LOT (Leak-Off Test)
A leak-off test (LOT) is a wellbore integrity test performed after drilling out a casing shoe cement plug and drilling a short distance into new formation, in which the well is pressure-tested by pumping fluid at a controlled rate while monitoring surface pressure versus volume pumped, with the objective of confirming casing shoe cement integrity and determining the maximum allowable mud weight (leak-off pressure) for the next hole section without initiating formation fracturing.
Key Takeaways
- The leak-off pressure (LOP) is identified as the point on the pressure-versus-volume plot where the curve deviates from linearity, indicating that fluid has begun entering the formation or micro-fractures at the shoe, and this pressure defines the maximum equivalent mud weight window for the next casing string.
- A formation integrity test (FIT) is a limited-pressure version of the LOT in which pumping stops at a pre-defined target pressure below the expected LOP, confirming only that the formation can hold the required mud weight without actually fracturing the shoe.
- The extended LOT (XLOT) continues pumping past the initial leak-off point to identify the formation breakdown pressure (FBP), fracture propagation pressure (FPP), instantaneous shut-in pressure (ISIP), and fracture closure pressure (FCP), providing a full characterization of the in-situ minimum horizontal stress for geomechanical modelling.
- AER Directive 008 and BSEE regulations under 30 CFR Part 250 both mandate leak-off or pressure integrity testing at each casing shoe before drilling ahead into a new hole section, with the test results submitted in the well completion or interim report.
- The fracture gradient derived from the LOT sets the upper bound of the mud weight window, while the pore pressure gradient sets the lower bound; the difference between the two defines the drilling margin available to prevent both kick and lost circulation simultaneously.
Fast Facts
A standard LOT is conducted with the drill bit 3 to 10 meters below the casing shoe in new formation, ensuring the test reflects competent rock rather than potentially weakened cement. Typical pump rates are 0.25 to 0.5 barrels per minute (bbl/min) to allow clear identification of the deviation from linearity. Test pressures at deepwater casing shoes may correspond to equivalent mud weights of only 10.0 to 10.5 ppg, providing very narrow drilling margins that limit deepwater well design. In tight formations, the LOT and pore pressure may be nearly identical, creating a near-zero drilling margin that requires managed pressure drilling (MPD) techniques.
Tip: When interpreting a LOT, always plot pressure against cumulative volume pumped rather than time. Plotting against time introduces pump rate variability that obscures the linearity break. The linear portion of the pressure-volume graph represents elastic compression of the wellbore fluid column and deformation of the borehole wall; the point where the curve bends away from this line is the true leak-off pressure, regardless of how many minutes of pumping it required to reach it.
What Is a Leak-Off Test
The leak-off test is one of the most operationally important pressure tests in drilling engineering. After setting and cementing a casing string, the driller must confirm two things before drilling the next hole section: first, that the cement bond is sound and the casing shoe is sealed, and second, that the formation just below the shoe is strong enough to withstand the maximum mud weight that will be needed in the next interval. The LOT answers both questions simultaneously by pressuring up the wellbore and observing whether pressure holds linearly (cement is intact, formation is elastic) or whether pressure deviates (leak-off has occurred at the shoe).
The test is performed with the casing shoe at the bottom of the cemented string. The well is closed in at surface, and a pump begins circulating fluid slowly into the closed wellbore. As fluid is injected, surface pressure rises proportionally to the volume pumped, following the elastic compressibility of the fluid column and borehole. When the pressure reaches the point at which the cement seal leaks, micro-fractures open at the shoe, or the formation begins to take fluid, the pressure-volume relationship deviates from linearity. This deviation point is the leak-off pressure (LOP), expressed in pounds per gallon (ppg) equivalent mud weight or kilopascals per meter.
How a Leak-Off Test Works
The standard LOT procedure involves closing the well-in with the drill string on bottom approximately 5 to 10 meters below the casing shoe. Surface valves are closed, and a low-volume, high-pressure pump (typically a cementing or testing pump, not the main rig mud pump) injects fluid at a controlled constant rate of 0.25 to 0.5 bbl/min. A chart recorder or data acquisition system logs surface pressure at 30-second intervals against cumulative pump strokes or volume injected. The operator watches for the straight-line region to bend toward a lower slope, marking this deviation as the LOP. Upon reaching LOP, pumping stops and the well is bled back to confirm the shoe is holding.
The distinction between a LOT and a formation integrity test (FIT) is operationally significant. An FIT stops pumping at a pre-calculated target pressure corresponding to the maximum mud weight expected in the next hole section plus a safety margin, typically 0.5 ppg above planned maximum mud weight. If the well holds that pressure with no deviation from linearity, the FIT is passed and drilling can proceed. The actual leak-off pressure is never determined. FITs are faster, less risky (no fracturing occurs), and sufficient when the operator is confident the formation is stronger than needed. LOTs are preferred when the actual fracture gradient needs to be quantified, such as when planning a high-mud-weight interval or when pore pressure uncertainty is high.
The extended LOT (XLOT) continues pumping past LOP to fracture the formation intentionally, recording the formation breakdown pressure (FBP), the fracture propagation pressure (FPP) as the fracture grows at lower pressure than it initiated, the instantaneous shut-in pressure (ISIP) immediately after pump shutdown, and the fracture closure pressure (FCP) as the fracture walls close. The ISIP and FCP closely approximate the minimum horizontal stress (Shmin), providing essential geomechanical data for wellbore stability modelling, casing design, and hydraulic fracture design in nearby wells. XLOTs are standard practice in appraisal wells and required by some regulators before sanctioning deepwater development plans.
