Undersaturated Oil Viscosity Correlation for Adverse Conditions

Abstract

Abstract The determination of viscosity is required for evaluation of the pressure drop resulting from flow through porous media, tubing or pipelines. Viscosity is a necessary property to ascertain well productivity or to properly size tubing, pipelines and pumps. Numerous methods exist to estimate viscosity for computer calculations. Oils encountered in deep water environments are often highly undersaturated - in some cases in excess of 15,000 psi. For transport, the dead oil must be pumped in an environment with temperatures as low as 35°F. At this temperature, the dead oil atmospheric viscosity can be in excess of 500 cp. The pressure required to pump oil through pipelines from deep water can exceed 3000 psi at the pump on the platform and over 5000 psi at the sea floor. The pressure effect on viscosity results in a significant additional increase in this property which can adversely affect pipeline performance. The existing methods for estimating undersaturated viscosity were not developed using data that encompasses the pressure or viscosity range that are currently encountered by the industry. A large database comprised of 1,399 oils and 10,248 data points was constructed to evaluate the accuracy of existing correlation methods. Pressure differentials up to 25,000 psi and viscosity in excess of 1000 cp are included in the database to ensure that viscosity at both typical conditions and the extreme conditions encountered in deep water are represented. The existing methods are shown to be inadequate over this wide range of conditions. A new method was developed that offers improved accuracy and consistency over the expanded range of viscosity and pressure differential. Introduction Viscosity is an important parameter necessary for the determination of the pressure drop associated with fluid flow. For radial flow through porous media, the relationship between flow rate and pressure drop is described by Darcy's law8. ........................... (1) For situations above bubblepoint in a reservoir, the fluid composition and temperature remain constant and properties vary only with pressure. Viscosity changes range from 5–35%/1000 psi as shown in Fig. 1. On the other hand, density or formation volume factor above bubblepoint changes range from 0.5–2.8%/1000 psi. An examination of Eqn. 1 shows pressure drop is directly proportional to change in viscosity and formation volume factor. Clearly viscosity changes are the most significant property and must be accurately quantified. Fluid flow in pipes8 is characterized by ............................ (2) where the three parts of the equation describe the hydrostatic, frictional and kinetic energy losses in the system, respectively. The friction factor, f, is defined by the Moody Friction factor chart as a function of Reynolds Number and pipe roughness. The Reynolds Number is calculated ......................................................................... (3) For laminar flow at Reynolds Numbers less than 2000, the friction factor is defined ........................................................................................ (4) Therefore for situations of single phase laminar oil flow in pipelines, the pressure drop is directly proportional to changes in viscosity and density. As viscosity is most sensitive to changing pressure, it is the most important term to accurately quantify with changing pressure. Numerous correlations have appeared in the literature for estimating the viscosity of undersaturated oil. These methods are reviewed by Lake30; however, a more complete summary table is provided in the Appendix because of updates due to recently published correlations.