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Summary. Service companies traditionally have used similar types of downhole tools to locate the lowest free point in a string of pipe. Advancing technology has encouraged new concepts and innovative tool designs that are changing this specialized service. Traditional tools make static measurements of incremental tension or torsional deviation on either side of the forecasted point at which the pipe is stuck. The speed with which the free point is located depends on the initial forecast and procedural requirements necessary for the measurement. In a sense, the operator approaches the free point by successive approximations.

This paper introduces two new tool designs to minimize interpretation error and to accelerate the location of the lowest free point. Each design provides a log record of the free-point service. One of these tools extends the concept of static measurements of incremental tensile and torsional deviation, while the other relies on the measurement of magnetic permeability and produces a continuous log. Example logs from each tool design are presented.

Introduction

Free-point tools may be divided into two general categories: those that directly measure the relative movement (or lack of movement) between two points of a pipe as a result of tensile or torsional forces applied uphole and those that indirectly measure pipe mobility by an analog representation.

Tools that directly measure pipe movement are equipped with a method of physically anchoring themselves to the pipe ID. Generally, these tools have an upper and a lower anchor. A sensor section is located in the tool center; one side of the sensor connects to the upper anchor, while the other side of the sensor connects to the lower anchor. In this arrangement (see Fig. 1). movement of the pipe resulting from applied stress forces-such as tension or torsion-will cause the upper and lower parts of the sensor to separate. Examples of devices providing a directly measured pipe movement include the Spring Tector and Magna Tector tools. Tools providing indirect measurements typically are not anchored to the pipe. The location of a stuck point in pipe by the latter tools is identified, for example, by changes in the attenuation of a reference signal or changes in magnetic characteristics above and below the suspected stuck point. To amplify the effect, especially for those tools sensing change in magnetic characteristics, stress can be applied to and removed from the pipe.

Indirect Measurement

An example of a tool that uses the indirect measurement technique is the Sonic Pipe Recovery (SPR) tool. This tool is used in a manner similar to that of a cement bond logging tool. Anchors are not used, and the tool is centralized and logs throughout the interval of interest. The sonde is equipped with a single transmitter and receiver. The time/amplitude measuring system is designed to give preference to the signal conveyed by the pipe rather than that by the formation. Surface equipment is designed to capture the pipe signal at the appropriate time, representing the velocity of sound through pipe. The amplitude attenuation of the received signal can he interpreted as a measure of pipe binding; i.e., a stuck point reduces the received signal amplitude. A significant benefit of this technique is its ability to identify stuck intervals, whereas direct measurement tools are limited to the identification of a point location. Fig. 2 shows the response of an SPR log in the presence of mud sticking in the interval from 6,150 to 6,190 ft [1875 to 1887 m]. This log was recorded at a wireline speed of 30 ft/min [0. 15 m/s] in 2 3/8-in. [6.03-cm] tubing inside of 4 1/2-in. (11.4-cm) casing. While this particular example is easily interpreted others require considerable skill for accurate appraisal. In openhole applications, the combination of the SPR toot with a gamma ray tool aids in the interpretation of probable causes of stuck pipe.

Direct Measurement

Direct measurements require the application of a force-e.g., tension or torsion-at the surface. Applied force causes the pipe to move. Above the stuck point, the pipe will respond to the applied force; below the stuck point, little or no response will occur. Skilled pipe recovery personnel use surface measurements to forecast the location of the stuck point. The tool is then lowered to the suspected area and measurements are taken. When a tensile force is applied to free pipe of uniform cross-sectional area, the pipe will stretch in accordance with the relationship

..........................................(1)

Keeping in mind the inherent stress limitation of the pipe, field practice is to apply enough force. F, to produce the proportionality

..........................................(2)

Therefore, in a tool measuring a 5-ft 11.5-m] span-e.g., the anchors are separated by 5 ft [1.5 m]-the sensor would experience a movement of about 0.018 in. [0.046 cm]. When a torsional force Tis applied to free pipe of uniform cross-sectional area, the pipe will twist in accordance with threlationship

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Again, keeping in mind the inherent stress limitation of the pipe, field practice is to apply enough torque to produce the proportionality

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With 1 3/8 turns per 1,000 ft [305 m] of free pipe, the sensor of a tool measuring a 5-ft [1.5-m] span experiences a rotational movement of 2.5deg..

When the stuck point is in drill collars below the pipe, the increased wall thickness reduces the incremental stretch and twist experienced in pipe-e.g., when surface forces are applied to produce 3 1/2 in. [9 cm] of stretch in 1,000 ft [305 m] of drillpipe, the attached drill collars stretch proportionally less because of the larger cross-sectional area. The effect can be approximated by the expressions

..........................................(5)

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