Near-critical Conditions Research Articles

Free-surface flows with near-critical flow conditions

Open channel flow situations with near-critical flow conditions are often characterized by the development of free-surface instabilities (i.e., undulations). The paper develops a review of several near-critical flow situations. Experimental results are compared with ideal-fluid flow calculations. The analysis is completed by a series of new experiments. The results indicate that, for Froude numbers slightly above unity, the free-surface characteristics are very similar. However, with increasing Froude numbers, distinctive flow patterns develop. Key words: open channel flow, critical flow conditions, free-surface undulations, flow instability, undular surge, undular broad-crested weir flow, culvert flow.

Interfacial Tension of Nitrogen/Volatile Oil Systems

Abstract This paper presents interfacial tension (IFT) measurements of a nitrogen/volatile oil system at conditions that are typical for North-Sea oil reservoirs. The volatile oil is a well-defined three-component oil, which can be displaced by nitrogen in a developed miscible manner. Using the pendant drop method, we have measured the IFT of the nitrogen/volatile oil system at four characteristic compositions at a constant temperature of 100°C and in the pressure range of 30 MPa to 40 MPa. For the analysis of the pendant drops we have used advanced image processing techniques. Depending on composition and pressure, we have measured IFTs as low as 0.05 mN/m with an accuracy less than 2%. Predictions of the IFT of our fluid systems with the parachor method and with Lee & Chien's method show large deviations from the experimental values. At near-critical conditions, the nitrogen/volatile oil system behaves in accordance with critical scaling theories.

Exploring The Fascinating World Of Reservoir Fluid Phase Behaviour

Introduction Reservoir fluid phase equilibria affect all aspects of petroleum industry operations, from reservoir development to fluid processing and pipelining. Fluid composition, phase equilibria and physical properties are needed on the upstream side in reserves evaluation, well testing, predicting reservoir performance and optimizing its recovery. As still deeper formations including off-shore structures are drilled, more exotic fluids are encountered. The recently considered plan to produce a reservoir in Alberta containing fluid with H2S in excess of 90% is one such example. The phase equilibria reflect the nature of intermolecular forces, and the behaviour of these highly non-ideal systems presents a severe test of correlations (including the most sophisticated one, the equation of state) developed to describe properties of the typical gas and oil. While a lot of progress has been made in modeling phase equilibria and properties of reservoir fluids, some laboratory measurements still form a basic part of fluid evaluation. This brief note selects to explore the special but quite frequently encountered features of reservoir fluid equilibria-the complex phenomena brought about by close proximity to the critical point. Near-critical fluids and mixtures are encountered in both primary and supplementary recovery processes. Some manifestations of near-critical behaviour are discussed. The reservoir engineer needs to be aware of them, as near-critical conditions have implications for fluid flow, multiple phase formation, data collection, and data interpretation. Reservoir Fluid Spectrum Figure 1, the pressure-temperature diagram, provides a suitable frame of reference for classifying different oil and gas field fluids. The classification is based on the position of a fluid two-phase envelope relative to reservoir and surface conditions of pressure and temperature. The shape of the two-phase region and the relative positions of the critical point (CP), the cricondenbar and cricondentherm (maximum two-phase pressure and temperature, respectively) are determined by fluid composition, specifically by the magnitude and makeup of the C7+ fraction. As the amount of the C7+ fraction increases, the fluid character changes from a dry gas to a wet gas, gas condensate, volatile oil, black oil (and ultimately, a bitumen). At the same time the two-phase envelope extends to higher temperatures. The different production paths implied in the figure by possible reservoir (R) and surface (S) conditions define different fluid types as follows: RI-SI is a dry gas, RI-S2 a wet gas, R2-S2 a gas condensate, R3-82 a volatile oil, and R4-83 a black oil. Turning attention to reservoirs at temperatures close to fluid critical temperature, these are either volatile oils (R3) with bubble point saturation pressures, or gas condensates (R2) with dew point saturation pressures. A rule-of-thumb usually applied in determining the fluid type considers fluids with C7+ content over 12.5% and of producing GOR under 570 m3/m3 to be oils. However, a hypothetical fluid with the phase diagram of Figure 1, i.e., of a fixed C7+ content and fixed surface GOR (corresponding to, e.g., 82) could be either an oil or gas depending entirely on whether the reservoir temperature is R2 or R3! This illustrates that this rule-ofthumb, while quite useful, has to be applied judiciously.

