The Properties of Well Fluids as Related to Hydraulic Pumping

Abstract

Regional Fall Meeting of the Los Angeles Basin Section of SPE, 22–23 October, Pasadena, California Introduction Successful hydraulic pumping of oil well installations depends on an accurate knowledge of the fluid properties. Such information enables us to select the proper tubing string sizes and to predict, and later check, the bottom-hole pump performance. In this paper, we will consider the effects of temperature and pressure on viscosity and specific gravity, and will present a convenient method for calculating these effects. An extensive study was made on crude oil data gathered from all over the world, and, on the basis of this work, empirical equations are developed. Based on these equations, four nomographs are included which facilitate the following calculations: the viscosity of any crude at any temperature and pressure; the specific gravity at any point in the well and the average pressure gradient; the viscosity at any point in the well; and the viscosity gradient of a well which may be used directly in the tubing friction formulae. Viscosity The viscosity of oils is the property subject to the greatest variation and must be considered the most important factor in hydraulic pumping or any other production method. The range varies from 1.0 centistokes or less, to as much as 1,000,000 centistokes under well conditions. Viscosity varies unpredictably when wax, water and gas are present, but this paper will not cover these problems. In any given well, the viscosity varies with depth, and this variation is dependent on the temperature and pressure gradient. Fig. 1 shows this variation for a typical crude—36.2° API gravity. The following equations and nomographs enable one to determine the viscosity gradient in the different tubing strings. Gravity Factor It has been shown by Beal that at normal temperature [usually 100°F for viscosity determinations], the viscosity of oils increases with gravity quite consistently, even though the compositions of the oils may differ. Paraffinic oils generally have somewhat higher viscosities than asphaltic crudes. Fig. 2is a plot of the viscosity at 100°F of a large number of oils — paraffinic, asphaltic and mixed base — from widely scattered fields, which shows a good correlation with specific gravity at 60°F and with the data of Beal. This chart has, as the ordinate, the log e-log e scale for kinematic viscosity, [same ordinate used for ASTM viscosity charts] and has, as the abscissa, the specific gravity, delta, scale.