Dandina N. Rao

A new technique of vanishing interfacial tension for miscibility determination

A new technique is presented in this paper to enable rapid and cost-effective determination of minimum miscibility pressure and composition. This new technique, called the Vanishing Interfacial Tension (VIT) technique, consists of measuring the interfacial tension between the injected gas phase and the crude oil at the temperature of the reservoir and at varying pressures and/or enrichment levels of the gas phase. These interfacial tension measurements are carried out by computer digitization of the image of the profiles of the sessile and pendant drops of crude oil enclosed in the surrounding medium of injection gas. By fitting these actual drop profiles with the iterative solution of the Laplace capillary equation, the value of interfacial tension is obtained at each pressure or enrichment level. By making a plot of the interfacial tension against the independent variable (either pressure or enrichment), accurate values for the minimum miscibility pressure (MMce:simple-para) or composition (MMC) are then obtained by extrapolation to zero interfacial tension. The paper provides experimental evidence for the validity of the new VIT technique by comparing the results with slim-tube displacement tests. While the slim-tube technique requires 4–6 weeks for obtaining MMP or MMC, the VIT technique yields the MMP in about 4–6 h, and MMC in about 16–24 h. The paper also presents an application of the VIT technique to optimize injection gas composition for an Alberta field project. The Alberta Energy and Utilities Board (a Government of Alberta agency) has accepted the optimized composition based on the VIT technique.

Application of the new vanishing interfacial tension technique to evaluate miscibility conditions for the Terra Nova Offshore Project

Abstract Terra Nova oil pool is the second largest oil pool discovered in the Grand Banks of the Canadian East Coast and is under development. As a part of the oil recovery process selection criteria, miscibility conditions were determined for Terra Nova oil with various enrichment levels of gas available from the offshore production facilities using the recently developed vanishing interfacial tension (VIT) technique. The VIT technique is based on the concept that the interfacial tension between the gas and crude oil phases at reservoir temperature must reduce to zero as these two phases approach the point of miscibility. The concept of zero-interfacial tension at miscibility is, in turn, based on the well-accepted fact that the interface between the phases must vanish, as they become miscible with one another. Thus, the minimum miscibility pressure (MMP) and minimum miscibility composition (MMC) can be determined precisely by measuring gas–oil interfacial tension as a function of pressure and gas composition, down to as low an interfacial tension as the measurement technique allows, and then extrapolating the data to zero-interfacial tension. This paper presents the details of this new VIT technique and its evaluation against slim-tube tests, and discusses its application to the Terra Nova gas injection scheme. The interfacial tension data obtained at reservoir conditions using the computerized axisymmetric drop shape analysis (ADSA) technique are presented as a function of pressure, gas composition, and the mode of gas–oil contact (first-contact or equilibrium). In addition to providing visual evidence of miscibility as the point of zero-interfacial tension is approached, the VIT technique is rapid in that it enables the experimental determination of MMP and MMC within about 2–3 days as against 4–6 weeks required by the slim-tube technique.

Determination of gas-oil miscibility conditions by interfacial tension measurements

Processes that inject gases such as carbon dioxide and natural gas have long been and still continue to be used for recovering crude oil from petroleum reservoirs. It is well known that the interfacial tension between the injected gas and the crude oil has a major influence on the efficiency of displacement of oil by gas. When the injected gas becomes miscible with the crude oil, which means that there is no interface between the injected and displaced phases or the interfacial tension between them is zero, the oil is displaced with maximum efficiency, resulting in high recoveries. This paper presents experimental measurements of interfacial tension between crude oil and natural gases (using a computerized drop shape analysis technique) as a function of pressure and gas composition at the temperature of the reservoir from which the crude oil was obtained. The point of zero interfacial tension was then identified from these measurements by extrapolation of data to determine minimum miscibility pressure (MMP) and minimum miscibility composition (MMC). The gas-oil miscibility conditions thus obtained from interfacial tension measurements have been compared with the more conventional techniques using slim-tube tests and rising-bubble apparatus as well as predictive correlations and visual observations. The miscibility pressures obtained from the new VIT technique were 3-5% higher than those from visual observations and agreed well with the slim-tube results as well as with the correlations at enrichment levels greater than 30 mol% C2+ in the injected gas stream. The rising bubble apparatus yielded significantly higher MMPs. This study demonstrates that the VIT technique is rapid, reproducible, and quantitative, in addition to providing visual evidence of gas-oil miscibility.

