Influence of the Crude Oil Characterization on Mmp Calculation

Oil & Gas Science and Technology, Jul 2018

This paper focuses on the influence of the crude oil characterization on minimum miscibility pressure (MMP) calculation. It is shown what kind of compounds may be lumped together in order to calculate the MMP with a good accuracy. In a first step, a selected crude oil is modeled with 37 pure compounds using the characterization procedure developed by Avaullée et al. (1997a) in which two parameters of the distillation residue are fitted on the experimental bubble points. In this context, all the experimental data (PVT, swelling test, slim tube test) are calculated with a very good accuracy. In a second step, the heavy intermediate, the middle intermediate and the light compounds are successively lumped. The influence of such lumping procedure on the calculated MMP is discussed.Dans cet article, nous étudions l'influence de la modélisation des coupes d'un pétrole brut sur la valeur calculée de la pression minimale de miscibilité (MMP). En particulier, il est expliqué quels types de constituants peuvent être regroupés sous forme d'un pseudo-constituant sans pour autant dégrader la valeur de la MMP. La procédure suivie dans cet article est simple : à partir d'un pétrole brut donné, nous utilisons dans un premier temps une représentation par 37 composants, exploitant l'information analytique standard disponible sur ce fluide. La modélisation des fractions lourdes utilisée est celle développée par Avaullée et al. (1997a) , dans laquelle nous avons ajusté deux paramètres du résidu de distillation C(20+) sur les pressions de saturation expérimentales. Sur le fluide étudié, nous mettons en évidence le bon niveau de restitution des grandeurs volumétriques, des données expérimentales de gonflement ainsi que de la MMP mesurée lors d'une expérience de déplacement en tube mince. Dans un deuxième temps, les composés intermédiaires lourds, les intermédiaires moyens et les composants légers font successivement l'objet de regroupement en pseudo-constituant. L'influence de tels regroupements sur la valeur calculée de la MMP est alors analysée.

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Influence of the Crude Oil Characterization on Mmp Calculation

C A L C U L AT I O N J.-N. JAUBERT 0 L. ARRAS 0 E. NEAU 0 0 (3) RŽsidence Michelet-Delattre , 5, boulevard du GŽnŽral-Koenig, 13009 Marseille - France Institut national polytechnique de Lorraine1 Faculté des sciences de Luminy2 - L. AVAULLÉE SE3T3 (1) Laboratoire de thermodynamique des sŽparations, ƒcole nationale supŽrieure des industries chimiques, 1, rue Grandville, 54001 Nancy Cedex - France (2) Laboratoire de chimie physique, 163, avenue de Luminy, 13288 Marseille Cedex 9 - France INFLUENCE DE LA CARACTƒRISATION D'UN PƒTROLE BRUT SUR LE CALCUL DE LA PRESSION MINIMALE DE MISCIBILITƒ Dans cet article, nous Žtudions lÕinfluence de la modŽlisation des coupes dÕun pŽtrole brut sur la valeur calculŽe de la pression minimale de miscibilitŽ (MMP). En particulier, il est expliquŽ quels types de constituants peuvent tre regroupŽs sous forme d'un peusdo-constituant sans pour autant dŽgrader la valeur de la MMP. La procŽdure suivie dans cet article est simple : ˆ partir dÕun pŽtrole brut donnŽ, nous utilisons dans un premier temps une reprŽsentation par 37 composants, exploitant lÕinformation analytique standard disponible sur ce fluide. La modŽlisation des fractions lourdes utilisŽe est celle dŽveloppŽe par AvaullŽe et al. (1997a), dans laquelle nous avons ajustŽ deux paramtres du rŽsidu de distillation C20+ sur les pressions de saturation expŽrimentales. Sur le fluide ŽtudiŽ, nous mettons en Žvidence le bon niveau de restitution des grandeurs volumŽtriques, des donnŽes expŽrimentales de gonflement ainsi que de la MMP mesurŽe lors d'une expŽrience de dŽplacement en tube mince. Dans un deuxime temps, les composŽs intermŽdiaires lourds, les intermŽdiaires moyens et les composants lŽgers font successivement lÕobjet de regroupement en pseudo-constituant. L'influence de tels regroupements sur la valeur calculŽe de la MMP est alors analysŽe. INFLUENCE OF THE CRUDE OIL CHARACTERIZATION ON MMP CALCULATION This paper focuses on the influence of the crude oil characterization on minimum miscibility pressure (MMP) calculation. It is shown what kind of compounds may be lumped together in order to calculate the MMP with a good accuracy. In a first step, a selected crude oil is modeled with 37 pure compounds using the characterization procedure developed by AvaullŽe et al. (1997a) in which two parameters of the distillation residue are fitted on the experimental bubble points. In this context, all the experimental data (PVT, swelling test, slim tube test) are calculated with a very good accuracy. In a second step, the heavy intermediate, the middle intermediate and the light compounds are successively lumped. The influence of such lumping procedure on the calculated MMP is