THE BEST EQUATION OF STATE AND CORRELATION DENSITY AND VISCOSITY

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1 Page 1 of 17 PROPOSED FOR PREDICTION OF IRANIAN CRUDE OIL DENSITY AND VISCOSITY INTRODUCTION Density and viscosity are two important physical properties which are highly used in chemical engineering calculations. For determination of these two physical properties two general types are applied: 1. Experimental type 2. calculation type The experimental cost is more expensive than that of calculation type, and preparation of the reservoir conditions in the laboratory is difficult. Further more due to different pressures and temperatures exist in reservoir conditions, applying the reservoir condition in laboratory causes many difficulties with more errors. The calculation methods which are based on several mathematical relationships may predict density and viscosity, if needed data are available. Due to the lower cost of this method than that of the experimental method, by computerization of the calculation method, if logical results with low errors is desired, it can be

2 Page 2 of 17 used insisted of experimental method. In this work with using of some needed experimental data from the "Iranian oil field PVT reports" are prepared and needed steps are applied to predict the oil density and viscosity, and compared with the experimental works. Finally the best calculation method which satisfies the experimental works with the lowest error is selected and recommended. APPLIED STEPS 1- Searching in several equations of state and correlation for determination of density and viscosity. 2- Preparation of required data(from the PVT reports). 3- Computerization of the defined available equations or using of the available packages. 4- Retrieve the determined results and compare with the experimental values with using of tables and graphs. EQUATIONS OF STATE AND CORRELATIONS FOR DETERMINATION OF DENSITY AND VISCOSITY

3 Page 3 of 17 Equations which are consisted of relations between pressure, volume, and temperature are called equations of state(eos). By using of these equations some fluid physical properties can be calculated. In this work one of the wanted physical properties is density which can be calculated by "EOS". The ability of these equations for determination of density differs from each other, so causes different error percentages. To decrease the error percentages, in this work, besides the EOS, some correlation are used. In other words, at first, some needed data are calculated from the EOS, then by applying the calculated data and another needed data in correlation the liquid density with lower error percentage may be calculated. For prediction of "liquid viscosity" some required calculated data from EOS and density correlation and the other required data are applied in the viscosity correlation. To have a good accuracy with lower error percent by using of a computer program "C7+" is break down in to nine pseudo components. USED EQUATIONS OF STATE AND CORRELATIONS In this work, it is attended to use the best known "EOS" and correlation. So, after a literature survey about different EOS and correlation their ability for needed calculations were studied. Finally, several available equations and

4 Page 4 of 17 correlation were selected. Six EOS which are tabulated as the following: Method Type PR EOS SRK EOS SRK(KD) EOS BWRST EOS LKP Corresponding state GS Corresponding state RECOMMENDATION OF THE BEST USED EOS AND CORRELATION FOR DENSITY PREDICTION After some studies and calculations, "API" correlation was selected as the best equation used with EOS for density prediction. In other words, due to the fact that the experimental densities are measured at variable pressures and constant temperature conditions, so, by using "EOS" the

5 Page 5 of 17 compositions and any other required data of the liquid mixture are calculated isothermally at any needed pressure, and then by using "API" correlation, liquid density is calculated. Among the mentioned EOS, Peng Robinson (PR), Soave- Redlich-Kwong(SRK), Kabadi-Danner modification of Soave-Redlich-Kwong [SRK(KD)], and Grayson-Streed(GS) equations of state when used with "API" correlation have good density prediction, which the best results are predicted by "SRK(KD)" and "GS" when used with "API" correlation. VISCOSITY PREDICTION The studies for viscosity prediction shows that if the "C7+"content is less than 45%, it is better to break down "C7+" in to some pseudo components and calculate their required physical properties. Then, with use of the viscosity correlation, liquid viscosity is calculated. There are some computer programs for break down of "C7+" in to some pseudo components. In this work the "C7+" break down package (which is installed on the NIOC main frame) has been used. The required data for using the package are: pressure, temperature, composition,and some data from the "Assay Book"in NIOC south oil field. In the mentioned package the "C7+" fraction