LOT Across International Jurisdictions
In Canada, the Alberta Energy Regulator (AER) requires pressure integrity testing at every casing shoe under Directive 008 (Casing Seat Selection) and Directive 036 (Drilling Blowout Prevention Requirements). The test must be conducted before drilling ahead into a new formation section, and the results must be entered into the drilling report submitted after well completion. The AER specifies minimum test pressures based on planned maximum mud weight for the next interval, and failure to conduct the test or record the data is a compliance violation. In the WCSB, LOT-derived fracture gradients are integrated with regional pore pressure models to design casing programs for deep, high-pressure wells in the Deep Basin and Montney formations where the difference between pore pressure and fracture gradient can be as little as 0.2 to 0.5 ppg.
In the United States, BSEE regulations under 30 CFR Part 250 mandate pressure integrity testing at each casing shoe for all OCS wells, with LOT or FIT results to be recorded in the Sundry Notice or Well Activity Report submitted to the appropriate BSEE district office. Onshore wells on federal land are subject to BLM Order 2 requirements for casing and cementing, which include shoe integrity testing. State regulations in Texas (Texas Railroad Commission), North Dakota, and other producing states have equivalent requirements. In deepwater Gulf of Mexico operations, the narrow drilling margins imposed by low LOT values at shallow casing shoes are a primary driver of the large number of casing strings (sometimes 8 to 10) required in deepwater wells compared to 4 to 5 strings in typical onshore wells.
In Norway, Sodir and the Petroleum Safety Authority Norway (PSA) require LOT or FIT at each casing shoe under the petroleum facilities regulations. Norwegian North Sea and Norwegian Sea wells operate in challenging environments with narrow drilling margins in overpressured Tertiary sequences, making accurate LOT-derived fracture gradients essential for well design. Norwegian operators including Equinor use XLOT data from appraisal wells to build geomechanical models applied to all development wells in a given field area, reducing the need to conduct XLOTs on every production well while maintaining accurate knowledge of the minimum stress state throughout the reservoir overburden.
In the Middle East, Saudi Aramco's drilling standards require shoe integrity tests at every casing point, with LOT data incorporated into wellbore stability models used to plan mud weights across the thick Paleozoic and Mesozoic sequences in Arabian Peninsula wells. Onshore wells in Saudi Arabia often encounter formations with extremely high fracture gradients due to naturally high horizontal stresses in the foreland basin setting, giving wide drilling margins. However, HPHT wells in the Khuff carbonate gas reservoirs beneath the Arab-D oil zones require careful LOT-based fracture gradient determination to prevent lost circulation while maintaining sufficient overbalance to control HPHT pore pressures.
Synonyms and Related Terminology
The LOT is also referred to as a shoe test, casing shoe test, or formation strength test in some operational contexts. The formation integrity test (FIT) is a less aggressive variant. The extended LOT (XLOT) is also called a mini-frac test or stress test when its primary purpose is minimum stress determination rather than shoe integrity. Fracture gradient is the drilling engineering parameter directly derived from LOT results. Mud weight window is bounded above by the fracture gradient and below by the pore pressure gradient. Equivalent circulating density (ECD) must remain below the LOT-derived fracture gradient during drilling to prevent lost circulation. Instantaneous shut-in pressure (ISIP) and fracture closure pressure (FCP) are XLOT-specific measurements.
FAQ
What does it mean if a LOT result is lower than expected based on offset well data?
A lower-than-expected LOT result indicates either poor cement quality at the shoe (cement channels allowing early leak-off), a mechanically weak formation immediately below the shoe (unconsolidated sands, naturally fractured zone), or that the formation stress state in that specific well location is lower than the regional trend. Before concluding the formation is weak, the cement bond log for the shoe interval should be reviewed to check for channeling. If cement quality is confirmed acceptable, the low LOT result restricts the maximum mud weight in the next section and may require an additional casing string to bridge to a competent formation with a higher fracture gradient.
How is the LOT result converted to an equivalent mud weight?
The LOT pressure recorded at surface (in psi or kPa) is converted to an equivalent mud weight (EMW) by adding the hydrostatic pressure of the mud column in the wellbore at the shoe depth to the surface pressure, then dividing by the shoe depth to obtain the gradient. In field units: EMW (ppg) = [Surface LOT pressure (psi) + mud hydrostatic pressure at shoe (psi)] / (0.052 x shoe depth in feet). The resulting EMW defines the maximum equivalent mud weight density that can be applied in the next hole section without fracturing the shoe formation.
Why the Leak-Off Test Matters
The LOT is a safety-critical test that defines the upper boundary of the drilling mud weight window for the next hole section. Without an accurate LOT result, the drilling engineer cannot confirm that the casing shoe can contain the wellbore pressure required to safely overbalance the highest-pressure formations in the next interval. If the shoe fails during drilling due to an insufficient LOT or an untested shoe, the resulting lost circulation event can lead to a well control emergency: pressure communication from the lost circulation zone to a shallower, lower-pressure formation can create an underground blowout. The LOT is also the primary input to fracture gradient profiles used in well design software, casing design, and mud weight schedules that protect the wellbore integrity from surface to total depth throughout the life of the well.