Determination of Oil and Water Volumes By the Capacitance Method
Authors: Burrus Bill 
Published: December 2007

This paper explains the capabilities and limitations of Net Oil Detection Systems in oilfield production for well test and commingling applications. The theory of capacitance measurement and instrumentation will be briefly discussed to establish a foundation for detailed explanations of typical systems with analysis of each for function and accuracy. Systems compatible with high-speed computer program testing will also be discussed.

Introduction

Net oil determination by the capacitance method is one of the most significant developments in recent years to the oil producing industry. It has made automation of smaller leases both practical and economical by replacing conventional heat treating and metering facilities with a simple and comparatively inexpensive device that can accurately determine the water-oil ratio in a fluid stream, and, in conjunction with a meter, display net oil and net water volumes without the need for physical separation of the two components.

The ability of the Automatic Net Oil Detector [ANOD] to handle water cuts ranging from O to 100 per cent makes its application ideal for simplified well testing, eliminating samplers and questionable sample analysis. Low range ANOD's of 0 to 5 per cent and 0 to 35 per cent provide highly accurate and inexpensive lease commingling measurement, reducing the number of treating systems in a field and allowing for the design of efficient central treating and Custody Transfer systems. Field unitization, increased operating efficiency by automation, and natural attrition of equipment are causing many individual studies of new producing and testing techniques. The ANOD and similar systems are streamlining old facilities.

The ANOD gives directly on digital counters instantaneous local water-oil ratios and displays cumulative net fluid volumes in barrels. For remote transmission to central computing or data processing stations, the percentage of cut and rate of flow may be converted to analog functions, or accumulated for periodic transmission as fluid totals.

Fig. 1 shows an ANOD in its simplest form. Three major components are present in all systems [1] an in-line capacitance probe, [2] a fluid meter, and [5] a Capacitance Product Analyzer [CPA]. The probe and CPA are coupled electronically to detect and respond to capacitance changes in the probe. The CPA s response is linear and proportional to changes in the oil-water ratio in the probe. The CPA receives meter pulses proportional to flow and mechanically gates them to Net Oil and Net Water counters in direct proportion to the oil-water mixtures for local readout.

THEORY OF OPERATION

The in-line probe [Fig. 2] is a capacitor cell with fixed physical dimensions. Fluid flows through the annular space separating the inner and outer electrode and the inner electrode terminal is insulated while the outer electrode is grounded. With the dimensions fixed, the capacitance value [expressed in Micro-Micro-Farads] can vary only with a change of the insulator fluid composition. This change can be predicted in terms of the Dielectric Constant [Dk] of the fluids.

Dielectric Constant is a dimensionless number related to several physical properties of a material. By careful component design most of the variables affecting the Dk can be eliminated and the Dk can be considered only as a function of the material's ability to be polarized. The standard measure of a capacitor's value is based on an insulator material of air which has a Dk of essentially 1.00.