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PRODUCTION LOGGING OF MULTIPHASE FLOW IN HORIZONTAL WELLS

PRODUCTION LOGGING OF MULTIPHASE FLOW IN HORIZONTAL WELLS
The advent of horizontal drilling brought new and unexpected challenges with respect to characterizing flow regimes in horizontal wells. New approaches to logging, fluid flow interpretation and evaluation, and wellbore configuration were developed in response to those challenges. One obvious problem presented by the horizontal wellbore was that wireline tools could no longer be pulled to total depth by gravity. As borehole inclination approaches sixty degrees from vertical, friction increases to the point that wireline tools cannot freely fall to the bottom of the well. In this horizontal environment, gravity is no help at all. This unit will discuss some of the methods that were devised to overcome this problem.

As with vertical wells, it was necessary to identify commercially productive perforations, as well as to determine which intervals produced water. However, the results obtained by conventional production logging tools were generally not sufficient to characterize horizontal flow regimes. These conventional tools were designed to measure vertical flows, in which fluid phases are dispersed with near uniformity across the wellbore. With few exceptions, conventional production logging tools were not capable of characterizing horizontal flow in any detail. This experience prompted the study of horizontal flows in order to develop logging tools capable of discerning each phase within the flow.
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The problems encountered in horizontal wells were not merely related to flow patterns and downhole tool designs. The broad application of slotted liners, gravel packs, and external casing packers pose problems in understanding and characterizing flow in horizontal wells. Through such configurations, fluids can move both within the casing or liner, and through the annulus. As a result, the point at which a logging tool detects a sudden inflow into the borehole may not, in fact, mark the actual source of the fluid entry. This unit focuses on current methods of measuring multiphase flow in horizontal wells. Though current technology has not addressed all of the problems associated with horizontal production logging, oil companies and logging companies continue to study the nature of multiphase flow downhole, and seek to develop more accurate tools to characterize flow in horizontal wells.

Horizontal Well Configurations

What is horizontal well ?
The term "horizontal well" is very loosely used in the industry, since no well is ever completely horizontal, except over the course of short intervals. In this discussion, the term "horizontal well" will encompass any well that is horizontal (90 degrees of deviation), near-horizontal (within 10 degrees of horizontal), or undulating (deviation fluctuates above and below 90 degrees). The important factor to recognize is that in a "horizontal well" a change of only a few degrees in inclination (e.g. changing from 88 to 90 or 92 degrees from vertical) can make a dramatic difference from a production logging perspective. The angle affects which fluid phase (heavy or light) will be dominant within the cross section of the wellbore. A slight change in well angle can dramatically affect individual phase velocities.
Fluid identification logging tools are sometimes used to determine which fluid phases are present within the wellbore, and to measure each fraction at any point along the wellbore. Under ideal conditions, if such a tool provided excellent and reliable information within the horizontal environment, then the character of the logs would be much more affected by the well inclination than the entry profile. In vertical wellbores, the fluid identification tool provides an important indicator of fluid entry, but in horizontal wells, these indications could be misleading in the absence of other amplifying information; especially the directional survey.


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