discussed. INFLUENCIA DE LA CARACTERIZACIîN DE UN PETRîLEO CRUDO RESPECTO AL CçLCULO DE LA PRESIîN MêNIMA DE MISCIBILIDAD En este art’culo se estudia la influencia de la modelizaci—n de las fracciones de un petr—leo crudo con respecto al valor calculado de la presi—n m’nima de miscibilidad calculada (MMP). B‡sicamente, REVUE DE L’INSTITUT FRANÇAIS DU PÉTROLE VOL. 53, N° 1, JANVIER-FÉVRIER 1998 se indica quŽ tipos de componentes se pueden reunir en forma de un seudocomponente sin por ello degradar el valor de la MMP. El procedimiento seguido para este art’culo es sencillo : tomando como punto de partida un petr—leo crudo determinado, utilizamos en una primera etapa una representaci—n por medio de 37 componentes, en cuyo caso se utiliza la informaci—n anal’tica est‡ndar disponible en este fluido. La modelizaci—n de las fracciones pesadas utilizada es aquella desarrollada por AvaullŽe y sus colaboradores (1997a), en la cual hemos ajustado dos par‡metros del residuo de destilaci—n C20+ en las presiones de saturaci—n experimentales. Se hace resaltar, para el fluido estudiado, el correcto nivel de restituci—n de las magnitudes volumŽtricas, de datos experimentales de aumento de volumen, as’ como de la MMP medida con motivo de una experiencia de desplazamiento en tubo delgado. En una segunda etapa, los compuestos intermedios pesados, los intermedios medianos y los componentes livianos, son sucesivamente objeto de reagrupaci—n en seudocomponente. Se analiza entonces la influencia de semejantes reagrupaciones sobre el valor calculado de la MMP. INTRODUCTION It is well known from the literature (Johns, 1992) that the miscibility process, when a gas is injected into a crude oil, is controlled by a sequence of nc-1 key tie lines if nc is the number of components present. There is the initial tie line (the tie line that extends through the original oil composition), the gas tie line (the tie line that extends through the injected gas composition) as well as nc-3 intermediate tie lines called crossover tie lines as initially defined by Monroe et al. (1990) . In this context, the minimum miscibility pressure (MMP) which is a key parameter in a gas injection project design must be defined as the lowest pressure at which any one of these nc-1 tie lines becomes a critical tie-line. However, in softwares currently available in petroleum companies, a crude oil is usually described by a maximum of 10 components or pseudocomponents meaning that some hydrocarbons are lumped together in order to reduce computing times. It is obvious that such a lumping procedure makes disappear many key tie lines and may lead to very different values of the MMP. The goal of this study is to test the influence of such lumping procedures on the calculated MMP value. 1 EXPERIMENTAL DATA A crude oil for which many PVT experiments were performed by the French Petroleum Company TOTAL SA was selected. The composition of the reservoir fluid and the results of the True Boiling Point (TBP) distillation are shown in Table 1. The following experimental data are available: – Bubble points, total densities and relative volumes measured during constant mass expansions performed at three different temperatures (t1 = 87.8°C, t2 = 103.3°C and t3 = 121.1°C). The reservoir temperature is t2. In such experiments, the relative volume (Vtot Vsat ) is defined as the total cell volume divided by the liquid volume at saturation pressure. – Liquid densities, compressibility factors of liberated gas and Bo measured during a differential vaporization at the reservoir temperature. It is recalled that Bo = (Vliq Vref ) is defined as the liquid volume divided by a reference volume (liquid volume at the final pressure measured under atmospheric conditions: 15°C, 1 atm). It is thus possible to calculate the relative volume ( Bo Bosat ) defined as (Vliq Vsat ) where Bosat is the Bo value at the bubble point. – Bubble points and densities of the reservoir oil swollen with a lean gas at the reservoir temperature (swelling test). – The minimum miscibility pressure MMP (slim tube test) when the lean gas used during the swelling test is injected into the reservoir crude oil. The composition of the injection gas is shown in Table 2. All the experimental data are summarized in Tables 3 to 6. RP2Tcc2 exp êëêém × èçae1 - èae TTcøö 0.5 ø÷ö úûúù In order to calculate correct densities, the molar volume v, solution of the EoS must be reduced using a volume translation c as proposed by Péneloux et al. (1982) . v correct = veos - c with c = RTc [0.1153758 - 0.440642ZRA ] P c In the previous equation, ZRA is the Rackett compressibility factor appearing in Spencer and Danner's (1973) modification of the Rackett Equation. For a mixture containing p compounds of mole fraction xi, classical mixing rules were used: p p bm = å xibi cm = å xici i=1 i=1 p p am ( T ) = å å [ ai(T ) × a j(T ) xi × x j(1 - kij(T ))] i=1 j=1 with kij(T ) = and d i = ai( T ) bi Eij(T ) - (d i - d j ) 2 2d i d j The binary interaction energies Eij were estimated using the group contribution method developed by Abdoul et al. (1992) . 