What is the significance of the leak-off test in primary well control?

The leak-off test (LOT) is a critical procedure in primary well control because it determines the fracture gradient of the formation at the shoe of the casing, which is the point where the casing is cemented to the wellbore. The fracture gradient is the pressure at which the formation will fracture, allowing drilling fluid to flow into the formation. Knowing the fracture gradient is essential for determining the maximum mud weight that can be safely used without fracturing the formation. The leak-off test is conducted after drilling out the cement plug inside the casing and drilling a short distance into the new formation below the casing shoe. The well is then shut in, and pressure is gradually increased in the wellbore by pumping fluid into the well. The pressure is monitored at the surface, and a plot of pressure versus pumped volume is created. Initially, the pressure will increase linearly with pumped volume. However, as the pressure approaches the fracture gradient, the pressure-volume plot will begin to deviate from linearity, indicating that the formation is starting to leak off or take fluid. The leak-off pressure is the pressure at which this deviation occurs. The leak-off pressure is then converted to a leak-off test (LOT) equivalent mud weight (EMW), which represents the maximum mud weight that can be safely used without fracturing the formation. The LOT EMW is used to design the mud program for the next section of the well. The LOT is a fundamental test for ensuring wellbore integrity and preventing underground blowouts. An accurate LOT provides critical information for maintaining primary well control by providing a safe operating window for mud weight selection. Therefore, the leak-off test is a key component of primary well control, providing essential data for safe drilling operations.