Phase Behavior Calculations Research Articles

Effects of the Characterization Methods of Heptane Plus Components (C7+) on the Phase Behavior of Volatile Fluids

For reservoir fluids with strong volatile tendencies, such as gas condensate and volatile oil, the determination of the characteristic parameters of the equation of state has a great influence on the fluid phase behavior and subsequent reservoir engineering, production engineering, and surface facilities design calculations. However, the process of characterizing reservoir fluid lacks a unified and standardized procedure, and the methods are largely empirical and rather complicated. In this paper, three main stages are defined for the characterization of the equation of state parameters for these volatile reservoir fluids, including (1) determining the C7+ distribution, (2) calculating the equation of state parameters for the C7+ single-carbon component, and (3) lump components. The typical calculation methods in each step and their effects on the fluid phase behavior calculations are quantitatively analyzed and compared. Finally, the workflow and the suitable characterization methods for gas condensate and volatile oil phase behavior calculations are selected. The results show that C7+ distribution and C7+ single carbon component properties have a greater influence on the phase behavior prediction of condensate gas and volatile oil, while the lumping of C7+ single carbon component has relatively little influence.

The Development and Application of EOS-based VT Phase Behavior Calculation Algorithms in Petroleum Industry

The use of equation of state (EOS)-based phase behavior calculations is widespread in the petroleum industry, including the calculation of oil and gas reserves, production forecasting, and optimization of enhanced oil recovery (EOR) plans, surface separator design, and pipe flow calculation. The most commonly used method for providing phase behavior information is PT phase-equilibrium-calculation algorithms, which have been extensively studied for decades. However, simulation and engineering design of these processes using VT phase-equilibrium- calculation algorithms is sometimes more convenient than using conventional PT algorithms and has distinct advantages. The VT algorithm has been continuously improved over the last decade to ensure calculation accuracy, robustness, and efficiency, and it has been gradually applied in the petroleum industry. This article provides an overview of research findings in the field of EOS-based VT phase behavior calculation algorithms and their applications in oil and gas engineering. The Helmholtz-free-energy minimization approach, the Gibbs-free-energy minimization approach, and the nested approach based on the PT algorithm are three typical VT algorithm approaches discussed. The petroleum industry’s main applications of phase equilibrium calculation using the VT algorithm are described. Furthermore, some existing problems are identified, and several prospects for the application of the VT algorithm in the petroleum engineering field are presented. A critical review of the current state of the VT algorithm process, we believe, will fill the gap by shedding light on the process’s flaws and limitations, future development areas, and new research topics.

A generalized flash algorithm to bridge stability analysis and phase split calculations

Phase split and phase stability algorithms traditionally have been the backbone of all phase behavior calculations in the presence of an Equation of State model. Phase stability examines whether a feed composition at given specifications is stable, or if it will split in two or more phases. In the latter case, phase split will further provide the composition and molar fraction of all equilibrating compositions.Despite the extension and improvement of the original algorithms developed by Michelsen as far as both speed and robustness are concerned, those two calculations are still treated as independent. In this work, we explore the relationship between them and develop a new algorithm called “the generalized flash” which acts as a bridge between stability and phase split. By utilizing suitable values of its parameters, namely, the stability distance θ and the molar fraction β, it is shown that the generalized algorithm can reduce to any of the two basic ones. Additionally, it can offer intermediate results between stability and phase split which also guarantee minimum Gibbs energy. The algebraic, geometric and computational aspects of the proposed algorithm are investigated. Finally, applications of the new algorithm with emphasis to negative flash calculations are discussed.