A Noncubic Equation of State for the Treatment of Hydrocarbon Fluids at Reservoir Conditions

A modified version of the Benedict-Webb-Rubin equation of state (EOS) has been developed. Attention was focused on near-critical conditions, where good behavior was obtained by forcing a tight fit to the experimental critical isotherms, including the critical density. The new model requires component critical constants T{sub c} and P{sub c} and acentric factor {omega}. Generalized expressions have been developed for the calculation of the EOS parameters from the reduced temperature and the acentric factor. Pure-component vapor-pressure and density data were reproduced accurately in a wide range of conditions (reduced temperatures down to 0.4 and pressures up to 1,000 bar, both for liquids and gases, including the critical region) and {omega} values up to 0.6. Mixtures are treated like pure compounds. Their T{sub c}, P{sub c}, and {omega} are calculated by new mixing rules derived from those of cubic EOS`s. With these rules, experimental VLE data were reproduced accurately, with no need of binary interaction constants for alkane-alkane pairs and one constant for the other pairs. Procedures were developed for a fast and safe solution of the equation of state.

Quantitative Spectroscopic Investigation of Enhanced Excited State Complex Formation in Supercritical Carbon Dioxide under Near-Critical Conditions: Inconsistency between Experimental Evidence and Classical Photophysical Mechanism

Quantitative Spectroscopic Investigation of Enhanced Excited State Complex Formation in Supercritical Carbon Dioxide under Near-Critical Conditions: Inconsistency between Experimental Evidence and Classical Photophysical Mechanism

Kinetics and Products from o-Cresol Oxidation in Supercritical Water

Dilute aqueous solutions of o-cresol (2-methylphenol) were oxidized in a tubular flow reactor at near-critical and supercritical conditions. The power-law rate expression that best correlates the kinetics of o-cresol disappearance is rate = 10{sup 5.7} exp({minus}29,700/RT)[o-cresol]{sup 0.57}[O{sub 2}]{sup 0.22}[H{sub 2}O]{sup 1.44}. The power-law rate expression that best correlates the experimental results for the conversion of organic carbon to CO{sub 2} is rate = 10{sup 6.8} exp({minus}34,000/RT)[TOC]{sup 0.34}[O{sub 2}]{sup 0.73}[H{sub 2}O]{sup 1.18}. All concentrations are in moles per liter, the activation energy is in calories per mole, and the rate is in moles per liter per second. The most abundant products from o-cresol oxidation were typically phenol, 2-hydroxybenzaldehyde, 1,3-benzodioxole, indanone, CO, and CO{sub 2}. 2-Hydroxybenzaldehyde was the major primary product. A reanalysis of published kinetics data for the oxidation of two other ring-containing compounds (pyridine and 4-chlorophenol) in supercritical water revealed that the rate laws previously reported for these two compounds do not provide the best correlation of the experimental data. The authors report the new rate laws, which are similar to those for o-cresol, 2-chlorophenol, and phenol in that the global reaction orders are between 0.55 and 0.9 for the organic compounds and between 0.2 and 0.5 for oxygen.

Direct evolution of brine from a crystallizing silicic melt at the Questa, New Mexico, molybdenum deposit

Molybdenum mineralization in the Southwest orebody at the Questa deposit is associated with crystallization of aplite porphyry, explosive brecciation, and exsolution of magmatic hydrothermal fluids. Quartz, K feldspar, fluorophlogopite, and molybdenite precipitated from exsolving aqueous fluids and cemented aplite porphyry and andesite wall-rock clasts forming a magmatic hydrothermal breccia ore.Fluid inclusions in quartz in the breccia matrix trapped aqueous fluids that transported and precipitated molybdenum. Two fluid inclusion populations associated with molybdenite mineralization were identified. The largest population commonly contains liquid, vapor, halite, sylvite, and one or more opaque daughter crystals. These inclusions homogenize between 200 degrees and 500 degrees C and have a prominent mode at 360 degrees to 400 degrees C; salinities vary from 31 to 57 wt percent NaCl equiv. Low first ice-melting temperatures suggest that additional fluid components, possibly calcium or magnesium chlorides, may be present. Approximately 80 percent of these inclusions homogenize by halite dissolution; phase equilibria constraints require that these inclusions trapped fluids in the vapor-absent field. The remaining saline inclusions homogenize to liquid at temperatures above halite dissolution temperatures. Significantly, the saline fluid did not coexist with a low-density aqueous fluid.A second population of inclusions contains fluids with salinities of 2.7 to 25.0 wt percent NaCl equiv and homogenizes by vapor bubble disappearance, vapor bubble expansion, or critical behavior. Textural relationships and phase behavior during microthermometry indicate that these inclusions constitute a single population which formed as pressure and temperature fluctuated at near-critical conditions.Pressure changes related to brecciation followed by crystallization and system resealing produced the observed fluids. Saline fluids were trapped as the system sealed following a major brecciation event. These high-salinity fluids were produced as the melt, under pressure that increased from hydrostatic to lithostatic, partitioned increasing amounts of chlorine into the exsolving aqueous fluid. Renewed fracturing and abrupt pressure decrease to hydrostatic conditions reduced the mass of chlorine partitioning to the exsolving fluid. Lower salinity fluids exsolved at near-critical conditions and were trapped by inclusions that homogenize to liquid, to vapor, or by critical behavior. The lack of cogenetic liquid- and vapor-rich fluid inclusions and phase equilibria constraints indicate that high-salinity fluids (< or = 57 wt % NaCl equiv) were generated directly by the crystallizing magma and were not a product of aqueous fluid immiscibility.

