Monitoring - Leakoff Testing (LOT)

Leakoff Testing (LOT)

Contacts

Ahmed Abou-Sayedn ahmed@advantekinternational.com Advantek International

Abstract

Conventional drilling leakoff testing is performed below the shoe. Commonly, this is in shale. It is primarily intended to evaluate casing integrity. Gross information can be determined about in-situ stress conditions. The best practice is to only use it for what it is intended (integrity). More refined measurements of in-situ stress are possible with Extended Leakoff Testing or microfracturing. These alternative tests do have drawbacks, but they will provide better stress estimates.

Key Issues

Procedures Drawbacks

Key Issue 1: Procedures

Procedures 1.1:

Generic procedures can be as follows:

After the cement sets, drill ahead for 10 to 20 feet of new interval.
Pump drilling fluid at between 0.25 and 1 BPM (0.04 to 0.16 m3/minute.
Continue pumping until the rate of pressure increase slows, possibly indicating formation breakdown.
After breakdown, shut-in and monitor the pressurefor ten minutes or so. Some people think that this may correlate to a fracture with a dimension of about 25 feet in diameter.

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Key Issue 2: Drawbacks

Drawbacks 2.1:

You need to be aware of the limitations:

Most of these tests incorporate only one cycle of injection and shut-in. This may be inadequate for stress measurement.
Commonly, pressures are only recorded every one-half barrel or every minute. This is commonly insufficient for stress evaluation.
Ten minute monitoring of pressure decline after shut-in is sometimes insufficient to reliably infer stress conditions.
If surface pressure alone is measured, accurate drilling fluid density is required.

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Drawbacks 2.2:

Can I learn anything about the in-situ stresses?

If the quality of the test is acceptable, inferences can be made of the maximum and minimum principal stresses.
Remember that these are only the stresses in the drilled out zone below the shoe.

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Alternatives 2.3:

What else could I have done?

Most other methods will involve additional expense. However, you need to rate the importance of stress information in your risk analyses and make the appropriate decision.
Infer stresses from acoustic logs - preferably where there is a reliable shear wave (can be difficult) and where there are other measurements that facilitate calibrating the logs.
Perform an Extended Leakoff Test.
Carry out microfracturing. In many instances, this will be the premium measurement, but it will be expensive and can be diffiuclt in soft formations. It is possible to do these measurements in cased hole if the situation arises.

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