3 CRUDE OIL MODELING According to the EoS described in the previous section, each component of a petroleum fluid is characterized by the following parameters: critical temperature Tc critical pressure Pc acentric factor w Rackett compressibility factor ZRA atomic group fractions allowing the calculation of the binary interaction parameters (bips). 3.2 The intermediate compounds In this study, each cut from C11 to C19 was modeled according to the method recently developed by Avaullée et al. (1997a). In this approach, each cut is in a first step modeled as a mixture of three pure compounds (one paraffin, one naphtene and one aromatic). The relative amount of these three molecules is determined in order to reproduce the experimental molar weight and density of the cut. The critical properties and the acentric factor of these three pure compounds are estimated using the group contribution method developed by Avaullée et al. (1997b). In a second step, these three compounds are lumped together in order to model a given cut by one pseudocomponent only. The critical temperature, critical pressure and acentric factor of this pseudo-component are solution of a non-linear three equation system (Avaullée et al., 1997a) . The Rackett compressibility factor and the group fraction allowing to estimate the bips are calculated by additivity balanced by the corresponding relative amounts. 3.3 The C20+ residue Once more, the method developed by Avaullée et al. (1997a) was used in order to model the C20+ residue by a single compound. In order to use the PR EoS, the critical temperature, critical pressure and the shape parameter m must be estimated. Avaullée et al. (1997a) developed a specific correlation allowing the calculation of the critical pressure knowing the critical temperature and suggest to tune the critical temperature and the m parameter on the experimental bubble points of the crude oil. Since in this study, three saturation pressures were experimentally determined, this method can be used. The Rackett compressibility factor of the residue is determined so that the density calculated using the cubic EoS matches the experimental one. Moreover, the knowledge of the experimental molar weight and density of the C20+ compound allows to determine the group fractions necessary to estimate the bips. 4 CALCULATION OF THE PVT EXPERIMENTAL DATA 3.1 The light compounds For light compounds (until C10), all these parameters are known in the literature. Before performing MMP calculations, the crude oil modeling previously described was used in order to check whether it was accurate to reproduce with a good accuracy the experimental data. The evolution of the relative volume versus pressure for the four depletions, the PT phase envelop and the P-X diagram corresponding to the swelling test are shown on Figures 1 to 6. Moreover, during the swelling test simulation, the densities of the saturated mixtures of reservoir oil and injected gas were calculated with a 0.8% deviation. The average overall deviation between calculated and experimental total densities obtained during constant mass expansions was 1.6%. During the differential vaporization simulation, the liquid phase densities and the liberated gas compressibility factors deviated respectively of 1.5% and 0.7% from experimental data. In conclusion, the characterization procedure used allows a very satisfactory estimation of the various PVT data. 5 MMP CALCULATIONS 5.1 Results using the complete crude oil compositional model In a first step, the previous crude oil modeling was used to calculate the MMP when the gas, the composition of which is given in Table 2, was injected in the reservoir crude oil. Before performing such a calculation, it is necessary to determine the process mechanism: pure vaporizing (VGDM), pure condensing (CGDM) or mixed condensing/vaporizing (C/VM). For the fluids investigated in this study, the mechanism was found to be a mixed C/V mechanism as discovered by Zick (1986) . In this case, the MMP may be determined using either a multicell approach developed by Zick but never published, or a classical commercial 1D simulator. The first solution was chosen in this study. The results of the calculations are shown in Table 7 and evidence a very good agreement between experimental and calculated MMP. Indeed, the deviation observed is less than 3% which is the experimental uncertainty in the determination of the MMP using a slim tube. Comparison between calculated and experimental MMP using a 37 component characterization for the crude oil 0.8 200 300 400 Figure 3 Relative volume Vtot/Vsat versus pressure during a constant mass expansion performed on the selected crude oil at t3 = 121.1°C. P (bar) Relative volume Vliq/Vsat versus pressure during a differential vaporization performed on the selected crude oil at t2 = 103.3°C. experimental saturation pressures