Phase Behavior Calculations Research Articles

Isothermal vapour–liquid equilibria in the ternary system tert-butyl methyl ether + tert-butanol + 2,2,4-trimethylpentane and the three binary subsystems

Vapour–liquid equilibrium data are reported for the ternary tert-butyl methyl ether+ tert-butanol+2,2,4-trimethylpentane and the three binary tert-butyl methyl ether+ tert-butanol, tert-butyl methyl ether+2,2,4-trimethylpentane, tert-butanol+2,2,4-trimethylpentane subsystems. The data were measured isothermally at 318.13, 328.20, and 339.28 K covering pressure range 15–100 kPa. Azeotropic data are presented for the tert-butanol+2,2,4-trimethylpentane system. Molar excess volumes at 298.15 K are given for the three binary systems. The binary vapour–liquid equilibrium data were correlated using Wilson, NRTL, and Redlich–Kister equations; the parameters obtained were used for calculation of phase behaviour in ternary system and for subsequent comparison with experimental data.

Phase behavior prediction of complex petroleum fluids

The new phase behavior prediction method of petroleum fluids, which was recently developed by Manafi et al. for simple petroleum fluids, has been modified and extended for application to complex petroleum fluids. A combination of the Peng–Robinson equation of state for phase behavior prediction and the Riazi and Mansoori equation of state for density prediction has been applied. The proposed method is applicable for various equations of state as long as they give accurate prediction of phase behavior and densities of hydrocarbons and their mixtures. The composition of petroleum fluids is described by discrete and plus-fraction parts. For the plus-fraction of petroleum fluids, appropriate distribution functions for molecular mass, true boiling point and specific gravity are used. The distribution functions required an average value of molecular mass, and specific gravity of the plus-fraction as input data. Phase behavior calculations are performed for prediction of saturation pressure as well as flash liberation, differential liberation and flash-separation, in a wide range of pressures and temperatures for six different complex petroleum fluids and compared with experimental data. It is demonstrated that the proposed method can predict the phase behavior of complex petroleum fluids mixtures quite accurately.

Phase behaviour of sterols and vitamins in supercritical CO2

Extraction with supercritical solvents has been used in different areas, such as petroleum desasphaltation, descaffeination of coffee and tea and in the separation of other types of natural products. The supercritical solvent most frequently utilized in the extraction of natural products is carbon dioxide (CO2) due to its several advantages over other solvents such as low cost, atoxicity and volatility. The design, evaluation and optimization of a supercritical extraction that is based on phase equilibrium require phase equilibrium data. This type of data is very scarce for natural compounds like sterols and vitamins. These natural compounds are produced synthetically, but nowadays interest in their extraction from natural sources is increasing. Therefore, the objective of this work is to study the thermodynamic modelling equilibrium of systems containing vitamins A, D, E and K, using the predictive LCVM model. The sensitivity of critical properties in the calculation of the phase behavior was also studied. This study proved that the choice of a group contribution method to calculate thermodynamic properties is very important for obtaining good results in the phase equilibrium calculations.

COMPOSITIONAL ANALYSIS OF NORTH SEA OILS

ABSTRACT The molar fluid composition of either the reservoir fluid or the well stream is determined by combining the true boiling point (TBP’) distillation data with gas chromatographic (GC) analysis of the light ends. For the purpose of thermodynamic simulation of phase behavior of petroleum reservoir fluids, in addition to the compositional data, physical properties of the pseudo fractions, i.e. density and molecular weight are required. A major drawback of the TBP distillation is the fact that the fractions contain typically 20 -30 % of the material outside the defined boiling range. Another significant issue is the use of generalized density and molecular weight data in the absence of experimentally determined values. This can introduce major inaccuracies in the phase behavior calculations because the generalized value of density and molecular weight significantly differ in each oil based on the paraffin-naphthene-aromatic distribution and its geographic origin. In this work we have performed the true TBP distillation of 7 stabilized North Sea oil samples. All the oils were distilled from carbon number 6 to 19 and the distillation was terminated at C20+, which was termed as the residue. We have performed analysis of the Cm fraction of each oil by gas chromatography. Subsequently, the specific gravity and molecular weights of the TBP fractions were determined and compared with the generalized values, which indicated major differences. In addition, the superiority of the PVT calculations for a volatile oil and a gas condensate using the experimentally determined specific gravity and molecular weight of the pseudo fractions against the generalized properties is also demonstrated.