Dimple and nose coalescences in phase-separation processes.

Coalescence processes are investigated during phase separation in a density-matched liquid mixture (partially deuterated cyclohexane and methanol) under near-critical conditions. As a result of the interplay between capillary and lubrication forces, ``nose'' coalescence appears to be always associated with the slow growth of isolated droplets (exponent \ensuremath{\approxeq}1/3), whereas ``dimple'' coalescence corresponds to the fast growth of interconnected droplets (exponent \ensuremath{\approxeq}1). At each stage of growth, the distribution of droplets trapped during dimple coalescence is reminiscent of all of the previous coalescence events.

Prediction/correlation of vapor-liquid equilibria in synthetic gas and oil systems

Anastasiades, K., Stamataki, S. and Tassios, D., 1994 Prediction/correlation of vapor-liquid equilibria in synthetic gas and oil systems. Fluid Phase Equilibria 93: 23-54. The performance of two cubic equations of state (EoS) — a translated version of a modified Peng-Robinson and the Jhaveri-Peng-Robinson — in the prediction/correlation of the phase behavior of gas and oil mixtures is presented. Problems associated with the uncertainty involved in the description of the heavy end of the mixtures are avoided by considering synthetic systems. Prediction results are fair for the gas condensate systems and become very poor when approaching near-critical conditions. The results are better for the oils but deviations are observed with increasing volatility. A systematic evaluation of the improvements obtained by tuning the thermodynamic model to match the saturation pressure alone or together with constant composition expansion data is carried out to determine the number and type of the EoS parameters that need to be tuned. For far-critical gas condensates and for oil mixtures the adjustment of one parameter is adequate to yield satisfactory results. This is not the case for near-critical gas condensates where adjustment of two parameters is necessary. The overall quality of the results obtained by prediction or tuning justifies the wide use of the aforementioned EoS — and probably other cubic EoS — in the petroleum industry.

A small angle x-ray scattering study of the effect of pressure on the aggregation of asphaltene fractions in petroleum fluids under near-critical solvent conditions

Small angle X-ray scattering was used to determine the effect of pressure on the extent of asphaltene aggregation for a system under near-critical conditions. A mixture containing 60 vol% Crude Oil A in n-pentane was studied at 110[degree]C, at pressures ranging from 25 to 400 bar. As the pressure of the near-critical solution is isothermally decreased, these results indicate (1) an increase in the extent of asphaltene aggregation and/or; (2) increased attractive interactions among aggregates. Information derived from different regions of the X-ray scattering curve indicate increasing aggregation with decreasing pressure. From these experimental results, together with theoretical interpretation, we infer that as the pressure is reduced, increased aggregation of asphaltenes results which may contribute to formation damage in hydrocarbon reservoirs, and to fouling in hydrotreatment and cracking catalysts. 71 refs., 10 figs., 2 tabs.

