tube results and the relative error was found 4.86% for recombined-oil and 23.36% for dead-oil. A huge deviation between VIT and slim-tube is noted while measuring MMP for dead-oil, due to deficiency of multiple contacts miscibility and stabilization of heavier fractions. subsequently, an already incorporated correlation for MMP is utilized, considering the effect of nanopore confinement. This study provides an appropriate technique for predicting MMP considering the capillary pressure and solubility on well performance of tight reservoirs.

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IOP Conference Series: Earth and Environmental Science
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Carbon Dioxide Minimum Miscibility Pressure with Nanopore
Confinement in Tight Oil Reservoirs
To cite this article: R S Mohammad et al 2018 IOP Conf. Ser.: Earth Environ. Sci. 167 012030
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1234567890 ‘’“”
8th International Conference on Environment Science and Engineering (ICESE 2018) IOP Publishing
IOP Conf. Series: Earth and Environmental Science 167 (2018) 012030 doi :10.1088/1755-1315/167/1/012030
Carbon Dioxide Minimum Miscibility Pressure with
Nanopore Confinement in Tight Oil Reservoirs
R S Mohammad1, *, S Zhang1, E Haq2, X Zhao3 and S Lu1
1. Oil and Gas Field Development Engineering Department, China University of
Petroleum-Beijing, Changping, China
2. Geological and Geosciences Department, China University of Petroleum-Beijing,
Changping, China
3. Centre of Geoscience Computing, School of Earth and Environmental Science,
University of Queensland, Brisbane, Australia
E-mail: 2846921094@qq.com
Abstract. CO2-injection is one of the capable processes in EOR from low-permeable
reservoirs and MMP determination is a key factor in estimating the displacement
efficiency of the CO2 in the EOR processes. The laboratory procedures for MMP
determination recognized in the oil industry are slim-tube, rising-bubble, or vanishing
interfacial tension (VIT). However, the presence of nanopores in tight formations
influences phase equilibrium, causing reduction in MMP. Instead, the existing MMP
correlations need to be modified for tight reservoirs that might result in reliable MMP.
Therefore, MMP measurement is performed using WinProp to validate correlations
for tight oil samples. This study presents MMP determination experimentally using
VIT for tight oil samples in both recombined-oil and dead-oil conditions.
Subsequently, results obtained from VIT are compared with slim-tube results and the
relative error was found 4.86% for recombined-oil and 23.36% for dead-oil. A huge
deviation between VIT and slim-tube is noted while measuring MMP for dead-oil, due
to deficiency of multiple contacts miscibility and stabilization of heavier fractions.
subsequently, an already incorporated correlation for MMP is utilized, considering the
effect of nanopore confinement. This study provides an appropriate technique for
predicting MMP considering the capillary pressure and solubility on well performance
of tight reservoirs.
1. Introduction
Carbon dioxide enhanced oil recovery (EOR) is the main choice to execute CO2 geological
sequestration due to the extra economic advantage, prevailing infrastructure and acquaintance of
petroleum and its operations. A CO2 Miscible injection process have become commonly used
procedure for the improved oil recovery worldwide. Thus, many CO2 field applications have been
recorded as successful [1], [2]. In miscible CO2 injection process, the main task is to displace the
trapped oil using miscible CO2 injection process, which increases the displacement efficiency and
enhance the oil recovery. minimum miscibility pressure (MMP) determination is a key parameter in
estimating the capability of injected fluid in the EOR processes. The miscibility is achieved at the
MMP, when interfacial tension among the CO2 and reservoir fluid is approaching zero, which