Generalized Flory-Huggins model for heat-of-mixing and phase-behavior calculations of polymer-polymer mixtures

An extended and generalized Flory–Huggins model for calculating the heats of mixing and predicting the phase stability and spinodal diagrams of binary polymer–polymer mixtures is presented. In this model, the interaction parameter is considered to be a function of both temperature and composition. It is qualitatively shown that the proposed model can calculate the heats-of-mixing curves containing exothermic, endothermic, and S-shaped or sigmoidal types and predict the spinodals, including the upper and lower critical solution temperatures, and closed-loop miscibility regions. Using experimental results of analog calorimetry for four polymer mixtures of polystyrene/poly(vinyl chloride) (PS/PVC), polycarbonate (PC)/poly(ethylene adipate) (PEA), polystyrene/poly(vinyl acetate) (PS/PVAc), and ethylene vinyl acetate copolymer (EVA Co)/chlorinated polyethylene (CPE), the capabilities of the proposed functionality for the interaction parameter was studied. It is shown that this function can be used satisfactorily for the heat-of-mixing calculations and phase-behavior predictions. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1328–1340, 2000

Complex Multiphase Equilibrium Calculations by Direct Minimization of Gibbs Free Energy by Use of Simulated Annealing

Summary The computational problems in reservoir fluid systems are mainly in the critical region and in liquid-liquid (LL), vapor-liquid-liquid (VLL), and higher-phase equilibria. The conventional methods to perform phase-equilibrium calculations with the equality of chemical potentials cannot guarantee a correct solution. In this study, we propose a simple method to calculate the equilibrium state by direct minimization of the Gibbs free energy of the system at constant temperature and pressure. We use the simulated annealing (SA) algorithm to perform the global minimization. Estimates of key parameters of the SA algorithm are also made for phase-behavior calculations. Several examples, including (1) VL equilibria in the critical region, (2) VLL equilibria for reservoir fluid systems, (3) VLL equilibria for an H2S-containing mixture, and (4) VL-multisolid equilibria for reservoir fluids, show the reliability of the method.

Predicting Phase Behavior of Water/Reservoir-Crude Systems With the Association Concept

Summary An association model has been incorporated in the Peng-Robinson equation of state (PREOS) to use the combined model for water/res-ervoir crudes phase-behavior calculations. The interaction coefficients between H2O and hydrocarbons follow a simple relationship. Such a relationship makes the use of the association equation of state (AEOS) both accurate and simple. The computed high-pressure crude/steam distillation results by the AEOS are in excellent agreement with the data.

Experimental studies and discrete thermodynamic modeling on supercritical CO2 extractions of a hexadecane and crude oil

Continuous multiple-contact experiments using a supercritical CO2 were performed to study the phase equilibrium behavior of the dynamic extractions of a hexadecane and crude oil. The extraction yields increased as CO2 density increased with a pressure rise at constant temperature. The rates of extractions were also greater at higher pressure. The simulated distillation analysis of extracted crude oil samples represented that the earlier extracts contained lighter compounds and the latter extracts contained progressively heavier compounds. These compositional changes occurring during a dynamic extraction were also ascertained by phase-equilibrium flash calculations using the equations of state and a pseudo-component lumping method. Two different equations of state, Soave-Redlich-Kwong and Peng-Robinson, were used to predict the equilibrium compositions of the extract phase that is a supercritical carbonic phase. The results of phase behavior calculations established the nature of the extraction and partitioning process as a function of time. These results also provided reasonable agreement between the experimental data and the calculated values.

