Impact of water on miscibility characteristics of the CO₂/n-hexadecane system using the pendant drop shape analysis method

2.1 Experimental materials

In this study, CO₂ with the purity of 99.9% is employed as the gas phase, and vacuumed deionized water is dissolved into CO₂ to provide the water component in the CO₂/oil system. The pure oil sample of n-hexadecane (n-C₁₆H₃₄, with molecular weight of 226.5, density of 770 kg/m³, and freezing point of 17–19 °C) is employed as the oil phase. The deionized water and pure oil sample are employed in this paper to focus on the mechanism behind the water present in the CO₂/oil system without involving the salinity as well as the oil composite’s influences.

2.2 Experimental apparatus

The KRUSS DSA100HP optical measurement system, consisting of the KRUSS DSA100 Droplet Shape Analyzer and the EuroTechnica PD-E1700 high-temperature high-pressure (HTHP) gas/liquid supply device, is employed to conduct the MMP measurements of the CO₂/oil system and the CO₂/oil/water system (Cui et al., 2022; Liu et al. 2022b). The chamber temperature is measured by a thermocouple (Thermoexpert) with the uncertainty of 0.2 °C at 80 °C circumstance, and the system pressure is measured with the built-in pressure gauge (Armaturenbau GmbH) with the calibrated accuracy of 0.1% in the range of 0–100 MPa.

The schematic diagram as well as the physical image of the experimental apparatus is shown in Fig. 1 (a) and (b) respectively.

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Fig. 1 The (a) schematic diagram and (b) physical picture of the MMP measurement apparatus.

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2.3 Experimental procedure

The procedure for measuring the MMP of the binary CO₂/oil system has been described in detail by Cui et al. (2022). In short, with the help of the scientific software of KRUSS ADVANCE, the pendant oil shape variation with time is recorded and based on the shrinkage behavior of the oil droplet in a certain period, the MMPs of the CO₂/oil system could be obtained. In this study, the MMPs of CO₂/n-C₁₆H₃₄ were measured at first as the control group. For the two-phase three-component CO₂/n-C₁₆H₃₄/H₂O system, there need extra steps to supply water component into the system. The detailed procedure for measuring MMPs of the water presence system is described as follows,

1. Remove the air from the test chamber with continuous CO₂ injection for 5 min with the gas inlet and outlet valve open. Then close the gas inlet valve and partially close the liquid outlet valve.

2. Inject deionized water from the gas outlet tube into the chamber. Continue the water injection until the water flow is observed through the liquid outlet and the accumulated water shows a clear surface at the bottom part of the chamber. Then close the liquid outlet.

3. Flood the chamber again at atmospheric pressure with CO₂ injection through the gas inlet to make sure the system contains only CO₂ and water. Then close all the valves connecting to the test chamber.

4. Heat the test chamber until the temperature inside the chamber reaches the designated value.

5. Pressurize the chamber with continuous injection of CO₂ through the gas inlet until the pressure inside the chamber approaches to the near-miscibility point of the studied system. The near miscibility pressure point is set after trial and error pre-tests.

6. Maintain the system under designated pressure and temperature for 8 h to attain the quasi-thermodynamic equilibrium condition in which CO₂ is assumed to be saturated with water component.

7. Inject the oil from the capillary burette to form a stable pendant drop. Record the oil droplet volume with a CCD camera and analyze its volume variation history in the period of 300 s at 15 s intervals with the help of the software of ADVANCE. Fig. 2 shows a snapshot of the formed CO₂/oil/water system inside the test chamber.

8. Repeat step 7) three times to obtain consistent pendant drop variation results.

9. Pressurize the chamber with CO₂ injection to a higher pressure level, then stabilize the system for 10 min to cope with the slightly variant water solubility in CO₂ at the elevated pressure (Zhao and Lvov, 2016). Then perform steps 7) and 8) again.

