Interfacial tension measurements and modelling of (carbon dioxide+n-alkane) and (carbon dioxide+water) binary mixtures at elevated pressures and temperatures

Molecular Insights into the Wettability and Adsorption of Acid Gas-Water Mixture

Sequestration of acid gas in geological formations is a disposal method with potential economic and environmental benefits. The process is governed by variables such as gas-water interfacial tension, wetting transition, and gas adsorption into water, among other things. However, the influence of the pressure and temperature on these parameters is poorly understood. This study investigates these parameters using coarse-grained molecular dynamics (CG-MD) simulations and density gradient theory (DGT). Simulations were carried out at 313.15 K and a pressure range of 0-15 MPa. A comparison was made against H2S-water systems to clarify the effects of adsorption on interfacial tension due to vapor-liquid-liquid equilibrium. The predicted H2S-water interfacial tension and phase densities by CG-MD and DGT matched the experimental values well. The adsorption can be quantified via the Gibbs Adsorption function Γ12, which correlated well with the three-phase transition. On the one hand, pressure increments below the three-phase transition revealed a significant adsorption of H2S. On the other hand, above the three-phase transition, the Gibbs Adsorption capacity remained constant, which indicated a saturation of H2S at the water surface due to liquid-liquid equilibrium. Finally, H2S behaves markedly differently in wetting transition, rather than the involved for CO2 to different molecular layers beneath the surface of aqueous solutions. In this respect, H2S is represented by a first-order wetting transition while CO2 presents a critical wetting. Finally, it has also been found that the preferential adsorption of H2S over the H2O interface is greater if compared to that of CO2, due to its strong interaction with water. In fact, we have also demonstrated that CO2 under triphasic conditions strongly influences the wetting of the ternary system.

Numerical analysis of pore-scale CO2-EOR at near-miscible flow condition to perceive the displacement mechanism

Gas flooding through the injection of [Formula: see text] is generally performed to achieve optimum oil recovery from underground hydrocarbon reservoirs. However, miscible flooding, which is the most efficient way to achieve maximum oil recovery, is not suitable for all reservoirs due to challenge in maintaining pressure conditions. In this circumstances, a near-miscible process may be more practical. This study focuses on pore-scale near-miscible [Formula: see text]-Oil displacement, using available literature criteria to determine the effective near-miscible region. For the first time, two separate numerical approaches are coupled to examine the behavior of [Formula: see text]-oil at the lower-pressure boundary of the specified region. The first one, the Phase-field module, was implemented to trace the movement of fluids in the displacement [Formula: see text]-Oil process by applying the Navier-Stokes equation. Next is the TDS module which incorporates the effect of [Formula: see text] mass transfer into the oil phase by coupling classical Fick's law to the fluids interface to track the variation of [Formula: see text] diffusion coefficient. To better recognize the oil recovery mechanism in pore-scale, qualitative analysis indicates that interface is moved into the by-passed oil due to low interfacial tension in the near-miscible region. Moreover, behind the front ahead of the main flow stream, the [Formula: see text] phase can significantly displace almost all the bypassed oil in normal pores and effectively decrease the large amounts in small pores. The results show that by incorporating mass transfer and capillary cross-flow mechanisms in the simulations, the displacement of by-passed oil in pores can be significantly improved, leading to an increase in oil recovery from 92 to over 98%, which is comparable to the result of miscible gas injection. The outcome of this research emphasizes the significance of applying the [Formula: see text]-EOR process under near-miscible operating conditions.

Understanding diffusion-controlled bubble growth in porous media using experiments and simulations

Nucleation and subsequent expansion of gas bubbles in porous media is relevant to many applications, including oil recovery, carbon storage, and boiling. We have built an experimental setup using microfluidic chips to study the dynamics of bubble growth in porous media. Visualization experiments of the growth of carbon dioxide bubbles in a supersaturated dodecane solution were conducted. We show that bubbles grow as dissolved gas molecules inside the oversaturated liquid diffuse to the gas-liquid interface. Bubbles expanding inside a porous medium displace the liquid phase until the cluster of the gas-filled pores becomes connected to the outlet at the critical gas saturation, which is used as a measure for the total liquid displacement. Our experiments uniquely focus on the growth of a single bubble and show that larger pressure drops lead to faster bubble growth while resulting in lower critical gas saturations. A nonlinear pore-network model is implemented to simulate bubble growth. We compare model predictions for bubble growth dynamics to our experimental results and present the need for further theoretical development to capture deviations from invasion-percolation when a large pressure drop is applied.

