A New Method in Calculating Water Cut and Oil and Water Volumes Using Coriolis Meter
Authors: AL-Mubarak A.M. 
Published: December 2007

A Coriolis device such as Micromotion massflow meter measures mass and density of the mixture flowing through it. By deriving the density of the pure components, namely oil density and water density, at the flowing pressure and temperature, the water cut can be calculated. Due to the presence of gas in solution and pressure effect on density after the two-phase separation process, correction of the density for oil and water must be applied. By using the principle of ideal solutions, physical properties such as density of oil at separator conditions can be calculated within accepted engineering accuracy. A fluid density is an indirect function of a pressure and a temperature. Once the density at standard conditions is obtained from a lab measurement, it must be corrected for compressibility and thermal expansion at separator conditions for both oil and water. In Saudi Aramco a method and software were developed to perform well rate test in all Northern Area fields for several real time computing platforms using Remote Terminal Unit, Application Module for Honeywell system and Personal Computer with data acquisition system.

Introduction

Measuring water for well under test often requires additional processing beyond gravitational separation. Two-phase separator with Coriolis meter simplifies the process by lessening the requirement of additional internals and eliminating the need of any crude oil treating system for oil and water separation. The Emulsion State has no effect in measuring water cut using Coriolis meter. It is critical to have accurate oil and water densities at standard conditions for each well and relevant PVT data to obtain accurate water cut measurement during test. The gas in solution Rs and oil formation factor Bo as a function of pressure and temperature are required for all possible operating ranges of a test facility. Knowing the gravity of the gas in solution is also required. Continuos capturing online data that include pressure, temperature, mass flow and mixture density enables accurate measurement of well test results very successful. The last three variables are obtained from a Coriolis meter via a communication protocol such as MODBUS or HART protocol.

As it was indicated in Ref. 1, a small amount of gas might not effect mass measurement accuracy; however, it will cause a decrease in density reading. A proper engineering design of test separator or fluid conditioner will eliminate the possible effect of entrained gas. Ref. 2 indicates the high accuracy of the emulsion density measurement obtained from the Coriolis meter technology. Based on Chevron's comprehensive performance evaluation, a density accuracy of 0.0002 g/cc could be achieved while the accuracy specified by most vendors is 0.0005 g/cc.

Densities Determination

By using the ideal-solution principles, the oil formation factor can be expressed as follow:

(1)

Equation (1) can be rewritten as

(2)

Equation (2) can be arranged to calculate the density of oil phase at processing pressure and temperature as

(3)

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