Exploring the effects of four important factors on oil–CO2 interfacial properties and miscibility in nanopores

In this paper, a modified Peng–Robinson equation of state (PR-EOS) coupled with the parachor model and a newly-developed diminishing interface method (DIM) are applied to predict the interfacial properties and minimummiscibility pressures (MMPs) of light oil–CO2 systems in nanopores. First, the modified PR-EOS is used to calculate the vapour–liquid equilibrium by considering the effects of capillary pressure and shifts of critical temperature and pressure. Second, a thermodynamic formula of the interfacial thickness (IT) between two mutually soluble phases is derived, based on which the novel DIM is developed. The MMP is determined by extrapolating the derivative of the IT with respect to the pressure (vd/vP)T to zero. It is found that at the pore radius of 10 nm, all three quantities, the interfacial tensions (IFTs), ITs, and MMPs, show obvious increases with temperature. The effects of the initial oil composition on the three quantities are measurable but marginal and the MMP is more sensitive to the initial oil composition at a higher temperature. Moreover, the IFTs and ITs are weakly dependent but the determined MMPs are strongly dependent on the injection gas composition. The presence of CH4 in the injection gas results in a substantial MMP increase in nanopores. At a constant temperature, the effects of the feed ratio of injection gas to oil on the IFTs, ITs, and MMPs are negligible with pure CO2 injection, especially at low feed gas–oil ratios (less than 0.50 : 0.50 in mole fraction), whereas they become much stronger and cause the MMPs with impure CO2 (0.65CO2 + 0.35CH4) injection to be considerably increased from 26.3 to 40.0 MPa by reducing the feed gas–oil ratio from 0.90 : 0.10 to 0.10 : 0.90 in mole fraction.


Introduction
In the petroleum industry, gas (e.g., CO 2 ) injection has been used for a long time to enhance oil recovery 1,2 by improving the uid microscopic displacement efficiency, whose mechanisms are attributed to, for example, interfacial tension (IFT) reduction and miscible displacement. 3 During the gas injection process, interfacial mass transfer always occurs across the interface between the oil and gas phases so that some interfacial properties (e.g., IFT) vary under different conditions. In general, the IFTs between the oil and gas phases can be measured at different pressures and reservoir temperatures by applying, for example, the axisymmetric drop shape analysis (ADSA) technique for the pendant drop case. 4 Alternatively, they can be predicted by using, for instance, the parachor model. 5 On the other hand, a distinct miscible state between the oil and gas phases can be developed as the mass transfer continues and the physicochemical properties of two phases become similar. 6 Accordingly, the minimum miscibility pressure (MMP) is dened as the lowest operating pressure at which the oil and gas phases can become miscible in any portions through a dynamic multi-contact miscibility (MCM) process at the reservoir temperature. 7,8 To ensure a miscible gas ooding process with a high oil production, an accurate determination of the MMP for a given oil-gas system is required in the oileld application. A number of theoretical models, 9,10 numerical simulations, 11,12 and experimental methods 13,14 have been developed to determine the MMPs of various oil-gas systems. However, most of them are applied to determine the MMPs in bulk phase rather than in porous medium.