Surface tensions at elevated pressure depend strongly on bulk phase saturation

HYPOTHESIS

Understanding interfacial phenomena at elevated pressure is crucial to the design of a variety of processes, modeling important systems, and interpreting interfacial thermodynamics. While many previous studies have offered insight into these areas, current techniques have inherent drawbacks that limit equilibrium measurements.

EXPERIMENTS

In this work, we adapt the ambient microtensiometer of Alvarez and co-workers into a high pressure microtensiometer (HPMT) capable of experimentally quantifying a wide range of interfacial phenomena at elevated pressures. Particularly, the HPMT uses a microscale spherical interface pinned to the tip of a capillary to directly measure surface tension via the Laplace equation. The stream of microscale bubbles used to pressurize the system ensures quick saturation of the bulk phases prior to conducting measurements. The HPMT is validated by measuring the surface tension of air-water as a function of pressure. We then measure the surface tension of CO₂ vapor and water as a function of pressure, finding lower equilibrium surface tension values than originally reported in the literature.

FINDINGS

This work both introduces further development of a useful experimental technique for probing interfacial phenomena at elevated pressures and demonstrates the importance of establishing bulk equilibrium to measure surface tension. The true equilibrium state of the CO₂-water surface has a lower tension than previously reported. We hypothesize that this discrepancy is likely due to the long diffusion timescales required to ensure saturation of the bulk fluids using traditional tensiometry. Thus we argue that previously reported elevated pressure measurements were performed at non-equilibrium conditions, putting to rest a long standing discrepancy in the literature. Our measurements establish an equilibrium pressure isotherm for the pure CO₂-water surface that will be essential in analyzing surfactant transport at elevated pressures.

On the Evaluation of Interfacial Tension (IFT) of CO2–Paraffin System for Enhanced Oil Recovery Process: Comparison of Empirical Correlations, Soft Computing Approaches, and Parachor Model

Carbon dioxide-based enhanced oil-recovery (CO2-EOR) processes have gained considerable interest among other EOR methods. In this paper, based on the molecular weight of paraffins (n-alkanes), pressure, and temperature, the magnitude of CO2–n-alkanes interfacial tension (IFT) was determined by utilizing soft computing and mathematical modeling approaches, namely: (i) radial basis function (RBF) neural network (optimized by genetic algorithm (GA), gravitational search algorithm (GSA), imperialist competitive algorithm (ICA), particle swarm optimization (PSO), and ant colony optimization (ACO)), (ii) multilayer perception (MLP) neural network (optimized by Levenberg-Marquardt (LM)), and (iii) group method of data handling (GMDH). To do so, a broad range of laboratory data consisting of 879 data points collected from the literature was employed to develop the models. The proposed RBF-ICA model, with an average absolute percent relative error (AAPRE) of 4.42%, led to the most reliable predictions. Furthermore, the Parachor approach with different scaling exponents (n) in combination with seven equations of state (EOSs) was applied for IFT predictions of the CO2–n-heptane and CO2–n-decane systems. It was found that n = 4 was the optimum value to obtain precise IFT estimations; and combinations of the Parachor model with three-parameter Peng–Robinson and Soave–Redlich–Kwong EOSs could better estimate the IFT of the CO2–n-alkane systems, compared to other used EOSs.

Molecular Dynamics Simulations Study on the Shear Viscosity, Density, and Equilibrium Interfacial Tensions of CO2 + Brines and Brines + CO2 + n-Decane Systems

