Extensive experimental data were acquired for oil-water flow in horizontal pipes for a very wide range of oil viscosity. Pressure drop, flow rate, input water fraction, in-situ holdup, mixture temperature, and flow pattern data were obtained for 612 oil-water tests in 1.5-in. pipe, and 587 tests in 1-in. pipe. Oils with viscosities of 4.7, 58, 84, and 115 cp were used in the 1.5-in. runs, while the 1-in. tests used 237-cp and 2116-cp oils, all measured at 70 degrees F. Mixture velocities varied from 1.5 to 12 ft/s, while input water fractions ranged from 0.05 to 0.90, and mixture temperatures were between 50 and 98 degrees F. A new correlation is proposed for the prediction of the inversion point of an oil-water dispersion. It was found that the input water fraction required to invert the dispersion decreases with increasing oil viscosity. Pressure drop due to friction was also found to increase Pressure drop due to friction was also found to increase abruptly when the flowing oil-water mixture reached the inversion point where the external phase inverted from water to oil. Two pressure-gradient prediction models are presented; one for stratified, and the other for homogeneously dispersed oil-water flows. Comparison between model predictions and experimental data shows satisfactory predictions and experimental data shows satisfactory agreement. Experimental oil-water flow pattern maps were developed. The existing flow pattern in an oil-water mixture depends primarily on mixture velocity, input water fraction, and primarily on mixture velocity, input water fraction, and oil viscosity (only when oil is the external phase). Introduction Cocurrent flow of two immiscible liquids such as oil and water in horizontal pipes is a common occurrence in the petroleum industry. The need to understand the nature petroleum industry. The need to understand the nature and flow behavior of this type of multiphase flow is very complicated due to the existence of various flow patterns and different mechanisms governing them. This phenomenon, coupling with the hard-to-predict rheological behavior of an oil-water system, have been the driving force behind a considerable research effort in this area. The results of these studies would lead to better predictions of the existing flow pattern and its associating pressure gradient, yielding a better designing scheme for such system. This paper investigates the simultaneous flow of different oil-water fluid systems. The study involves gathering approximately 1,200 oil-water experimental data points in 1-in. and 1.5-in. horizontal pipes, for a wide range of flow conditions (flow rates, temperatures, input water fractions, etc.), and also for a wide range of oil viscosities. A correlation is presented, based upon this study and some other published results, for the prediction of the inversion point of an oil-water dispersion system. Two pressure-gradient point of an oil-water dispersion system. Two pressure-gradient prediction models were also developed for two different prediction models were also developed for two different oil-water flow patterns; namely, stratified and homogeneous. In addition, typical experimental oil-water flow pattern maps are also presented. LITERATURE REVIEW An oil-water mixture flow presents a unique and complex problem for pipeline transportation of heavy crude oils in problem for pipeline transportation of heavy crude oils in the petroleum industry due to its complicated rheological behavior, and the vast difference in pressure gradients encountered for different flow patterns. For the homogeneous flow pattern, the system of two immiscible liquids, such as oil and water, could become even more complex since the resulting mixed fluid can turn into an emulsion. An emulsion is a dispersed system which consists of two immiscible liquids. An unstable emulsion, or a dispersion, is an emulsion which can separate into the original phases within a reasonable period of time at rest. These dispersions can also exhibit Newtonian or non-Newtonian rheological behavior; depending on each specific oil-water system. Another phenomenon that further complicates an oil-water dispersion system is the phase inversion phenomenon, in which the dispersed phase inversion phenomenon, in which the dispersed phase switches to the continuous phase. phase switches to the continuous phase. P. 155