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XLOT Formation Integrity Post Test
25 February 2020
We are planning an exploration well in the Norwegian sea.

To plan for a potential later injector well we require an XLOT at the 9 5/8" shoe above the reservoir.

Concern has been raised over formation integrity of the cap rock formation after performing this XLOT.
- Is the integrity of the rock weakened after performing an XLOT to the point that it could lead to increased risk drilling to TD?
- Is there a risk that the fracture propagates to the reservoir sands and later causes increased losses to TD?
- What is the experience of people performing an XLOT after drilling out the shoe and then drilling through reservoir to TD?

It has been suggested we would be better performing the XLOT during the P&A phase after setting reservoir P&A plugs and dressing off (obviously would take more time). Any experience with this?

Thanks,
Mikaela
6 answer(s)
mikaela.clare
Senior Drilling Engineer
Spirit Energy
Total Posts: 2
Join Date: 25/02/20
Thanks for the detailed response Peter.

We are planning for an XLOT above the reservoir to aid in planning for a later injector well and the placement of that well. I am not a geomechanical expert but I am told this is why it is required and that on the closest offset field they performed an XLOT in this formation and later got good use of the closure pressure data for that reason.

It has been recommended to perform the XLOT as close to the top reservoir as possible but not within 30m to prevent the risk of the test going anywhere near the reservoir sands.

We have a detailed test procedure that was successful for another operator on the offset field and so we are using that and adapting it for our planned rig.

We are consulting with geomechanical experts on the pocedures and methods. I have been told that as long as ECD in the following section is kept below the minimum horizontal stress then drilling ahead should not present additional issues caused by the XLOT. 


ianpetitt
(retired) Well Fluids Team leader
SPREAD Associates
Total Posts: 48
Join Date: 14/06/06
Mikaela,

Please first check to see whether your company actually has a Rock Mechanics or Wellbore Stability Manual - this may be confidential document.

The XLOT has been around for many years. Most rock mechanics or Wellbore Stability focal points would like an XLOT test after a dry hole well has been drilled and logged just prior to abandonment. 
The difficulty has all was been A) having the test in the drilling programme initially and
B) persuading the drilling staff to spend more time with the abandonment as you may increase the risk of loss circulation during the post XLOT well abandonment. 

I have once come across a XLOT performed prior to drilling a section when the rig site drilling supervisor made a mess of a conventional LOT which then developed into a XLOT with the final pressure being well below the fracture closure pressure.  
 
The section has to be abandoned because we could not drill the section with the now lower exposed leak off pressure (the well was drilled in the late 90s before MPD or wellbore stability mud additive increases were around).  

Regards Ian


RuariTruter
Norwell Engineering
Total Posts: 24
Join Date: 17/09/07
Mikaela

Following on from Pete's summary, I'd also look at the SPE papers on the background of XLOT, it can get quite contentious.  Good place to start is "Improving Formation-Strength Tests and Their Interpretation" by Eric Van Oort & Richard Vargo (SPE 105193-PA)

AKA All you ever wanted to know about formation tests but didn't have nine hours to talk about it.

Personally I agree with Pete, a test at the shoe is unlikely to be indicative of later performance!!

Cheers

Ruari



ADunsmuir
Mud Engineer
MI-Swaco
Total Posts: 4
Join Date: 10/11/16
A big thanks to Peter for posting details on the term XLOT. It was not a term with which I was familiar. There is a vast wealth of knowledge within forum members and it is great that this can be shared around. Another "Gold Star" to Dave and his team for continuing to manage and support this.
Companyrep
Drilling Specialist/Well Engineer/Training Consultant
Kingdom Drilling
Total Posts: 441
Join Date: 10/01/05

Guidelines for Extended Leak-off Tests

Extracted from a technical review document.

Executive Summary

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.

Background

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 drilling operations.

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.

Purpose

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.

Conclusions in Brief

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.

Significance

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.

Recommendations

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.

Companyrep
Drilling Specialist/Well Engineer/Training Consultant
Kingdom Drilling
Total Posts: 441
Join Date: 10/01/05
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. 

 


Posted by

Mikaela Clare

Senior Drilling Engineer

Spirit Energy

Total Posts: 2
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