6 Page 6 of 17 is broken down in to some pseudo components. The results of running this program are "Normal Boiling Point", "Molecular Weight", and "Specific gravity" of the determined pseudo components. There is some data about the Iranian oil fields in the "Assay Book". In this work the effect of some parameters such as temperature and pressure, are considered in calculations and the following methods are studied to find the best method for prediction of oil viscosity. "Beal" method, "Beggs and Robinson" method, "Lohrenz and Bray" method, "Abbot-Kaufmann-Domash" correlation, Internally consistent correlation, and "API" method. REQUIRED DATA PREPARATION Due to the fact that the prediction of "Oil density and viscosity of the Iranian oil fields" are to be considered, so, the existing "PVT" reports have been used. The required data are retrieved from the Ahwaz, Aghajari, Gach saran, and Marun reservoirs PVT Wells Reports. The under saturated reservoir oil composition and the density and viscosity at variable pressures and constant temperature can be retrieved from the PVT Reports. In PVT reports for identification of "C7+", it's molecular weight and specific gravity are given. As an example for viscosity and density prediction, the retrieved data from "Well AZ - 34" are as follows:

7 Page 7 of 17 Component Composition Mole % C C C ic nc ic nc C C CO H2S The "C7+ required existing data are Molecular Weight of C7+ = 265 Specific Gravity of C7+ = , So the density of C7+ = lb/ft^3 COMPUTERIZING USED EQUATIONS There are some suitable packages for prediction of the fluid physical properties

8 Page 8 of 17 that have been used in this work. Among available packages the most perfect one is "Pro2 Package". For prediction of "density and viscosity", the selected EOS and correlation have been used to apply on this package. For calculation of the viscosity, as previously mentioned, the "C7+" component is broken in to nine pseudo components by using the " C7+ break down package". For comparison of the calculated data with the experimental data, they have been tabulated and graphed by using the "Quatroporo Package (QP)". The recommended type and it s errors are given in the last columns. DISCUSSION AND RECOMMENDATIONS Due to the difference in the compositions of the oil fields and the formations (Asmary and Bangestan formations)from which oil is produced,the data are divided in to four groups based on the ratio of the amount of "C1","C2",and "C3" to that of "C7+". The ratio may be written as the following: X M +X M +X M C1 C1 C2 C2 C3 C3 R = X M

9 Page 9 of 17 C7+ C7+ Where: R=Ratio, Xi=Mole fraction of component "i", Mi=Molecular weight of component "i" The groups can be classified as follows: 1 - R < 10 = 2-10 < R < 12 = 3-12 < R < 14 = 4 - R > 14 GROUP 1 R < 10 = In this case "C7+" content is higher than that of light components. So, the dependence of the oil density to the light component physical properties is decreased,and all of the selected equations predict the liquid density with good accuracy. Among these equations, SRK(KD) is the best and is recommended for prediction of liquid density of this group.

10 Page 10 of 17 GROUP 2 10 < R < 12 = In this case, the light components of the oil are a little higher than that of the previous cases, This causes that the calculated physical properties by some of the selected equations differs from the experimental values. For example the "LKP" equation doesn't predict logical results, because of a calculated "bubble point much higher than that of experimental values. The other equations predict good results up to the bubble point. But after matching the bubble point "BWRST" method is failed. The liquid density prediction by the "SRK" "PR", "SRK(KD)",and "GS" methods are good. Among these methods "SRK(KD)" is the best. To have the best results a combination of "SRK(KD)" and "GS" (after the bubble point for the pressure lower than 3000 PSI)is recommended. GROUP 3 12 < R < 14 = In this case the oil light components are more than that of the 2nd group. So, the calculated physical properties by some of the equations is far from the experimental values. In this case the "LKP" and "BWRST" methods, theoretically flash the oil at a pressure much higher than that of the experimental values.

