(640f) Effect of Iodide-Doped Fluids on the Interfacial Tensions of CO2, N2, Decane and H2o at Elevated Temperatures and Pressures

CO₂, N₂, decane and H₂O are common analogues to represent reservoir fluids in laboratory experiments. The accurate knowledge of interfacial tension (IFT) of them is essential for efficient oil and gas extraction and carbon geological storage [1, 2]. X-ray CT-imaging is a helpful technique combined with core flooding experiments, as it enables the visualization of the multiphase flow process as well as the pore-scale phenomena. To enhance the contrast between fluids, iodized organics and iodide-containing salts, known as the contrast agents, are usually added to the fluids [3]. In this work, the effect of the iodide-doped fluids on the interfacial tension at reservoir conditions are studied experimentally and numerically.

Iododecane was selected as a representative of the contrast agents added to the oil phase and potassium iodide was selected as a representative contrast agent added to the aqueous phase. The IFTs between gas (CO₂ and N₂), 7wt% KI (aq) and decane-iododecane mixtures were measured by the pendant drop method. The high-pressure view cell was firstly filled with gas or the lighter liquid. A drop of the denser fluid was then suspended at the tip of a capillary. The profile of the pedant drop was captured by a CCD camera and analyzed according to the Axisymmetric Drop Shape Analysis (ADSA). The experimental temperature was kept by a thermostat with the standard uncertainty of 0.025 K and the experimental pressure was kept by the bulk phase with the standard uncertainty of 0.035 MPa. The densities of pure decane, N₂ and CO₂ were taken from REFPROP 10.0 software [4]. The densities of pure iododecane and decane-iododecane mixtures were measured by a DMA HP densimeter (Anton Paar), which was newly calibrated under vacuum and with ultrapure water at the pressures from 0.1 MPa to 65 MPa and the temperatures from 283 K to 473 K. The densities of the fluid samples are determined by measuring the resonance periods of the vibrating tube filled with the fluid samples. An empirical equation was developed to correlate the densities of the decane-iododecane mixture with temperature, pressure and composition. The densities of 7wt% KI (aq) was obtained by an empirical equation [5]. The saturated densities were calculated by the volume translated Peng-Robinson equation of state, with the binary interaction coefficients regressed according to the phase equilibrium data. The volume translation values of CO₂, N₂, decane and iododecane at each isotherm were regressed based on the pure densities. The interfacial tension was then calculated by iterative fitting of the Young-Laplace equation [6].

Calibrations were carried out with a spherical calibration tool in air (for gas-liquid system) and in DI water (for liquid-liquid system) to minimize the effect of the bulk phase refractive index changes. It is reported that there are surface-active impurities in the normal alkanes and purification processes are needed prior to the measurement [7]. Therefore, decane was passed through a 500 mm long chromatography column filled with activated alumina (100 mesh). It was found that activated alumina can catalyze partial decomposition of iododecane, thus iododecane was purified by passing it through the same column packed with activated charcoal (100 mesh). For each liquid drop, IFTs were measured and recorded for at least 600 s and there was no drift of IFTs over time, which proved the success of the purification operations.

In addition, calculations were implemented by using the predictive volume-translated Peng-Robinson equation of state (VTPR) coupled with the square gradient theory (SGT) to compare with and supplement the measurement results. The bulk phase equilibria properties were firstly determined through VTPR equation of state. The molar density distribution of each component across the interface were determined by minimization of the Helmholtz free function and the interfacial tension between the two phases were obtained accordingly. Prior to the interfacial tension calculation, the influence parameters of H₂O, decane and iododecane were regressed according to the respective surface tension. The surface tensions of iododecane were measured in this work at the temperatures ranging from 298 K to 353 K. The influence parameters of N₂ and CO₂ were obtained from literature [8] as they are supercritical over the entire or part of the temperature range considered. This method was extended to mixtures by using the quadratic mixing rule (QMR).

We report the IFTs between CO₂, N₂, 7wt% KI (aq) and decane-iododecane mixtures with iododecane mass fraction of 0, 50%, 70%, 90% and 100%. Experimental measurements were carried out at four isotherms (298 K, 313 K, 333 K and 353 K) and pressures up to 30 MPa, or near the miscibility pressures for CO₂ and hydrocarbon systems. The expanded relative uncertainty at 95% confidence is about 1%, except in the (CO₂ + decane-iododecane mixtures) system, the expanded relative uncertainty at 95% confidence is up to 8% near miscibility pressures. The IFTs of (N₂-7wt% KI (aq)) systems and (N₂-hydrocarbon) systems are both observed to decrease with increase of either pressure or temperature. The IFTs between CO₂ and decane-iododecane mixtures decrease with the increasing pressure, until the miscibility pressure is reached. The IFTs between H₂O and decane-iododecane mixtures decrease with the increasing temperature and exhibit slight increase with the increasing pressure.

The IFTs between gas and brine have a linear relationship with the brine salinity, and the IFTs between N₂ and 7wt% KI (aq) are about 2.5 mN/m greater than those in the salt-free system over the entire temperature and pressure ranges. This difference exceeds the measurement uncertainty. In the (N₂ + decane-iododecane mixture) system, the IFTs increase with the iododecane mass fraction. IFTs between N₂ and iododecane are about 6.5 mN/m greater than those between N₂ and decane over the entire temperature and pressure ranges. In the (CO₂ + decane-iododecane mixture) system, IFTs increases linearly with the iododecane mass fraction at the same experimental temperature and pressure conditions. The miscibility pressure of CO₂ with decane-iododecane mixture increases with the iododecane mass fraction. In the (H₂O + decane-iododecane mixture) system, the IFTs decreases with the iododecane mass fraction and the IFTs between H₂O and iododecane are about 4.5 mN/m lower than those between H₂O and decane. Empirical equations were developed to correlate all of the measured data in terms of temperature, pressure, KI molality and iododecane mass fraction.

The IFTs of the binary systems (N₂-H₂O, N₂-decane, N₂-iododecane, CO₂-decane, CO₂-iododecane) and ternary systems (N₂-decane-iododecane and CO₂-decane-iododecane) were fully predicted by the van der Waals Square Gradient Theory coupled with the volume translated Peng Robinson EOS. The models can provide accurate predictions of the interfacial tensions of the systems studied over the entire temperature and pressure scope. Therefore, the VTPR + SGT approach can be recommended as a feasible modeling method for the interfacial tensions of systems comprising N₂, CO₂, decane, iododecane and H₂O at elevated temperatures and pressures.

In this work, we present the experimental results of high-pressure high-temperature IFTs of CO₂, N₂, decane and H₂O with iodide-containing fluids and evaluated the ability of the VTPR + SGT method of representing the data. The IFT data obtained can support and assist the interpretation of multiphase flow in porous media for enhanced oil recovery and carbon geological storage and provide fundamental information for reliable numerical simulation.

References

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