The IFT and MMP of the oil-gas system strongly depend on the temperature/pressure and initial overall uid composition, i.e., the initial oil and injection gas compositions as well as the feed ratio of injection gas to oil. 15 In the literature, the IFT of an oil-gas system is decreased with pressure at a constant temperature, whereas it is increased with temperature if the pressure keeps constant. 16 The MMP also becomes higher at a higher temperature. In addition, the initial overall uid composition is found to have a foremost and direct effect on the IFT and MMP. 17 In practice, the effect of initial oil composition was studied by comparing the measured IFTs/MMPs of different oil samples with the same solvent phase (e.g., CO 2 ) while the effect of injection gas composition was studied by choosing different solvent phases with the same oil sample. For example, the CH 4 -dominated HCs pre-saturated live light oil-CO 2 system was found to have a higher IFT/MMP, whereas the intermediate HCs pre-saturated one has a lower IFT/MMP in comparison with that of the dead light oil-CO 2 system. 18 Similar results were also found in terms of the effect of injection gas composition on the IFT/MMP studies. 15 Moreover, the effect of feed ratio of injection gas to oil on the measured IFT/MMP cannot be ignored although there is no general consensus on it. In some early studies, the feed ratio was considered to only affect how quickly the equilibrium state could be achieved and have no effect on the IFT/MMP. 19 Later, it was found that the measured IFT/MMP reached a minimum value when the feed ratio of the injection gas to oil equals to 1 : 1 in volume and was slightly increased at an increased injection gas concentration. 20 The presence of nanopores in tight formation and its effect on the liquid phase behaviour, IFT and MMP of the light oil-gas systems have been introduced in the literature. 21 Cubic equation of state (EOS) is usually treated as an available and appropriate approach to calculate the vapour-liquid equilibrium (VLE) properties in nanopores. 22 The conned space or pore proximity are found to cause variations of the phase behaviour and IFT/MMP due to the effects of capillary pressure and shis of the critical properties. 23 More specically, the bubble-point and dew-point pressures and temperatures are suppressed or increased to different extents. The IFT and MMP are generally decreased with the reduction of pore radius at a constant temperature. 24 For example, the bubble point pressure of a Bakken oil-CO 2 system is found to be signicantly decreased while its upper dew-point pressure is increased and its lower dew-point pressure is decreased with an increasing effect of connement. 25 A reduction of 1.38 MPa (200 psi) in MMP for a light oil-pure CO 2 system is found when the pore radius is decreased from 1000 nm to 4 nm at a constant temperature. Moreover, for a light oil-CO 2 /CH 4 system, the MMP in bulk phase can be up to 3.45 MPa (500 psi) higher than that at the pore radius of 4 nm. It is obvious that the injection gas composition affects MMPs. However, to the best knowledge of the authors, no study so far has systematically studied the effects of the initial overall uid composition (e.g., initial oil composition, injection gas composition, or feed ratio of injection gas to oil) on the IFTs and MMPs in nanopores.
In this paper, rst, a PR-EOS is modied for the VLE calculations by considering the effects of capillary pressure and shis of the critical temperature and pressure, which is also coupled with the parachor model to predict the IFTs in nanopores. Second, the interfacial thickness between two mutually soluble phases (e.g., light oil and CO 2 phases) is determined by considering the two-way mass transfer, i.e., CO 2 dissolution into the oil phase and light hydrocarbons (HCs)-extraction from the oil phase by CO 2 . Based on the determined interfacial thicknesses, a new technical method, namely, the diminishing interface method (DIM), is proposed and applied to determine the MMPs. Finally, the following four important factors are specically studied to evaluate and compare their detailed effects on the IFTs, interfacial thicknesses, and MMPs at the pore radius of 10 nm: temperature, initial oil composition, injection gas composition, and feed ratio of injection gas to oil. It should be noted that most pore sizes of the middle Bakken formation are distributed in the range of 10-50 nm 26 and the effect of pore radius on the phase behaviour of oil-gas system is substantially increased in smaller pores. 25 Hence, 10 nm is particularly selected as the target pore radius in this study. Also, the feed ratio of injection gas to oil is always equal to 0.90 : 0.10 in mole fraction for all cases in this paper except for Section 4.4 Effects of feed ratio of injection gas to oil.

Experimental
In Table 1, the detailed compositions of the Pembina dead and live oil samples and Bakken live oil sample used in this study are listed. The properties of the Bakken live oil-CO 2 system were introduced in the literature. 25 Furthermore, a Pembina dead light oil sample was collected from the Pembina oileld, cardium formation in Alberta (Canada). The gas chromatography (GC) compositional analysis of the cleaned Pembina dead oil was performed and the detailed results can be found elsewhere. 27 The Pembina live oil with the gas-oil ratio (GOR) of 15 : 1 sm 3 /sm 3 was reconstituted by saturating the Pembina dead oil sample with the produced HC gas. The actual composition of the produced gas was equal to 66.50 mol% A mercury-free pressure-volume-temperature (PVT) system (PVT-0150-100-200-316-155, DBR, Canada) was used to measure the PVT data of the Pembina dead light oil-CO 2 system with four different CO 2 concentrations at T ¼ 53.0 C. 27 The measured PVT data are summarized in Table 2. The experimental setup and procedure of the PVT tests were described previously. 27 It is found that the experimentally measured saturation pressure, oil density, and oil-swelling factor (SF) increase with CO 2 concentration due to the CO 2 dissolution. In this work, the measured Pembina oil PVT data were used to tune the modied Peng-Robinson EOS (PR-EOS) for VLE calculations. Besides, a series of bubble-point pressures of the Bakken live oil-pure CO 2 system at different pore radii and T ¼ 116.1 C are obtained from the literature and plotted in Fig. 1.