10. Repeat steps 7) to 9) until the set of measurements has been finished under all the designated pressure levels. Determine the MMP of the CO₂/oil/water system based on the quantitative criteria in the ODVM method, that is, 10% oil droplet reduction rate within a 300 s measurement period (Cui et al., 2022).

11. Repeat steps 1) to 10) at another designated temperature to study the miscibility characteristics of the three component two phase system at a wider temperature range.

12. After finishing one round of MMP measurements at different temperature conditions, the measurement is performed again by repeating steps 1) to 11) to make sure the consistency of the measured MMPs of the CO₂/oil/water system.

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Fig. 2 A snapshot of the CO₂/oil/water system inside the test chamber.

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As an example of the measurement procedure, Fig. 3 depicts the dynamic oil droplet volume variation results at system condition of 12.2 MPa and 72.1 °C. Following above mentioned step 8), three times of measurements (a, b and c) were performed to obtain reliable results. Table 1 lists in detail the pendant oil droplet volume values at different times in the three measurements, showing the consistent droplet volume shrinkage rate of over 10% within the 300 s’ measurement period that complies with the MMP determination criteria for the CO₂/C15 + oil systems (Cui et al., 2022). Based on the repeatable results, the MMP of the CO₂/n-C₁₆H₃₄/H₂O system is determined, therefore, to be 12.2 MPa at 72.1 °C.

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Fig. 3 Three-time measurement results (a, b and c) on oil droplet volume variation behavior of the CO₂/n-C₁₆H₃₄/H₂O system at 12.2 MPa and 72.1 °C.

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3.1 Validation study based on MMP results of the binary CO₂/n-C₁₆H₃₄ system

To validate the accuracy of the measurement results, we measured the MMPs of the binary CO₂/n-C₁₆H₃₄ system at different temperature conditions and compared the measured results with the reported experimental results (Cui et al., 2022) and the predictions from the phase equilibrium data (D’souza et al., 1988). To make comparisons with the phase equilibrium data, at first we estimate the CO₂ molar fraction in the binary mixture interface region based on the equal-volume ideal mixing assumption under the measured MMP conditions, then determine the phase equilibrium predicted MMPs of the CO₂/oil system as the pressure where the CO₂ solubility equals to the interfacial CO₂ molar fraction.

Fig. 4 plots the measured (in upper triangular dots) together with the fit line (in dashed line) of the MMP results of the binary system in the temperature range of 40∼82 °C. A clear linear relationship between system MMPs and temperatures is observed and can be fit as,

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Fig. 4 Validation study based on comparisons between measured MMP values and reported results (Cui et al., 2022; D’souza et al., 1988) of the binary CO₂/n-C₁₆H₃₄ system.

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To make clear comparisons, the literature reported MMP measurement results (in solid line by Cui et al., 2022), as well as the phase equilibrium calculated MMP values (in lower triangular dots by D’souza et al., 1988) of the binary CO₂/n-C₁₆H₃₄ system are also plotted in Fig. 4. The Absolute Deviation (AD) and the Average Absolute Deviation (AAD), which are defined as Eqs. (2) and (3) respectively (Wang et al., 2021), are employed as the evaluation indicator of the accuracy of the measurement results in this study.

Based on the comparative results, as shown in Fig. 4, it is calculated the maximal AD value is less than 6.6% and the AAD value is less than 4%, which validates, therefore, the reliability and the accuracy of the measurement technique employed in this paper.