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8th International Conference on Environment Science and Engineering (ICESE 2018) IOP Publishing
IOP Conf. Series: Earth and Environmental Science 167 (2018) 012030 doi :10.1088/1755-1315/167/1/012030
consequences in potential transfer of molecules across the interface leading to mutual miscibility and
homogeneous fluid formation [3].
There are many experimental procedures for the determination of MMP such as Slim-tube (ST),
vanishing interfacial tension (VIT) and raising bubble (RB). A number of studies presented the slim-
tube for different gas-oil systems for calculation of MMP and formed various standards for its
prediction. However, VIT experiment is achieving cohesion within the oil industry because it is
requiring extremely short time and less expenses in estimating CO2-Oil MMP while slim-tube needs
extended time. The concept of VIT is to predict the miscibility situations by calculating the interfacial
tension through the fluid phases against differing injectivity pressures or composition of the injected
fluid and subsequently calculates the MMP by extrapolation [4]. The miscibility mechanism is
achieved, as interfacial tension (IFT) among injected CO2 and the reservoir fluid reaches zero and
transmission of molecules take place between fluid bulks that results in miscibility development. Holm
et al. introduced the MMP as oil recovery reaches 80% at gas breakthrough [5]. Waqar et al.
conducted experimental work and compared VIT and ST test, however they did not consider the
nanopore confinement effect in their study [6]. Rao et al. applied VIT to realize the miscibility
conditions for a couple of gas–oil samples by altering the compositions of the injected fluid and
injectivity pressure [7], [8]. Orr & Jessen have questioned the viability of this approach, but recent
studies by Ayirala and Rao [9] showed that the experimental VIT MMP results were within 5 to 8
percent of the slim-tube MMP values [10]. Zhang et al. proposed correlation for CO2-Oil MMP with
nanopore confinement effect which lacks experimental analysis [11] in contrast to this study.
On the other hand, a diversity of corrections is in appreciation of CO2-Oil MMP, which help engineers
in making decisions to develop injectivity status and to design appropriate downstream facilities [12].
The central factors impacting CO2-Oil MMP correlations are reservoir fluid compositions, reservoir
temperature and CO2 injectivity conditions [13]. In the previous studies, reservoir temperature was
considered the most significant parameter influencing the CO2-Oil MMP. However, few techniques
have huge inconsistencies with the data obtained from laboratory work. It was noted that MMP is
further affected by molecular weights of crude oil [14]. Subsequently, the prognosis of CO2-Oil MMP
is required to deem crude oil composition for computing MMP. Higher concentration of lighter
components in the crude oil increases the MMP, while higher concentration of intermediate
components reduces the MMP [15]. Most of the pores in tight reservoirs are on the order of a few
nanometers varying from 3nm to 100nm and from 40nm to 1500nm for shales and sandstones,
respectively [16].
Many researches pointed that the fluid phase behavior in these nanopores diverges notably then that of
macro pores, which cause variations in Van der Waals forces, molecules orientation in such a
nanoscale pore structure [17], as presented in Figure 1. The molecular orientation can be transformed
due to nanopore confinement effect as proposed by Pitakbunkate et al [18]. They show that the
conventional liquid-vapor equilibrium (VLE) calculations could be extended to unconventional
reservoirs by including capillary pressure and shift in critical temperature and pressure in VLE
calculations. A mathematical algorithm is used to calculate VIT by integrating the extended VLE
calculation to any MMP algorithm, such as the multiple mixing cells (MMC) model of Ahmadi and
Johns [19]. In this approach, incremental pressure increase as IFT approaches zero.
Nanopore confinement can cause fluid phase equilibrium alteration with shifting critical properties [20]
resulting in different fluid behavior. Yangyang et al., observed that the CO2 density near pore walls is
higher than that in pore centres [21]. Since the crude oil phase equilibrium and fluid properties in a
confined system deviate from those at their bulk state, CO2-Oil MMP is decreased with confinement
due to critical properties shift. Therefore, the consequence of nanopore confinement on CO2-crude oil
MMP is studied and incorporated into a new novel CO2-Oil MMP correlation. The recommended
screening technique is carried out by phase behavior module of CMG (Winprop) using tight reservoir
crude oil samples.

