Monitoring - Extended Leakoff Testing (XLOT)

Extended Leakoff Testing (XLOT)

Contacts

Ahmed Abou-Sayed ahmed@advantekinternational.com Advantek International

Summary

Extended leakoff testing ideally differs from conventional leakoff testing in the following ways:

At least two and preferably three injection-shut-in-leakoff cycles are performed.
It is desirable to use electronic pressure gauges to measure surface and ideally bottomhole pressure. Five second data collection (or more frequent) has been advocated.
Shut-in is monitored for a period of time longer than 10 minutes. Some people say 30 minutes.
If bottomhole pressure is inferred, fluid densities are measured accurately. Follow API RP 13B-2 for oil-based fluids.
Recorded data are processed using fundamental reservoir engineering concepts to determine when the created fracture has closed and to consequently infer the minimum stress.

Key Issues

Key Issue 1: Procedures

Procedures 1.1:

Generic procedures can be as follows:

Drill out cement and 10 feet of new formation.
RU surface transducer to choke manifold for annular measurements.
Circulate at least bottoms up. Check densities
Pull drill bit ten feet into casing and hang off drill string.
If a wireline downhole gauge is used, rig up a pump-in sub, wireline BOP, and pressure lubricator with the downhole gage assembly. Connect surface transducer to pump-in sub for drillpipe pressure measurement.
RU cement pumper.
RIH WITH downhole gauge to top of bit or baffle plate on wireline. Pull up 25 feet. Hang off.
Shut-in annular BOP.
Pump at 0.25 bpm (0.04 m³/minute) or lower - keep the rate constant. Pump until the pressure rise shows a definite change in the rate of increase. Pump an additional 0.25 bbl.
Shut-in and monitor the pressure for 30 minutes.
Bleed off pressure. Record the volume of fluid returned.
Repeat steps 9 through 11 for cycles 2 and 3.

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Key Issue 2: Results

Drawbacks 2.1:

You need to be aware of the limitations:

Some additional expense for equipment, particularly if bottomhole pressure is measured.
More time is required than for a conventional leakoff test. You must assess the value of the in-situ stress information. Don't hesitate to ask a specialist in your company. There are intangible benefits that you may not be aware of.
Interpretation can be complicated and it might not be possible to have a reliable interpretation on the spot.
If surface pressure alone is measured, accurate drilling fluid density is required.

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Results 2.2:

What can I learn?

Inference of in-situ stress conditions in the zone that was fractured (usually shale).
Inference of fluid loss in the zone that was fractured (usually shale).
Specialists might be able to infer something about Young's modulus or fracture toughness but this is always difficult.

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Interpretation 2.3:

What interpretation is done?

Quantitative stress measurement is quite feasible with extended leakoff testing. However, it is commonly done incorrectly or important information is missed. Both fracture mechanics and reservoir engineering concepts are used.
Three references are:
- McLennan, J.D. and Roegiers, J-C.: “How Instantaneous are Instantaneous Shut-In Pressures,”SPE 11064, SPE 57th Annual Fall Technical Conference and Exhibition, New Orleans, LA (Sept. 26-29, 1982).
- Kunze, K.R., and Steiger, R.P.: "Extended Leakoff Tests to Measure In Situ Stress During Drilling," in Rock Mechanics as a Multidisciplinary Science, Roegiers, J-C. (ed) Balkema, Rotterdam, ISBN 90 6191 194X (1991).
- Guo, F., Morgenstern, N.R., and Scott, J.D.: “Interpretation of Hydraulic Fracturing Pressure: A Comparison of Eight Methods Used to Identify Shut-in Pressure,” Int. J. Rock Mech. Min. Sci. & Geomech. Abstr. 30, No 6, (1993) 627-631.
Refer also to microfracturing.

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