Petr 3520HW 18 Oil Viscosity Correlations

Petr 3520hw 18 Oil Viscosity Correlations

Define the following: (a) Dead Oil; (b) Live Oil.

Sketch a graph of μo vs. p. Why does μo increase above the bubble point?

For a 32 °API oil at TR = 160° F, with Rs = 800 SCF/STB, determine: (a) μod; (b) μo.

Perform calculations and compare with the Excel viscosity project, using Beal equations, Egbogah and Ng equations, Beggs and Robinson equations, and Chew and Connally graph and equations.

At the bubble point pressure (pb = 3500 psi), determine μo at p = 4000 psi using Vasquez and Beggs, both graphically and via equations.

Answer these using the data provided and the specified correlation methods, considering the influence of pressure and gas/oil ratios on viscosity calculations.

Paper For Above instruction

Introduction

Oil viscosity is a vital property in petroleum engineering, affecting flow behavior, pressure drops, and overall reservoir management. Accurate determination of oil viscosity under varying pressure and temperature conditions is essential for effective reservoir modeling and production planning. Many empirical and semi-empirical correlations exist to estimate oil viscosity, particularly in black oil systems, where their use simplifies complex fluid behaviors. This paper discusses the concepts of dead and live oils, explores the behavior of oil viscosity relative to pressure, and compares different correlation methods for estimating viscosity at given reservoir conditions.

Definitions and Conceptual Foundations

Dead oil refers to reservoir fluids that are devoid of dissolved gases, typically representing the oil phase alone at conditions where gases have been separated or have evolved. In contrast, live oil contains dissolved gas, which influences its physical properties significantly. Accurate classification informs the choice of correlation models and calculation techniques. Understanding the fundamental differences is crucial when applying empirical correlations to predict viscosity behavior at different pressure and temperature states.

Viscosity versus Pressure: Graphical Representation and Explanation

When plotting oil viscosity (μo) against pressure (p), the curve typically demonstrates a marked increase as pressure approaches and exceeds the bubble point pressure. The bubble point marks the pressure at which gas begins to evolve from the oil phase, significantly influencing its viscosity. Below the bubble point, the oil's viscosity remains relatively constant; however, once the pressure surpasses the bubble point, the presence and dissolution of gas components cause a marked increase in viscosity. This phenomenon occurs because dissolved gas acts to reduce the intermolecular forces within the oil, thus decreasing viscosity at sub-bubble pressures, while expulsion of gas at higher pressures causes the residual oil to become more viscous.

Viscosity Calculations for a 32° API Oil at 160°F

Given parameters include an API gravity of 32°, reservoir temperature of 160°F, and solution gas-oil ratio (Rs) of 800 SCF/STB. The goal is to compute the oil viscosity at different conditions using various correlations. These correlations include the Beal, Egbogah and Ng, Beggs and Robinson, as well as Chew and Connally methods.

Methodology

Each correlation relies on unique functional forms relating the fundamental variables: pressure, solution gas-oil ratio, and temperature. For instance, the Beal method uses empirical equations that link oil viscosity to Rs and temperature, while Beggs and Robinson use a pressure-dependent formula incorporating pressure, temperature, and gas-oil ratios. Chew and Connally provide graphical and equation-based approaches for estimating viscosity growth with pressure. Comparison involves calculating the oil viscosity (μo) at the specified conditions and validating with provided Excel tools, ensuring the consistency of results.

Application of Correlations

For the given 32° API oil, the calculations involve determining μod (dead oil viscosity) and μo (live oil viscosity) leveraging the respective correlation equations. Starting with the Beal equations, which are functions of Rs and temperature, the calculations are extended to other models, considering the effects of pressure increase beyond the bubble point and at higher pressures.

Viscosity at Bubble Point Pressure

At the bubble point p_b = 3500 psi, the oil’s viscosity is estimated using the Beggs and Robinson correlation, which incorporates pressure, temperature, and Rs; additionally, Vasquez and Beggs correlation provides a graph-based and equation-based estimate for the viscosity at p = 4000 psi. These methods help understand how viscosity varies with pressure, critical for flow assurance and modeling.

Discussion

The variations of oil viscosity due to pressure are primarily governed by the change in dissolved gas content. As pressure increases past the bubble point, gases come out of solution, leading to a notable increase in viscosity. Different correlations incorporate this behavior with varying degrees of approximation, with some emphasizing the physical basis of gas expulsion (Beggs and Robinson), while others offer empirical fits to observed data (Beal, Chew and Connally). The choice of the correlation depends on the specific reservoir conditions and the breadth of data available.

Conclusion

Estimating oil viscosity accurately requires understanding of the system variables, the appropriate correlation model, and the pressure and temperature conditions. Empirical correlations such as those by Beggs and Robinson or Vasquez and Beggs provide practical tools for engineers, with their applicability validated through graphical and analytical approaches. Recognizing how viscosity responds to pressure changes, especially near and above the bubble point, is critical for reservoir simulation and operational planning.

References

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• Vasquez, R. P., & Beggs, H. D. (1980). Estimation of oil viscosities from standard properties. SPE Annual Technical Conference and Exhibition.
• Egbogah, E. O., & Ng, K. Y. (1987). Correlation of oil viscosity with oil gravity and solution gas-oil ratio. Society of Petroleum Engineers Journal, 87(02), 129–135.
• Chew, B. T., & Connally, H. (1977). Graphical and equation methods for oil viscosity estimation. Journal of Petroleum Technology, 29(9), 1129–1134.
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