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IOP Conf. Series: Earth and Environmental Science 167 (2018) 012030 doi :10.1088/1755-1315/167/1/012030
Figure 1. Molecular orientation in nanopore confined system [18].
As a result of the above-mentioned experimental MMP approaches, a study for MMP measurements
using VIT experimental procedure is performed. The crude oil of tight reservoir is being utilized with
CO2 in both the stock tank oil (dead oil) and recombined oil (live oil) forms individually for
displacement study. Furthermore, a comparative estimation of CO2-crude oil MMP with confinement
effect calculating correlations with VIT measured MMP was also carried out using CMG WinProp to
find an appropriate correlation for precise MMP prediction.
2. Methodology
2.1. MMP Correlation
Many researchers developed correlations for computing crude oil- CO2 MMP. Holm & Josendal (1974)
modified correlation to predict the CO2 MMP based on molecular weight of C5+ and the desired
temperature of the reservoir. Lee J. provided a model based on reservoir temperature and CO2 vapor
pressure for estimating MMP. Furthermore, procedure by Orr and Jensen was almost suitable
correlation for predicting MMP of low temperature reservoirs. Yuan et al. presented correlation to
predict CO2-oil MMP using intermediates and heavier mole frictions of reservoir fluid and reservoir
temperature. Shokir developed a new model for the prediction of both impure and pure CO2
displacements. However, none of above-mentioned CO2-crude oil MMP determining correlations
might prove to be reliable MMP with confinements effect. Therefore, a novel correlation is proposed
to take into account the effect of nanopore confinement in determining the CO2-oil MMP.
The solubility parameters for hydrocarbon components can be measured by [22]:

 Eq. (1)
Where
δ is solubility parameters,
σ is interfacial tension,
V is molar volume,
α is constant and
A is coefficient for solubility parameters.

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IOP Conf. Series: Earth and Environmental Science 167 (2018) 012030 doi :10.1088/1755-1315/167/1/012030
Furthermore, the oil solubility parameters can be determined from average molecular weights and
reservoir temperature [23].
  Eq. (2)
Where T is reservoir temperature,
M is molecular weight.
The solubility parameters for CO2 can be estimated by the following expression [24]

 Eq. (3)
Where Pc is critical pressure,
α is constant,
ρr is reduced density and
ρr(liq) is reduced density of gas compressed to a liquid state.
The pressure condition that corresponds to |δo- δg|≈3.0 (cal/cm3)0.5 is the CO2-crude oil MMP [23].
At reservoir temperature, the relationship between gas solubility and reduced density can be calculated
by Equation (3). Then the CO2 density can be obtained. According to Figure 2, the CO2-oil MMP can
be estimated.
Figure 2. Density of CO2 required for miscible displacement [17].

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Taking the confinement effect into consideration, a critical property shift is given by the following
equations [20]:

 

 Eq. (4)

  

 Eq. (5)


 Eq. (6)
Where ΔPc and ΔTc are the critical pressure and critical temperature shift due to nanopore
confinement effect, respectively. Pc and Tc are the critical pressure and critical temperature,
respectively. Pcp and Tcp are the pore critical pressure and pore critical temperature, respectively.
Where, rp is the pore radius and σ is the Lennard-Jones size parameter.
Subsequent to the above steps, the CO2 density can be obtained. Consequently, it requires to be
modified by the following equation at frequent pore radius in 50 nm or less as proposed by Zhang et al
[11]. Then the modified CO2-Oil MMP corresponding to Figure 2 with the nanopore confinement
effect can be achieved.