3.2 Water presence effect on MMPs of the CO₂/n-C₁₆H₃₄ system

To make clear vision of the water presence effect on the miscibility characteristics of the CO₂/n-C₁₆H₃₄ system, Fig. 5 shows, as a representative case, the oil droplet volume variation history of the CO₂/oil system and CO₂/oil/water system in the very close pressure conditions of 12.6 MPa and 12.2 MPa at temperature of 72.1 °C. It is clearly observed from Fig. 5 (a) that the oil droplet increases from 1.099 μL to 1.364 μL in the CO₂/oil system at 12.6 MPa, indicating the immiscible condition between CO₂ and n-C₁₆H₃₄. Whereas in the water presence system, as shown Fig. 5(b), the oil droplet volume decreases from 1.776 μL to 1.596 μL in 300 s, showing the obvious miscible behavior based on the quantitative criterion of 10% volume reduction in 300 s as adopted by the ODVM technique (Cui et al., 2022). It is concluded, therefore, the presence of water results in the decreased MMP of the CO₂/n-C₁₆H₃₄ system to 12.2 MPa, under which the water-free CO₂/n-C₁₆H₃₄ still stands as an immiscible system.

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Fig. 5 Water presence effect on oil droplet volume variation behaviors at 72.1 °C. (a) CO₂/n-C₁₆H₃₄ system at 12.6 MPa (b) CO₂/n-C₁₆H₃₄/H₂O at 12.2 MPa.

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To reveal the MMP determination processes for the water presence CO₂/n-C₁₆H₃₄ system, Fig. 6 (a)-(e) plot the oil volume variation histories at different pressure levels under various temperatures of 40.5 °C, 51.7 °C, 61.2 °C, 72.1 °C and 82.3 °C respectively. The detailed oil droplet volume variation results in three time measurements, as mentioned as step 8) in Section 2.3, are supplemented in Appendix as Tables A1–A5 for the five different temperatures.

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Fig. 6 MMP determination for CO₂/n-C₁₆H₃₄/H₂O system based on oil droplet volume variation history under different pressures at temperature of (a) 40.5 °C, (b) 51.7 °C, (c) 61.2 °C, (d) 72.1 °C and (e) 82.3°C.

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In Fig. 6 (a), it is observed at 6.4 MPa and 7.0 MPa, the pendant oil swells with the elapsing time, showing the obvious immiscible behavior. Whereas under the pressure of 8.4 and 9.0 MPa, the oil droplet shrinks so fast that the system is expected to well exceed the MMP point. Under the pressure of 7.8 MPa, on the other hand, oil droplet shows the volume reduction rate of 10% in the period of 300 s, which complies with criterion as employed by the ODVM technique (Cui et al., 2022). Accordingly, the pressure of 7.8 MPa is determined as the MMP of the CO₂/n-C₁₆H₃₄/H₂O system at temperature of 40.5 °C. In the similar manner, the MMPs of the CO₂/n-C₁₆H₃₄/H₂O system under different temperatures of 51.7 °C, 61.2 °C, 72.1 °C and 82.3 °C have been determined to be 8.8 MPa, 10.2 MPa, 12.2 MPa and 13.2 MPa respectively based on measurement results in Fig. 6 (b)-(e).

Fig. 7 compares the MMPs of the CO₂/n-C₁₆H₃₄ system with or without water presence at five temperatures in the range 40–82 °C. The measurement error of $\pm 0.1\mathrm{M}\mathrm{P}\mathrm{a}$ , which corresponds to the pressure gauge accuracy as pointed out in previous Section 2.1, is plotted together with the measurement results as well in the figure. It is clearly observed the water presence could unanimously reduce the MMPs of the CO₂/n-C₁₆H₃₄ system, namely, from 8.2 to 7.8 MPa, from 9.6 to 8.8 MPa, from 11.6 to 10.2 MPa, from 13.0 to 12.2 MPa, and from 14.6 to 13.2 MPa at temperature levels of 40 °C, 51 °C, 61 °C, 72 °C and 82 °C respectively.

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Fig. 7 Comparisons of the MMP values between the CO₂/n-C₁₆H₃₄ system and the CO₂/n-C₁₆H₃₄/H₂O system at different temperatures.