11 Page 11 of 17 The difference is about 2000 PSI. So, the predicted density by these equations cannot be accepted(even though the results match experimental density values). "GS" method, theoretically flashes the oil at a pressure about " " PSI higher than the that of the experimental values. So, the "GS" method density results cannot be accepted, but if it is used after the bubble point, predicts good results. "PR","SRK",and "SRK(KD) prediction is good and among these methods "SRK(KD)" is the best. To have the best results a combination of "SRK(KD)" and "GS"(after the bubble point for the pressure lower than 3000 PSI)is recommended. GROUP 4 R > 14 In this case because of high content of light components, the dependence of the oil density to the light components physical properties plays an important role. As previously described, if the content of the oil light components is increased the accuracy of the equations for density prediction is decreased. In this case the "BWRST","LKP", and "GS" methods don't have a logical prediction because of calculating the bubble point much higher than that of experimental values, but "PR", "SRK", and "SRK(KD)" equations of state predict acceptable bubble point. Among these EOS "SRK(KD)" is the best, but for the best results a combination of "SRK(KD)" and "GS" (after the bubble point for the pressure lower than 3000 PSI)

12 Page 12 of 17 is recommended. It is important to know that with the equations mentioned in the all discussed groups the "API" density correlation is used. RECOMMENDED VISCOSITY METHOD Reservoir oil is consisted of hydrocarbon components ranges from "C1" to heavy hydrocarbon components. Since there are high contents of "C1"and"C2"in reservoir oil, (especially at high pressure reservoir conditions) and individual viscosity determination of these two components isn t available, so, the methods based on the data of individual component viscosity cannot predict the reservoir oil viscosity with good accuracy. In addition, the presence of the high contents of the heavy components causes an approaching to the real oil viscosity prediction. Especially, if "C7+" is broken down to several pseudo components the approaching to the real oil viscosity would be increased. In the other hand one of the most important parameter for prediction of oil viscosity is density. So, by using the mentioned equations for density prediction, the oil viscosity is calculated. Inspite of logical approaching oil viscosity predicted by "Lohrenz and Bray" method to the real oil viscosity, due to the fact that this method is based on individual components viscosity, it cannot predict accurate viscosity. Viscosity determination by "Beal method" which is based on graphs, cannot be computerized with good accuracy for viscosity determination,because of different

13 Page 13 of 17 ranges. Viscosity prediction by "Beggs and Robinson"which is based on the liquid "API gravity"is not more accurate than the similar methods. The method of "Abott -Kaufmann-Domashm", TWU", and "API" which are the same, predict liquid viscosity with good accuracy but "API" method prediction is better than the others. The calculated viscosity without broken down of the "C7+" have an average error of 20%. So, for the best prediction of the oil viscosity, a "C7+" broken down in to nine pseudo components applied in the "SRK(KD)"and"GS" EOS with the mentioned condition, and finally use of the "API" viscosity correlation is recommended.

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15 Page 15 of 17 PRESSURE VISC VISC VISC VISC VISC VISC VISC VISC (L) (L) (L) (L) (L) (L) (L) (L) VISC (L) PSIA C.P. C.P. C.P. C.P. C.P. % % % ERROR ERROR ERROR %ERROR V-130 LAB PR SRK SRK GS REC.TYPE PR (KD) (KD) GS REC.TYPE AVERAGE %ERROR COMPARISON OF THE EXPERIMENTAL WITH PREDICTED VISCOSITY (FAJR-44)

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17 Page 17 of 17 PRESSURE DENSITY DENSITY DENSITY DENSITY DENSITY DENSITY DENSITY DENSITY DENSITY DENSITY DENSITY DEN PSIA LB/FT3 LB/FT3 LB/FT3 LB/FT3 LB/FT3 LB/FT3 LB/FT3 %ERROR %ERROR %ERROR %ERROR %ER D LAB PR SRK SRK SRK GS BWRST LKP PR SRK (KD) (KD) GS BW AVERAGE %ERROR COMPARISON OF THE EXPERIMENTAL WITH THE PREDICTED CRUDE OIL DENSITY IN DIFFERENT P