Flash Calculations Research Articles

Prediction of Asphaltene Precipitation by Using an Association Equation of State

In this work, a new and efficient algorithm for asphaltene deposition modeling through application of an association thermodynamic model is introduced. It is assumed that pure asphaltene component only participates in the deposition phase. For characterization of asphaltenes, a two-phase flash calculation and cubic equation of state (Peng-Robinson equation of state) were used to describe asphaltene-toluene mixture (non-association mixture) and vapor-liquid equilibria, in which the critical properties of pure asphaltene component for the initial guess are adjusted with experimental data. Some of the association equation of state (AEOS) are selected for asphaltene modeling in a crude oil system as association mixture, which gives accurate results for water. To extend the model to crude oil mixtures, AEOS and vapor-liquid-deposition phase three-phase flash calculation are used to obtain the amount of asphaltene deposition and association model parameters. The authors’ experimental data from the Iranian crude oil reservoir, in this study, were used to examine the proposed asphaltene association model. It has been shown that the calculated results are in good agreement with the experimental data.

Phase-state identification bypass method for three-phase thermal compositional simulation

In this paper, we propose a strategy to bypass the phase identification of fluid mixtures that can form three, or more, phases. The strategy is used for reservoir simulation of multicomponent, three-phase, thermal compositional displacement processes. Since the solution path in compositional space is determined by a limited number of “key” tie-simplexes, the proposed “bypass” method uses information from the parameterized tie-simplexes and their extensions. The tie-simplex parameterization is performed in the discrete phase-fraction space. Once the phase-fraction space is discretized, a conventional three-phase flash is used adaptively to compute the phase states at the discretization nodes. If all discretization vertices of a given discrete cell, in phase-fraction space, have the same phase state, then this state is assigned to the entire cell and expensive flash calculations are bypassed. We demonstrate the robustness and efficiency of our phase identification bypassing strategy for several cases with three-phase flow, including a six-component ES-SAGD (enhanced solvent SAGD) model.

A least-squares support vector machine approach to predict temperature drop accompanying a given pressure drop for the natural gas production and processing systems

Precise estimation of temperature variations throughout gas production systems can enhance designing the production amenities. Routine methods for determining the temperature profiles in gas production systems are based on the gas composition and flash calculations. However, if the gas compositions are not available, the gas production system can be modelled by employing a black-oil approach, which is also a method for calculating the oil/gas resources and for modelling the gas reservoir operation. Accordingly, for black-oil models and when the natural gas compositions are not accessible, applying robust predictive tools in this research is of high interest in natural production systems. The current study places emphasis on applying the predictive model based on the least- squares support vector machine (LSSVM) to estimate precisely the proper temperature drop associated with a given pressure drop throughout the natural gas production systems based on the black-oil approach to acquire an accurate result for the temperature drop of natural gas streams. Genetic algorithm was used to optimise hyper-parameters (γ and σ2) which are embedded in the LSSVM model. Using this method is simple and it accurately determines the temperature drop through the natural gas stream with minimum uncertainty.

An improved component retrieval method for cubic equations of state with non-zero binary interaction coefficients for natural oil and gas

Volumetric and equilibrium calculations for the natural gas and oil defined by a large number of components are not feasible in applications like compositional reservoir simulation. Therefore, the fluid mixture is grouped to reduce computational load and to make faster calculations. However, for several reasons, it is required to have the detailed fluid composition rather than the lumped one. In this work, an improved delumping method is presented to retrieve the phase composition of the detailed mixture based on the grouped mixture thermodynamic calculations. The method is based on previously proposed delumping techniques for non-cubic equation of state (Assareh et al. in Fluid Phase Equilib 339:40–51, 2013). To prepare lumped mixtures, a grouping technique, based on the components similarity, is used to classify the components with close critical properties and binary interaction coefficients in a pseudo-component (Assareh et al. in Int J Oil Gas Coal Technol 7(3):275–297, 2014). Afterward, a number of delumping parameters calculated from lumped system flash calculation are assigned to the components in a specific pseudo-component. The detailed mixture equilibrium ratios based on fugacity coefficient for a common cubic equation of state are calculated using these delumping coefficients. The accuracy of the method is verified on two petroleum reservoir fluids, a gas condensate and an oil reservoir fluid. The delumped equilibrium ratios were in good agreement with detailed ones with the absolute deviation of less than 2 %. The results confirm the applicability and accuracy of the presented method for detailed composition retrieval while simulating with pseudo-components.