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It has to be pointed out in the water-CO₂ coexistence system, CO₂ chemistry in aqueous solutions always rise as an important factor. For instance, the sufficient dissolution of CO₂ in the water phase could reduce the PH values of the aqueous phase and the PH decrease effect relates tightly to system temperatures (Hunter and Savage, 2003). In our study, on the other hand, the MMPs of the CO₂/n-C₁₆H₃₄/H₂O system are determined based on pendant drop volume analysis, where the oil droplet directly contact the CO₂-rich phase instead of the aqueous phase, therefore the PH variation in aqueous phase on MMPs of the CO₂/oil system can be reasonably neglected. The reasons behind the dissolved water on the MMP variations of the CO₂/oil system are analyzed in Section 3.3 based on the thermodynamic calculations, and further discussions on significances of the research are provided in the following Section 3.4.

3.3 Thermodynamic calculations

To understand the water presence effect on lowering MMPs of the CO₂/oil system, thermodynamic phase equilibrium calculations are carried out and the corresponding results are provided in this section. Based on the physical model in our previous work (Wang et al., 2021), as shown in Fig. 8 the three-component two-phase system in the interfacial region, the phase behavior of the CO₂/oil/water system is investigated.

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Fig. 8 The physical model employed in thermodynamic calculation studies of the miscibility characteristics of the CO₂/n-C₁₆H₃₄/H₂O system (Wang et al., 2021).

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In the thermodynamic calculations, we employed the P-R EOS model (Peng and Robinson, 1976). Although was most commonly employed in the nonpolar substances system such as CO₂/hydrocarbons, P-R EOS has found wide applications on the water containing system as well. The typical operation is to modify the alpha term and the binary interaction parameter (BIP) in the P-R EOS, thus to reproduce accurately the phase behavior of the gas/water or gas/brine system (Liu et al., 2022a; Søreide and Whitson, 1992). In our previous work (Wang et al., 2021), we have testified the specified alpha term and BIPs for the CO₂/oil, CO₂/water and oil/water couples, as well as the way to introduce water component in the two-phase three-component system. In consistent with experimental observations that there is no water condensation, the thermodynamic calculation doesn’t encounter the multiphase situation either. Specifically, we employed the following modified alpha and BIP terms for the calculation of the mutual solubility between the CO₂ and water.

The alpha function of H₂O is listed in Eq. (4) (D’souza et al., 1988),

We employed two independent BIPs (Poirier, 2012)(kij ≠ kji,), as listed in Eq. (5) and Eq. (6), in the thermodynamic calculations for the binary CO₂/H₂O system (Wang et al., 2021).

To show clearly the water effect on miscibility characteristics of the CO₂/n-C₁₆H₃₄ system, Table 2 and Table 3 comparatively list the MMP results, calculated together with measured, of the binary CO₂/oil systems and the water presence CO₂/oil system in the temperature range 40–82 °C. In these two tables, the comparisons between the calculated and the measured MMP results show the AAD of 6.2% and 6.3% respectively, both lie below the acceptable discrepancy range of 8% (Chabab et al., 2019), therefore validates the calculation results. Through detailed comparisons between Table 2 and Table 3, calculation results clearly show as well the water presence could reduce the MMP values of the CO₂/n-C₁₆H₃₄ system, namely, from 8.95 to 8.5 MPa, from 10.6 to 9.85 MPa, from 12.3 to 10.7 MPa, from 13.5 to 11.6 MPa and from 14.0 to 13.1 MPa at different temperature levels of 40 °C, 51°C, 61°C, 71 °C and 82 °C respectively.

As revealed by the thermodynamic calculations, the lower CO₂ mole fraction in the water presence system could be one of the main mechanisms behind the MMP reduction behavior of the CO₂/oil system. Jessen and Orr (2008) has reported the unanimously decreasing saturation pressure, which equals to the slim tube MMP at certain CO₂/oil mixture composition, with the decreasing CO₂ fraction in the system. As displayed in Fig. 8, it is deduced the system containing water component would pull down both the molar fraction of CO₂ and oil phase as compared to the system without water. The lower molar fraction of CO₂ could be more easily accommodated in the intermolecular alkane spaces, thus to form the stable single phase system at lower pressure levels (Han et al., 2015; Liu et al., 2017).

