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

An improved component retrieval method for cubic equations of state with non-zero binary interaction coefficients for natural oil and gas Mehdi Assareh 0 1 2 Cyrus Ghotbi 0 1 2 Gholamreza Bashiri 0 1 2 Emad Roayaie 0 1 2 0 NIOC EOR Research Institute , Tehran , Iran 1 Department of Chemical and Petroleum Engineering, Sharif University of Technology , Tehran , Iran 2 Faculty of Chemical Engineering, Iran University of Science and Technology , Tehran , Iran 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. retrieval; Cubic EOS; Petroleum fluid characterization; Component; Equation of state; Lumping; Phase behavior - Abbreviations AAD % Average absolute deviation percent E Extended analysis L Lumped analysis MW Molecular weight yi Mole fraction fi Fugacity /i Fugacity coefficient Ki Equilibrium ration T Temperature (K, C) P Pressure (KPa) TC Critical temperature (K) VC Critical volume [m3/Kmol] PC Critical pressure (MPa) w Acentric factor EOS Equation of state Xdel Value in the lumped system Xdet Value in the extended system Petroleum reservoirs fluids composition undergoes considerable changes through the field exploitation (Danesh et al. 1992; Kuntadi et al. 2012; Rastegar and Jessen 2009). This composition modification, resulted from different oil and gas velocities in porous medium for regular production scenarios. Reservoir fluid type and its thermodynamic condition may enhance the composition change. For example, volatile oil and gas condensate reservoir fluids experience larger composition changes for the same production scenarios than dry gas and black oil reservoir fluids. This phenomenon is accelerated in the case of gas injection processes wherein hydrocarbons and non-hydrocarbons with different compositions are mixed with inplace reservoir fluids. There are many components in petroleum reservoir fluids. These components are usually grouped due to computational resource limitations and calculation performance. In a typical grouping technique, grouped reservoir fluids are prepared in two steps. In the first step, components should be classified into different groups and in the second step, the pseudo-components (groups) critical and thermo-physical properties are calculated (Briesen and Marquardt 2004). Reservoir fluid composition plays a key role in properties calculations. Therefore, oil and gas fields compositional simulation with grouped reservoir fluid descriptions lose their validity after a period (as pseudo-components properties are changing with composition change). Therefore, it is necessary to prepare a grouped fluid description for which composition changes have minimal impact to pseudo-components properties. Besides this, detailed composition changes must be controlled to check the accuracy of the compositional simulation with a lumped fluid description. In addition, there are several cases in which it is required to retrieve the detailed fluid composition from equilibrium calculations performed on lumped fluid description (Kuntadi et al. 2012). For example, to simulate a surface facility, for which the inputs are the compositions of the reservoir outputs provided by the reservoir simulation, those must usually be known with precision. Surface facility simulations can be performed for a larger number of components (Nichita et al. 2007; Nichita and Graciaa 2011). The detailed equilibrium ratios estimation from the results of a lumped system flash calculation is referred to as delumping or inverse lumping process (Schlijper and Drohm 1988). Several delumping methods are reported in the literature for cubic equations of state like PengRobinson (PR) (Peng and Robinson 1976). Danesh et al. (1992) implemented a modified Wilson equation for K value variations as a function of equations of state (EOS) parameters (Danesh et al. 1992). In their work, they mentioned that in gas-injection processes in which fluid compositions vary considerably, the use of group properties commonly generated from the original oil composition is inaccurate for prediction of the phase behavior. They made composition retrieval of each phase after equilibrium calculations and formed the new groups for the next cell calculations in a reservoir simulator. In this delumping approach, they determined the constants by nonlinear least squares for the lumped equilibrium data. The original component K values were subsequently determined from the equation to retrieve the detailed compositions of the relevant phases. They have presented the application of this method for gas injection processes simulation. Based on this, they tracked the overall composition using material balance calculations. They expressed the log of equilibrium ratio as a linear function of: LnKi c0 c11 xi 1 where xi is the component acentric factor and Tri is the reduced temperature, while c0 and c1 are constants determined from equilibrium information from lumped fluid equilibrium calculations. Stenby et al. (1996) proposed a delumping method for a two-ase flash calculation. The idea was to analyze the fugacity coefficients derived from EOS according to Michelsen reduction approach (Michelsen 1986; Stenby et al. 1996). In this method, a flash calculation is performed on the lumped system to obtain the lumped component equilibrium ratios. The delumping coefficients calculated from a regressio (...truncated)