A new generalized alpha function for a cubic equation of state Part 2. Redlich-Kwong equation

The approach described in the previous paper (Twu, C.H., Coon, J.E. and Cunningham, J.R., 1994a. A new generalized alpha function for a cubic equation of state. Part 1. Peng-Robinson equation. Fluid Phase Equilibria, 105: 49–59) is applied to the Redlich-Kwong cubic equation of state (RK CEOS) to develop a new generalized alpha function for this equation. The new generalized alpha function for the RK CEOS reproduces the vapor pressure for hydrocarbons from the triple point to the critical point with almost identical accuracy to the generalized alpha function used for the Peng-Robinson CEOS in the previous paper. This indicates that the approach developed should be a general one, applicable to any cubic equation of state. The alpha function has been generalized in terms of the reduced temperature and acentric factor, so it can be used for any hydrocarbons and petroleum fractions, with no additional characterization to the standard methods required (Twu, 1984). The new alpha function is more appropriate for the RK CEOS than Soave's alpha function, which has been widely used in phase behavior calculations in the petroleum production and refinery industries over the last twenty years.

Compositional simulation of reservoir performance by a reduced thermodynamic model

In compositional reservoir simulations, it is usually assumed that a local thermodynamic equilibrium between all phases exists throughout the reservoir within each time step. Phase behavior calculations are conducted based on an equation of state (EOS) in each block. This computation normally takes a very significant percentage of the total CPU time. In this work, an extension of the reduced thermodynamic model (Wang and Stenby, Computers chem. Engng 16, 5449-5456, 1992) to the processes in which both pressure and feed composition vary is demonstrated. This modified model has been tested against the experimental data of gas condensates and the PVT data computed from the EOS. The results indicate that the new model can well represent the influences of changes in the feed composition and the pressure on the K-values. The new model has been implemented into a compositional reservoir simulator, UTCOMP (Chang Ph.D. Thesis, Univ. of Texas, Austin, 1990). Simulations of natural depletion for three well-defined mixtures and of a dry gas recycle process for a gas condensate in the North Sea are performed. The results simulated with the new method are comparable to those computed by the PR EOS. But the CPU time is reduced by approximately 50% by means of the new method compared to the one required by the PR EOS. In addition, this work shows that the CPU time can be further decreased by using the newly suggested procedure (Leibovici and Neoschil, Fluid Phase Equil. 74, 303-308, 1992) for solving the Rachford-Rice equation for phase split.

New applications of Kahl's VLE analysis to engineering phase behavior calculations

Abstract Nitsche, J.M., 1992, New applications of Kahls' VLE analysis to engineering phase behavior calculations. Fluid Phase Equilibria , 78: 157-190. In a little known but profound paper, G.D. Kahl (1967) (Phys. Rev., 155: 78-80) developed a compelling argument questioning the validity of isothermal integration of the van der Waals equation between the spinodal points in accordance with Maxwell's equal-area construction. This work generalizes Kahl's approach and shows conclusively that it is entirely possible to describe vapor-liquid equilibria without lending credence to the thermodynamic properties of unstable states. The analysis is structured around a new free energy parameter Δ( T ) that allows adjustment of calculated vapor pressures without alteration of the equation of state itself, as is demonstrated using a novel modification of the Redlich-Kwong equation. The analysis opens the door to greater flexibility in using excess free energy models to devise mixing rules for equations of state. An extension of the theoretical development leads to a new type of VLE procedure whereby unrelated equations for liquid and vapor phases can be combined for essentially exact volumetric description of the pure fluids comprising a mixture. Introduction of a parameter Δ( T ) also makes it possible to formulate a conjecture explaining the apparent nonexistence of liquid-solid critical points.

A comparison of the finite-difference and the finite-element methods for simulating unstable displacements

The finite-difference and the finite-element methods are the two most commonly used numerical methods in reservoir simulation. A comparative study of the two methods in compositional and noncompositional simulations is presented. The comparisons are based on grid orientation effect and computational efficiency. In the grid orientation study, the finite-element model with Gaussian quadrature showed less grid orientation than either the finite-difference model or the finite-element model with Lobatto quadrature. In heterogeneous media, the finite-element model with Gaussian quadrature suffered from severe numerical oscillations inspite of upstream weighting. The finite-element model with Lobatto quadrature was stable in heterogeneous media; however, the results were essentially identical to those obtained by the finite-difference method. In simulating two-phase noncompositional displacements, the finite-element models took twice as much computing time as the finite-difference model due to the lengthy process of formulating and assembling the transmissibility matrices. In compositional systems, however, the finite-element method took only 50% more time than the finite-difference method because the bulk of the computational effort was concentrated in the phase behavior calculations rather than in the formulation of the system matrices.

