Monitoring Casing Condition

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Once a casing string has been successfully cemented in place and tested, subsequent operations must be planned to ensure that the casing remains fit for purpose. Condition monitoring systems should be implemented to highlight the potential problems so that timely action can be taken.

1 Drillpipe tool joint hardfacing

Tool joint hardfacing on drillpipe should be designed so as to minimise casing wear (see section on casing wear). Acceptable specifications, hardfacing techniques, and inspection procedures have been developped.

These requirements should be included in all drilling contracts, and the condition of the hardfacing should be checked once the rig comes under contract, and at regular intervals thereafter. Field re-hardfacing is a problem and can now be eliminated by the use of a new material.

This new hardfacing material - Armacor - has been shown in tests to reduce both casing wear and drillstring friction relative to current hardfacing materials. The material forms a very hard, thin, glass-like layer. When the surface is worn away a new layer is formed, i.e. it is self repairing. The manufacturer is Amtech, based in Houston.

2 Drillpipe casing protectors

Rubber casing protectors can be placed close to the drillpipe tool joints with the aim of preventing contact between the tool joint hardfacing and the casing wall. Alternatively, they can be placed in the middle of the pipe, thus increasing the number of contact points with the casing wall and hence lowering the contact forces that lead to wear.

Tests and field studies have lead to the understanding that drillpipe protectors will not prevent severe casing wear when placed next to the tool joint. It has been found that the protectors themselves wear quickly and deform enormously under loading.

Loss of protectors is common, due to the weak gripping mechanism and the "snowballing" of protectors can lead to well control problems due to annulus pack-off. They also create complications when closing pipe rams in a well control situation. If the protector location is not measured, or the protector has slipped, there is a risk of closing the pipe rams on a protector and deforming the pipe body. The duration of stripping operations will be extended by the presence of protectors and the task made more complex if their location is not accurately known.

Hence, in general, drill pipe protectors should not be used for casing wear prevention. Alternative approaches to reducing casing wear - such as the use of machined-smooth tool joint hardfacing - are preferable. Protectors may have an application, however, in reducing torque and drag in extended reach drilling. Specially the development of non-rotating protectors is mentioned here.

3 Monitoring and predicting wear while drilling

Computer programs can be used to predict casing wear at the well design stage and to monitor casing wear while the well is being drilled. It can model the wear distribution, wall thickness reduction and metal recovery at surface.

The program should be re-run once the casing has been installed and a survey has been performed to determine the actual wellpath. Magnets should be installed in the mud flowline upstream of the shale shakers to collect steel. They should be cleaned regularly to avoid the collected steel being washed off. Recovered steel should be weighed and compared with the computer predictions.

Removal of steel filings from the mud will have the additional benefits of improved survey quality and increased pump life.

Once casing wear has been identified as a problem, appropriate drilling techniques should be implemented to minimise wear (e.g. use of downhole motors). However, it is essential that this is considered in the well design phase.

4 Wear/corrosion logging

Reduction in casing wall thickness due to wear or corrosion can be established using wireline logging tools and calipers. They are used mainly as repair-decision tools. In areas where casing corrosion is a known or potential problem, such logs can be included in workover programmes as appropriate.

These logging tools fall into three categories:

4.1 Electromagnetic tools

Electromagnetic tools are further divided into three types:

·Cathodic protection profile tools: Such tools are used to predict rather than measure corrosion. This is achieved by measuring the casing potential drop between the tool sensors and from this, calculating the current flow in the casing wall. The presence of such current flow indicates that corrosion is taking place.

These tools can be used to determine a potential profile for casing in its native state and also when protection systems (such as cathodic protection [524]) are in place. For the latter case, they will give an indication of the effectiveness of the protection.

Examples of such tools are Schlumberger's Corrosion and Protection Evaluation Tool (CPET) and the Atlas Casing Potential Profile (CPP) instrument.

·Flux leakage tools: These tools use a combination of electromagnetic flux leakage and induced eddy current measurements to detect localised problems (such as pits and holes) on both internal and external surfaces. The size and depth of pits can be determined.

Examples of such tools are Schlumberger's Pipe Analysis Log (PAL) and Atlas's Vertilog.

The PAL tool has an outer diameter of 33/8 in (0.0857 m) and can be used in casing sizes up to 10.2 in (0.2591 m) internal diameter. It will detect defects greater than 0.3 in (7.6 mm) in diameter. The flux leakage response is proportional to the depth of the defect.

The Vertilog instrument comes in a number of sizes and can be run in casing ranging from 41/2 in (0.1143 m) to 133/8 in (0.3397 m) outer diameter. The depth of any defect is expressed as a percentage penetration.

·Electromagnetic thickness tools: These tools utilise an induced eddy current system to detect areas of general metal loss both internally and externally. Low vertical resolution means this method is best suited for detection of large scale wall loss such as generalised thinning, large holes, or vertical splits.

When combined with acoustic thickness measurement, this device can be used to detect metal loss from casing outside the one in which the tool is run.

Examples of such tools are Schlumberger's Multifrequency Electromagnetic Thickness Tool (METT) and Atlas's Magnelog.

