Experimental investigation of the development of surfactant stabilized oil-water pipe flow downstream of a choke valve

Abstract

In wells and multiphase flowlines the presence of valves, pumps and other complex geometries will considerable contribute to dispersion of the phases. In combination with the droplet stabilizing effect of natural surfactants contained in the crude oil and other production chemicals, this may lead to a considerable development length of the flow. Such effects should be understood and considered by future simulation tools. In the presented work, the described scenario was experimentally tested under controlled conditions. The development of dispersed oil-water flow downstream of a valve was monitored. Surfactant was added to the mineral oil to obtain a suitable droplet stability. The near horizontal test section was 220 m long and had an inner diameter of 109 mm. The vertical phase fraction distributions were measured at six locations along the pipe with traversing gamma densitometers. The test section was further instrumented with nine pressure transducers, four optical sections, and three droplet sampling points to monitor flow development. The flow development showed a characteristic behavior of a rather sudden separation after a certain length with stable dispersed flow conditions. The degree of inlet mixing and thus initial droplet size governed by the pressure drop over the mixing valve was found to be crucial for the development length. Flow rate and initial water fraction were of minor importance. The separation behavior was coalescence controlled as it would be expected for a surfactant stabilized system. Still, even if static separation bench tests resulted in very long separation time scales, the in-flow separation happened much faster. This indicates the importance of dynamic flow conditions and associated effects such as shear and a certain mixing to be preferential for the separation. The measured pressure drop of the dispersed flow was more than 20% higher than separated flow with otherwise the same conditions. As the flow separates, the pressure drop decreases and, as expected, finally approaches the value for separated flow. The work presents a detailed data set as basis for model development.