Leak off Test (LOT)

Formation Strength Test

A LOT, or Leak-Off Test, is a procedure conducted during drilling operations to determine the strength of the formation at the casing shoe. Essentially, it measures the pressure at which the formation begins to fracture (or leak off), providing critical data for wellbore integrity and safe drilling practices.

In order to calculate an LOT, the casing shoe is drilled out and approximately 10-15ft of new formation drilled. The string is pulled back into the shoe and the well is circulated to a clean and uniform fluid. This is essential to ensure an accurate pressure reading is obtained downhole. BOPs are closed and surface pressure is gradually applied and recorded until it is clear that the pressure change is no longer linear thus indicating that the formation is begining to fracture and take fluid.

The LOT pressure is calculated by the addition of the surface pressure plus the hydrostatic of the mud column.

LOT Provides Insights On:

• Formation Integrity: LOT confirms the pressure capacity of the formation, ensuring that further drilling won't exceed its fracture gradient.

• Maximum Allowable Mud Weight: LOT data helps establish the upper limit for mud weight without risking formation breakdown.

• Kick Tolerance: The results inform well control measures and the ability to handle formation fluid influxes

In areas where formation strength is well know, a Formation Integrity Test (FIT) may be performed instead of a LOT as a safer way to obtain the necessary data.