Kinetic Modeling of Vacuum Residue Thermal Cracking in the Visbreaking Process Using Multiobjective Optimization

AbstractA discrete six‐lump kinetic model for the thermal cracking of vacuum residue in the visbreaking process has been developed, and the time‐dependent behavior of each individual lump has been determined. A combination of the heuristic method and gradient‐based method (hybrid method) has been taken into account to optimize the parameters of the model. A new concept of objective functions has been presented and applied for optimization, so more acceptable results have been obtained. Such an approach has not been presented before. A number of 60 parameters was considered primarily, which has been reduced to 36 with the aim of flash calculation information, and optimized. The obtained results are in good agreement with experimental data. The analysis of the estimated rate constants showed that the cracking of vacuum residue to lighter products was the more dominant reaction pathway during the visbreaking process over the temperature range of 400–430 °C.

Multiphase equilibrium calculations using a reduction method

Phase equilibrium problems (phase stability testing and multiphase flash) have to be solved repeatedly, sometimes a very large number of times, in process simulators and in compositional reservoir simulation. The computational effort can be significant, particularly when a detailed description (with a large number of components nc) of the mixture is required. The reduction method represent an attractive technique in the attempt to reduce the computational time, by significantly reducing the dimensionality of the problem, from nc×(np−1) in the conventional methods to a maximum of (2c+3)×(np−1), where np is the number of equilibrium phases and c the number of components with non-zero binary interaction parameters (BIP). The number of independent variables does not depend on nc, but only on c, and the computation time increases linearly with nc. Traditionally, the reduction approaches use the reduction parameters and the phase mole fractions as independent variables. In this work, a recent improvement of two-phase reduced flash calculations (Nichita and Graciaa, Fluid Phase Equilib. 302, 2011, 226–233) is extended to multiphase equilibrium calculation with any number of phases. Two approaches are proposed: (i) a direct extension of the reduction method for two-phase flashes and (ii) a constrained minimization Gibbs free energy with respect to a specific set of variables and constraints (taking advantage of symmetry). The proposed algorithms are tested for several multiphase systems (with up to four phases and exhibiting complex phase envelopes) containing hydrocarbon components, carbon dioxide and hydrogen sulfide. Numerical experiments show that the reduction methods for multiphase flash calculations are robust and they become faster than the conventional methods when (i) the number of components increases, (ii) the number of equilibrium phases increases and (iii) the number of BIP families decreases. For mixtures with many components and few BIP families, the reduction method may be at least one order of magnitude faster than conventional methods.

Fully implicit compositional simulator for modeling of asphaltene deposition during natural depletion

This paper describes development of a fully implicit compositional simulator for modeling of asphaltene deposition during natural depletion. In this paper, a new approach for multiphase flash calculation has been developed. This approach provides a more detailed description of the kinetic part of asphaltene deposition which needs to be explained more clearly. Due to a large number of unknowns, there are many ways to solve such a system by choosing different sets of independent variables. A new set of independent variables in a fully implicit model is considered for asphaltene deposition modeling. By incorporating an asphaltene precipitation model into a compositional simulator where the phase equilibrium equations, the volumetric constraint equation, the component transport equations, the multiphase flash equations and the deposition equation are solved simultaneously; a simulator for asphaltene deposition was developed with respect to natural depletion. The pure solid model is used to model asphaltene precipitation. The solid particles are considered to be separated into three parts: precipitated, flocculated and deposited solid. A first-order chemical reaction is used which models forward and reverse rates for the conversion of precipitated asphaltene to flocculated asphaltene. Also, a deposition model including adsorption, pore throat plugging, and re-entrainment was used. The simulator can also predict formation damage including porosity and permeability reduction in each block.

