A Consistent and Accurate Dead-Oil-Viscosity Method

Summary The calculation of pressure drop resulting from the flow of oil through porous media or pipes requires the evaluation of viscosity. This is the single most important transport property necessary to calculate pressure drop accurately. The basis for oil-viscosity calculations using a traditional black-oil approach is the determination of dead- or gas-free-oil viscosity. A total of 23 dead-oil-viscosity calculation methods have been identified from the literature and evaluated in this paper. A large database consisting of data from conventional pressure/volume/ temperature (PVT) reports, crude-oil assays, and the literature was compiled from more than 3,000 samples from around the world. The number of actual viscosity measurements exceeded 9,800. An evaluation of the correlations yielded unacceptable results largely because of the failure of the methods to properly account for the physics of the problem. In general, this results from the methods’ failure to properly account for the change in viscosity with temperature and to address the chemical nature of the oil. A significant improvement in results can be realized through the use of the Watson characterization factor in addition to oil API gravity and temperature in the correlation of viscosity. This work has identified the character of the crude to have a significant effect on oil viscosity, especially for oils with gravities less than 25°API. Methods have been proposed in the literature that use the Watson characterization factor; however, these have been largely ignored in the upstream oil industry. Therefore, a new method has been developed that shows significant improvement over existing methods. At reservoir conditions, a 2- to 13-fold reduction in average absolute error was noted when compared with the error observed from traditional methods. At surface process conditions, this improvement ranged 3- to 60-fold. In addition, an updated correlation for Watson characterization has been developed. The ASTM density correction for varying temperature has been examined. Revised coefficients were developed that enhance the method’s accuracy for both oils and pure components and provide a suitable means to convert kinematic viscosity to absolute viscosity.