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Casing Inspection : Corrosion Investigation

Corrosion logs include mechanical, electromagnetic, acoustic, and electropotential measurements. These are used to :
• monitor pipe wear caused by continued drilling operations
• detect corrosion on the inside or the outside of the pipe
• locate holes and pits
• detect split or parted pipe
• detect collapsed pipe
• locate perforations
• determine where electrochemical corrosion is likely to occur

The tools for these logs are generally sized to match the casing to be inspected, and hence are not through-tubing devices.
Mechanical Calipers

Mechanical calipers are of two basic types. The bow-spring type caliper ( Figure 1 ) is typically run with a flowmeter, and is used to monitor the inside of the pipe.
Mechanical Caliper - Figure 1
Its use is critical when restrictions such as asphalt, paraffin, or scale buildup are likely. It is also routinely used in openhole completions where a flow profile with a flowmeter is required. The other is the multifinger type, with anywhere from 40 to 80 individual fingers. As these fingers scrape the pipe wall, their maximum deflection is monitored. A single measurement of the maximum deflection among all of the fingers is most common, although some tools are capable of providing a maximum and minimum indication, or of examining individual angular sectors of the wellbore—e.g., a minimum and a maximum for each 120o (one-third) of the casing wall. Figure 2 shows a schematic of this type of tool, along with a typical log response.
This example is a Dialog Company multi finger caliper recorded with a pen recorder, and hence the deflections are arced in character. The multi finger calipers offer good detail of the inside of the casing and are accurate for measurement of percent wall penetration.

Multifinger Type Caliper - figure 2

Electromagnetic Casing Inspection
The electromagnetic tools fall into two categories: those that saturate the casing with magnetic flux lines and measure the distortion of those lines by a defect, and those that measure the amount of metal remaining by measuring the phase shift between two coils. Both types of tool inspect both the inside and the outside of the pipe. The tools that measure the distortion of flux lines by defects in the pipe wall are padtype devices. The most modern of these devices record each pad directly. The service companies and their trade names for this service are as follows:
· Schlumberger Wireline and Testing Pipe Analysis Log (PAL or PAT)
· Western Atlas Vertilog
· Halliburton Energy Services Pipe Inspection Tool (PIT)

These devices are hereafter referred to as pad-type casing inspection tools. The schematic of Figure 3 shows the pad tool.
Pad-type Casing Inspection Tool - figure 3
Inside the tool is a coil, which generates a magnetic field whose flux lines are parallel to the casing axis. Inside each pad is a coil which generates a current as it passes over a point where the flux lines are distorted into the wellbore. This occurs at a pit or hole in the pipe, even if the pit is located on the outside of the pipe. Surface roughness has the same effect, appearing as a lot of pits. The pads are also equipped to highlight defects appearing on the inner surface. With this information, defects on the inner or outer surface of the pipe can be detected.
The test, which responds to defects on either the inner or outer surface of the pipe (or within the metal), is sometimes called the flux leakage test. These tools have an upper and lower array of pads to ensure complete wall coverage. The flux leakage for the upper and lower pad arrays are labeled FL-1 and FL-2 on the log. The track labeled "discriminator" shows the measurement of the internal wall condition only. It is apparent that the interval from 4793 to 4870 ft (1401 to 1484 m) shows general external corrosion, since the inner wall is clear of defects except for the intervals noted. The track labeled "average" shows the average of all of the FL-l and FL-2 responses as seen by the pads. If the defect is large, it is detected by many or most of the pads and therefore shows a large average. A single-point defect shows a small average reading.
Casing Inspection : Corrosion Investigation Reviewed by Industri Migas on 4:55 PM Rating: 5

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