The shear viscosity, density, and interfacial tensions (IFT) of two systems, namely, brine and brine/n-decane, blended with carbon dioxide (CO₂) were investigated via molecular dynamics simulations over broad ranges of temperature, pressure, CO₂ mole fraction, and brine concentration. The operating conditions for the molecular simulations to be studied are similar to the CO₂ geological storage processes. The effects of temperature, pressure, and concentrations on the viscosity and IFT have been investigated and analyzed. All four influencing parameters affect the shear viscosity and IFT. The pressures and temperatures up to 1000 bar and 573 K, respectively, were used for predicting the viscosity and IFT by considering intermolecular interactions, while salinities up to 32 000 ppm and CO₂ mole fractions between 0 and 0.5 were used in the simulations. Comparisons were made between simulated values and the predicted results of an empirical correlation, both against experimental data. Both monovalent and divalent ions and their mixtures were used in the simulations, and the results showed that monovalent ions impose stronger interactions in the solution than divalents. The results have revealed that the supercritical CO₂'s capability to reduce the IFT of the brine/n-decane interface is remarkable, which makes it a promising agent for underground geological injection for enhanced oil recovery. Also, viscosity and density ratio analysis have confirmed the viability of CO₂ storage in deep saline aquifers, where harsh geothermal conditions of high salinities limit the extent of the experiments. The molecular simulation results are in good qualitative agreement with the experimental data available in the literature for the viscosity, density, and IFT.

Interfacial properties of binary mixtures of Lennard-Jones chains in planar interfaces by molecular dynamics simulation

Binary mixtures of fully flexible linear tangent chains composed of bonded Lennard-Jones interaction sites (monomers) were studied using the molecular dynamics simulation in the NVT ensemble. Their interfacial properties were investigated in planar interfaces by direct simulation of an explicit liquid film in equilibrium with its vapor. A method for the calculation of long-range interactions in inhomogeneous fluids was implemented to take into account the potential truncation effects. Surface tension and the pressure tensor were calculated via the classical Irving-Kirkwood method; vapor pressure, orthobaric densities, density profiles, and Gibbs relative adsorption of the volatile component with respect to the heavy component were also obtained. The properties were studied as a function of the temperature, molar concentration of the heavy component, and the asymmetry of the mixture. According to the results of this work, the temperature loses influence on the surface tension, vapor pressure, and Gibbs relative adsorption curves as the molecular length of the heavy component increases. This suggests that the universal behavior observed in pure fluids of Lennard-Jones chains also holds for binary mixtures. The contribution of the long-range interactions turned out to account for about 60%, 20%, and 10% of the surface tension, vapor pressure, and orthobaric density final values, respectively. This contribution was even larger at high temperatures and for large molecules. Strong enrichment of the volatile component at the interface was observed in the asymmetric mixtures. One of these mixtures even showed a barotropic effect at elevated pressures and a class III phase behavior.

Pore-Scale Simulations of CO2/Oil Flow Behavior in Heterogeneous Porous Media under Various Conditions

Miscible and near-miscible flooding are used to improve the performance of carbon-dioxide-enhanced oil recovery in heterogeneous porous media. However, knowledge of the effects of heterogeneous pore structure on CO2/oil flow behavior under these two flooding conditions is insufficient. In this study, we construct pore-scale CO2/oil flooding models for various flooding methods and comparatively analyze CO2/oil flow behavior and oil recovery efficiency in heterogeneous porous media. The simulation results indicate that compared to immiscible flooding, near-miscible flooding can increase the CO2 sweep area to some extent, but it is still inefficient to displace oil in small pore throats. For miscible flooding, although CO2 still preferentially displaces oil through big throats, it may subsequently invade small pore throats. In order to substantially increase oil recovery efficiency, miscible flooding is the priority choice; however, the increase of CO2 diffusivity has little effect on oil recovery enhancement. For immiscible and near-miscible flooding, CO2 injection velocity needs to be optimized. High CO2 injection velocity can speed up the oil recovery process while maintaining equivalent oil recovery efficiency for immiscible flooding, and low CO2 injection velocity may be beneficial to further enhancing oil recovery efficiency under near-miscible conditions.

Three-phase flow displacement dynamics and Haines jumps in a hydrophobic porous medium

We use synchrotron X-ray micro-tomography to investigate the displacement dynamics during three-phase—oil, water and gas—flow in a hydrophobic porous medium. We observe a distinct gas invasion pattern, where gas progresses through the pore space in the form of disconnected clusters mediated by double and multiple displacement events. Gas advances in a process we name three-phase Haines jumps, during which gas re-arranges its configuration in the pore space, retracting from some regions to enable the rapid filling of multiple pores. The gas retraction leads to a permanent disconnection of gas ganglia, which do not reconnect as gas injection proceeds. We observe, in situ, the direct displacement of oil and water by gas as well as gas–oil–water double displacement. The use of local in situ measurements and an energy balance approach to determine fluid–fluid contact angles alongside the quantification of capillary pressures and pore occupancy indicate that the wettability order is oil–gas–water from most to least wetting. Furthermore, quantifying the evolution of Minkowski functionals implied well-connected oil and water, while the gas connectivity decreased as gas was broken up into discrete clusters during injection. This work can be used to design CO₂ storage, improved oil recovery and microfluidic devices.

