Digital Instrumentation for Net Oil Measurement in Well Testing

Abstract

Abstract A capacitance probe type of net oil analyzer (NOA) system for automatic well testing is described. Changes in water content of the separator discharge stream cause capacitance changes that modulate the frequency of a high frequency oscillator. The latest solid state integrated circuit devices have been combined with digital circuit techniques. This system attains high accuracy and fast response to changes in character of the flow stream by making up to 36,000 discrete incremental analyses of the stream per barrel of liquid passing the probe. Flow stream conditions have been investigated, and their effect upon capacitance probe measurements is reported and discussed. The results of both laboratory and field tests are presented. presented. Introduction The capacitance probe technique for measuring the BS and W content of crude oil has become well known during the past 10 years, and numerous papers on it appear in the literature. The large difference between the dielectric constants of crude oil (2 to 2.7) and of water (80) makes the change in capacitance of a coaxial cell through which a stream of oil emulsion flows a sensitive indicator of the water content of the stream. Wood' found that if the emulsion has a continuous oil phase, measurements with the capacitance probe are reliable indicators of water cut; but if the continuous phase is brine, the relationship between dielectric constant and water cut established for oil external emulsions does not apply. He also indicated that the discontinuity occurs in the region of 50 to 60 percent water. Rarely does the water bound in emulsion with crude oil in field production approach 50 percent. Our experiments in emulsifying numerous crude oils without the use of special additives have produced no emulsions in excess of 50 percent water cut without free water fall-out. Droemer reports field emulsions ranging up to 12 percent water cut after removal of free water, and Woodhall found field emulsions to be below 20 percent water cut even when wells being tested were producing 90 percent water. Even a short resident time in a test separator is usually sufficient for free water fall-out from relatively high water cut emulsions. Although the literature deals largely with the use of capacitance probes to measure oil external emulsions, the method may also be used to meter emulsions and free water in the same stream, provided they exist in separate slugs of sufficient length and time duration while passing through the probe to be individually resolved by the probe and its associated instrumentation. There are certain requirements for good accuracy in analyzing and metering intermittent flow of relatively small volume dumps that may contain slugs of free water interspersed with slugs of emulsion. These requirements are (1) a probe antenna shorter than the slugs to be resolved, (2) a fast instrument response to the passage of fluid interfaces and to changes in water cut of the emulsion, and (3) a sampling rate great enough to obtain several measurements on successive increments of the stream that are equal to the probe antenna length. The ideal application of and least severe test for a capacitance probe is in the measurement of net oil in the emulsion discharge of a three-phase separator after knockout of the free water. The most severe test lies in its application to the discharge of a small two-phase separator, in which the resident time of the fluid is short, and in which the input contains a considerable amount of free water. If the filling time is short, slugs of free water- and perhaps even a continuous stream of water-appear at the discharge as the well continues to produce while the separator dumps. The simultaneous passage of continuously flowing streams of free water and oil through a capacitance probe must be avoided if satisfactory measurement accuracy is to be attained. Principles of the Capacitance Probe Principles of the Capacitance Probe The capacitance cell through which the liquid stream flows consists of an insulated probe antenna arranged coaxially within a section of pipe (Fig. 1). JPT P. 1141