Modied equation of state
In this study, a modied PR-EOS is proposed to calculate the VLE properties at the pore radius of 10 nm. 24 More specically, rst, the shis of critical properties (i.e., critical temperature and pressure) of the conned uids are considered in nanopores as follows, 23 where s LJ is the Lennard-Jones size diameter, s LJ ¼ 0:244 ; r p is the pore radius; T cp is the critical temperature in nanopores; T c is the critical temperature in bulk phase; P cp is the critical pressure in nanopores; P c is the critical pressure in bulk phase. In addition, the liquid and vapour phases are assumed to be the wetting phase and non-wetting phase, respectively. 26 Thus the capillary pressure (P cap ) is, where P V is the pressure of the vapour phase and P L is the pressure of the liquid phase. On the other hand, the capillary pressure can be expressed by the Young-Laplace equation, where g is the interfacial tension and q is the contact angle of the vapour-liquid interface with respect to the pore surface, which is assumed to be 30 according to the experimental results in the literature. 21 Therein, the IFT is estimated by means of the Macleod-Sugden equation, 28 where r L and r V are the respective molar densities of the liquid and vapour bulk phases; x i and y i are the respective mole percentages of the i th component in the liquid and vapour bulk Table 2 Measured 27 and calculated saturation pressures, oil densities, and oil-swelling factors (SFs) of the Pembina light oil-pure CO 2 systems at the reservoir temperature of T res ¼ 53.0 C a Test no. a x CO 2 : weight or mole percentage of CO 2 dissolved into in the dead light oil, P m sat : measured saturation pressure, P c sat : calculated saturation pressure, r m oil : measured oil density, r c oil : calculated oil density, SF m : measured oil-swelling factor, SF c : calculated oil-swelling factor, 3: relative error between the calculated and measured data. phases, i ¼ 1, 2,., r; r is the component number in the mixture; and p i is the parachor of the i th component.
The VLE calculations based on the modied PR-EOS require a series of iterative computation through, for example, the Newton-Raphson method. Fig. 2 shows the owchart of the VLE calculation process. The predicted PVT data for the Pembina dead oil-pure CO 2 system and Bakken live oil-pure CO 2 system are summarized and compared with the measured and/or literature date in Table 2 and Fig. 1, respectively. Overall, the modied PR-EOS in this study is capable of predicting the phase behaviour of the two light oil-CO 2 systems in bulk phase and/or nanopores, whose results agree well with the measured/literature data.

Interfacial thickness
In this study, a formula for determining the interfacial thickness between two mutually soluble phases (e.g., oil and CO 2 phases) is derived by taking account of the two-way mass transfer, where d is the distance between two mutually soluble phases, which is also denoted as the interfacial thickness. The detailed derivations for the interfacial thickness are stated in the ESI. † It should be noted that the sign of d is determined by the characteristics of the two bulk phases. More specically, if the two phases are barely mutually soluble and repulsive intermolecular interaction dominates in the interfacial region, d > 0 . If the two phases are mutually soluble and two-way mass transfer occurs across the interface, d < 0. In this study, the interfacial tension of the light oil-CO 2 system is decreased with the pressure so that d is negative. Although the sign of d can be positive, zero, or negative, the physical interfacial thickness has to be positive. 29

DIM for MMP determination
In theory, the interfacial thickness (s) is dened as the partial derivative of the IFT (g) with respect to the pressure (P) at a constant temperature, i.e., d ¼ (vg/vP) T . In this study, the interfacial thickness is obtained by using the forward nite difference approximation (FDA) of the partial derivative of the IFT (g) with respect to the pressure (P) at a constant temperature, i.e., d ¼ (Dg/DP) T . The IFTs and interfacial thicknesses between the oil and CO 2 phases as well as the FDA of the partial derivative of the interfacial thickness (second derivative of the IFT) with respect to the pressure at a constant temperature, i.e., (Dd/DP) T , for example, the Pembina live oil-pure CO 2 system at T ¼ 15.6 C is plotted in Fig. 3a. It is found that the IFT of the oil-CO 2 system is reduced with the pressure since the live light oil and pure CO 2 phase are mutual soluble. Thus the sign of d is negative in this study. However, it should be noted that the physical interfacial thickness is always positive even if the sign of d could be positive, zero, or negative. 29 In terms of the DIM method, the MMP is determined by linearly regressing and extrapolating the derivative of the interfacial thickness with respect to the pressure (Dd/DP) T vs. pressure data to zero. Physically, (Dd/DP) T ¼ 0 means the interfacial thickness between the oil and CO 2 phases becomes constant and remains unchanged with the pressure. Thus it is inferred that a stable interfacial thickness between the crude oil and CO 2 phases rather than a zero-IFT condition is obtained when the miscibility is achieved. Mathematically, the linearity of such a linear regression can be represented by the so-called linear correlation coefficient (LCC) or R 2 . More specically, the LCC of the linear regression of the data points from the highest (Dd/DP) T point at the lowest pressure to any point at an arbitrarily higher pressure is obtained for the MMP determination. In the previous study, R c 2 ¼ 0.990 is considered to be a critical value of the LCC criterion. 20 Hence, in Fig. 3a, the MMP of the Pembina live oil-pure CO 2 system is determined to be 7.0 MPa by using the LCC criterion from the DIM method at T ¼ 15.6 C.

