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Formation integrity test
A Formation Integrity Test (FIT) is a test of the strength and integrity of a new formation and it is the first step after drilling a casing shoe track. An accurate evaluation of a casing cement job and of the formation is extremely important during the drilling of a well and for subsequent work.  The Information resulting from Formation Integrity Tests (FIT) is used throughout the life of the well and also for nearby wells. Casing depths, well control options, formation fracture pressures, and limiting fluid weights may be based on this information.
The main reasons for performing a formation integrity test are to:
- Investigate the strength of the cement bond around the casing shoe and to ensure that no communication is established with higher formations.
- Determine the fracture gradient around the casing shoe and therefore establish the upper limit of the primary well control for the open hole section below the current casing.
- Investigate well bore capability to withstand pressure below the casing shoe in order to validate or invalidate the well engineering plan regarding the casing shoe setting depth.
FITs also serve several other purposes, including:
- Knowing optimum mud weight and equivalent mud weights for drilling the next section
- Minimizing risk of loss circulation in drilling trouble zones
- Deepening the next casing set point, perhaps eliminating a casing string
- Determining whether planned casing running speeds will destabilize the wellbore
- Acquiring actual (e.g., not predicted) data inputs for cementing simulating models
- Reducing the risk of inducing a fracture during cementing operations.
For a holistic view of the wellbore conducting FITs more often than is considered the industry norm can be helpful. Offset well information, geomechanics data, drilling fluid hydraulics, borehole imaging, and formation evaluation data lead to a competent wellbore, maintain stability, manage pore pressure, and optimize drilling, casing-running, and cementing operations.
FIT methods are included in plans for a drilling program to reduce formation-related uncertainties. However, with complex wells the integrity of the wellbore in respect to its ability to withstand the planned fluids, casing, and cementing programs changes with each new formation. Regular underreaming, wellbore strengthening operations, and use of loss-circulation materials (LCMs) contribute to unknown factors. Some cementing programs assume that the weakest component is the last casing shoe; however, this may not be the case. Fracture pressure predictions typically assume shale and tend to be misleading when drilling in sands, and there are inevitable margins of errors involved with such predictions.
FITs vs LOTs
Two methods of testing wellbores are used within the industry: FIT and leak-off test (LOT). The industry often confuses LOTs and FITs. But understanding the difference is important in appreciating the benefits of frequent dynamic FITs when drilling in trouble zones. A FIT is comparable to testing a pressure vessel to its rated operating pressure, which includes a safety factor and in which no damage to future pressure containment capability is expected. Conventional FITs require use of the rig’s mud pumps, closing the BOP and incurring non-productive time (NPT). The operator determines the FIT “test-to” value. If no leak-off occurs, the test helps assure that the mud in the hole at the time is suitable for the anticipated fracture gradient. The test also helps when optimizing casing-running speeds and planning a cementing program that will not induce a fracture. FITs are considered good drilling practice because of the information obtained from them. However, resistance to conducting FITs is typical because of the NPT required. In contrast, a LOT is comparable to testing a pressure vessel to leak, rupture, or become permanently deformed. The pressure is raised until the last casing shoe or formation is fractured as indicated by leak-off. In either case, future pressure containment capability may decrease, requiring less pressure to open the fractures back up again. LOTs deserve careful weighing of risk versus benefit in respect to making the fracture pressure gradient more of a relative unknown than it may already be. However, LOTs provide valuable information to determine the maximum wellhead pressure that could be sustained in case a kick occurs and has to be circulated out. Confusion between the two tests lies in the casual use of the terms and could be further compounded if leak-off occurs in advance of reaching the test-to pressure during a FIT. If the integrity test reveals the leak-off value, the FIT inadvertently becomes a LOT and likely recorded as such on the daily rig report.
- Guide to Blowout Prevention. 2002. Houston: Well Control School.
- Hannegan, D. and Arnone, M. 2012. Dynamic FITs verify changing integrity of complex wellbores. Drilling Contractor. http://www.drillingcontractor.org/dynamic-fits-verify-changing-integrity-of-complex-wellbores-19314 (accessed 1 December 2014).
Noteworthy papers in OnePetro
Akers, J. and Sellers, J. 2005. Use of Pressure-While-Drilling Tools to Improve Formation Integrity Test Interpretation. Presented at the SPE/IADC Drilling Conference, Amsterdam, 23-25 February. SPE-91852-MS. http://dx.doi.org/10.2118/91852-MS.
Burgess, K.A., MacDougall, T.D., Siegfried, R.W., et al. 2001. Wireline-Conveyed Through-Casing Formation Tester Preserves Casing Integrity . Presented at the SPE Eastern Regional Meeting, Canton, Ohio, 17-19 October. SPE-72371-MS. http://dx.doi.org/10.2118/72371-MS.
Goud, M.C., Joseph, G. 2006. Drilling Fluid Additives and Engineering to Improve Formation Integrity. Presented at the SPE/IADC Indian Drilling Technology Conference and Exhibition, Mumbai, India, 16-18 October. SPE-104002-MS. http://dx.doi.org/10.2118/104002-MS.
Hannegan, D.M. 2013. Closed-Loop Drilling, Cementing, and Frequent Dynamic Formation Integrity Testing. Presented at the Offshore Technology ConferenceSource Offshore Technology Conference, Houston, 6-9 May. IDOTC-24097-MS. http://dx.doi.org/10.4043/24097-MS.
Karimi-Jafari, M., Berest, P., Brouard, B., et al. 2008. Interpretation Of Mechanical Integrity Tests. Presented at the ISRM International Symposium - 5th Asian Rock Mechanics Symposium, Tehran, Iran, 24-26 November ISRM-ARMS5-2008-154.
Peuchen, J., Klein, M. 2011. Prediction of Formation Pore Pressures for Tophole Well Integrity. Presented at the Offshore Technology Conference, Houston, 2-5 May.OTC-21301-MS. http://dx.doi.org/10.4043/21301-MS.
Postler, D.P. 1997. Pressure Integrity Test Interpretation. Presented at the SPE/IADC Drilling Conference, Amsterdam, 4-6 March. SPE-37589-MS. http://dx.doi.org/10.2118/37589-MS.
Rezmer-Cooper, I.M., Rambow, F.H.K., Arasteh, M., et al. 2000. Real-Time Formation Integrity Tests Using Downhole Data. Presented at the IADC/SPE Drilling Conference, New Orleans 23-25 February,SPE-59123-MS. http://dx.doi.org/10.2118/59123-MS.
Williams, S.M., Ardila, M., Vladislav, V.A., et al. 2009. Formation Testing, Completion Integrity Evaluation and Geomechanical Applications Using a Wireline Cased-Hole Formation Tester. Presented at the Offshore Europe, Aberdeen, UK, 8-11 September. SPE-125106-MS. http://dx.doi.org/10.2118/125106-MS.
Wikipedia. 2014. Well control (10 September 2014 revision). http://en.wikipedia.org/wiki/Well_control#cite_note-5 (accessed 1 December 2014).
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