Acid Gas Re-Injection System Design Using Machine Learning

An “energy evolution” is necessary to manifest an environmentally sustainable world while meeting global energy requirements, with natural gas being the most suitable transition fuel. Covering the ever-increasing demand requires exploiting lower value sour gas accumulations, which involves an acid gas treatment issue due to the greenhouse gas nature and toxicity of its constituents. Successful design of the process requires avoiding the formation of acid gas vapor which, in turn, requires time-consuming and complex phase behavior calculations to be repeated over the whole operating range. In this work, we propose classification models from the Machine Learning field, able to rapidly identify the problematic vapor/liquid encounters, as a tool to accelerate phase behavior calculations. To set up this model, a big number of acid gas instances are generated by perturbing pressure, temperature, and acid gas composition and offline solving the stability problem. The generated data are introduced to various classification models, selected based on their ability to provide rapid answers when trained. Results show that by integrating the resulting trained model into the gas reinjection process simulator, the simulation process is substantially accelerated, indicating that the proposed methodology can be readily utilized in all kinds of acid gas flow simulations.

METHOD OF DETERMINATION OF THERMODYNAMIC AND FILTRATION PARAMETERS FOR HYDRODYNAMIC COMPOSITIONAL MODELS

The calculation of phase behavior is important for choosing a rational mode of development of oil and gas fields. However, solving equations of state and material balance by traditional methods is associated with difficulties: Cardano’s formula allows finding only the roots of a cubic equation, the dichotomy method requires the selection of a segment on which there is only one root. Newton’s iterative method allows finding only one root under given conditions. Another important problem in hydrodynamic simulation is the uncertainty of the relative phase permeability of oil during three-phase filtration. It is proposed for the first time to use the algorithm of sequential division of Euclidean polynomials and Sturm’s theorem to calculate phase behavior. The aim of the work is to develop a methodology for calculating phase behavior and determining the area of increased uncertainty of relative phase permeability of oil. This area is defined as a set of phase saturation values at which relative phase permeabilities of oil according to Stone 1st and 2d models differ by more than 10%. The proposed methodology makes it possible to predict areas of metastable states. The possibility of calculating the roots of material balance equations that do not have physical meaning is excluded. For a model three-phase system, an area of increased uncertainty of relative phase permeability of oil is determined. It is shown that it is possible to determine all real roots of the equations. The proposed metho­dology makes it possible to refine forecast calculations for gas condensate fields.

Higher-order compositional simulation of asphaltene damage and removal in the wellbore by the CPA-EOS

Simulation of asphaltene damage around the wellbore is important in relation to well productivity. Existing numerical approaches rely on the surface adsorption of asphaltene on the rock using a Langmuir isotherm. We demonstrate that asphaltene accumulation around the wellbore can be modeled as an asphaltene-rich liquid phase without the need to include surface adsorption explicitly. The relative permeability concept accounts for surface adsorption as well as pore-plugging. Our approach is fully verified with recent experimental data. This work is based on the fact that below the asphaltene onset pressure, an asphaltene-rich phase is formed with much higher viscosity than the oil phase from which it evolves. We perform phase behavior calculations using the cubic-plus association equation of sate (CPA-EOS). Flow and species transfer between the phases are simulated based on the higher-order method with high accuracy and low numerical dispersion. The algorithm includes full 3D unstructured gridding to model well damage and frequency and extent of asphaltenes removal in field scale applications. An example with a fracture crossing the producing well is included to examine deposition in the fracture and the matrix.

Vapor-liquid equilibrium and criticality of CO2 and n-heptane in shale organic pores by the Monte Carlo simulation

Thanks to the application of CO2 enhanced oil recovery (EOR) technology in shale oil production, phase behavior of CO2 and alkanes in nanopores has received considerable attention. Understanding the confinement effect on the vapor–liquid equilibrium and critical state of CO2 and alkanes is imperative to the calculation of phase behavior and prediction of oil recovery. In this work, the vapor–liquid equilibrium of pure CO2 and n-heptane (nC7) and their mixtures in shale organic pores are simulated using the Monte Carlo simulation. Compared to the bulk phase, the density of equilibrium gas is larger due to adsorption while the liquid phase density is lower because of capillary condensation. The influence of the confinement effect on the equilibrium phase composition results in the lower bubble point pressure and higher dew point pressure of the binary system. Changes in the equilibrium phase lead to a decrease in the critical temperature and pressure of single-component fluids and the minimum miscible pressure of binary system in nanopores. We fitted the critical property shift and analyzed the influence of pore size on fluid properties such as equilibrium pressure, composition, critical parameters, density distribution, etc. This study provides better insight into vapor–liquid equilibrium and critical state of CO2 and alkane in shale organic pores.