Temperature effect
It is seen from Fig. 3a-e that the IFTs and interfacial thicknesses of the Pembina live oil-pure CO 2 system at the pore radius of 10 nm and ve different temperatures of T ¼ 15.6, 30.0, 53.0, 80.0, and 116.1 C are different, but overall, they are decreased with the pressure. More specically, in comparison with those at higher temperatures, the IFTs at lower temperatures are decreased much more rapidly with the pressure increase. This is because CO 2 solubility in oil phase is decreased with the temperature if the pressure is kept constant. 30 Accordingly, a larger density difference between the oil phase with less CO 2 dissolution and CO 2 phase is caused at a higher temperature and the corresponding IFT becomes higher. In addition, the interfacial thickness at a lower temperature is increased initially to different extents, that is, a substantial increase at T ¼ 15.6 and 30.0 C and a slight increase at T ¼ 53.0 C. Aerwards, it is quickly decreased and nally tends to be stabilized. However, the interfacial thickness of the oil-CO 2 system at a higher temperature (i.e., 80.0 and/or 116.1 C) is continuously decreased with the pressure. It is worthwhile to mention that among the ve different temperatures, the interfacial thickness of the oil-CO 2 system at the lowest temperature (i.e., 15.6 C) is quickly reduced to be the smallest at a high pressure while at the highest temperature (i.e., 116.1 C) it is decreased slowly and becomes the largest at a high pressure. In a similar manner with the Pembina live oil-CO 2 system, the IFTs and interfacial thicknesses of the Bakken live oil-pure CO 2 system at T ¼ 53.0 and 116.1 C are decreased with the pressure and shown in Fig. 4a and b.
By means of the DIM method, the determined MMPs of the Pembina live oil-pure CO 2 system at T ¼ 15.6, 30.0, 53.0, 80.0, and 116.1 C are shown in Fig. 3a-e, which increase from 7.0, 8.5, 13.7, 22.5, to 29.7 MPa, respectively. Moreover, the MMPs of the Bakken live oil-pure CO 2 system at T ¼ 53.0 and 116.1 C are determined to be 14.5 and 30.7 MPa, respectively. With the above-mentioned results, the temperature effects on the determined MMPs of the Pembina and Bakken live oil-pure CO 2 systems at the pore radius of 10 nm from the DIM method are summarized in Table 3 and plotted in Fig. 5. On a basis of the data points in this gure, the MMPs for the two oil-CO 2 systems at different temperatures are correlated to the temperature T (C) by using the linear regression: Pembina live oil-pure CO 2 system (7) It is found that in general, the determined MMP increases almost linearly with the temperature in the range of 15.6-116.1 C. In the literature, a linear correlation between the MMP and temperature in bulk phase was also recorded 15 and a test temperature was reported even up to 150 C. 31 More specically, in this study, the determined MMP increases with the elevated temperature at the respective rates of 0.2384 and 0.2531 MPa C À1 for the Pembina and Bakken live oil-pure CO 2 systems at the pore radius of 10 nm. Hence, it is concluded that the temperature has a strong effect on the MMP of the light oil-CO 2 system in nanopores. Fig. 6a and b show the predicted IFTs and interfacial thicknesses of the Pembina dead oil-pure CO 2 system at T ¼ 53.0 and 116.1 C, both of which are decreased with the pressure. In comparison with those of the Pembina live oil-pure CO 2 system, the IFTs and interfacial thicknesses are slightly lower/smaller and their rates of reductions are relatively higher with respect to the pressure. This is because the addition of CH 4 -dominated produced gas into oil sample reduces the CO 2 solubility in the oil phase. 