A new parametric algorithm for isothermal flash calculations by the Adomian decomposition of Michaelis–Menten type nonlinearities

We develop a robust algorithm for isothermal flash calculations of multi-component mixtures. The algorithm provides an explicit, parametric solution for a recent variation of the Rachford–Rice equation by employing a powerful analytical technique, namely the Adomian decomposition method. The computational speed of the algorithm is shown to be further enhanced by applying the iterated Shanks transformation. Unlike previous methods, our algorithm obviates the troublesome need for an initial guess, guarantees an excellent rate of convergence and is demonstrated to be more computationally economic than prior art. Several reliable examples from the literature, including the flash calculation for a mixture with nineteen components, as well as an extensive set of hypothetical, randomly-generated problems are presented to illustrate the exceptional efficacy of our new approach.

Application of Multiple-Mixing-Cell Method To Improve Speed and Robustness of Compositional Simulation

Summary Standard equation-of-state-based phase equilibrium modeling in reservoir simulators involves computationally intensive and time-consuming iterative calculations for stability analysis and flash calculations. Therefore, speeding up stability analysis and flash calculations and improving robustness of the calculations are of utmost importance in compositional reservoir simulation. Prior knowledge of the tie-lines traversed by the solution of a gas-injection problem translates into valuable information with significant implications for speed and robustness of reservoir simulators. The solution of actual-gas-injection processes follows a very complex route because of dispersion, pressure variations, and multidimensional flow. The multiple-mixing-cell (MMC) method, originally developed to calculate minimum miscibility pressure of a gas-injection process, accounts for various levels of mixing of the injected gas and initial oil. This observation suggests that the MMC tie-lines developed upon repeated contacts may represent a significant fraction of the actual simulation tie-lines encountered. We investigate this idea and use three tie-line-based K-value-simulation methods for application of MMC tie-lines in reservoir simulation. In two of the tie-line-based K-value-simulation methods, we examine tabulation and interpolation of MMC tie-lines in a framework similar to the compositional-space adaptive-tabulation (CSAT) method. In the third method, we perform K-value simulations based on inverse-distance interpolation of K-values from MMC tie-lines. We demonstrate that for the displacements examined, the MMC tie-lines are sufficiently close to the actual simulation tie-lines and provide excellent coverage of the simulation compositional route. The MMC-based methods are then compared with the computational time by use of other methods of phase-equilibrium calculations, including a modified application of CSAT (an adaptive tie-line-based K-value simulation), a method using only heuristic techniques, and the standard method in an implicit-pressure/explicit-concentration-type reservoir simulator. The results show that tabulation and interpolation of MMC tie-lines significantly improve phase equilibrium and computational time compared with the standard approach, with acceptable accuracy. The results also show that computational performance of the MMC-based methods with only prior tie-line tables is very close to that of CSAT, which requires flash calculations during simulation. The K-value simulations by use of MMC-based tie-line-interpolation methods improve the total computational time up to 51% in the cases studied, with acceptable accuracy. The results suggest that MMC tie-lines represent a significant fraction of the actual tie-lines during simulation and can be used to significantly improve speed and robustness of phase-equilibrium calculations in reservoir simulators.

Speeding up the flash calculations in two-phase compositional flow simulations – The application of sparse grids

Flash calculations have become a performance bottleneck in the simulation of compositional flow in subsurface reservoirs. We apply a sparse grid surrogate model to substitute the flash calculation and thus try to remove the bottleneck from the reservoir simulation. So instead of doing a flash calculation in each time step of the simulation, we just generate a sparse grid approximation of all possible results of the flash calculation before the reservoir simulation. Then we evaluate the constructed surrogate model to approximate the values of the flash calculation results from this surrogate during the simulations. The execution of the true flash calculation has been shifted from the online phase during the simulation to the offline phase before the simulation. Sparse grids are known to require only few unknowns in order to obtain good approximation qualities. In conjunction with local adaptivity, sparse grids ensure that the accuracy of the surrogate is acceptable while keeping the memory usage small by only storing a minimal amount of values for the surrogate. The accuracy of the sparse grid surrogate during the reservoir simulation is compared to the accuracy of using a surrogate based on regular Cartesian grids and the original flash calculation. The surrogate model improves the speed of the flash calculations and the simulation of the whole reservoir. In an experiment, it is shown that the speed of the online flash calculations is increased by about 2000 times and as a result the speed of the reservoir simulations has been enhanced by 21 times in the best conditions.

