What does a Leak-Off Test (LOT) primarily measure?

A Leak-Off Test (LOT) primarily measures the surface pressure at which drilling fluid (mud) begins to leak into the surrounding formation. This test is crucial in evaluating the integrity of the wellbore and the formation's ability to withstand the pressure exerted by the mud column without allowing fluid communication between the well and the formation. By determining this pressure, operators can better assess the mechanical properties of the rock and make informed decisions regarding drilling practices, such as weight and type of mud to use, and mitigating the risk of well control issues.

The emphasis on measuring leakage into the formation is vital because it helps identify potential problems such as formation fractures or inadequate cementing that could lead to well control incidents. Understanding the pressure at which mud begins to leak provides insights into the pressure limits of the formation being drilled, thus ensuring that operators maintain safe drilling practices.