18 Two respective MMPs are determined to be 11.6 and 26.5 MPa from the DIM at T ¼ 53.0 and 116.1 C, both of which are slightly lower than 13.7 and 29.7 MPa of the Pembina live oil-pure CO 2 system at the same temperature. The effects of initial oil composition on the determined MMPs are further summarized in Fig. 7. First, it is found that the effects of initial oil composition on the determined MMPs are measurable but marginal at a constant temperature. This is attributed to the high feed ratio of the injection gas to initial oil (i.e., 0.90 : 0.10 in mole fraction). In this case, the initial oil composition has a weak effect on the IFTs, interfacial thicknesses, and MMPs due to the small feed quantity. Second, the MMP increases from 11.6 to 13.7 MPa at T ¼ 53.0 C and from 26.5 to 29.7 MPa at T ¼ 116.1 C when the oil sample is changed from dead to live oil. A larger MMP increase occurs at a higher temperature and the Table 3 Summary of the predicted minimum miscibility pressures for the Pembina dead and live light oil-pure and impure CO 2 systems 27 and Bakken live light oil-pure and impure CO 2 systems 25 at the nanopore of 10 nm, different temperatures, and different feed ratios of gas to oil  pore radius of 10 nm. Hence, the MMP is found to be more sensitive to the initial oil composition at a higher temperature and it is better to control the temperature when a miscible gas injection project is conducted in the tight oil formation.

Effects of initial oil and injection gas compositions
In the oileld application of a CO 2 injection project, the injected pure CO 2 will likely contain some solution gas, the primary component of which is CH 4 . 32 In this study, the effects of CH 4 in an impure CO 2 sample on the IFTs, interfacial thicknesses, and MMPs are purposely studied by adding respective 0.10, 0.35, and 0.50 CH 4 in mole fraction into the pure CO 2 to obtain three different impure CO 2 samples. Fig. 8ac indicate that the IFTs and interfacial thicknesses for the Pembina live oil at T ¼ 53.0 C have a slightly decreasing rate of reduction with respective to the pressure if CH 4 content increases from 0.10, 0.35, to 0.50 in mole fraction. Meanwhile, the MMPs substantially increase from 17.2, 26.3, to 31.0 MPa  Obviously, the addition of CH 4 can signicantly increase the MMP, plus the high feed ratio of injection gas to oil (i.e., 0.90 : 0.10 in mole fraction) maximizes the effect of injection gas composition. In comparison with CO 2 , CH 4 has a much lower solubility in the oil and a rather weaker ability for light and intermediate-HC extractions. 33 Thus the addition of CH 4 Fig. 9 Predicted interfacial tensions, interfacial thicknesses, and minimum miscibility pressures of the (a) Bakken live oil-90 mol% CO 2 + 10 mol% CH 4 system; (b) Bakken live oil-65 mol% CO 2 + 35 mol% CH 4 system; and (c) Bakken live oil-50 mol% CO 2 + 50 mol% CH 4  into a pure CO 2 sample can severely prevent the two-way mass transfer between the oil and gas phases and result in a lower rate of IFT/interfacial thickness reduction with pressure as well as a higher MMP.
