Leakoff Test (LOT)

What Is a Leakoff Test?

Leakoff test (abbreviated LOT) is a pressure integrity test performed immediately after drilling out a casing shoe cement plug, in which the formation directly below the shoe is pressured up with drilling mud to identify the pressure at which fluid first migrates into the rock or a fracture initiates. The leakoff pressure (LOP) defines the upper boundary of the mud weight window for the next hole section and provides the primary field calibration point for fracture gradient and minimum horizontal stress estimation.

Key Takeaways

A leakoff test pressures the formation 10 to 15 feet below the casing shoe at a slow pump rate, typically 0.25 to 0.5 bbl/min, while recording surface pressure against volume pumped.
The leakoff point is identified where the pressure-volume curve departs from linearity, indicating fluid entry into the formation or fracture initiation at the shoe.
Leakoff pressure converted to equivalent mud weight (EMW) sets the maximum safe mud density for the next hole section and feeds kick tolerance and kill-sheet calculations.
An extended leakoff test (XLOT) cycles past leakoff to measure fracture closure pressure, which is the best direct field estimate of minimum horizontal stress.
A formation integrity test (FIT) stops pumping at a predetermined target pressure without inducing leakoff, confirming the shoe can hold a specified mud weight without fracturing.

How a Leakoff Test Works

After drilling out the float collar and casing shoe cement, the driller advances 10 to 15 feet into fresh formation to expose uncemented rock. The drill string is pulled back above the shoe, the blowout preventer is closed, and the annulus is isolated. The rig pump then injects mud at a slow, controlled rate while a data recorder or chart recorder logs surface pressure every 0.5 to 1 barrel pumped. During the early phase, pressure rises linearly because the wellbore fluid and casing are compressing elastically; this is called the linear compressibility trend.

The leakoff point occurs where the pressure-volume plot deviates from the straight line, signaling that fluid is entering the formation through micro-fractures or a pre-existing natural fracture is beginning to open at the shoe. Pumping stops at or just after this deviation. The surface pressure at leakoff is the leakoff pressure (LOP). Fracture gradient at the shoe is then calculated: FG (ppg) = LOP (psi) / (0.052 x TVD ft). This value defines the maximum equivalent circulating density the shoe can sustain without lost circulation when drilling deeper.

Wellsite engineers compare the LOT-derived fracture gradient against the pore pressure gradient predicted for the next section to confirm an adequate mud weight window. If the window is less than roughly 0.5 ppg, the well design may require an additional casing string, a liner, or a managed pressure drilling system to reach the planned total depth safely.

Fast Facts: Leakoff Test

Test interval: 10 to 15 ft of new formation drilled below casing shoe
Pump rate: 0.25 to 0.5 bbl/min to minimize friction effects on pressure
Key output: Leakoff pressure (LOP) in psi at surface
Derived value: Fracture gradient in ppg equivalent mud weight
LOT vs. FIT: LOT finds actual leakoff; FIT confirms shoe to a preset limit only
XLOT outputs: Fracture reopening pressure, closure pressure, minimum horizontal stress
Typical deepwater fracture gradient: 13 to 16 ppg EMW near mudline, increasing with depth
Regulatory requirement: BSEE, NSTA, AER, and most national regulators require documented LOT before drilling ahead

Field Tip:

After stopping the pump at leakoff, bleed off pressure slowly and monitor the bleed-off rate carefully. If wellbore pressure does not fall steadily to zero or if returns appear at the surface, the shoe may be communicating with a permeable zone. Document bleed-off volume and time in the daily drilling report; a pressure that stabilizes at a value above zero indicates the formation is still taking fluid and that the true LOP may have been called early.

LOT, XLOT, and Formation Integrity Test Compared

The standard LOT stops at the departure from linearity, inducing only a micro-fracture or the earliest crack opening. An extended leakoff test (XLOT) continues pumping past fracture initiation through a clear propagation phase, then shuts in and records pressure decline as the fracture closes. Multiple pressurization cycles are often run. The closure pressure, measured at the inflection point of the decline curve, is the best available field estimate of minimum horizontal stress (Shmin). In geomechanical terms, Shmin from XLOT is the anchor point for wellbore stability models and hydraulic fracture design across an entire field development. In tectonically relaxed basins, Shmin typically runs 60 to 75 percent of overburden stress at the same depth.

A formation integrity test is more conservative: engineers calculate the minimum acceptable shoe strength based on the planned mud weight for the next section, pump only to that equivalent pressure, hold briefly, and stop. No fracture is induced and no leakoff point is identified. FITs are preferred in deepwater wells where shallow fracture gradients are very narrow near mudline, because deliberately fracturing a weak shallow formation reduces the kick tolerance for all subsequent sections. Where regulations permit FITs in place of full LOTs, operators typically run them at the shallowest casing seats and switch to full LOTs at deeper seats where formation integrity is better established.

LOT: universal abbreviation on well reports, morning reports, and regulatory filings
shoe test: informal field term reflecting that the test validates the casing shoe and adjacent cement bond
XLOT (extended leakoff test): enhanced version cycled past fracture initiation to measure closure pressure and minimum horizontal stress
FIT (formation integrity test): conservative variant that confirms shoe strength to a preset limit without intentionally fracturing the formation

Frequently Asked Questions About Leakoff Test

Why is the test conducted immediately after drilling out the shoe rather than deeper in the section?

The casing shoe is the shallowest exposed point in the open-hole interval and therefore the weakest link for pressure containment. Testing at this moment establishes the actual pressure limit before the operator drills deeper, increases mud weight, or encounters a kick. Waiting until deeper in the section means the well has already been drilled on unverified pressure assumptions, which defeats the purpose of the test.

How does leakoff pressure affect kick tolerance?

Kick tolerance is the maximum gas influx volume that can be circulated out without fracturing the shoe during well control operations. It is calculated directly from the LOP: higher leakoff pressure means the shoe can sustain greater annular pressure during a kill, allowing larger influxes to be safely managed. Operators update kick tolerance calculations after every LOT and use the result to set the maximum permissible influx volume before a well control response is required.

Can the leakoff test be run in a deviated or horizontal well?

Yes. Procedure is identical, but engineers use true vertical depth of the casing shoe rather than measured depth when converting LOP to a fracture gradient in ppg. In highly deviated wells, stress anisotropy can cause fractures to initiate at lower pressures than in a vertical well at the same TVD. XLOT data from nearby vertical offset wells is particularly valuable for calibrating geomechanical models before drilling a deviated section in a new area.

Why Leakoff Test Matters in Oil and Gas

The leakoff test is one of the most operationally consequential measurements taken during drilling because its result governs mud weight limits, casing design, kick tolerance, and wellbore stability planning for the entire section below each shoe. An inaccurate or absent LOT can lead to lost circulation from mud densities exceeding the true fracture gradient, or to kicks and blowouts from mud held too light. On multi-string wells, each casing seat has its own LOT, and the cumulative pressure-depth profile of all LOT results determines whether the planned total depth is reachable with the current casing program. Regionally, LOT data archives are used to calibrate pore pressure and fracture gradient prediction models, reducing geological uncertainty and improving well design across entire plays.