 Eq. (7)
2.2. Experimental Work
2.2.1. Recombination of Downstream Fluids
The live crude oil sample utilized in this study were collected from downstream system (gas from
first-stage separator and dead oil from stock tank) and experimentally recombined at reservoir
conditions based on the saturation pressure to reproduce a representative fluids of tight oil reservoir as
validated in our previous studies [25], [26]. This study does not explain the recombination related
processes, though it represents the reservoir fluid and recombined fluid compositions used in this work
in Table 1. The carbon dioxide utilized in this study has a purity > 99.95%. The rest of PVT properties
of tight reservoir fluid such as oil viscosity, density, gas-oil ratio and formation volume factor were
measured using CMG WinProp as shown in Table 2 resulting in a good correlation with tight reservoir
fluid data. Table 1. Reservoir fluid and recombined fluid components.
Comp.
Pc
(psi)
Tc
(oK)
Mol.
Weight
Reservoir Fluid
(Mole %)
Recombined
Fluid (Mole %)
Average Absolute
Error (%)
N2
492.31
126.2
28.013
1.3643
1.3843
1.4691
CO2
1069.86
304.2
44.010
6.7853
6.8817
1.4213
C1H4
667.19
190.6
16.043
27.034
27.426
1.4514
C2H6
708.34
305.4
30.070
5.5795
5.6450
1.1733
C3H8
615.76
369.8
44.097
5.7348
5.7831
0.8422
iC4H10
529.05
408.1
58.124
1.8855
1.8839
0.0828
nC4H10
551.09
425.2
58.124
3.3868
3.3806
0.1839
iC5H12
490.84
460.4
72.151
2.6760
2.6544
0.8066
nC5H12
489.37
469.6
72.151
3.7068
3.6693
1.0108
C6H14
477.03
507.5
86.010
5.1205
5.0579
1.2230
C7+
145.69
905.1
274.00
36.755
36.233
1.4193
AARE (%)
0.0828

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Table 2. Comparison between PVT properties of tight oil reservoir and predicted properties using
WinProp.
PVT Properties.
Sample
Model
(Winprop)
Average Absolute
Error (%)
Bubble Point (psi)
2652
2651.864
0.005130
Viscosity (cp)
0.270
0.2698
0.074070
Oil Density (lb/ft3)
36.849
36.900
0.138402
Gas-Oil Ratio (scf/stb)
76.50
76.84
0.444444
Oil FVF (bbl/stb)
1.317
1.319
0.151860
AARE (%)
0.162780
2.2.2. Vanishing Interfacial Tension Test
Vanishing interfacial tension (VIT) is a fast route to calculate the minimum miscibility pressure to
economically recover oil during CO2 injection process. VIT offers a method to directly measure the
IFT between the two phases. The experimental schematic demonstration utilized in this work is
illustrated in Figure 3. The apparatus comprises of two high pressure and temperature PVT cells and
syringe pump (ISCO-260D). The upper cell contains recombined fluid phase and CO2 phase is injected
from syringe pump until a little head of free CO2 is seen at the top, confirming that the recombined
fluid is fully saturated with CO2 phase. The measurement system is consisting of high pressure syringe
pump to inject CO2 as a solvent into the lower cell at stable desirable temperature. The PVT cell
contains a moveable piston to manipulate the pressure inside the cell by pumping CO2 on to the back
of PVT cell as a pressurizing fluid. The valve between both the cells allows us to displace the system,
while keeping the upper cell at a relatively higher pressure than the lower cell pressure. As the valve is
opened the upper cell system is likely to pump the recombined fluid into the lower cell and once the
droplet is formed the valve is closed or the moveable piston is utilized to pressurize and depressurize
the system. The IFT measurements are conducted with few test droplets until the system have excess
oil at the bottom of the lower cell, this is to achieve a stable droplet at the tip of the capillary tube. As
the stabilized droplet of recombined fluid is introduced in the lower cell which is fully saturated with
CO2, in terms the CO2 instantly swells the droplet which dissolves in the oil, thus significantly
extending the volume of the oil phase, leading the lighter components of the oil phase to isolates into
the vapor phase and achieve solubility through CO2 phase. In the process one may achieve slight
stability in terms of droplet. Through saturating/presaturating the oil phase with CO2 (upper cell) and
CO2 phase with the oil (lower cell), some of that mass transfer upon the formation of the droplet at the
tip of capillary is removed. However, the lower cell which has two lateral windows for optical
observations; one of the lateral windows is utilized for lighting source and the other for camera to
detect the shape of droplet and the magnified picture is digitized and stored in the operating system.
Afterwards, the digitized droplet analyzes drop shapes through Laplace equation to fit the
experimental drop shape for the determination of interfacial tension with the knowledge of density
difference (density of the oil saturated with CO2 at desired pressure and temperature and density of the
CO2 phase saturated with oil at desired pressure and temperature).