A soft computing method for rapid phase behavior calculations in fluid flow simulations

To describe the flow of hydrocarbons in the reservoir and in pipelines upstream petroleum engineers utilize compositional simulation models which allow for the accurate description of complex phase behavior phenomena by means of EoS models and detailed compositional characterization. To speed up the conventional time-consuming phase stability and phase split calculations, soft computing has been recently utilized by means of methods such as proxy models and on-the-fly tabulation of phase behavior results.

An effective asphaltene precipitation modeling approach using PC-SAFT with detailed fluid descriptions for gas injection conditions

Fluid characterization plays a prominent role in identifying the properties of the crude oil, particularly the equilibrium conditions. It gains more importance, especially in problematic crudes that, for instance, contain precipitating constituents, like Asphaltene. Aside from the pressure depletion, enhanced Asphaltene precipitation during the miscible gas injection causes many problems to the production system. Therefore, the employed characterization procedure must be able to predict the Asphaltene instability conditions effectively. Recently a class of solution techniques using the perturbed chain statistical associating fluid theory (PC-SAFT) and dealing with Asphaltene phase as a liquid-like phase have gained attractions in literature. However, such techniques could not consider the effect of the injected gas composition and structure, because they lumped all the heavy gas components in a single pseudo-component. Hence, in this study, a detailed characterization method is proposed that can account for the heavier gas components. The modeling approach in this work, assumes the liquid-like Asphaltene framework and PC-SAFT for phase behavior calculations. In this work, the three PC-SAFT parameters for Asphaltene and Aromatic + Resin are simultaneously regressed with gas injection, to match the saturation and Asphaltene onset pressures with experimental data. The outcomes of this study are compared with the experimental data and previous characterization models. The results prove the ability of the detailed characterization approach for predicting the bubble point and Asphaltene onset pressures. It not only leads to lower estimation errors in most cases but also can capture the actual trend of bubble and Asphaltene onset curves, which makes it even more reliable for the prediction of instability conditions.

Coupling solidification kinetics with phase-behavior computations in hydrodynamic simulations of high-pressure, dynamic-compression processes

In this study, we report a numerical scheme to integrate models for the kinetics of solidification processes together with phase-behavior computations in the context of continuum-scale hydrodynamic simulations. The objective of the phase-behavior computations is to determine the pressure and temperature, given the following three sets of inputs: (1) an appropriate equation of state to describe our system, (2) the phase fraction(s) produced by the kinetic models, (3) and the volume and internal energy obtained by solving the conservation equations that govern the hydrodynamic behavior. The kinetics are assumed to be governed by the Kolmogorov–Johnson–Mehl–Avrami equation, and the nucleation and growth rates that enter into that equation are functions of the pressure and temperature produced by the phase-behavior computations. Our formulation allows for the fluid and solid phases to be at different temperatures (thermal nonequilibrium) and pressures (arising from surface-tension-induced Laplace contributions). The formulation is presented in a fairly general setting that is independent of any particular material, although we demonstrate it in some examples related to high-energy-density science applications where materials are rapidly compressed to pressures exceeding several gigapascals in less than a microsecond. We conclude with a critical evaluation of our approach and provide suggestions for future work to improve the predictive capabilities and generality of the models.

