Guidelines for Extended Leak-off Tests
Extracted from a technical review document.
Extended Leak-off Test (XLOT), is an enhancement of the standard Leak-off Test (LOT) practices. An XLOT aims at obtaining the minimum in situ stress (or closure stress), in addition to the customary leak-off pressure used in casing design. The closure stress is increasingly important for the well control of deviated wellbores, because the leak-off pressure approaches the closure stress (from above) as well inclination increases. XLOT's have the following significant advantages:
• Extends the data collection of the standard LOT and increases its reliability.
• Determines an estimate of the formation closure stress, the minimum stress which must be exceeded to propagate a fracture in the formation. This formation parameter is independent of the drilling fluid rheology and is a lower bound of the leak-off pressure. Knowledge of the closure stress is required for reliable wellbore stability analysis and for effective well control and formation stimulation operations.
• Requires little additional time while drilling, while acquired data are useful during the entire well and field life.
• May be used to quantify better the risk associated with drilling and wellbore stability.
• Can be applied optionally, particularly in exploration, appraisals new field development, when in situ stress data is not well restablished with minimal deviation from standard drilling practices.
Drillers can perform a Leak-off Test after the casing shoe is
cemented to test the integrity of the cement job and the capacity of the
formation to support the mud weight for controlling the well during subsequent
One important requirement is to drill at least 10 to 15ft (3-5m) of new formation before conducting the LOT, to ensure that the formation is subjected to pressure, without the protection of the casing.
Historically, industry practice has been to conduct a leak-off test by closing the well at the surface (with a blowout preventer) and pumping into the closed well at a constant rate until the test pressure is reached or until the well begins to take whole mud, causing a pressure departure from an increasing linear pressure trend. The pump is then stopped for at least 5-10mins to determine the rate of pressure decline.
This standard LOT procedure determines the leak-off pressure, which by design is not to be exceeded during subsequent drilling to ensure shoe integrity. Note that the leak-off pressure depends on mud properties, formation properties and the in situ stresses.
Extending the leak-off test (i.e. through pumping slightly larger fluid volumes, recording data longer, and more frequently, and if/as necessary, repeat the test) the closure stress of the formation being tested can be determined, reliably. The closure stress is the least pressure that must be exceeded to propagate a fracture.
It is desirable to undertake a XLOT programme as early as possible in the life of a field to determine the closure stress variation though the zones of interest (closure stress profile).
The closure stress profile is necessary for the evaluation of the risks related to well control and wellbore stability, especially for deviated wellbores, and will be useful for all subsequent drilling and fracturing operations.
Extended Leak-off Tests can offer significant advantages over the standard LOT practice as successfully applied by several operators to gather in situ stress information in new fields. Adapting and adopting guidelines offered could result in LOT practices thast can provide more data of better reliability, quality and value. For each applicaiotn the physics of the leak-off test the rationale of the proposed XLOT practice needs to be fully understood.
Examples shouled be reviewed and assessed to demonstrate the important points of test interpretation and its limitations.
Propagating a small fracture (by increasing slightly the injected volume during an XLOT)
provides better, more reliable data
(closure stress of the formation at the casing shoe) for drilling and production operations without compromising shoe integrity. Fracture re-opening pressure is smaller than the original
fracture initiation pressure by a
small amount (equal to the tensile strength of
the formation). Therefore, the XLOT is associated with a negligible reduction of casing shoe
integrity, because the mud filter plugs the fracture as it closes. The common field practice to stop the test
once the leak-off pressure is
reached should be carefully re-evaluated on
a per well basis given that very important information is being sacrificed
that could significantly impact the
well and the development of the field.
Preliminary guidelines for extended leak-off tests which provide (in addition to the leak-off pressure of the standard LOT) estimates of in-situ closure stress which can impacts reservoir evaluation, completion operations and wellbore stability.
It is recommended to:
• Review the XLOT procedure with field personnel, and put the associated risks into perspective using the physics of the test.
• Agree on an XLOT procedure for application to new fields and to older fields where in-situ stress information does not exist.
• Standardize the interpretation of the LOT and recording of the data on the daily drilling reports.
• Collect closure stress data (by
conducting a systematic XLOT
programme) as early as possible in the
life of a field, necessary for well control,
stability and production operations.
To plan for a potential later injector well we require an XLOT at the 9 5/8" shoe above the reservoir.
why do you require a xlot? I don’t get any of this rhyme or reasoning?
As the cap rock in unlikely to be the weakest formation exposed in this section?It’s the weakest formation that exists would be my main concern. In this case it is unlikely the shoe?
secondly further questions asked should be addressed by geomechanical geotechnical bods to give this test a green light or not for all the right reasons. Eg this is value adding data.
xlots can be value adding in certain formations that attribute desired characteristics to conduct such a test. In Some formations not.
Such decisions of well integrity testing, method, reason, purpose, objectives, value etc needs to be taken via a multidisciplinary meeting to assure all issues are addressed.