The Porgera gold mine, Papua New Guinea; magmatic hydrothermal to epithermal evolution of an alkalic-type precious metal deposit

The Porgera gold mine, Papua New Guinea; magmatic hydrothermal to epithermal evolution of an alkalic-type precious metal deposit

On the nonlinear dynamics of free bars in straight channels

A simple morphological model is considered which describes the interaction between a unidirectional flow and an erodible bed in a straight channel. For sufficiently large values of the width-depth ratio of the channel the basic state, i.e. a uniform current over a flat bottom, is unstable. At near-critical conditions growing perturbations are confined to a narrow spectrum and the bed profile has an alternate bar structure propagating in the downstream direction. The timescale associated with the amplitude growth is large compared to the characteristic period of the bars. Based on these observations a weakly nonlinear analysis is presented which results in a Ginzburg-Landau equation. It describes the nonlinear evolution of the envelope amplitude of the group of marginally unstable alternate bars. Asymptotic results of its coefficients are presented as perturbation series in the small drag coefficient of the channel. In contrast to the Landau equation, described by Colombini et al. (1987), this amplitude equation also allows for spatial modulations due to the dispersive properties of the wave packet. It is demonstrated rigorously that the periodic bar pattern can become unstable through this effect, provided the bed is dune covered, and for realistic values of the other physical parameters. Otherwise, it is found that the periodic bar pattern found by Colombini et al. (1987) is stable. Assuming periodic behaviour of the envelope wave in a frame moving with the group velocity, simulations of the dynamics of the Ginzburg-Landau equation using spectral models are carried out, and it is shown that quasi-periodic behaviour of the bar pattern appears.

A generalized sideband stability theory via center manifold projection

A center manifold theory is used to determine all modes that contribute significantly to the leading-order sideband stability of finite-amplitude monochromatic waves. The classical multiscale theories based on the Ginzburg–Landau equation are extended away from near-critical conditions and are shown to have omitted an important contribution from nonlinear interactions with low wave-number modes. Stability bounds on stable monochromatic waves are reported for dispersive systems that extend the classical Eckhaus bound for nondispersive systems and the Lange–Newell and Benjamin–Feir stability conditions for monochromatic waves with critical wave numbers. These new stability bounds are verified numerically by computing the evolving spectrum of a model equation.

Numerical experiments on convective heat transfer in water-saturated porous media at near-critical conditions

Fluid and heat flow at temperatures approaching or exceeding that at the critical point (374 °C for pure water, higher for saline fluids) may be encountered in deep zones of geothermal systems and above cooling intrusives. In the vicinity of the critical point the density and internal energy of fluids show very strong variations for small temperature and pressure changes. This suggests that convective heat transfer from thermal buoyancy flow would be strongly enhanced at near-critical conditions. This has been confirmed in laboratory experiments. We have developed special numerical techniques for modeling porous flow at near-critical conditions, which can handle the extreme nonlinearities in water properties near the critical point. Our numerical simulations show strong enhancements of convective heat transfer at near-critical conditions; however, the heat transfer rates obtained in the simulations are considerably smaller than data reported from laboratory experiments by Dunn and Hardee. We discuss possible reasons for this discrepancy and develop suggestions for additional laboratory experiments.

Solute–solvent interactions in infinitely dilute supercritical mixtures: A molecular dynamics investigation

A molecular dynamics investigation of the environment surrounding an infinitely dilute Xe-like Lennard-Jones atom in a supercritical Ne-like Lennard-Jones fluid shows enhanced solute–solvent interactions (clustering) in the vicinity of the solvent’s critical point. Under near-critical conditions, the solute is at all times surrounded by an environment which is greatly enriched in solvent with respect to bulk conditions, but the identity of the solvent molecules in the cluster changes continuously. The enhancement of solute–solvent interactions in the Xe-in-Ne near-critical system is in contrast with the behavior exhibited by the symmetric Ne-in-Xe near-critical system (i.e., infinitely dilute Ne in near-critical Xe). In this case, the environment surrounding Ne atoms tends to be solvent lean with respect to bulk conditions. As Ne’s critical point is approached, Ne–Xe interactions become progressively irrelevant with respect to Ne–Ne interactions, giving rise to pronounced density fluctuations around the Xe atoms. Differences between Xe-in-Ne and Ne-in-Xe near-critical systems are confirmed by cluster statistics and stability analysis.