Arc Flash Calculations for a 1.3-MW Photovoltaic System

When planning for the installation of a 1.3MW photovoltaic system, and its integration into an existing facility's electrical system, an arc flash study was performed to determine the best protective device settings to minimize the arc flash energy in the photovoltaic system, and to determine the effect of the additional short circuit current on the existing system arc flash calculations. The existing system has distribution at 13.8kV and two 10MVA transformers to a 2.4kV feeder system. The photovoltaic system adds the hazard of 500 VDC sub-array collection wiring and terminations. The DC arc flash exposure of PV systems, including those in the inverter enclosures, can be complicated, and this paper outlines the basic data collected for the study, the steps taken to complete the calculations, the methodology used, the results, and the team's learnings.

A comparison of conventional and reduction approaches for phase equilibrium calculations

A major part of the computational effort in chemical process simulation and compositional reservoir simulation is spent by phase equilibrium calculations: phase split and phase stability analysis. The calculation algorithms must be efficient and highly robust. The reduction methods offer an attractive alternative to conventional methods. Several recent papers have questioned on the efficiency of the reduction methods as compared to conventional methods, concluding that reduction methods become faster than conventional ones starting from a certain number of components. In this work, besides evaluating the computational effort required by various conventional and reduction formulations, the efficiency (per iteration and global) of stability testing and flash calculations is evaluated separately, the condition of the linear system of equations is analyzed, as well as the influence of symmetry properties and of the linear solver. Some important links are formally established between conventional and reduced methods and between different reduction methods. Numerical experiments carried out with several mixtures confirm that flash calculations using the reduction methods are more efficient than conventional methods only for mixtures with many components (more than 20) and few non-zero binary interaction parameters; this means that reduction methods for phase split are suitable for process simulation (mixtures may contain hundreds of components), but there are not suitable for compositional simulation, where the number of components is limited, typically to a dozen. On the contrary, for phase stability testing, it is shown that reduction methods are more efficient than conventional ones even for mixtures with few components, thus they are suitable for compositional simulation. This observation is very important since, in most compositional reservoir simulators, no explicit phase split is required, flash calculations being part of a larger problem (coupled with flow equations), while phase stability testing must be performed in most grid blocks.

Effect of oil composition on fluid substitution in heavy oil reservoirs

ABSTRACTUnlike light oils, heavy oils do not have a well‐established scheme for modelling elastic moduli from dynamic reservoir properties. One of the main challenges in the fluid substitution of heavy oils is their viscoelastic nature, which is controlled by temperature, pressure, and fluid composition. Here, we develop a framework for fluid substitution modelling that is reliable yet practical for a wide range of cold and thermal recovery scenarios in producing heavy oils and that takes into account the reservoir fluid composition, grounded on the effective‐medium theories for estimating elastic moduli of an oil–rock system. We investigate the effect of fluid composition variations on oil–rock elastic moduli with temperature changes. The fluid compositional behaviour is determined by flash calculations. Elastic moduli are then determined using the double‐porosity coherent potential approximation method and the calculated viscosity based on the fluid composition. An increase in temperature imposes two opposing mechanisms on the viscosity behaviour of a heavy‐oil sample: gas liberation, which tends to increase the viscosity, and melting, which decreases the viscosity. We demonstrate that melting dominates gas liberation, and as a result, the viscosity and, consequently, the shear modulus of the heavy oils always decrease with increasing temperature. Furthermore, it turns out that one can disregard the effects of gas in the solution when modelling the elastic moduli of heavy oils. Here, we compare oil–rock elastic moduli when the rock is saturated with fluids that have different viscosity levels. The objective is to characterize a unique relation between the temperature, the frequency, and the elastic moduli of an oil–rock system. We have proposed an approach that takes advantage of this relation to find the temperature and, consequently, the viscosity in different regions of the reservoir.

