Ultrasonic Flow Measurement in Horizontal Wells
Authors: Ravenscroft F.A. , Gulliver J. , Hayes D. 
Published: October 2008

Abstract
In highly deviated or horizontal wells, the performance of existing flow meters for production logging is severely limited. Oil and water tend to separate and flow at different rates up the borehole. Ultrasonic flow measurement technology has been developed which can measure over a large velocity range from I mm/s to 5m/s. Ultrasonic beams projected into the surrounding medium can resolve the different velocities of the components of the flow. A prototype production logging tool for the measurement of liquid flows in horizontal or deviated wells using ultrasound has been constructed. The tool is able to measure the variation of velocity over the crosssection of the borehole in oil/water flows. Results of flow measurements in oil and water flow covering stratified to mixed flow and at different pipe inclinations are included in this paper. The tool can also provide diagnostic information relating to the identification of the flow regimes, reverse flow and the location of the oil/water/gas interfaces. The prototype production logging tool includes a fixed array of flow measurement transducers with signal processing carried out downhole. It is designed to be positioned at any point along the string. The tool has been tested in the laboratory and in the field.

Introduction

The Requirement.
Accurate quantitative determination of the rate of the fluid moving into or out of the production zones of a reservoir is important for reservoir modelling and production planning. Flow rate data can also give indications of other operational parameters such as leaks in the borehole, or cross flow situations where fluid may be flowing out of one producing zone and into another. If problems in the borehole can be identified, and remedial actions taken, then operators can benefit from large cost savings Improvements in drilling technology has led to an increase in the number of horizontal production wells being drilled. In horizontal wells phase separation can occur along the well bore, which makes interpretation of data from production logging tools, especially flow meters, increasingly difficult. An instrument capable of measuring flows downhole would ideally be required to provide a reliable and repeatable measurement of the individual flow rates of the fluids in the borehole (oil, water and gas) with depth. Ideally, it would also identify the perforated regions from which the fluids are flowing into the well. Current Downhole Technology. Current methods of flow measurement downhole mainly involve mechanical spinners placed in the flow where the rotation rate of the spinner is related to the fluid velocity. These devices have their limitations I; one major drawback is that they do not have a predictable response in multiphase flow, especially in inclined wells or horizontal wells. In addition, the spinner rotation rate is not only dependent on fluid velocity but also fluid viscosity. Moreover the technique relies on moving mechanical parts that are prone to clogging and failure.

Ultrasound for flow measurement.
The limitations that current downhole flow meters have in their ability to measure flow rate accurately and reliably has led to investigations into a number of other methods and a range of development projects.