International Journal of Petroleum and Geoscience Engineering Volume 03, Issue 01, Pages ISSN: 2289-4713 Investigation of Under-Saturated Oil Viscosity Correlations under Reservoir Condition; A Case Study on Libyan Crude Oil Khulud Rahuma a, *, Elmahboub Edreder b a Department of Petroleum Engineering, University of Tripoli Tripoli, Libya b National Oil Company Tripoli, Libya * Corresponding author. E-mail address: khulud782000@yahoo.com A b s t r a c t Keywords: Crude oil viscosity, Under-saturated oil, Fluid properties, PVT analysis. Accepted: 22 April2015 Evaluation of under-saturated oil viscosity is one of the important factors in designing many oil field operations. Oil viscosity can be obtained in two methods, either by carrying out experimental measurements or estimated by a proper model. Viscosity usually reported in standard PVT analyses and its correlations can be categorized to either dead or live oil viscosity. Live oil viscosity correlations can be further classified to saturated (at bubble point pressure) or under-saturated (above bubble point pressure). The purpose of this study, testing the performance of the under-saturated oil viscosity correlations existed in the literature such as: Beal, Vasques and Begs, Labedi, Elsharkawy, and Khan and Ali for some Libyan oil samples under reservoir conditions. Statistical analysis is investigated to test the applicability of the models. Good agreements between the predicted and experimental values have been observed. Furthermore, the best evaluation of under-saturated oil viscosity was obtained by Beal correlation with an absolute average deviation percent (% AAD) of 19.95 %. Academic Research Online Publisher. All rights reserved. 1. Introduction Reservoir oil viscosity is an important property to understanding the reservoir flow behaviour. Direct viscosity measurements are expensive or sometimes unavailable, hence empirical correlations are often used for predictions. However, several international published correlations were used in computation of dead, saturated, and under-saturated oil viscosities. The under-saturated oil viscosity is defined as the viscosity of crude oil at a pressure above the bubble point, (Pb), and reservoir temperature, (T). To predict under-saturated oil viscosity it is necessary to use saturated crude oil viscosity and pressure above the bubble point. This work concentrated on the prediction of under-saturated oil viscosity under reservoir conditions, in order to expert oil viscosity correlations for some Libyan crude oil samples and checking results obtained with experimental data for under saturated oil. 2. Effect of Some Properties on the Viscosity Viscosity of fluid changes with temperature, pressure and composition. In the gaseous state the viscosity is much lower than in the liquid state, due to the distance between the molecules which
greater than in the liquid phase. In the liquid phase, required transport and separation equipments from the transfer of momentum is mainly due to the wellhead to the refinery. According to the intermolecular effects between the dense packed Pressure oil viscosity classified to: 1) Dead oil molecules, whereas in the gaseous phase the viscosity, which defined as the viscosity of crude momentum is transferred by collisions of the freely oil at atmospheric pressure (no gas in solution) and moving molecules. Generally, the temperature of system temperature. 2) Saturated oil viscosity, the reservoir is approximately constant during the which is defined as the viscosity of the crude oil at depletion. The production of oil reservoir is started the bubble-point pressure and reservoir at the initial reservoir pressure P res, and as the temperature.3) Under-saturated oil viscosity, pressure is reduced the viscosity decreases until the defined as the crude oil at a pressure above the saturation pressure is reached. In case the pressure bubble-point pressure and reservoir temperature. in the oil reservoir drops below the saturation Beside these properties, reservoir fluid pressure, the viscosity of the oil (liquid phase) is composition, pour point temperature, molar mass, increased (lower mobility), resulting in a lower normal boiling point, critical temperature and a production. The reason is that the oil separates into centric factor of the components effect on the a liquid phase and a gaseous