Simulation of Petroleum Residue Hydroconversion in a Continuous Pilot Unit Using Batch Reactor Experiments and a Cold Mock-Up

The kinetics of atmospheric petroleum residue hydroconversion with a dispersed catalyst was studied. A methodology has been developed in order to transpose the chemical kinetics (reaction network, stoichiometry, and kinetic constants) obtained with a batch reactor by Nguyen et al. to a continuous reactor [Nguyen, T. S.; Tayakout-Fayolle, M.; Ropars, M.; Geantet, C. Chem. Eng. Sci. 2013, 94, 214]. Their five-lump kinetic model takes into account vapor–liquid mass transfer, vapor–liquid equilibriums, and hydrogen consumption. Consequently, hydrodynamics and vapor–liquid mass transfer of the micropilot unit’s reactor were studied in a cold mock-up by tracer experiments. The same thermodynamic model given by Nguyen et al. was used, and the flash calculations were performed using ProSimPlus software. Experimental data were obtained in the micropilot unit at 420, 430, and 440 °C with a dispersed catalyst for residence times of 1 and 2 h. The catalyst precursor, an oil-soluble molybdenum naphthenate, was added to obtain a molybdenum concentration of 600 wt ppm in the feedstock. The total pressure was 12 MPa with a hydrogen-to-feed ratio of 500 N m3/m3. The methodology was validated by comparing the model’s output with the experimental results.

Comparison of Reduced and Conventional Two-Phase Flash Calculations

Summary Phase-equilibrium calculations become computationally intensive in compositional simulation as the number of components and phases increases. Reduced methods were developed to address this problem, where the binary-interaction-parameter (BIP) matrix is approximated either by spectral decomposition (SD), as performed by Hendriks and van Bergen (1992), or with the two-parameter BIP formula of Li and Johns (2006). Several authors have recently stated that the SD method—and by reference all reduced methods—is not as fast as previously reported in the literature. In this paper we present the first study that compares all eight reduced and conventional methods published to date by use of optimized code and compilers. The results show that the SD method and its variants are not as fast as other reduced methods, and can be slower than the conventional approach when fewer than 10 components are used. These conclusions confirm the findings of recently published papers. The reason for the slow speed is the requirement that the code must allow for a variable number of eigenvalues. We show that the reduced method of Li and Johns (2006) and its variants, however, are faster because the number of reduced parameters is fixed to six, which is independent of the number of components. Speed up in flash calculations for their formula is achieved for all fluids studied when more than six components are used. For example, for 10-component fluids, a speed up of 2–3 in the computational time for Newton-Raphson (NR) iterations is obtained compared with the conventional method modeled after minimization of Gibbs energy. The reduced method modeled after the linearized approach of Nichita and Graciaa (2011), which uses the two-parameter BIP formula of Li and Johns (2006), is also demonstrated to have a significantly larger radius of convergence than other reduced and conventional methods for five fluids studied.

English

The production of natural gas (NG) is increasing considerably, contributing to the energetic matrix of the world. New reserves of NG have been discovered and explored regularly. Despite the various applications of natural gas, which allow its use as a raw material for the production of synthesis gas (syngas), for instance, there are remote wells that demand treatment before use. In this work, a simplified approach to remove the heavy species of natural gas (C5+) is presented. The specification of the treated gas is based on the methane index to use the remote NG for fuel in the drive of engines of compressors and boilers. Thus, the present approach proposes the application of two simple operations, i.e., absorption followed by a flash, to regenerate the solvent. This approach may be applied in remote production fields. Based on process simulations and physicochemical properties, 1-octanol was selected as absorbent for this separation. New measurements of absorption in laboratory scale are reported using a synthetic NG and an on-line chromatography analysis. Soave-Redlich-Kwong equation of state (EoS) with Mathias-Copeman alpha function and MHV2 mixing rule with UNIFAC was applied to describe the required equilibrium data. In order to validate this model for the absorption calculations, predictions of vapor-liquid equilibrium of alkane and alcohol binary mixtures obtained from literature data were tested and evaluated. Based on the absorption measurements and SRK-MHV2 flash calculations, curves of operation and equilibrium were provided. Using n-heptane as key-component it was demonstrated that one or up to two theoretical stages are enough to proceed the separation. This work also presents a gas absorption pilot set-up up to 70 bars that is able to evaluate scale up and pressure effect. It is demonstrated, experimentally and computationally, that the proposed approach of C5+ species absorption from NG is simple and feasible to specify the methane index of the gas for remote combustion applications.