3.4 Discussions

In this chapter, further discussions are carried out to highlight the significances of this research on guiding the CO₂ EOR and CCUS applications.

As the common case in CO₂ EOR practices, the miscibility characteristics of CO₂/oil/water systems have been encountered frequently and therefore got extensive research attentions. Although it is well recognized the water presence may decrease the diffusion rate between CO₂ and oil due to the bridging and blockage effect affect (Askari et al., 2022; Wylie and Mohanty, 1997), lot of researchers have reported the same or even better recovery results in water presence environments at near miscible conditions, that is, at pressure slightly lower than the MMP of the system. It is noted the dynamic diffusion characteristics of the CO₂/oil/water system, although being important for the dynamic flooding processes, does not necessarily related to the thermodynamic equilibrium MMP results (Dindoruk et al., 2021).

Shyeh-Yung (1999) conducted comprehensive experiments on CO₂ flooding decane in Berea sandstone as well as on CO₂ flooding degassed realistic oil in Texas carbonate at near-miscible conditions with 3.5 wt% total dissolved solids (TDS) brine presence. They didn’t observe the abrupt decline on oil recovery efficiency when system switching from the miscible to the near-miscible condition, which substantially contradicts the slim tube results. Sohrabi et al. (2008) investigated in pore-level the near-miscible gas flooding oil (mixture of n-decane and methane) mechanisms with distilled water presence by visualizing the displacement behavior in high-pressure glass micromodels. Their results showed the near miscible gas flooding is a very effective oil recovery technology. Bui et al. (2010) conducted CO₂ flooding to displace Arbuckle oil from Berea sandstone at the near miscible condition and they reported over 50% of residual oil after the brine (1 wt% TDS) flooding could be recovered by the CO₂ injection. Alhosani et al. (2019) studied near-miscible CO₂ injection in the Ketton limestone saturated with oil (80% wt% decane and 20 wt% iododecane) and reservoir brine (70% brine + 30% Sodium Iodide) using X-ray micro-tomography. They reported up to 80% of the residue oil could be recovered with the supercritical CO₂ injection. Ajoma et al. (2021) demonstrated through laboratory works the superiority of pure water saturated CO₂ (wsCO₂) injection over pure CO₂ injection under both miscible and near-miscible conditions in Bentheimer cores. They reported wsCO₂ injection could provide higher oil (65 mol% hexane and 35 mol% decane) recovery (4–11%) and higher net CO₂ storage amount (4–9%) at miscibility and near-miscibility conditions rather than that obtained by CO₂ injection.

Although the favorable water presence effects on oil recovery and CO₂ storage at near miscible conditions have been widely reported, the reasons provided by the researchers behind their experimental observations are quite different. Some researchers attributed the high oil recovery below MMP to the low gas-oil interfacial tension, which reduces the pore level bypassing flow and therefore enhances the gas sweep efficiency (Alhosani et al., 2019; Shyeh-Yung, 1999; Sohrabi et al., 2008). Some researchers attributed the experimental phenomena at near miscible conditions to the substantial CO₂ dissolution in the oil phase, which reduces significantly the oil viscosities to help the oil recovery processes (Bui et al., 2010). Some researchers, on the other hand, attributed the favorable water presence effect at miscible and near miscible conditions to the significantly reduced mobility ratio between CO₂ and water (Ajoma et al., 2021). So far, few researchers have addressed specifically the impact of water on miscibility characteristics of the CO₂/oil system.

It is expected, therefore, the research work reported in this paper could provide an innovative viewpoint on understanding the mechanisms behind the water presence effect in the miscible and near miscible CO₂ flooding processes, thus help substantially the CO₂ EOR and CO₂ geological storage practices. The further systematic investigations on the salinity and oil compositions effects would be the challenging research topics in the future works.