A compositional numerical study in hydraulic fractured tight reservoir: Considering capillary pressure in phase behavior calculation

The compositional numerical simulator is a critical method for simulating the development of oil and gas production. In the simulation of conventional reservoir, capillary pressure is relatively small and often been ignored in phase behavior calculations. But In tight reservoirs, the capillary pressure is so large that can seriously affect the fluid phase behavior. The existing compositional numerical simulators cannot accurately calculate oil and gas production of tight reservoirs because the effects of capillary pressure on phase behavior are ignored. In this paper, we developed a full compositional numerical simulator that simulates the production in tight reservoirs with the consideration of capillary effect on the phase behavior of fluid. To compare the simulation results with and without considering capillary pressure in phase behavior calculations, we respectively set the capillary pressure as a function of gas saturation or zero in matrix and fractures. Three capillary pressure assignment methods for tight reservoirs were performed to illustrate the influence of capillary pressure on production. The simulation results indicate that the consideration of capillary pressure on phase behavior lead to more favorable for oil production, such as lower cumulative gas production and gas-oil ratio, accelerated pressure drop rate, increased oil saturation, and higher recovery compositions of heavy components. This study provides reasonable simulation results for the actual production of tight reservoirs, giving important insights into the evaluation of the production and development from tight reservoirs.

Improved Characterization of Heptane Plus Fraction of Niger Delta Light Crudes

Accurate determination of molecular weight of heptane plus( ) fractions is essential in reliable phase behavior calculations and compositional EOS modeling studies. Empirical correlations provide cheaper alternatives in time and cost, to obtaining reliable molecular weight data than by experiments, though with compromised accuracies. Several empirical correlations developed to predict molecular weight of fractionsof petroleum were reviewed. A new correlation for calculating molecular weight of heptane plus fractions of light crudes was developed from a database compiled from Niger Delta fields for over 1,200 light crude oil assays, conventional PVT reports, and literature data. The correlation was developed using rational multiple regression analysis method. The new correlation’s performance was compared with five others which do not depend on boiling point temperatures. Results showed that the new correlation has superior performance with the lowest absolute average and relative mean square errors. The new correlation had an average relative error of 8.15%, root mean square error of 0.09% and correlation coefficient of 0.955.

Integrated reservoir/wellbore production model for oil field asphaltene deposition management

Asphaltene deposition prevention, mitigation, and management remains a major challenge to the oil industry due to its complexity, poor understanding, and inadequate predictive tools. A literature review study on asphaltene deposition revealed a lack of integrative models that link reservoir, wellbore, and surface facility to predict asphaltene deposition taking into account the effect of their interaction on asphaltene deposition. In addition, most of the existing studies are focused on either modeling the thermodynamics aspects of asphaltene precipitation, or single-phase asphaltene deposition. Therefore, it is critical to model asphaltene deposition under multiphase flow conditions to, accurately, develop prevention, mitigation, and management strategies, which depends on not only asphaltene thermodynamics, but also multiphase flow hydrodynamics and behavior. The objective of this study is to develop a robust systematic approach for predicting asphaltene deposition in production system through coupling reservoir and wellbore production models, which provides a cost-effective optimal mitigation and management strategies. The proposed work in this study integrates five models, namely reservoir asphaltene deposition model, equation-of-state (EOS) model, asphaltene thermodynamics precipitation model, mechanistic multiphase flow model, and asphaltene deposition transport model. The above-mentioned models are integrated using developed workflow platform, which enables compositional tracking throughout the entire production system. Furthermore, experimental fluid characterization data was used to tune the EOS model to ensure accurate phase behavior and volumetric calculations, and to tune the thermodynamic asphaltene precipitation model. A field case input data is used to evaluate the proposed integrated model, which indicates severe asphaltene depositions in production tubing. The proposed model predicted the location and growth of asphaltene deposition thickness with time and space in the inner production-tubing wall. The model results also show that local asphaltene deposition reduced tubing cross-sectional area, increasing in-situ superficial oil and gas velocities, thus increasing pressure drop and decreasing flowrate. Sensitivity analyses to investigate several parameters such as depletion drive mechanism, asphaltene particle size, and injection of CO2 rich gas on asphaltene deposition show excellent results that are aligned with the physical and theoretical understanding of asphaltene deposition. These results are critical in selecting, optimizing, and implementing asphaltene deposition mitigation and management strategies, which affects project economics and safety.

