Leak-off Test (LOT): Procedure, Formula and Example

The leak-off test (LOT) is the most reliable way to find the maximum mud weight allowed in the open-hole section of a well. The crew runs it in the first few feet of new hole drilled after a casing point, which is the likely weakest point of the open hole if there are no highly permeable formations deeper down. The test result, converted to an equivalent mud density, gives the maximum mud weight the section can withstand without losing circulation.

Leak-off tests are usually run on the first few wells in a new block. The test closes the hole at surface and applies pressure until mud just begins to inject into the formation.

Leak-off Test Procedure

After cementing the casing, run in hole with the bit and drillstring.
Pressure test the casing, then drill out the casing shoe and a further minimum of 10 ft of new formation.
Pull the bit up to the casing shoe.
With the bit at shoe depth, shut off the pumps, wait for flow to cease, then close the kelly cock and the blowout preventer (mainly the annular preventer).
Use the cementing unit to pump drilling mud slowly through the choke line into the annulus. While pumping, always monitor the pressure build-up and the volume pumped.
The pressure build-up should be more or less linear until mud begins to bleed into the formation. The pressure at which the curve departs from linearity is the leak-off pressure (PLOT).
As pumping continues, the curve flattens until pressure no longer increases. The pump is now injecting mud into the formation pores and fractures. The mud pressure at this point is the injection pressure.
At the injectivity point, shut off the pump and close the choke line.
Monitor the pressure. The shut-in pressure normally falls until it reaches an equilibrium point slightly above the leak-off pressure. This is the bleed-off point.
Hold the bleed-off pressure for several minutes to confirm that no breakdown has taken place. If it remains steady, open the choke valve to vent the rest of the pressure.

Leak-off test pressure versus volume plot — Typical leak-off test plot of pressure against volume pumped.

Bottom Hole Pressure at Leak-off

The leak-off pressure gives the bottom hole pressure at leak-off. From it, the maximum mud weight or ECD permitted can be calculated. Leak-off pressure is interpreted together with formation pressure to set safe drilling limits.

BHP (at leak-off) = 0.052 × ρm × D + PLOT

PLOT = leak-off pressure, psi
ρm = mud density, lb/gal (ppg)
D = true vertical depth of the casing shoe, ft

The maximum mud weight permitted is then:

ρmax = BHP / (0.052 × D)

Example: at a shoe depth of 5,000 ft with 10 ppg mud, the leak-off pressure is 800 psi. BHP = 0.052 × 10 × 5,000 + 800 = 3,400 psi. The maximum mud weight is 3,400 / (0.052 × 5,000) ≈ 13.1 ppg. In practice, a safety margin is kept below this value.

LOT vs. Formation Integrity Test (FIT)

A formation integrity test pumps only up to a pre-set pressure to prove the formation can hold a required mud weight, without reaching leak-off. A leak-off test goes further and finds the actual limit.

Frequently Asked Questions

Why is the test run just below the casing shoe?

That is usually the weakest point of the open hole, so it limits the mud weight for the whole section.

What is the difference between leak-off pressure and injection pressure?

Leak-off pressure is where the curve first leaves a straight line. Injection pressure is where the pressure stops rising because mud is entering the formation.

Key Takeaways

The LOT sets the maximum mud weight for the open-hole section.
PLOT is the point where the pressure curve leaves the straight line.
ρmax = (0.052 × ρm × D + PLOT) / (0.052 × D).