A Robust Prediction Method for Rapid Phase-Behavior Calculations

Summary The success of phase-behavior calculations based on an equation of state (EOS) depends to a great extent on the quality of the selected initial estimates of the independent variables. Estimates considerably different from the final answer often lead to either trivial solutions or convergence problems. The method presented in this paper consists of obtaining the maximum possible information concerning the vapor/liquid equilibrium of the mixture by directly solving the selected EOS before the start of the main iterative calculation. The information obtained, with minimal computational cost, is subsequently used to initiate a Newton-type iterative method that is stable and converges rapidly toward the solution when good estimates are provided.

Predicting Yield of Revaporized Condensate in Gas Storage

Summary Phase behavior calculations for gas-condensate systems Phase behavior calculations for gas-condensate systems were used to predict depletion yields of condensates. The same procedures may be used for predicting revaporization and depletion yields of condensates in gas storage cycles. Data on two Niagaran reef storage reservoirs in Michigan show yields less than ideal for complete mixing and revaporization in the reservoir. Phase Calculations for Gas Phase Calculations for Gas Condensate Reservoir Depletion of gas-condensate reservoirs of fixed volume is accompanied by retrograde condensation of liquid in the reservoir. Calculations of phase behavior during primary production is needed to verify the surface yield primary production is needed to verify the surface yield of condensate and to obtain the composition and quantity of retrograde liquid in the reservoir. The depletion program used for studying Michigan Silurian reef fields for program used for studying Michigan Silurian reef fields for conversion to storage is that of Firoozabadi et al. based on the Peng-Robinson equation of state and interaction parameters for methane and the C6+ constituents. parameters for methane and the C6+ constituents. A first step in these phase-behavior calculations is to obtain a recombined separator gas and liquid well-stream analysis at an early date in the life of the reservoir along with the reservoir temperature and pressure. The extended analysis basic to these calculations should include constituents with boiling points as high as C13 to C20 groups. The gas and condensate production vs. reservoir pressures is needed for material balance of yields and determination of reservoir size. Fig. 1 for Cold Springs 12 and Fig. 2 for Rapid River 35 reservoirs show a comparison of primary yield of condensate. The agreement verifies that the phase relationships, composition of well fluid, and depletion balances are reliable for gas-storage vaporization and depletion calculations. The calculated composition of the reservoir gas and retrograde liquid (some 3 to 4% by volume of pore space) are needed in gas-storage calculations. pore space) are needed in gas-storage calculations. Gas Storage Underground storage fields are equipped to inject gas in summer at periods of low demand and to produce gas at high rates for peak or high-demand periods. With pressures above discovery, storage reservoirs may pressures above discovery, storage reservoirs may deliver 80 to 100% of the primary content in a 100-day withdrawal period. When condensate is present in the reservoir because of retrograde condensation, a second purpose is to produce the condensate by vaporizing it purpose is to produce the condensate by vaporizing it with gas pressure, followed by depletion or displacement. Modes of Operation There are alternate modes of operation for injecting and withdrawing gas, three of which are discussed. Mode 1 is to inject into all wells and withdraw gas from all wells. One might assume a completely mixed fluid of dry gas, reservoir gas, and reservoir liquid. In the initial withdrawal, the gas will be leaner than calculated in that the condensate near the wellbore will have been stripped before full pressurization. JPT P. 1173

Calculation of gas-gas phase equilibrium for the mixture He-Fl2

Equilibrium conditions are determined by use of the Redlich-Kwong equation of state and two different methods for numerical calculation of the mixture phase behavior.