Measurements of dynamic contact angles in solid-liquid-liquid systems at elevated pressures and temperatures

Abstract This paper presents a recently developed technique, which has been used successfully in several oil-field applications, to generate reproducible measurements of both the water-advancing and -receding contact angles at reservoir conditions of temperature and pressure using live reservoir fluids. Unlike the conventional sessile drop and modified sessile drop contact angle tests, which require 30–40 days to complete, the new technique enables these measurements within 2–3 days while assuring measurement accuracy within about 2–3°. The new technique is called the dual-drop–dual-crystal (DDDC) technique, and involves the equilibration of two parallel solid surfaces immersed in reservoir brine with two crude oil drops placed on them before creating the advancing and receding interfaces. The fluids–solid equilibrium is attained quickly due to the effect of buoyancy forces that help in draining the water film trapped between the crude oil drop and the solid surface. Dynamic measurements are presented to demonstrate the dynamic behavior of the oil–water–rock three-phase boundary in water-, intermediate- and oil-wet solid–fluid systems. The paper includes the methodology and salient features of the new technique, some of the recent results to demonstrate its accuracy and short run times, and actual reservoir cases involving serious wettability shifts caused by fluids incompatibility and temperature variations. Due to its accuracy, short run duration and operability at reservoir conditions, the new technique offers excellent potential for detailed understanding of the influence of various surfactants used in oil-field operations on reservoir wettability alteration, screening and optimization of fluids for field applications, optimization of production strategies, and for many other practical applications involving solid–liquid–fluid systems.

Comparative Evaluation of a New Gas/Oil Miscibility-Determination Technique

A new experimental technique of vanishing interfacial tension (VIT) has been reported in recent literature for quick and cost-effective determination of gas/oil miscibility. However, this technique has been criticized because of the perceived absence of compositional-path specification and lack of confirmation against standard gas/oil systems. In this paper, we address these concerns by conducting interfacial-tension (IFT) measurements at elevated pressures and temperatures in two standard gas/oil systems and at varying molar compositions of gas and oil in feed mixtures. Though gas/oil ratio was found to have an impact on mass-transfer rates, the IFT between gas and oil was unaffected in the two standard gas/oil systems as the fluid phases approached equilibrium. This indicates compositional-path independence of gas/ oil IFTs measured at near-equilibrium conditions; hence, miscibilities determined using the VIT technique. The minimum miscibility pressures (MMPs) determined using the VIT technique acceptably matched (within 5 to 8%) with the reported slim tube miscibilities for both the standard gas/oil systems. These experimental results clearly support wide use of the VIT technique for rapid and cost-effective determination of MMPs and minimum miscibility enrichments (MMEs) in improved-oil-recovery applications.

FLUID–FLUID AND SOLID–FLUID INTERFACIAL INTERACTIONS IN PETROLEUM RESERVOIRS

The distribution and flow behavior of crude oil, gas and brine in the porous rock medium of petroleum reservoirs are controlled largely by the interactions occurring at the interfaces within the various fluids and by the interactions between the fluids and the rock surface. With an objective to correlate the macroscopic multiphase flow behavior with fundamental interfacial interactions, the recent developments in the field of fluid–fluid and solid–fluid interactions and their applications in petroleum engineering are presented in this contribution. A computerized drop shape analysis technique and its application to the measurement of fluid–fluid interfacial tension at elevated pressures and temperatures are discussed. A recently developed technique that is capable of measuring dynamic (advancing and receding) contact angles at realistic conditions encountered in petroleum reservoirs is presented. Its effectiveness in making reproducible and rapid measurements relative to the conventional techniques is demonstrated with several reservoir case studies. Attempts are made to correlate the interfacial phenomena of adhesion and spreading in solid–liquid–liquid systems with dynamic contact angles as well as to extend the applicability of the critical surface tension concept from the conventional solid-liquid-vapor systems to the rock-oil-brine systems of interest in petroleum engineering. These interfacial concepts have been applied to the practical problems of asphaltene destabilization from crude oils and the effect of temperature on wettability alteration in heavy oil fields. A simple procedure is outlined to enable the estimation of interfacial adhesion forces and to demonstrate the significant role they play relative to the capillary forces in retaining the fluids within the porous rock medium.

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.