 Eq. (8)

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Figure 3. Vanishing Interfacial Tension measurement apparatus.
3. Results and Discussion
3.1. VIT measured MMP
MMP measurements using VIT technique are done in a high-pressure PVT cell. Crude oil is
introduced as a drop phase into the chamber filled with the injected fluid. Pendant drop shape analyses
are utilized to calculate the interfacial tension. The pressure is gradually increased by pumping more
injection fluid on the back of PVT cell as pressurizing fluid. The interfacial tension is determined at
different pressures at reservoir temperature. VIT technique for estimating MMP between reservoir
fluid and injected fluid (CO2) with the nanopore confinement effect of tight reservoir besides the slim-
tube test data is validated at the reservoir temperature of 238 °F. The reservoir fluid compositions and
critical properties are presented in Table 1. Vanishing interfacial tension measurement for minimum
miscibility pressure data for both of live oil (recombined oil) and dead oil (stock tank oil) samples are
illustrated in Figures. 4 and 5, respectively. However, it was observed that the live oil MMP estimated
using VIT procedure is 3478 psi, which is comparatively higher in comparison with the calculated data
of 3316 psi utilizing slim-tube technique with AARE of 4.86%. The close agreements designate that
VIT procedure can precisely predict the MMP of the reservoir fluid in confined phase. The
measurements are done at 9 different pressures, after which the line is extrapolated to zero IFT value.
In contrast, stock tank oil (dead oil) MMP determination using VIT was 1821 psi, whereas ST
measurement was 2376 psi with AARE of 23.36%. It is noted that a huge deviation between VIT and
ST data occurred due to heavier fractions of dead oil. Thus, light fractions of dead oil are isolated into
CO2 gas phase, considered too low to be enriched for solubility with crude oil. In case of stock tank oil
with high stabilized heavier fractions, this difference can be correlated to lack of multiple contact
miscibility development between crude oil and gas phases compared to achieved miscibility in slim-
tube case at respective injection pressure.
In order to consider the capillary pressure effect on the minimum miscibility pressure, the MMP is
calculated with different pore sizes. The MMP for the pore size 5 nm is 2810 psi, whereas MMP
increased to 3104 psi with the pore size of 10 nm. Therefore, the minimum miscibility pressure
reduces as the pore size is reduced. Thus, the reservoir fluid and injected fluid (CO2) will achieve
miscibility at lower pressures and subsequently is helpful for CO2 EOR. Figure 6 shows the
determined IFTs and MMP considering capillary pressure effect for tight oil reservoirs.

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Figure 4. MMP calculation using VIT and
comparison with ST test for live oil sample.
Figure 5. MMP calculation using VIT and
comparison with ST test for dead oil
sample.
Figure 6. Determined IFTs and MMP with capillary pressure effect for live crude oil of tight
reservoirs.
3.2. Correlation calculated MMP
A CO2-Oil MMP can be measured in several methods and the correlation provides one of the easiest
and attractive solutions to estimate MMP. Though, the correlations are used in certain conditions,
however they are yet to be considered as an efficient method to roughly determine CO2-Oil MMP.
Mostly, the MMP correlations are function of the few parameters including reservoir temperature,
molecular weights and mole fractions. Most of CO2 MMP correlations are summarized in Table 3.
Table 3. Common CO2 MMP Correlations.
Author
Correlation
Holm & Josendal
Graphical correlation as shown in Figure 2.
Cronquist