A Corresponding States Correlation for Calculating Gas-Condensate Phase Equilibria

Abstract A correlation has been developed for calculating the phase behavior of gas-condensate systems at reservoir conditions. The correlation is based on the principle of corresponding states and has been coded for an IBM 7094. Experimental K-values were determined for several gas-condensate systems at reservoir conditions to evaluate various semiempirical parameters of the correlation. The approximate range of application of the correlation is 150 to 300F and 1,500 to 6,000 psi. Introduction The rapid development of digital computers during the past several years has made feasible the calculation of hydrocarbon phase behavior by methods based on rigorous thermodynamic principles. Good correlations have been developed for low to moderate pressures, but these techniques have not yet been extended successfully to reservoir fluids at high pressures. Consequently, the determination of phase behavior of oil and gas systems at reservoir conditions is still based almost entirely on generalized data correlations or on experimental studies of the fluid in question. While these methods have been used successfully many times, they do have inherent limitations that restrict their applicability. Generalized correlations, such as the NGSMA K-charts, are limited to the range of pressure, temperature and components for which pressure, temperature and components for which the data were determined. The accuracy of these correlations is often questionable because the effect of total system composition is not well defined. Experimental studies offer a reliable method for determining phase behavior, but usually the studies are costly and time consuming. Recently, Leland and coworkers presented a new approach to calculating phase behavior from the principles of corresponding states. Corresponding states methods determine the thermodynamics properties of a given system by comparison with a reference substance whose properties are known. The accuracy of data properties are known. The accuracy of data approach depends on close chemical and structural similarity between the reference substance and the system in question and between components within the system itself. For high accuracy, it is usually necessary to correct for chemical and structural dissimilarities. In principle, however, the corresponding states method should be no less accurate at high pressures than at low pressures, provided reference substance properties are known. provided reference substance properties are known. This paper describes an empirical modification of the basic correlation proposed by Leland, et al. for the specific purpose of calculating the phase behavior of gas-condensate fluids at reservoir conditions. The modified correlation, which has been Programmed for an IBM 7094, may be used for either approximate or precise determination of fluid behavior depending on the amount of analytical and, experimental data available for the system. BASIC THEORY The basic theory of the corresponding states phase equilibria correlation was first published by phase equilibria correlation was first published by Leland, Chappelear and Gamson. Subsequently, Leland, Chappelear and Leach published methods for improving me accuracy of the original theory. The aim of the correlation is to calculate the K-value of each component of a given system as a function of pressure, temperature, and over-all composition, where ..........................................(1) Once the K-values are known, the phase behavior may be determined directly by an appropriate flash calculation. The basic equation for calculating component K-values was taken from the work of Joffe. For any component i of a mixture, the K-value is given by ..........................................(2) SPEJ P. 281

