On-line measurement of oil/gas/water mixtures, using a capacitance sensor

Abstract

This paper describes the use of a non-intrusive capacitance transducer for the simultaneous on-line measurement of water and undissolved gas in crude oil. The water concentration of the flow is determined from the mean capacitance of the flowing mixture. The same transducer can be used simultaneously to determine the void fraction of the flow by measuring the instantaneous variation in the permittivity of the mixture created by fluctuations of the gas component. There is an interaction between these two measurements but this can be decoupled. This measurement principle was investigated experimentally, using process oil/water/air mixtures. Results are reported for mixtures with water contents up to 40% v/v and void fractions up to 15% v/v. A three-component volumetric flowrate measurement system, based on the capacitance transducer discussed in this paper was proposed. The work reported and the conclusions drawn relate to tests using mixtures of lubricating oil/fresh clean water/air. No work has so far been carried out in mixtures of crude oil/saline waters/hydrocarbon gases typical of the intended applications, nor have estimates been made of the effects that the variable nature of these actual components might have on the performance of the technique. Also, the tests were made on a Perspex section of pipe at nominal pressure whereas the real-life application would require measurements on a high-pressure/high-integrity stell pipeline; the practical problems of adapting the technique to the latter situation have not been examined though they are considered to be resolvable.