A New Microwave-Based Water-Cut Monitor Technology
Authors: Hatton G.J. , Helms D.A. , Marrelli J.D. , Durrett M.G. 
Published: October 2008

A water-cut monitor suitable for use on a wide range of production fluids has been developed and tested. The monitoraccurately measures water cut from 0-100t without operator recalibration despite changes in temperature, water salinity, crude properties, and the presence of gas. Approved by Factory Mutual Research for usein Class 1, Division 1 areas, this microwave technology based, water-cut monitor is designed for unattended operation at remotelocations -- subsea, topside, or onshore.

INTRODUCTION

The objective of the Texaco Exploration and Production Technology Division (EPTD) team responsible for developing the water-cut monitor was to provide a means for the accurate measurement of water/oil ratios for a wide range of production fluids despite changes in oil composition, emulsion state, water conductivity and geochemistry, temperature and flow rate. This has been realized with the successful long term field test installation of Texaco Microwave Water cut Monitors (TMWM).

The installed units are designed for standalone, unattended, remote operation in hostile environments such as subsea North Sea, Gulf of Mexico or high temperature desert environments. The unit has also met American Bureau of Shipping (ABS) and Factory Mutual Research (FM) standards required for operation in critical areas such as offshore platforms zoned as Class 1, Division 1. Two basic models have been put into operation, the ALL-LIQUID Model designed for liquid streams - e.g. the liquid leg of test separator - and the. GAS-LIQUID Model designed to monitor gas/liquid streams - e.g. full wellhead production.

A number of TMWM field tests have been performed in the Middle East, on-shore 1n the U.S. and in the North Sea. Dur1ng these field tests instantaneous water cuts ranged from 0 to 1OO% while the average water cutover a well test period ranged from 0 to 80%. The results of the TMWM agree with thealternative water-cut measurement (usually a test separator system) to the accuracy of the alternative system.

During these field tests, two types of field problems previously unknown to the operators were uncovered by the ALL-LIQUID TMWM systems.The first was the presence of several percent free gas in the presumed gas free liquid leg of gas/liquid separators, andthe second was the frequent change (several times a minute for periods of hours) of the continuous phase of the emulsion in some operations.

Typical water-cut monitors will read erroneously in the above cases. The TMWM detects and estimates gas carry under and provides quantitative correction of water cut under many conditions of gas entrainment. This correction can be carried out in both oil-continuous and water continuous emulsions. Efforts to extend gascorrection to greater gas fractions are in progress. The TMWM also correctly predicted water cut despite frequent changes between emulsion states

THEORY OF OPERATION

The TMWM electronics measure absolute dielectric values of fluids in the sensor cell allowing accurate measurement of several fluid properties (in addition to water cut) such as water salinity, free gas fraction, and crude average paraffin number, without operator recalibration.