A Critical Pressure Correlation for Gas-Solvent-Reservoir Oil Systems

Abstract This paper presents a correlation for predicting the critical pressures of gas-solvent-reservoir oil systems of the type encountered in gas and solvent injection processes. Experimental data from 14 systems were used to develop the correlation which expresses critical pressure as a function of composition, C7+ mol weight, and C7+ characterization factor. Critical pressures predicted by the correlation for the 14 gas-solvent-reservoir oil systems have an average deviation of 2.8 per cent of the experimental values.The correlation was used to study a wide variety of other hydrocarbon mixtures reported in the literature whose critical conditions are in the range of interest in oil recovery work (100 to 280F and 2,000 to 6,000 psia). Critical pressures predicted by the correlation had an average deviation, expressed as per cent of experimental value, of 5 per cent for 20 miscellaneous mixtures from the literature, and 6.4 per cent for 17 methane-butane-decane mixtures reported by Sage and Lacey. Introduction Predicting critical points of hydrocarbon mixtures is an important part of phase behavior calculations. If the critical point can be predicted for a specific mixture, the separation between the bubble-point and dew-point regions will be defined, and physical properties of the mixture can be calculated by using the principle of corresponding states.Numerous correlations for predicting critical points of hydrocarbon mixtures have been published. Six of these correlations were for light hydrocarbons, and two correlations were for high boiling-point petroleum distillates. Organick and Eilerts presented correlations which combined the published data from both light and heavy mixtures, and these correlations became the only available methods for predicting critical pressures for wide boiling range mixtures.In the course of the authors' work on secondary recovery by gas and solvent injection, experimental data were obtained for 14 mixtures of dry gas, ethane-propane-butane solvents, and various reservoir oils. The critical pressures and critical temperatures of these gas-solvent-reservoir oil systems were in the range of 1,900 to 4,800 psia and 180 to 265F. The critical pressures predicted for the 14 mixtures by the correlations of Organick and Eilerts had an average deviation from the measured values of about 13 per cent. The authors felt that it was necessary to develop a better correlation for engineering calculations and the resultant correlation, which is presented on Figs. 1 and 2, predicts the critical pressures of the 14 mixtures with an average deviation of 2.8 per cent. The validity of the correlation was tested by applying the correlation to other hydrocarbon mixtures reported in the literature. This subject is discussed later in the paper.The authors feel that there is also a need for an improved critical temperature correlation for gas-solvent-reservoir oil systems. However, developing a critical temperature correlation is a significant study in itself and is not included in this paper. EXPERIMENTAL DATA The compositions and critical points obtained by the authors for 14 gas-solvent-reservoir oil mixtures are shown on Table 1. The methane concentrations in these mixtures varied from 20 to 63 mol per cent. The intermediate hydrocarbon group, consisting of ethane, propane, and butane, varied from 20 to 64 mol per cent, and the concentration of the C5+ fraction from 10 to 20 mo per cent.The physical properties of the C5+ fraction covered the range from light, paraffinic, nonasphaltic (Mixtures 5, 6, 7, 8, 9 and 10) to heavy, aromatic, and asphaltic Mixtures 13 and 14). JPT P. 556^

Nonequilibrium Gas Displacement Calculations

Abstract The effects of phase behavior on reservoir economics is a function of both fluid composition and flow properties of the reservoir rock. In some operations such as gas injection into volatile crude-oil reservoirs, the stock-tank recovery derived from vaporization of the reservoir oil by the displacing gas can approach the recovery derived from the displacement mechanism. In other cases of gas-cycling reservoirs, the dry injected gas can effectively vaporize a retrograde liquid phase, allowing a substantial reduction in cycling pressure with no appreciable loss in recoverable liquids. Because of these effects, it is essential that phase behavior be considered in developing the economics of any secondary recovery program in which gas is the injected fluid. The inclusion of phase behavior in conventional calculation procedures has been retarded because of the complexity of the calculation and the large volume of laboratory data required. However, by applying high-speed computers and new laboratory techniques, the phase-behavior displacement calculation has been developed to economically include this basic fundamental of gas displacement. The calculation is performed by reducing the reservoir to a series of one-dimensional segments. Phase changes, saturation distributions and changing fluid properties are evaluated for each segment as a function of time. The relative volumes of equilibrium liquid and gas displaced from each segment are determined by a trial-and-error procedure to meet the requirements of phase equilibrium and two-phase flow. The produced fluids are flashed through conditions of surface separation approximating those in the field to obtain actual values for stock-tank liquid recovery and producing gas oil ratios. Based on the results of successful field studies, the technique appears to have application to reservoirs producing under a nonmiscible gas-displacement mechanism and should substantially increase the reliability of this type of analysis when phase behavior is an important factor.

