Downhole Multiphase Flow Measurement in Horizontal Wellbores
Authors: Ünalmis Ö. Haldun , Trehan Sumeet 
Published: June 2013

Abstract
A new approach in multiphase flow measurement is introduced in this work. The approach is built on the field-proven in-well optical flow measurement technology capable of resolving two-phase flow through direct measurements of flow velocity and fluid mixture speed of sound, and auxiliary measurements of absolute pressure and temperature at meter location. The new multiphase solution is achieved by adding a third measurement capability to the flowmeter: differential pressure across the flowmeter is measured by an integrated optical differential pressure sensor or a secondary optical pressure gauge that can be used in conjunction with the existing auxiliary pressure gauge.
The new approach requires the measurement of frictional pressure drop across the flowmeter which was positioned horizontally in this work, although the method is applicable to general case with any orientation of the flowmeter. The measured pressure drop is then used to calculate the fluid mixture density based on the momentum equation and the commonly used theoretical models for the friction factor. The challenge of this approach is that the experimental pressure drop and the prediction by theoretical models are usually not balanced due to the inaccuracies inherent to the theoretical modeling and thus a closure is necessary to establish the balance in the momentum equation. The closure is achieved in two different ways in this work: 1) Measuring an "experimental" surface roughness term in laboratory through flow loop tests to balance the momentum equation; 2) Forming a complementary friction factor for cases where the surface roughness is known but not sufficient to balance the momentum equation. Once the momentum equation is balanced and the density is calculated, the speed of sound measurement and the calculated density are used to determine the phase fractions (holdup and water cut) of the fluid mixture. Along with the knowledge of the total flow rate through velocity measurement, the phase flow rates are calculated.
The solution method has been successfully demonstrated in flow loop tests using a two-phase optical flowmeter and an electronic differential pressure sensor. Both closure models can be used in this approach, although from a manufacturing point of view, it is more practical to determine the surface roughness term experimentally during the initial calibration of the flowmeter. The flow measurement is bidirectional and the method can be implemented for both producers and injectors. The measurement technology and the full-bore design allow a high turndown ratio when compared to other technologies.
Multiphase flow measurement in extreme environments such as in-well is a difficult task for many reasons including reliability/survivability/longevity issues, accessibility to the equipment, and complexity of the varying flow field as a function of position and time. These challenges are substantial when compared to the relatively benign environments for surface flow measurement, but the benefits are also substantial: zonal contribution from multi-zone wells, production optimization in intelligent wells equipped with ICVs, and reduction of surface tests & facilities are some examples demonstrating the need for in-well flow measurements. The current work aims to provide a solution for this need.