phase below the viscosity of oil. saturation pressure. This phase split will result in changes in the composition of both the gas and the 3. Correlation Overview liquid as the pressure is further reduced, because Mathematically engineer can use some models the volatile or light hydrocarbons go into the which vary in complexity and accuracy to compute gaseous phase, whereas the heavy hydrocarbons are viscosity of oil depending on data available, in lack left behind in the liquid phase leading to an and difficulties of laboratory measurements at increase in the viscosity of the liquid as the reservoir temperature and pressure. Some published pressure is reduced. Therefore, it is important to correlations applied to estimate under-saturated oil keep the pressure in the reservoir above the viscosity to some Libyan crude oils. Beal [1], saturation pressure. Even if the oil is produced Vasquez and Beggs [2], Labedi[3], Elsharkawy & from the reservoir as a single phase, the oil will Alikhan [4], and Khan& Ali [5] correlations undergo compositional changes during the pressure mathematically expressed as shown in Table 1. and temperature reductions occurring through the Table 1: Under-saturated Correlations. Correlation Year of Mathematical Equation Development Beal's 1946 µ o = µ ob +0.001(p-p b )(0.024µ 1.6 ob +0.038µ 0.56 ob ) Vasquez and Beggs 1976 µ o = µ ob ((p/p b ) m where m = 2.6(p 1.187) ( 10 a ), a = 3.9(10-5 ) p 5 Khan and Ali 1987 µ o = µ ob exp [ 9.6 * 10-5 (p-p b )] Labedi 1992 µ o = µ ob -µ au [1- (p/p b )] Elsharkawy& Alikhan 1999 µ o = µ ob + 10-2.0771 (p-p b )(µ 1.19279-0.40712 od - µ ob p -0.7941 b ) 57 P a g e
4. Mathematics The percent absolute deviation (%AD) and the percent absolute average deviations (% AAD) are used to subject the test and can be defined as: (1) (2) Where n is the number of experimental points, µ exp the experimental viscosity and µ cal the calculated viscosity. The % AD indicates how close the calculated values are to experimental values. Correlations results used for predicting undersaturated oil viscosity are shown in figures 1,2,3,4 and 5. Statistical analysis in terms of absolute deviation percent (% AAD), and the average deviations percent (AD%) were used to predict the accuracy of models abilities Table 2. Fig. 1: Beal's correlation. Fig. 2:Vasques and Beggs correlation. Fig. 3: Labedi correlation. 58 P a g e
Fig. 4: Elsharkawy & Alikhan correlation. Fig. 5: Khan and Ali correlation. Table 2: results of average deviation and absolute average deviation. Model AD(%) AAD(%) 1 Beal 2.11563 19.9546 2 Labedi 1.76922 20.5122 3 Elsharkawy & Alikhan -4.8769 23.1614 4 Khan & Ali -7.7498 24.4371 5 vasques &Beggs -33.576 48.7385 5. Conclusions In this study a number of empirical correlations are applied to evaluate viscosity of under-saturated oils. results compared with experimental values have been observed. From comparison we noticed, that small error obtained. The best estimate of the under-saturated oil viscosity (µ o) for Libyan crudes examined in this study was found by Beal, Labedi, that showing the smallest average absolute deviation (AAD %) within 19.95, and 20.51 respectively. Acknowledgement The authors would like to express their deep appreciation to Libyan Petroleum Institute (LPI) for their help and ]providing the necessary data. 59 P a g e
Nomenclatures [2] Vazques and Beggs, H.D., Correlations for crude API - Oil API gravity physical property prediction. J. Pet. Technol., 1980; Μd - Dead oil viscosity (cp) 32(6): 986-970. Μob - Saturated oil viscosity (cp) [3] Labedi, R., Improved correlations for predicting the Μo - Undersaturated oil viscosity (cp) viscosity of light crudes. J. Pet. Sci. Eng. 1992; 8: 221 T - Temperature (R, F) 234. P - Pressure (psia) [4] Elsharkawy, A.M., Alikhan, A.A., Models for AD- Average deviation predicting the viscosity of Middle East crude oils. Fuel, Pb - Saturation pressure oil at bubble point 1999; 78: 891 903. P- pressure (psia) [5] Khan, S. A., et al., Viscosity Correlations for Saudi AAD - Average absolute deviation Arabian Crude Oils, SPE Paper 15720, Presented at the Fifth SPE Middle East Conference held in Manama, References Bahrain, March 1987; 7-10,. [1] Beal, C., Viscosity of air, water, natural gas, crude oil and its associated gases at oil field temperature and pressures. Trans. AIME, 1946; 165:114 127. 60 P a g e