Oil-Water flow patterns, holdups and frictional pressure gradients in a vertical pipe under high temperature/pressure conditions

We conducted experimental measurements to examine the upward oil-water flow patterns in a vertical pipe (0.02 m I.D.) under high temperatures up to 130 °C and high pressures up to 20.51 MPa. The viscosity of the oil sample was 85.113 mPa·s at 40 °C. All the experiments were conducted with an in-house-built high pressure/temperature flow apparatus. This apparatus is equipped with a view window that allows us to visually observe the upward flow patterns of oil-water two-phase flow in a 0.02 m I.D. stainless steel pipe. The effects of pressure, temperature, input water fraction (IWF) and mixture flow rate on the flow patterns and friction pressure gradients were systematically investigated. We plot the observed flow patterns on a flow pattern map in which input water fraction was used as the y-axis and the mixture flow velocity was used as the x-axis. Based on the measurement results, we observed nine flow patterns including four water-in-oil flow, transition flow and four oil-in-water flow patterns. With an increase in temperature at a given input water fraction, slug flow, plug flow with large water drops tended to transform into bubbly flow and dispersed flow with smaller water drops, water-in-oil very dense bubbly flow (VDB W/O) was observed at high temperature due to the significantly increasing of in-situ water holdups. The transition flow and the boundaries in the flow pattern maps tend to occur at lower input water fraction values. The effect of pressure on the flow patterns was found to be opposite to that of temperature. The possible causes leading to the changes in the flow patterns subjected to pressure/temperature variations were provided in terms of changes in density ratio, interfacial tension, and viscosity ratio between oil and water. The phase inversion points at different temperatures and pressures reflected from friction pressure gradients were found to be in agreement with the results of flow patterns. This work contributes to a better understanding of oil-water two-phase flow behavior under high temperature/pressure.