Influence of magma chamber geometry on hydrothermal activity at mid-ocean ridges

Two-dimensional models of the ridge environment are utilized in this study to examine the behavior of ridge hydrothermal systems, with particular emphasis on the influence of magma chamber geometry. Seismological evidence supports a dike-like 2 km half-width chamber, and models of this chamber indicate that: (1) complete crystallization of the magma requires 30,000 years, (2) hydrothermal upflow and hot springs are concentrated in a narrow band within 1.5 km of the ridge axis for the lifetime of the system, (3) a large hydrothermal cell forms and remains centered 2 km from the axis for the lifetime of the system, (4) hydrothermal activity ends by 70,000 years. Petrological evidence supports a wide sill-like chamber up to 15 km in half-width, and models of this chamber indicate that: (1) complete crystallization of the magma requires 120,000 years, (2) hydrothermal vents are present at the ridge axis, but most of the vents are located 5–10 km away from the axis, (3) one large hydrothermal cell develops 15 km from the axis, while a series of small vigorous cells form directly above the intrusion, all of these features migrate toward the axis as the magma solidifies, (4) hydrothermal activity ends by 170,000 years. Substantially different hydrothermal systems develop around these two chamber geometries because they generate different distributions of near-critical P-T conditions for H 2O. These zones of near-critical conditions are the primary control on driving forces for ridge hydrothermal systems, and provide the primary link between magmatic and hydrothermal events. Narrow magma chambers produce a very narrow near critical zone, and therefore produce hydrothermal and hot-spring activity that is strong focused at the ridge axis. Wide chambers produce broad near-critical zones, and widely distributed hot-spring activity that sweeps towards thèaxis with time.

Modelling pore-scale displacement processes at near-critical conditions

The importance of critical phenomena in EOR is reviewed and the merits and limitations of conventional experiments briefly discussed. In order to develop an adequate description of the significant flow phenomena involved in field-scale displacement processes the pore-scale mechanisms underlying the macroscopic behaviour must be identified and understood. Therefore, an approach which complements conventional experiments and provides the necessary insights into the pore-scale phenomenology is required: details of a new physical modelling approach are presented. The effects of near-critical behaviour (i.e. low interfacial tension and small density contrasts) on displacement processes are modelled by the critical solution behaviour of partially miscible binary liquids. In particular, the interfacial tension can be varied precisely, and over a wide range, by controlling the temperature deviation from the critical solution temperature. Micromodels, transparent networks etched in glass, replace cores as realisations of porous media. They permit great flexibility and control over the pore-space morphoogy, allow fluid distributions to be observed, and provide the best means of assessing the influence of the pore-space geometry and topology on the processes of interest. The key role of interfacial phenomena in determining the flow characteristics is emphasised. Experimental observations are presented which illustrate pore-scale behaviour when the interfacial tension is very low. In particular, the importance of gravity is highlighted; effects are seen even on the pore scale where the influence of gravity is usually ignored. Finally, the implications of these observations for EOR are discussed and further topics for study suggested, including the development of this versatile modelling approach.

Delineation and Evaluation of a North Sea Reservoir Containing Near-Critical Fluids

SummaryA newly delineated reservoir in the North Sea Viking graben contains fluids that are at near-critical conditions. This paper presents a global approach for the delineation program, fluid correlation, and subsequent reservoir simulation studies. The delineation history and fluid sampling are detailed first. Fluid data correlation is presented, based on the use of an equation-of-state (EOS) program. Then, the compositional simulator, which provided satisfactory fluid representation for full-field studies and proved to be reliable and cost-efficient, is described.

Gravity effects in near-critical fluids in porous media

Critical scaling laws indicate that gravity effects can be important on the pore scale for near-critical fluids in porous media. Experimental studies, using 2,6-lutidine-water mixtures, demonstrate the increasing dominance of gravity over capillary forces as the critical point is approached. This is illustrated by observations of a pendant drop at near-critical conditions and by slow gravity segregation through pore to pore fluid movement in a transparent model porous medium.

Critical effects in the infrared spectrum of hydrogen

Measurements of absorption in the fundamental (4000 to 5000 cm−1) vibrational band of H2 have revealed new features in the collision-induced spectrum which attain their maximum intensity under near-critical conditions (Tc=32.98 K for para-H2). The new features are relatively sharp ‘‘spikes’’ (FWHM≊5 cm−1) which occur at the frequencies of the Q1(0) and/or Q1(1) transitions, and the effect is most prominent in pure para-H2. The behavior of these features as a function of temperature and of ortho-/para-H2 ratio has been investigated, they have also been detected in the spectrum of D2 near the critical point. An interpretation is proposed on the basis that long-lived spatial fluctuations in density give rise to a spoiling of the normally active mechanism of destructive intercollisional interference.