LOT (drilling)

Understanding LOT (Leak-Off Test) in Drilling Operations

In the world of oil and gas exploration, LOT (Leak-Off Test) plays a crucial role in determining the integrity of the wellbore and the pressure characteristics of the surrounding formations. It's an essential drilling test conducted to identify the leak-off point, the pressure at which drilling fluid starts to leak into the formation.

What is a LOT?

A LOT is a controlled pressure test performed during the drilling process. The test involves increasing the pressure within the wellbore until the formation fluid starts to penetrate the formation, indicating a leak-off point. This pressure is known as the Formation Integrity Test (FIT) pressure, also referred to as Fracture Gradient.

Purpose of LOT:

• Wellbore integrity: LOT helps determine the safe operating pressure for the wellbore. By understanding the leak-off point, drillers can ensure that the wellbore is strong enough to withstand the pressure of the drilling fluid and prevent potential blowouts or fluid losses.
• Formation pressure characteristics: The test provides information about the pressure gradients of the surrounding formations, which is crucial for planning and executing future drilling and completion operations.
• Hydraulic fracturing: LOT data can be used to estimate the pressure needed for hydraulic fracturing, a technique used to increase the permeability of the formation and enhance production.

How is LOT conducted?

• A specially designed tool is deployed into the wellbore and connected to the drill string.
• Pressure is gradually increased within the wellbore while monitoring the pressure readings and flow rates.
• The leak-off point is identified when the pressure reaches a point where the formation starts to accept fluid.
• The pressure reading at this point represents the formation integrity test (FIT) pressure.

Comparing LOT with FIT:

LOT and FIT are closely related. The FIT is the actual test pressure value determined during the LOT. LOT refers to the procedure and test methodology itself, while FIT represents the final result, the pressure at which the formation leaks off.

Significance of LOT:

Understanding the leak-off point and formation integrity is critical for safe and efficient drilling operations. LOT data guides crucial decisions related to:

• Mud weight selection: Determining the appropriate density of drilling mud to prevent fluid losses and maintain wellbore stability.
• Fracturing design: Predicting the pressure required to fracture the formation and optimize hydraulic fracturing operations.
• Wellbore design and casing setting depths: Ensuring adequate wellbore integrity and minimizing the risk of formation damage.

Conclusion:

LOT is an essential drilling test that plays a vital role in understanding the characteristics of the surrounding formations and ensuring the integrity of the wellbore. By determining the leak-off point and FIT pressure, LOT empowers operators to make informed decisions for safe and successful drilling operations. This data is crucial for planning future activities like hydraulic fracturing, well completion, and production optimization.