With the above-mentioned MMPs of the Pembina and Bakken live oil-impure CO 2 systems at T ¼ 53.0 C, the effects of injection gas composition on the MMPs are compared in Fig. 10. On a basis of the data points in this gure, the MMPs are linearly correlated to the amount of CH 4 addition in an impure CO 2 sample in mole fraction: Pembina live oil-impure CO 2 system (9) MMP/MPa ¼ 26.5056y CH 4 + 14.8176 Bakken live oil-impure CO 2 system (10) The above two correlations show that the MMP is rather sensitive to the CH 4 content in gas phase and it is increased linearly with CH 4 content up to 0.50 mole fraction. More specically, the MMP increases linearly with an increasing CH 4 content at the approximate rates of 0.3497 MPa and 0.2651 MPa per 0.01 CH 4 for the Pembina and Bakken live oil, respectively. It is found that by comparison, the unit change of injection gas composition affects the interfacial properties and MMPs to the most extent among the temperature, initial oil composition, and injection gas composition. Thus it is necessary to control the CH 4 content in the injection gas to ensure a miscible CO 2 injection in the oileld application for a tight oil formation. On the other hand, some addition of the intermediate HCs (e.g., C 2 -C 6 ) into the gas phase may be benecial for the miscibility development and MMP reduction, 34 which is recommended to  be implemented in the actual CO 2 -EOR project for the tight oil formation.  Fig. 3c, it is found that the IFTs and interfacial thicknesses at the ve feed ratios are fairly similar but their rates of reductions with respect to pressure is slightly increased by reducing the feed gas-oil ratio. In Fig. 3c and 11a- Table 3 and plotted in Fig. 13. Hence, it is concluded from the gure that at a constant temperature, the effects of the feed gas-oil ratio on the IFTs, interfacial thicknesses, and MMPs with the pure CO 2 injection is negligible, especially at low feed gas-oil ratios (less than 0.50 : 0.50). However, the IFTs, interfacial thicknesses, and MMPs of the impure CO 2 case are strongly dependent on the feed gas-oil ratio. More specically, the MMP is considerably increased if the feed gas-oil ratio reduces from 0.90 : 0.10 to 0.10 : 0.90 in mole fraction. Thus in the oileld application for a tight oil formation, a lower feed gas-oil ratio (i.e., smaller than 0.50 : 0.50 in mole fraction) is suggested for a pure CO 2 injection project, whereas a higher feed gas-oil ratio is preferred for a mixed CO 2 -CH 4 gas injection.

Conclusions
The following seven major conclusions can be drawn from this work: The modied Peng-Robinson equation of state (PR-EOS) coupled with the parachor model is found to be accurate for vapour-liquid equilibrium (VLE) calculations and interfacial tension (IFT) predictions in nanopores.
A thermodynamic formula of the interfacial thickness between two mutually soluble phases is derived, based on which the novel diminishing interface method (DIM) is developed. The minimum miscibility pressure (MMP) at the pore radius of 10 nm is determined by extrapolating (vd/vP) T to zero.
A higher temperature results in a higher IFT/interfacial thickness and a lower rate of the IFT/interfacial thickness reduction in nanopores. The MMPs of the Pembina and Bakken live oil-pure CO 2  The effects of initial oil composition on the IFTs, interfacial thicknesses, and MMPs are measurable but marginal in nanopores. The MMP increases from 11.6 to 13.7 MPa at T ¼ 53.0 C and from 26.5 to 29.7 MPa at T ¼ 116.1 C when the oil sample is changed from Pembina dead to live oil. It is found that the MMP is more sensitive to the initial oil composition at a higher temperature and the pore radius of 10 nm.
The IFTs and interfacial thicknesses are weakly but the MMPs are strongly dependent on the injection gas composition. The IFTs and interfacial thicknesses for the Pembina and Bakken oil cases at T ¼ 53.0 C have a slightly decreasing rate of reduction with respective to pressure if CH 4 content increases from 0.10, 0.35, to 0.50, whereas the MMPs substantially increase from 17.2, 26.3, to 31.0 MPa for the Pembina live oil and from 17.5, 24.1, to 28.1 MPa for the Bakken live oil.
The effects of the feed gas-oil ratio on the IFTs, interfacial thicknesses, and MMPs are found to be negligible with pure CO 2 injection, especially at low feed gas-oil ratios (less than 0.50 : 0.50), but become much stronger for the impure CO 2 injection. The MMP of the Pembina live oil-0.65CO 2 + 0.35CH 4 system increases from 26.3 to 40.0 MPa when the feed gas-oil ratio reduces from 0.90 : 0.10 to 0.10 : 0.90 in mole fraction.
For a miscible CO 2 injection project in the tight formation, the following strategies are effective to enhance tight oil recovery: a lower reservoir temperature, some additions of intermediate hydrocarbons (e.g., C 2 -C 6 ) into the injection gas sample, and a lower feed gas-oil ratio for pure CO 2 injection or a higher ratio for impure CO 2 injection.

Conflicts of interest
There are no conicts to declare.