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Orr & Jensen
 

Emera & Sarma
 

Yelling &Metcalfe



Shokir
 
Alston
 


Lee
 

Glaso
 

T
Shengli
 
Yuan
    


 

Recombined crude oil sample shown in Table 1 is used to validate the CO2-Oil MMP correlations.
Results in Table 4 are shown based on the correlation parameters. In general, the correlations
involving molecular weights and temperature give a good prediction of CO2-Oil MMP. The
correlations only taking temperature into consideration significantly deviates from the results provided
by the Winprop simulator. On the contrary results of MT and MTF correlations are similar but the
equations become more complex if the crude oil composition is involved in the correlations.
Table 4. CO2–Oil MMP correlation validation for each parameter.
Recombined Fluid
(Live Oil)
Stock Tank Oil
(Dead Oil)
Temperature (MMP in psi)
2892.052
2240.832
Temp. MW (MMP in psi)
3250.295
2630.984
Temp., MW & MF (MMP in psi)
3150.219
2570.068
CMG Winprop (MMP in psi)
3319.913
2619.381

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8th International Conference on Environment Science and Engineering (ICESE 2018) IOP Publishing
IOP Conf. Series: Earth and Environmental Science 167 (2018) 012030 doi :10.1088/1755-1315/167/1/012030
Afterwards, CO2 MMP is predicted with a confinement effect at 20nm and 10nm pore radii as shown
in Table 5. The correlation proposed by Zhang K., [11] gives a reasonable prediction of CO2-Oil MMP,
which also incorporates the alteration of CO2–Oil MMP with nanoscale pore confinement.
Comparison between experimental MMP and calculated MMP ITF values shown in Figure 7 suggest
that the results from the correlation considering confinement effect have good coincidence which
measured values.
Table 5. CO2–Oil MMP correlation validation for nanopore confinement.
Correlation (psi)
r = 20 nm
CMG (psi)
r = 20 nm
Correlation (psi)
r = 10 nm
CMG (psi)
r = 10 nm
Recombined Fluid
3343.118
3505.561
2968.154
3084.046
Stock Tank Oil
2214.726
2407.626
1849.760
2086.213
Figure 7. Calculated MMP IFT by confined correlation vs. experimental MMP IFT.
4. Conclusion
The VIT technique is efficient, less expensive and a reproducible technique in determining the MMP
compared to irreproducible, most expensive and time consuming ST procedure. VIT test needs at most
one day to estimate one MMP, whereas slim-tube requires a minimum of two weeks for one MMP
determination. For recombined crude oil, VIT estimated MMP was found significantly good in
contrast to slim-tube determined MMP that validates the multiple contact miscibility between
recombined fluid and injected CO2 gas. while in case of dead oil sample, higher deviation was noticed
between ST and VIT estimated MMPs. This may be due to deficiency of multiple contacts miscibility
and stabilization of crude oil heavier fractions.
With reference to common CO2-Oil MMP correlations, it was observed that for both of the crude oil
samples (recombined oil and dead oil samples), no correlation is able to calculate MMP in close
consent with VIT and ST determined MMP. It emphasizes the correlations validity only for certain
number of crude samples considered during correlations development. Therefore, most of the common
CO2-Oil MMP correlations are validated by Winprop simulator. Subsequently, a solubility depended
technique is proposed and confirmed to predict the CO2-Oil MMP considering the modifications by
nanopore confinement. Commonly, the correlations with molecular weights and reservoir temperature
can give a rough estimation of CO2-Oil MMP. For tight oil reservoirs, taking confinement effect into

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IOP Conf. Series: Earth and Environmental Science 167 (2018) 012030 doi :10.1088/1755-1315/167/1/012030
account, CO2-Oil MMP is reduced. This study entails the combination of solubility techniques and the
nanopore confinement effect that provide an efficient method for MMP calculations.
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