Improved Interpretation of Casing Logs for Casing Failure
- G.R. Wooley (Enertech Engineering and Research Co.) | M. Hatcher (NL McCullough)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling Engineering
- Publication Date
- March 1989
- Document Type
- Journal Paper
- 66 - 70
- 1989. Society of Petroleum Engineers
- 1.14 Casing and Cementing, 4.3.4 Scale, 1.6 Drilling Operations, 2.2.2 Perforating, 3 Production and Well Operations, 4.1.5 Processing Equipment, 5.6.1 Open hole/cased hole log analysis, 4.1.2 Separation and Treating, 4.2.3 Materials and Corrosion
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Casing inspection logs may be used to interpret the nature of a casing failure downhole. This paper provides data to improve interpretation for casing failures and presents results of laboratory tests of four commercially available tools. The tools are tested in five casing specimens, corresponding to five types of failures.
Results indicate that although standard casing inspection logs can detect the existence of a casing failure in a well and define its location, interpretation of additional details with regard to the type of failure requires special efforts. Calibration procedures and interpretation techniques that are not commonly practiced in the field are presented to allow distinction between a split and a part and to define details of the split or part. Also, recommendations are made with regard to running speeds and sensitivity settings.
Electromagnetic casing inspection logs are commonly used in the industry to survey the condition of casing. Logs may be used to estimate the amount of pitting, degree of corrosion, wall thinning, changes in diameter, and other casing features. Occasionally, casing inspection logs are used to investigate a casing failure in a well.
Interpretations of casing inspection logs may be used to determine the type of remedial work on a well where a casing failure has occurred, or they may be an important factor in a commercial casing failure claim. Understanding of the capabilities and limitations of casing inspection logs to detect and define a casing failure has been incomplete. Common interpretations of logs for defining details of casing failures have not been supported by laboratory verification.
A dramatic kick in a log reading may be labeled in the field as a "split" casing without much attention to the details of the log reading. Was the failure really a split casing, a fractured pin, or a jumpout? Did the coupling split? Did a pin failure occur on a mill or field end? Answers to such questions may be available from casing inspection logs, and may play an important role in determining cause of failure and remedial steps to take.
This paper reports on laboratory tests of four types of casing inspection logs offered by two companies.Logging Tools
Four types of electromagnetic logs are tested. One measures changes in wall thickness, another measures ID changes, another locates defects, and another measures the electric potential between two depths in casing.
Wall Thickness Tool. The casing inspection tool that is used to measure wall thickness comprises a pair of low-frequency magnetic coils: a transmitter and a receiver about 2 ft [0.6 m] apart. An electromagnetic signal is transmitted from one coil to the other through the casing, and the transmitted and received signals are compared. The shift in signal phase between the transmitter and receiver is proportional to the casing wall thickness.
Fig. 1 is a schematic of a typical four-coil system run in the field. Low-frequency coils that read wail thickness are at the bottom. Fig. 2 is a photograph of such a tool as it enters the test well used in this work. Available wall thickness tools include NL McCullough Casing Inspection Tool TM, Dresser Atlas Magnelog TM, and the Schlumberger Electronic Thickness Tool TM (ETT).
ID Caliper Tool. The electromagnetic casing caliper tool is a high-frequency inductance coil with a signal proportional to the circumference of the casing, which is converted to ID for recording. The caliper tool is normally run with the thickness tool and is placed above the two thickness coils and an electronics section as shown in Figs. 1 and 2. The inductance coil of the caliper tool has a white band in the photograph.
Example caliper tools are the Dresser Atlas Magnelog, which includes a caliper coil, and the NL McCullough Casing Caliper Tool TM, which is often run with the Casing Inspection Tool. Mechanical caliper tools are not addressed here.
Magnetic Flux Lines Tool. There are at least two logs available that measure disturbances in magnetic flux lines caused by defects in pipe. Dresser Atlas offers the Vertilog TM and Schlumberger has the Pipe Analysis Log TM (PAL).
The tool tested contains two rows of magnetic sensors, each row consisting of six sensors. Each sensor inspects approximately 60 deg. of the casing circumference. Four curves are recorded: (1) the sum of Row 1 signals, (2) the maximum of Row 1 signals, (3) the maximum of Row 2 signals, and (4) the maximum eddy current readings from all 12 sensors.
Electric Potential Tool. There is a casing inspection tool that does not use electromagnetic signals, but instead measures electric potential or resistance between two points in the casing. Casing potential tools are available from Schlumberger, Dresser Atlas, and NL McCullough.
This toot is used to detect the occurrence of cathodic corrosion in casing and to provide a measurement of rate of corrosion. For purposes of interpreting casing failures, this device would be useful for distinguishing between split pipe and a part, such as a connection jumpout. The electrical resistance is discontinuous for parted casing, but not for split pipe.
Casing Test Specimens
A casing manufacturer provided five 5 1/2-in., 17-lbm/ft [14-cm, 25-kg/m] N-80, 8-round LTC test specimens. The specimens were prepared to simulate five types of failures: (1) narrow split in pipe, (2) wide split in pipe, (3) pin jumpout, (4) fractured pin, and (5) jumpout from split coupling. Fig. 3 is a diagram of the five casing specimens.
Specimen 1 has an axial split approximately 3/16 in. [0.48 cm] wide and 4 ft [1.22 m] long cut in the pipe body beginning at the last scratch in the threaded region. The narrow split simulates the condition that would occur for a burst failure in a casing with external support from cement or formation.
Specimen 2 dimensions are the same as for Specimen 1, except that the split is 2 in. [5.08 cm] wide. A wide-split specimen simulates the condition in which a burst failure occurs without backup support from cement or formation. The irregular edge that might occur in an actual burst is not expected to change log readings. Also, the effect of removing metal from the specimen is small compared with the deflection caused by the opening.
Specimen 3 simulates parted casing in a well. The mill end contains 14 1/3 ft [4.37 m] of pipe assembled into a coupling, and the field end is a 5-ft [1.52-m] -long joint of pipe with pin threads. Between the two joints of pipe is a 4.4-ft [1.34-m] separation.
The ability of logs to detect a "dutchman" (fractured section of the pin) in the coupling is evaluated with Specimen 4. A section of threads has been cut off a casing joint and screwed into the coupling below the face of the coupling. This specimen simulates a tension failure at a connector by fracture of the pin near the last engaged thread.
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