Production Logging in Horizontal Wells Using Ultrasonics
Authors: Nyhavn Fridtjof , Bang Jon , Oyangen Terje 
Published: December 2007

Abstract
This paper discusses technical and practical aspects on the future needs for monitoring of flow conditions in deviated and horizontal production and injection wells. A monitoring technique based on ultrasound has been developed. Examples of measurements in various North Sea wells are presented. The results are promising, and show the potentials of non intrusive techniques for flow monitoring. It is expected that a combination of several sensor principles would facilitate the interpretation of downhole flow conditions.

Introduction
There is a trend to use more horizontal wells in today's oil and gas business community. Although many challenges are still to be overcome within drilling and completion of these wells, the monitoring of downhole conditions while draining/injecting seems to be a major bottleneck to obtain optimal production and recovery of reservoirs, specially as the wells get old. Among the reasons is the fact that the majority of sensors are developed for operation in vertical wells.
Horizontal wells have some properties that differ from the properties of vertical wells:Boreholes are drilled parallel to bed boundaries. The result is longer production or injection zones.Gravity is perpendicular to the borehole. This results in more complex cross sectional liquid composition (segregation). Furthermore, unstable flow due to holdup is induced, and wells are not easily entered by gravity based methods.New well and completion types are introduced.
However, as with vertical wells, the main objective of logging or monitoring horizontal production/injection wells is to monitor what flows from where to where. In addition, diagnostics of more advanced wells seem to increase in importance.
Refs. 5 through 7 discuss horizontal wells and the specific challenges when monitoring and managing these. Bay et al. states in 1993: "…development of production logging sensors and interpretation methods have not kept pace with drilling and completion advances, and future sensors that are being developed with horizontal flow in mind are some way off."
Refs. 4 and 8 through 12 discuss the challenge of downhole multiphase monitoring. Both ultrasonic and other sensor methods are focused. Although some of the discussions address surface equipment, these are still of value for solving downhole multiphase monitoring problems.
Non intrusive sensor techniques
Some energy carriers, like ultrasonic waves, can interact non intrusively with the medium it travels through. This means that characteristics of the medium can be estimated by analyzing the carrier after the travel. The received carrier energy is a fraction of the emitted carrier energy. It has been scattered by impedance discontinuities in the medium, deviated, attenuated, etc., see Fig. 1. The received carrier may carry information on medium density, medium velocity, medium composition, etc.
These features are illustrated in Fig. 1. To highlight the conceptual advantages of non intrusive monitoring, this and the following Figs. 2–4 depict an ideal future completion, with the sensor system integrated in the tube wall.
Non intrusive techniques offer valuable properties for logging of well fluids. Viewed from the production technology side, there is a strong motivation to locate sensors outside the well flow. Among the motives are the avoidance of erosion damage to the sensor and the minimization of the disturbance to flow induced by the sensor itself. Via the efforts towards permanently installed monitoring systems, non intrusive sensors are more focused. The potentials in avoiding decreased well productivity and sensor erosion suggest that these techniques be given high attention.
P. 277^