Molecular-Scale Considerations of Enhanced Oil Recovery in Shale

With only less than 10% recovery, the primary production of hydrocarbon from shale reservoirs has redefined the energy equation in the world. Similar to conventional reservoirs, Enhanced Oil Recovery (EOR) techniques could be devised to enhance the current recovery factors. However, shale reservoirs possess unique characteristics that significantly affect the fluid properties. Therefore, we are adopting a molecular simulation approach that is well-suited to account for these effects to evaluate the performance of three different gases, methane, carbon dioxide and nitrogen, to recover the hydrocarbons from rough pore surfaces. Our hydrocarbon systems consists of either a single component (decane) or more than one component (decane and pentane). We simulated cases where concurrent and countercurrent displacement is studied. For concurrent displacement (injected fluids displace hydrocarbons towards the production region), we found that nitrogen and methane yielded similar recovery; however nitrogen exhibited a faster breakthrough. On the other hand, carbon dioxide was more effective in extracting the hydrocarbons when sufficient pressure was maintained. For countercurrent displacement (gases are injected and hydrocarbons are produced from the same direction), methane was found to be more effective, followed by carbon dioxide and nitrogen. In all cases, confinement reduced the recovery factor of all gases. This work provides insights to devise strategies to improve the current recovery factors observed in shale reservoirs.

Effect of CO2/N2 Mixture Composition on Interfacial Tension of Crude Oil

CO₂-enhanced oil recovery (EOR) has demonstrated significant success over the last decades; it is one of the fastest-growing EOR techniques in the USA accounting for nearly 6% of oil production. A large quantity of CO₂ gas is required for the EOR process and sometimes other gases such as hydrocarbons, air, flue gases, CO₂, N₂, and mixtures of two or more gases are used for injection. It is also realized that the injection of CO₂ and N₂ combines advantage in reducing CO₂ concentrations in the atmosphere and improving the oil recovery by sequestering it underground. However, there are a number of variables involved in the successful design of the CO₂-EOR process. The objective of this study is to investigate the effect of CO₂/N₂ mixture composition on interfacial tension (IFT) of crude oil. Experiments were performed to measure the IFT of the CO₂/N₂ mixtures and crude oil for different compositions of gas by varying the system pressure at a fixed temperature. The effect of CO₂/N₂ mixture composition and pressure on the IFT of crude oil is evaluated. The experimental results show that an increase in the mole fraction of CO₂ in the gas mixture results in a decrease in IFT between CO₂-oil, irrespective of the system pressure. However, because of an increase in the mole fraction of N₂ in the gas mixture, an increase in IFT was observed and this change is opposite to the effect of the CO₂ mole fraction. Also, the change in IFT is consistent with the pressure, which means that the IFT decreases with an increase in the pressure at a given temperature. The effect of the CO₂ mole fraction is more profound compared to the N₂ fraction and with the pressure at which experiments were conducted in this study. The finding of this study helps in designing the CO₂-EOR process in which achieving miscibility conditions is vital for taking advantage of the CO₂ injection. Also, the presence of N₂ and its influence on the IFT that must be considered in the CO₂-EOR were addressed in this study.

Dynamics of fluid displacement in mixed-wet porous media

We identify a distinct two-phase flow invasion pattern in a mixed-wet porous medium. Time-resolved high-resolution synchrotron X-ray imaging is used to study the invasion of water through a small rock sample filled with oil, characterized by a wide non-uniform distribution of local contact angles both above and below 90°. The water advances in a connected front, but throats are not invaded in decreasing order of size, as predicted by invasion percolation theory for uniformly hydrophobic systems. Instead, we observe pinning of the three-phase contact between the fluids and the solid, manifested as contact angle hysteresis, which prevents snap-off and interface retraction. In the absence of viscous dissipation, we use an energy balance to find an effective, thermodynamic, contact angle for displacement and show that this angle increases during the displacement. Displacement occurs when the local contact angles overcome the advancing contact angles at a pinned interface: it is wettability which controls the filling sequence. The product of the principal interfacial curvatures, the Gaussian curvature, is negative, implying well-connected phases which is consistent with pinning at the contact line while providing a topological explanation for the high displacement efficiencies in mixed-wet media.