Deposition of heavy oil droplets onto a circular disk at elevated temperatures

Fouling of equipment surfaces affects the efficient processing of heavy hydrocarbons. An experimental study is reported on the deposition of bitumen‐containing droplets from a gas‐vapour mixture at temperatures up to incipient coking conditions. A heavy oil‐diluent feed mixture was atomized with nitrogen in a vertical spray chamber, and flowed upwards normal to a circular disk. The disk was attached to a load cell to measure total deposited mass with time, and at the end of each experiment the amount deposited was determined gravimetrically. The effects on deposition rate of heavy oil concentration in the feed mixture, temperature, and flow‐rates of feed mixture and nitrogen were determined. Different surfaces and coatings were tested. The deposition rate was found to depend most strongly on the droplet concentration, which was predicted with HYSYS, using an isothermal flash calculation. As temperature was increased, deposit H/C atomic ratio decreased and ash content increased.

Modeling and Simulation of a Multibed Industrial Hydrotreater with Vapor–Liquid Equilibrium

Dealing with vapor–liquid equilibrium (VLE) in hydrotreating reactors is a major obstacle to realistic process simulation. The present study focused on the modeling and simulation of a commercial light-cycle-oil (LCO) hydrotreater with rigorous vapor–liquid equilibrium (VLE) calculations. The hydrotreater was represented as a one-dimensional plug-flow adiabatic reactor with interbed gas quenching. The model accounted for the hydrodesulfurization (HDS) and hydrodearomatization (HDA) reactions. VLE calculations were performed in situ with a calibrated flash calculation program developed in-house specifically for hydrogen–petroleum systems. Significant differences in reactor temperature profiles, hydrotreating conversions, fluid rates, and phase compositions were observed between the simulations with and without VLE. Analysis of the effect of temperature indicated that reactor performance depends heavily on the axial temperature distribution, which is established based on the catalyst bed layout and the sele...

Multiphase equilibrium flash with salt precipitation in systems with multiple salts

A new methodology for determining simultaneous chemical and phase equilibrium of mixtures of light gases and aqueous electrolyte solutions with the potential for multiple salt deposition is proposed. The novel aspects of this new approach include, but are not limited to (1) a novel tearing algorithm for determining equilibrium ion solubility limits, (2) rigorous proof that the proposed tearing algorithm generates a Cauchy sequence and is therefore guaranteed to converge to the correct equilibrium ion solubility limits (3) and a unique formulation of the combined chemical and multi-phase equilibrium flash problem that accounts for salt deposition but decouples the chemical and phase equilibrium aspects of the flash.Examples from real EOR and CO2 sequestration applications are presented. Results clearly show that the proposed numerical approach is reliable, robust, and efficient and can be used to determine salt deposition in multi-phase flash problems. Several geometric illustrations and numerical details are used to elucidate key points of the proposed approach.

Calculation of phase equilibria for multi-component mixtures using highly accurate Helmholtz energy equations of state

To test the thermodynamic stability and to determine the equilibrium phase compositions in case the original phase is found unstable is one of the greatest challenges associated with calculating thermodynamic properties of multi-component mixtures. The minimization of the tangent plane distance function is a widely used method to check for stability, while different approaches can be chosen to minimize the Gibbs energy in order to find the phase equilibrium. While these two problems have been applied to several different thermodynamic models, very little work has been published on such algorithms using multi-parameter Helmholtz energy equations of state. In this work, combined stability and flash calculation algorithms at given pressure and temperature (p,T), pressure and enthalpy (p,h), and pressure and entropy (p,s) are presented. The algorithms by Michelsen et al. (1982, 1982, 1987) are used as basis and are adapted to multi-parameter Helmholtz energy models. In addition, a robust and sophisticated density solver is proposed which is necessary for the calculation of properties from the Helmholtz energy model at given state variables other than temperature and density. All partial derivatives necessary to solve the isothermal, isenthalpic and isentropic flash problems using numerical methods based on the Jacobian matrix are derived analytically and given in the supplementary material to this article. Results for some multi-component systems using the GERG-2008 model (Kunz and Wagner, 2012) are shown and discussed.