calculated phase envelop 500 700 P-T phase envelop of the selected reservoir crude oil. experimental bubble pressures calculated phase envelop 0.5 0.0 5.2 Results using different compositional models Critical point In this section it was decided to lump together successively the middle intermediate components (from C11 to C19), the light intermediate components (from C5 to C10), the light hydrocarbons (from C1 to C4) and to compute the MMP. A similar lumping procedure was applied to the injected gas composition. The pseudoization (lumping procedure) used in this paper was the very simple API method. In order to lump together n compounds of internal mole fractions xi in a pseudo-component k, the following formulae were used: n Vck = å xiVci with Vci = i=1 RTci Zci Pci (in this study Zci was merged with ZRAi ) Tc k n å xiVc i Tc i = i=1 Vc k n w k = å xiw i i=1 n ZRAk = å xiZRAi i=1 é ae n ê ç Pc k = åi=1 xi Pc i êê1 - b × çç1 è ê ë ö ù n Tc k ÷÷ úúú å xiTc i ÷ø ú i=1 û with b = 5.808 + 4.93w k On the other hand, atomic groups of pseudocomponent k were calculated knowing those of compounds i and the internal mole fractions xi. The complete results are shown in Table 8. Table 8 clearly evidences that it is not conceivable to lump together the light components to calculate the MMP. Indeed, the calculated value is very far from the one obtained using the complete modeling of the crude oil and the process mechanism is not the same. Effect of various clustering configurations on the calculated MMP Using 37 components a crossover tie line controls the miscibility process whereas the initial oil tie line does control the miscibility when light compounds are lumped together. On the other hand, lumping together either the compounds from C5 to C10 or from C11 to C19 gives a calculated MMP close to the one obtained with the complete compositional model. It is interesting to notice that lumping together the middle intermediate compounds makes decrease the MMP whereas lumping together the light intermediate components makes increase the MMP. In a last step, it was decided to model the crude oil with only 12 components introducing simultaneously both pseudo-components "C5-C10" and "C11-C19". The calculated MMP was thus MMP = 325 bars, what is very close (1.9% deviation) to the reference value. In this later case, the number of components has been reduced by a factor three and computing times by a factor 10. CONCLUSION This paper showed that: – the characterization procedure developed by Avaullée et al. is suitable for MMP calculation; – The MMP may be calculated with the same accuracy, modeling the crude oil with 37 or 12 compounds. To do so, the light intermediate compounds and the heavy intermediate compounds have to be lumped into two pseudo-components; – Lumping together the light compounds is not sui Abdoul W. , Rauzy E. and Péneloux A. ( 1992 ) A group contribution equation of state for correlating and predicting thermodynamic properties of weakly polar and non-associating mixtures. I. Binary and multicomponent systems . Fluid Phase Equilib., 68 , 47 - 102 . Avaullée L. , Neau E. and Jaubert J.N. (1997a) Thermodynamic modeling for petroleum fluid . II. Prediction of PVT properties of oils and gases by fitting one or two parameters to the saturation pressures of reservoir fluids. Fluid Phase Equilib ., 139 , 171 - 203 . Avaullée L. , Neau E. , Trassy L. and Jaubert J.N. (1997b) Thermodynamic modeling for petroleum fluid. I. Equation of state and group contribution for the estimation of thermodynamic parameters of heavy hydrocarbons. Fluid Phase Equilib ., 139 , 155 - 170 . Johns R.T. ( 1992 ) Analytical theory of multicomponent gas drives with two-phase mass transfer . PhD Stanford University, Stanford, CA. Monroe N.N. , Silva M.K. , Larsen L.L. and Orr F.M. ( 1990 ) Composition paths in four component systems: effect of dissolved methane on 1D CO2 flood performance . SPERE , 423 - 432 . Péneloux A. , Rauzy E. and Frèze R. ( 1982 ) A consistent correction for Redlich-Kwong-Soave volumes . Fluid Phase Equilib., 8 , 7 - 23 . Peng D.Y . and Robinson D.B. ( 1976 ) A new two-constant equation of state . Ind. Eng. Chem . Fund., 15 , 59 - 64 . Rauzy E. ( 1982 ) Les méthodes simples de calcul des équilibres liquide-vapeur sous pression . PhD University of AixMarseille III. Spencer C.F. and Danner R.P. ( 1973 ) Prediction of bubble point density of mixture . J. Chem. Eng. Data. , 18 , 230 - 234 . Trebble M.A. and Bishnoï P.R. ( 1987 ) Development of a new four-parameter cubic equation of state. Fluid Phase Equilib ., 35 , 1 - 15 . Zick A.A. ( 1986 ) A combined condensing/vaporizing mechanism in the displacement of oil by enriched gases . SPE 15493 , 1 - 11 . Final manuscript received in December 1997


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E. Neau, J. N. Jaubert, L. Arras, L. Avaullee. Influence of the Crude Oil Characterization on Mmp Calculation, Oil & Gas Science and Technology, 13-20, DOI: 10.2516/ogst:1998003