The METT comes in two sizes. The 23/4 in (0.0699 m) OD tool is suitable for casing sizes up to 95/8 in (0.2445 m) OD, while the 41/2 in (0.1143 m) OD tool is suitable for casing up to 133/8 in (0.3397 m) OD. The tools can measure casing internal diameter with an accuracy of ±0.025%.

The Magnelog also comes in two sizes. The 31/2 in (0.0889 m) OD tool can be used in casing ranging from up to 75/8 in (0.1937 m) OD, while the 51/2 in (0.1397 m) tool can be used in casing sizes from up to 133/8 in (0.3307 m) OD. Accuracy of measurement of the casing diameter is again ±0.025%.

4.2 Ultrasonic tools

These are often based on tools originally designed for other purposes, e.g. cement bond evaluation.

Acoustic cement evaluation tools can be used to determine the location and extent of metal loss by analysis of the waveforms of reflected signals. They are most suited to the detection of general wall loss, large holes, internal scaling and casing deformation. An example of these tools is Schlumberger's Cement Evaluation Tool (CET).

Other ultrasonic acoustic tools measure transit time and amplitude of a reflected signal and provide information on the condition of the internal casing surface. No information is given on wall thickness (although see below). These tools are generally used to evaluate short sections of corroded or damaged casing that have been identified as such from earlier runs with other tools. An example of this type of tool is Schlumberger's Borehole Televiewer Tool (BTT), which provides a "visual" image of the casing internal surface. It can only be run, however, in solids-free fluids (e.g. brine).

Schlumberger Ultrasonic Imager (USI) combines the capabilities of the CET with those of the BTT. As a result wall thickness information is available as well as a "visual" image of the internal casing surface.

4.3 Mechanical tools

These are mechanical caliper tools which directly measure the internal diameter of the casing at a number of points around its circumference (depending on the number of caliper arms). No direct information is provided on wall thickness. The vertical resolution is dependent on the running speed.

Examples are Schlumberger's Multi-Finger Caliper (MFC) and Tubing Geometry Tool (TGS), and Atlas's Multi-Finger Caliper. The Kinley caliper is also available from a number of sources.

The Schlumberger MFC tool is available in three sizes which cover casing ranging rom 5 in (0.1270 m) to 133/8 in (0.3397 m) OD. Smallest vertical resolution is 0.2 in (5.1 mm), while maximum radial accuracy is ±0.01 in (0.3 mm).

The Atlas' MFC tool comes in five sizes and covers casing from 23/8 in to 103/4 in OD. Radial accuracy is ±0.02 in (0.5 mm) for the smallest tool and ±0.05 in (1.3 mm) for the other sizes.

4.4 Visual techniques

In a limited number of cases downhole video camera techniques can be used. These cameras can be run on coiled tubing or wireline. Applicability is restricted to cases where a clear fluid is present (e.g. gas or clear brine) and work best when detecting "large" damage or leaks.

The application of the tools mentioned above can be summarised as follows:

Corrosion prediction

  • ·CPET, CPP

Small localised pitting or small holes

  • ·PAL, Vertilog (internal and external surfaces).
  • ·BTT, USI (internal surface and qualitative only).

General metal loss, large holes, or splits

  • ·METT, Magnelog, USI, CET (internal and external surfaces).
  • ·MFC, TGS, Kinley (internal surface only).

Since each of the above tools is generally more suited to detecting one form of metal loss than another, some prior knowledge of the nature of the problem is necessary when selecting logging tools. It may be necessary to combine several tools or to make a number of runs with different tools to properly determine the condition of the casing.

Consideration should also be given to the capability for real-time display. Some tools (e.g. Kinley caliper) have no real-time output and cannot be readily processed on site. The importance of this capability will depend upon the operation. Real-time display does allow extra passes to be made over intervals that are identified as experiencing metal loss.

If casing wear or corrosion is anticipated from the outset, it is advisable to run a base log immediately after the casing is installed. This will allow manufacturing defects to be differentiated from in-situ wear or corrosion.

5 Pressure testing

Pressure testing of worn or corroded casing is often a cost-effective alternative to wireline logging. However a successful pressure test only indicates the minimum casing strength (and thus the minimum wall thickness) at the time of the test. No information is gained on the presence or rate of any corrosion which may reduce the casing strength with time.

The test pressure should be chosen so as to ensure that the casing is capable of withstanding any loads to which it may be subjected in subsequent operations, together with a margin to allow for any further wall thickness reduction during that time.

6 Casing patches

Internal casing patches generally must have an ID large enough to allow the passage of large tools or packers. As a result the relatively thin wall means that they have little collapse resistance. The patches thus collapse or leak when exposed to external pressure or drawdown and must be avoided where these conditions will or can exist.

An alternative solution to an internal patch is to cut and retrieve (if possible) the damaged casing and then run new casing with an external casing patch. Unless such a patch contains a tested metal-to-metal seal, it cannot be considered gas tight.

Tristate (amongst others) can provide selective back-off tools that enable damaged joints to be unscrewed and retrieved and then new joints run and screwed back in.

 

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