Subhash C. Ayirala

Interfacial behavior of complex hydrocarbon fluids at elevated pressures and temperatures

Unlike all the physical properties of the bulk fluid phases, interfacial tension (IFT) is unique in the sense that it relates to the interface between the two immiscible fluid phases. Hence, interfacial tension between the fluid phases can be used to infer a great deal of information about solubility, miscibility and mass transfer interactions between the two bulk fluid phases in contact.In this paper, we examine the utility of interfacial tension to characterize miscibility and mass transfer mechanisms using complex hydrocarbon fluids at elevated pressures and temperatures.For CO2/n-decane system at 37.8 /spl deg/C the minimum miscibility pressure (MMP) determined using the VIT technique (7.9 MPa) matched well with the reported MMPs by slim-tube (8.2-8.6 MPa) and rising-bubble techniques (8.8 MPa). For CO2M-C1+ n-C4+ n-CW techniques system at 71.1 /spl deg/C the VIT technique resulted in an MMP value of 12.2 MPa, which is in good agreement with the published values of slim-tube and phase diagram measurements (11.7 MPa) and analytical model predictions (11.7 MPa). This paper discusses the multiple roles of interfacial tension with supporting experimental data obtained at elevated pressures and temperatures and emphasizes the need to recognize interfacial tension as a good phase behavior indicator in fluid-fluid phase equilibria for more efficient use of this fundamental property in several other applications.

Application of the parachor model to the prediction of miscibility in multi-component hydrocarbon systems

While most thermodynamic properties refer to individual fluid phases, interfacial tension (IFT) is unique in the sense that it is a property of the interface between the fluid phases. The IFT, being a sensitive property strongly dependent on the composition of the interacting phases, is a good indicator of mass transfer effects between the phases. Furthermore, a condition of zero interfacial tension is essential to attain miscibility of the fluid phases in contact. Based on this concept, a new technique of vanishing interfacial tension (VIT) has been reported recently for experimental determination of fluid–fluid miscibility. Similar to the VIT technique in concept, a computational model based on parachor IFT calculations has been proposed in the present study for miscibility prediction. This model has been compared with VIT experiments and EOS calculations. For this purpose, Rainbow Keg River (RKR) reservoir fluids have been used, since the phase behaviour data necessary for miscibility calculations and the VIT experimental results were readily available. The parachor computational model resulted in over-predictions of miscibility when compared to VIT experiments and EOS calculations. These over predictions appear to be due to the inability of the parachor model to account for counter-directional mass transfer effects that can occur in reality between the fluids. Thus, in addition to demonstrating the importance of counter-directional mass transfer effects on fluid–fluid miscibility, this study has identified the need to incorporate these mass transfer effects in the proposed parachor computational model to compute fluid–fluid miscibility.

The multiple roles of interfacial tension in fluid phase equilibria and fluid-solid interactions

The tension at the interfaces separating the three phases of matter is a unique property in that it can reveal a great deal of information about the phases in contact, including the direction and extent of mass transfer of components, their proximity to equilibrium, the nature of fluids distribution relative to one another, the contact angle, and the spreading and adhesion behavior of liquids on solid surfaces. In this paper we examine, with supporting experimental data, the multitude of roles played by interfacial tension in establishing (1) the phase behavior characteristics of solubility, miscibility, and the associated mass transfer mechanisms in multicomponent fluid systems, (2) the nature of fluids distribution in gas–oil–water systems in porous solid substrates and (3) the spreading and adhesion characteristics in solid–liquid–liquid systems through dynamic contact angles.

Wettability alterations due to crude oil composition and an anionic surfactant in petroleum reservoirs

In this study, dual-drop dual-crystal (DDDC) contact-angle measurements have been made using dolomite rock and fluid samples from the Yates reservoir (West Texas) and in the presence of an anionic (ethoxy sulfate) surfactant. The experiments have been conducted at Yates reservoir conditions (4.8 MPa and 27.8°C) and using live synthetic oil to provide realistic measurements of in situ reservoir wettability. Stocktank crude oil has also been used at reservoir conditions to study the oil compositional effects on wettability. An advancing contact angle of 152° measured for Yates dolomite rock, stocktank oil and synthetic reservoir brine showed a strong oil-wet nature. However, experiments with Yates live synthetic oil resulted in an advancing contact angle of 55°, indicating a weakly water-wet behavior. In the rock-fluids system consisting of Yates stocktank oil, the surfactant altered the wettability to less oil-wet by decreasing the advancing contact angle to 135°. For rock-fluids system with Yates live synthetic oil, the surfactant altered the wettability from weakly water-wet to strongly oil-wet by increasing the advancing contact angle from 55° to 165°. The oil-wet behavior observed with Yates live synthetic oil due to the surfactant indicates a significant wettability altering capability of the surfactant.

Influence of solid-liquid-liquid interactions on multiphase transport behavior in porous media

This paper examines the effect of two types of surfactants (one nonionic and the other anionic) on wettability and multiphase flow characteristics by conducting contact angle measurements and flow tests through porous media using three different rock-fluids systems. The contact angle measurements have been made using the Dual-Drop Dual-Crystal (DDDC) technique and the multiphase flow characteristics are reported as oil-water relative permeabilities computed using a simulator by history matching the pressure drop and production data obtained during flow tests.

Compositional dependence of wetting and contact angles in solid-liquid-liquid systems under realistic environments

The wetting and contact angles in porous media are important for the characterization of multi-phase flow behavior. The polar components such as asphaltenes in the oil-phase, which have been widely believed to be a major source of oil-wetting characteristics. The effect of light gaseous ends in crude oil on wetting is studied by depleting the pressure at regular intervals. The variation of contact angles observed with the pressure during the pressure depletion clearly showed the influence of light ends on wetting. The water-advancing contact angle of live crude oil gradually increased from 55/spl deg/ at bubble point pressure to 154/spl deg/ at ambient pressure. The de-asphaltened and the deresined crude oils showed strong oil-wet characteristics as stocktank crude oil. The light gaseous ends in oil phase appear to peptize the asphaltene molecules by surrounding them, thereby preventing their agglomeration and migration to the interface. The differences observed in the behavior of dynamic interfacial tensions between live and stocktank crude oils further substantiates the influence of light gaseous ends on the activity of polar components in live oil. The influence of solid surface roughness, mineralogy of rock substrates as well as the brine composition on wetting and contact angles have also been investigated. For highly smooth substrates, carbonate and silica showed relatively similar high water-advancing contact angles, while for rough substrates, the advancing contact angles on silica were much lower compared to the carbonates.