What Does Leak-Off Test (LOT) Mean?
A leak-off test (LOT) is used to determine the fracture pressure of an open formation. The test is conducted immediately after drilling below a new casing shoe and verifies whether the casing, cement and formation are able to withstand the pressure needed to drill the next section of the well. The LOT is also used to determine the maximum pressure that can be imposed on the formation without fracturing it as well as the maximum equivalent mud weight that should be applied to the well during drilling operations.
Trenchlesspedia Explains Leak-Off Test (LOT)
During the leak-off test (LOT), the well is shut and fluid is pumped into it, increasing the pressure gradually. The test is conducted to ensure the cement bond around the casing shoe is strong and that there is no flow path established to the formations to the previous annulus or above the casing shoe.
The LOT is conducted as described below:
- The new formation is drilled out a few feet and a sample is collected to confirm that the new formation is drilled. The string is then pulled into the casing.
- The annular preventer or pipe rams are closed and a cement pump is lined up. Drilling fluid is circulated through an open choke line to ensure that the surface line is filled with drilling fluid.
- Once done, the pump is stopped and the choke valve is closed. Using constant pump strokes, small amounts of drilling fluid are gradually pumped.
- Total pump strokes, drill pipe pressure and casing pressure are recorded.
The drill pipe pressure and casing pressure increase continually when pumping is in progress. A plotted graph between strokes pumped and pressure will show a straight-line relationship if formation is not broken. A deviation from the straight line will show that the formation is broken and drilling fluid has leaked into the formation. The pressure at which deviation from a straight line on the graph occurs is called leak-off test pressure. Once the test is complete, the pressure is allowed to bleed and the well is opened for the drilling operation.