Phase Behavior and DFT Calculations of Laterally Methyl Supramolecular Hydrogen-Bonding Complexes

Four new series of laterally methyl-substituted hydrogen-bonded supramolecular complexes were prepared. The prepared complexes were thermally investigated by differential scanning calorimetry (DSC) and phases identified by polarized light microscopy (PLM). Supramolecular hydrogen-bonded complexes formed from a 1:1 mixture of any two derivatives, bearing different alkoxy chains, of 4-alkoxyphenylazobenzoic acid and 4-(2-(pyridin-4-yl)diazenyl-(2-(or 3-)methylphenyl) 4-alkoxybenzoate. The investigated 1:1 mixture made by introducing a lateral methyl group by different spatial orientation angles into pyridine-based components. All new complexes were confirmed by Fourier-transform infrared spectroscopy (FTIR) and computational calculations used to study their stabilities. It is found that the prepared complexes are dimorphic, exhibiting smectic C and enhanced nematic phases. A comparison was made between the new series and previously investigated simpler complexes, revealed that the incorporation of the phenylazo group elongate the mesogenic part and hence broad nematic phases were obtained with high stability.

Calculation of Bubble Pressure for Crude Oils: The Effect of q-Weibull Distribution for Splitting the Heavy Fraction

Thermodynamic calculations involving crude oils require a proper characterization of the heavy-fraction in order to describe the phase behavior. Bubble pressure is a key property, and its calculation depends on splitting the heavy-fraction of the crude oil through a distribution function to obtain properties such as molecular weight and specific gravity. Small changes in these properties can significantly affect phase behavior calculations. The exponential and gamma distributions are the most used for splitting but they fail in characterizing many oils. Alternatively, the q-Weibull distribution is proposed to the splitting step instead of the exponential and gamma distributions in order to calculate the molecular weight and specific gravity. These three distributions are critically tested and compared for 13 oils with extended composition selected from the literature, stressing the effect of the characterization methods on the calculation of bubble pressure for reservoir fluids.

Complex Coacervation in Polyelectrolytes from a Coarse-Grained Model.

Complex coacervation refers to the formation of distinct liquid phases that arise when polyelectrolytes are mixed under appropriate polymer and salt concentrations. Molecular-level studies of coacervation have been limited. In this work, a coarse-grained model of the polymers and the corresponding counterions is proposed and used to simulate coacervation as a function of polymer length and overall salt concentration. Several sampling methods are used to determine the phase behavior of the underlying polymers. In particular, the results of simulations in different ensembles are shown to be consistent and to reproduce a number of phenomena observed in experiments, including the disruption of complexation by increasing ionic strength or by decreasing molecular weight. The coacervate concentrations determined from phase behavior calculations are then used to examine the rheology of the corresponding materials. By relying on long dynamic simulations, we are able to generate the dynamic response of the material in the form of dynamic moduli as a function of frequency, which are also found to compare favorably with experimental measurements.

Nanopore Confinement and Pore Connectivity Considerations in Modeling Unconventional Resources

Gas and liquid production from nanopore shale resources substantially increased during the past decade due to the advances in horizontal drilling and multistage hydraulic fracturing. Transport properties and mechanisms deviate from their bulk behavior when the pore sizes in unconventional formations are in the order of nanoscale. This is due to the dominant molecule–pore wall interaction effects comparing to molecule–molecule interactions in nanopores. Thus, the physics of multiphase flow in current commercial simulators should be changed to include the effect of pore size on both transport mechanisms and fluid properties. In this study, we analyze the effect of fluid confinement on phase behavior, fluid properties, and condensate banking around the hydraulic fracture where nanopores perform as the dominate storage region and dispersed with pores with bulk behavior. We modified critical properties of the fluid components for different pore sizes in the phase behavior calculations. Using experimental results, we developed a new correlation for estimating mean pore size as a function of permeability and porosity. Moreover, we considered pore size distribution of a shale sample to divide the reservoir into different regions. For each region, a specific permeability is assigned using the new developed correlation. Three different types of connectivity are considered between pores and its impact on production mechanisms is analyzed. Results of this study indicated that neglecting nanopore confinement effect on phase behavior results in an underestimation of the production while neglecting permeability change with pore size results in an overestimation of hydrocarbon production. The connectivity of different pore sizes has a significant impact on reservoir performance and determines the dominant factor.