The Uses and Limitations of Computers in Petroleum Engineering Work

Abstract Computer usage in the oil industry has progressed to the point where every engineer should be familiar with calculation procedures that can be applied to his work. This knowledge will allow him to evaluate the procedures published in the literature and determine the advantages and disadvantages of using a computer to do a certain job. In petroleum engineering work computers have been used to solve problems in:surface separation;primary reservoir performance;pressure maintenance and secondary recovery operations;gas-field operations including retrograde behavior, cycling, deliverability, gas plant operations, gas pipeline operations and compressor requirements;economics;maximizing profits or minimizing costs;statistical analysis; andgeneral engineering practices such as drilling, casing design, compressor design and log interpretation. There are advantages and disadvantages in the use of a computer for performing the calculation work that may be required in solving specific problems, and certain of the calculation procedures are not available without the aid of a computer. Because its petroleum engineers may be scattered over a large operating territory, a company can greatly enhance the efficient use of its computing facilities by a well planned organizational set-up. These computing facilities may be company-operated, company rented or handled by a consultant. The important thing is that the engineer be given free access to the use of the computer, either directly or through close coordination with a computing group comprised of engineers with backgrounds similar to his own who are trained in the use of computing equipment. Introduction Developments in computing equipment and procedures during recent years have caused some marked changes in basic engineering responsibility. The development has occurred at a rapid pace, and the oil industry has been quick to take advantage of these new tools. Today it has become apparent that every engineer should know something about the application of computing machines for solving problems in his field. There certainly are many valid reasons for not employing a computer to do certain jobs, and it definitely is true that a company can become over-expended with expensive equipment and an inefficient computing organization. The purpose of this paper is to discuss the efficient use of computers in petroleum engineering work. This discussion will includethe uses and limitations of a computer for solving various classes of petroleum engineering calculations anda summary of the basic requirements of an efficient engineering computing organization. For the purpose of discussion, the various calculation procedures have been grouped into the following categories:phase-behavior flash calculations,reservoir performance calculations not requiring phase-behavior considerations,reservoir performance calculations requiring phase-behavior considerations,two-dimensional reservoir studies,gas-well deliverability studies,economic analyses,optimization studies,statistical studies andgeneral engineering analyses. In a large engineering organization, various engineers in different locations will spend a great deal of time in simply equipping themselves with the correct equations, factors and experience necessary to perform certain calculations for the first time. A computer program, once prepared with a correct set of equations and factors, leaves all of the time open for gaining experience. There is no substitute for engineering experience in analyzing a problem and deciding whether or not detailed calculations are warranted. Along this line, it is essential for effective computer utilization that the validity of the basic data and the end use of the answer justifies the expense of a better study than can be obtained by an intuitive analysis or a good engineering estimate. This point cannot be overemphasized and should be kept in mind during the discussion of the various calculation procedures. Phase-Behavior Flash Calculations The flash calculation is an extremely useful engineering tool for which there is no complete substitute or short-cut. JPT P. 625^

Effect of Composition and Temperature on Phase Behavior and Depletion Performance of Rich Gas-Condensate Systems

Abstract The experimental phase behavior of several field gas-condensate systems, one field volatile oil system, and a series of synthetic systems having gas-oil ratios from 2,000 to 20,000 scf/bbl stock tank oil was measured. The data were used to calculate the depletion performance of the field systems at their respective reservoir temperatures and of the synthetic systems at various temperatures. The results of the performance calculations were used to prepare correlations of total stock tank oil and separator gas in-place, and the amount of each recovered by primary depletion. Data needed to use the correlations are initial gas-oil ratio, initial tank oil gravity, reservoir temperature and reservoir pressure. The correlations presented should be useful for estimating recovery from reservoirs producing volatile oils or rich gas condensates having gas-oil ratios from about 2,000 to 30,000 scf/bbl of stock tank oil. Introduction Prediction of depletion performance for gas-condensate reservoirs usually requires extensive laboratory work or lengthy phase-behavior calculations. For many small gas-condensate bearing zones the expense of such work is often not justified. Furthermore, where large holdings are involved, it is useful to have estimates of future oil and gas production in advance of the results of a laboratory analysis, even if such work is ultimately planned. For these reasons, it would be of value to have a method for rapidly estimating the depletion performance of gas-condensate reservoirs from the type of field data usually available. Few data on the reservoir depletion performance are available for the purpose of developing correlations. The increasing number of gas-condensate discoveries and the advent of high-speed computers have stimulated the development of methods for predicting performance. Such calculated performance data have been shown to be reliable and are becoming accepted as a basis for reserve estimates and for engineering studies. It therefore seems satisfactory to use results of performance predictions themselves for the purpose of developing the desired correlations.