Modeling the Interfacial Tension of Water-Based Binary and Ternary Systems at High Pressures Using a Neuro-Evolutive Technique

In this study, artificial neural networks (ANNs) determined by a neuro-evolutionary approach combining differential evolution (DE) and clonal selection (CS) are applied for estimating interfacial tension (IFT) in water-based binary and ternary systems at high pressures. To develop the optimal model, a total of 576 sets of experimental data for water-based binary and ternary systems at high pressures were acquired. The IFT was modeled as a function of different independent parameters including pressure, temperature, density difference, and various components of the system. The results (total mean absolute error of 3.34% and a coefficient of correlation of 0.999) suggest that our model outperforms other habitual models on the ability to predict IFT, leading to a more accurate estimation of this important feature of the gas mixing/water systems.

Bulk and interfacial properties of decane in the presence of carbon dioxide, methane, and their mixture

Molecular dynamics simulations were performed to study the bulk and interfacial properties of methane + n-decane, carbon dioxide + n-decane, and methane + carbon dioxide + n-decane systems under geological conditions. In addition, theoretical calculations using the predictive Peng-Robinson equation of state and density gradient theory are carried out to compare with the simulation data. A key finding is the preferential dissolution in the decane-rich phase and adsorption at the interface for carbon dioxide from the methane/carbon dioxide mixture. In general, both the gas solubility and the swelling factor increase with increasing pressure and decreasing temperature. Interestingly, the methane solubility and the swelling of the methane + n-decane system are not strongly influenced by temperature. Our results also show that the presence of methane increases the interfacial tension (IFT) of the carbon dioxide + n-decane system. Typically, the IFT of the studied systems decreases with increasing pressure and temperature. The relatively higher surface excess of the carbon dioxide + n-decane system results in a steeper decrease in its IFT as a function of pressure. Such systematic investigations may help to understand the behavior of the carbon dioxide-oil system in the presence of impurities such as methane for the design and operation of carbon capture and storage and enhanced oil recovery processes.

CO2 activating hydrocarbon transport across nanopore throat: insights from molecular dynamics simulation

In tight oil reservoirs, nanopore throat acting as the narrowest section of fluidic channel determines the oil transport performance; injecting CO₂ is found to significantly promote the oil flow.

Resolving Discrepancies in the Measurements of the Interfacial Tension for the CO2 + H2O Mixture by Computer Simulation

Literature values regarding the pressure dependence of the interfacial tension of the system of carbon dioxide (CO2) + water (H2O) show an unexplained divergence and scatter at the transition between low-pressure gas-liquid equilibrium and the high-pressure liquid-liquid equilibrium. We employ the Statistical Associating Fluid Theory (SAFT) and canonical molecular dynamics simulations based on the corresponding coarse grained force field to map out the phase diagram of the mixture and the interfacial tension for this system. We showcase how at ambient temperatures a triple point (gas-liquid-liquid) is expected and detail the implications that the appearance of the third phase has on the interfacial tensions of the system.

Pore-scale micro-computed-tomography imaging: nonwetting-phase cluster-size distribution during drainage and imbibition

We investigated the cluster-size distribution of the residual nonwetting phase in a sintered glass-bead porous medium at two-phase flow conditions, by means of micro-computed-tomography (μCT) imaging with pore-scale resolution. Cluster-size distribution functions and cluster volumes were obtained by image analysis for a range of injected pore volumes under both imbibition and drainage conditions; the field of view was larger than the porosity-based representative elementary volume (REV). We did not attempt to make a definition for a two-phase REV but used the nonwetting-phase cluster-size distribution as an indicator. Most of the nonwetting-phase total volume was found to be contained in clusters that were one to two orders of magnitude larger than the porosity-based REV. The largest observed clusters in fact ranged in volume from 65% to 99% of the entire nonwetting phase in the field of view. As a consequence, the largest clusters observed were statistically not represented and were found to be smaller than the estimated maximum cluster length. The results indicate that the two-phase REV is larger than the field of view attainable by μCT scanning, at a resolution which allows for the accurate determination of cluster connectivity.