New electrolyte SAFT-VR Morse EOS for prediction of solid-liquid equilibrium in aqueous electrolyte solutions

This work presents a new SAFT-VR Electrolyte equation of state (SAFT-VRE Morse EOS) that can predict solid-liquid equilibrium and second order derivative properties of electrolyte solutions. In this study, variable range statistical associating fluid theory (SAFT-VR Morse) EOS was extended to electrolyte solutions. In this model short-range interactions were mimicked by SAFT-VR Morse EOS, and long-range and solvation interactions were modeled by Debye-Hückle and Born terms respectively. Global ion parameters were adjusted using single electrolyte mean ionic activity coefficient (MIAC) and solution densities data at 25 °C and 1 atm. Using obtained ion-specific parameters, thermodynamic properties of aqueous electrolyte solutions such as MIAC, osmotic coefficient, vapor pressure, Gibbs free energy of hydration and excess enthalpy of solution were predicted without any adjustable parameter. Then, the model capability was evaluated in phase behavior calculations over a wide range of salt concentrations. In this respect, solid-liquid equilibrium (SLE) phase diagrams for mixed-electrolyte were predicted up to high temperatures. The results show that the model can predict the MIAC, osmotic coefficient, vapor pressure, and SLE of mixed electrolyte solutions up to 100 °C. Obtained results prove that SAFT-VRE Morse can efficiently be utilized for electrolyte solutions. We attribute this capability to utilizing the SAFT-VR Morse EOS as dispersion term.

Rapid phase stability calculations in fluid flow simulation using simple discriminating functions

This paper presents a new phase stability method that is applicable when repeated phase behavior calculations are needed as it is the case with multiphase fluid flow compositional simulation in upstream petroleum engineering. Two discriminating functions act as classifiers in such a way that a positive value of one of the two functions determines the stability state of the mixture. The two functions are generated off line, prior to the simulation, and their expressions are very simple so that they can be evaluated rapidly in a non-iterative way for every discretization block and at each timestep during the simulation. The CPU time required for phase stability calculations is dramatically reduced while still obtaining correct classification results corresponding to the global minimum of the system Gibbs energy function. The method can be applied to any chemical engineering problem where the class of several objects needs to be determined repeatedly and quickly.

Phase Behavior and Thermodynamic Model Parameters in Simulations of Extractive Distillation for Azeotrope Separation

Extractive distillation (ED) processes for separating ternary mixtures of benzene-cyclohexane-toluene with dimethyl formamide (DMF) and N-methyl-2-pyrrolidone (NMP) were studied using Aspen Plus and PRO/II simulators. The Aspen Plus built-in binary interaction parameters for the toluene-DMF, benzene-NMP and cyclohexane-NMP systems resulted in inaccurate phase behavior calculations. The vapor-liquid equilibrium (VLE) for the three binary systems was regressed to illustrate the importance of using accurate model parameters. The obtained binary interaction parameters described the phase behavior more accurately compared with the built-in binary interaction parameters in Aspen Plus. In this study, the effects of the regressed and built-in binary interaction parameters on the ED process design are presented. The total annual cost (TAC) was calculated to further illustrate the importance of the regressed binary interaction parameters. The results show that phase behavior and thermodynamic model parameters should receive more attention during the research and development of ED processes.