Drilling & Well Completion

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.


Test Your Knowledge

LOT Quiz

Instructions: Choose the best answer for each question.

1. What is the primary purpose of a Leak-Off Test (LOT)?

a) To determine the volume of drilling fluid needed for the well. b) To identify the pressure at which drilling fluid starts to leak into the formation. c) To measure the temperature of the formation. d) To analyze the composition of the formation fluids.

Answer

b) To identify the pressure at which drilling fluid starts to leak into the formation.

2. What is another term for the pressure determined during a LOT?

a) Wellbore pressure b) Formation Integrity Test (FIT) pressure c) Drilling mud weight d) Fracture gradient

Answer

b) Formation Integrity Test (FIT) pressure

3. Which of the following is NOT a benefit of conducting a LOT?

a) Determining the safe operating pressure for the wellbore. b) Assessing the formation pressure characteristics. c) Predicting the pressure needed for hydraulic fracturing. d) Determining the exact location of oil and gas reserves.

Answer

d) Determining the exact location of oil and gas reserves.

4. How is the leak-off point identified during a LOT?

a) By measuring the temperature of the drilling fluid. b) By monitoring the pressure readings and flow rates. c) By analyzing the composition of the drilling fluid. d) By observing the color of the drilling fluid.

Answer

b) By monitoring the pressure readings and flow rates.

5. What is the relationship between LOT and FIT?

a) LOT is a type of FIT. b) FIT is a type of LOT. c) LOT and FIT are completely separate tests. d) LOT and FIT measure the same thing, but in different units.

Answer

b) FIT is a type of LOT.

LOT Exercise

Instructions: You are the drilling engineer on a new oil well. The LOT results show a Formation Integrity Test (FIT) pressure of 10,000 psi. The current mud weight is 12 ppg (pounds per gallon).

Task:

  1. Determine if the current mud weight is sufficient to prevent fluid losses. Explain your reasoning.
  2. If the current mud weight is not sufficient, calculate the required mud weight to ensure wellbore stability.

Exercice Correction

1. The current mud weight is NOT sufficient to prevent fluid losses. The FIT pressure is 10,000 psi, which is higher than the hydrostatic pressure exerted by the current mud weight. This means that the pressure exerted by the formation is greater than the pressure exerted by the mud column, creating a potential for fluid losses into the formation. 2. To calculate the required mud weight, we need to convert the FIT pressure into mud weight equivalent. This can be done using the following formula: **Mud weight (ppg) = FIT pressure (psi) / (0.052 x Depth (ft))** Assuming the depth of the well is 10,000 ft, the required mud weight would be: **Mud weight (ppg) = 10,000 psi / (0.052 x 10,000 ft) = 19.23 ppg** Therefore, the mud weight needs to be increased to approximately 19.23 ppg to ensure wellbore stability and prevent fluid losses.


Books

  • "Petroleum Engineering: Drilling and Well Completion" by John Lee: This comprehensive textbook covers various aspects of drilling, including wellbore stability, formation pressure, and leak-off tests.
  • "Drilling Engineering: Principles and Practices" by Robert F. Mitchell: Another well-respected textbook that discusses LOT and its importance in drilling operations.
  • "Drilling and Well Completion: A Practical Approach" by Donald E. Woods: This practical guide provides insights into various drilling techniques and procedures, including LOT.

Articles

  • "Leak-Off Test (LOT): Understanding the Basics and Applications" by SPE: This article provides a detailed explanation of LOT, its purpose, and its significance in drilling operations. Available online on the SPE website.
  • "Formation Integrity Test (FIT) and Leak-Off Test (LOT): Understanding the Difference" by Schlumberger: This article clarifies the difference between FIT and LOT and highlights their importance in wellbore stability analysis. Available on Schlumberger's website.
  • "Leak-Off Test Analysis for Shale Gas and Tight Oil Reservoirs" by Society of Petroleum Engineers: This article discusses the application of LOT in unconventional reservoirs and the challenges associated with it. Available on the SPE website.

Online Resources

  • SPE (Society of Petroleum Engineers): The SPE website offers numerous resources related to drilling engineering, including articles, presentations, and technical papers on LOT.
  • Schlumberger: Schlumberger's website provides detailed information about their drilling services and tools, including LOT-related equipment and procedures.
  • Halliburton: Halliburton's website offers various resources on drilling technology, including information about LOT and its importance in wellbore integrity assessment.

Search Tips

  • Use specific keywords like "leak-off test," "LOT," "formation integrity test," "FIT," "drilling engineering," "wellbore stability," and "pressure gradient."
  • Combine keywords with specific reservoir types, such as "shale gas," "tight oil," or "unconventional reservoirs."
  • Use quotation marks around specific phrases to find exact matches, e.g., "leak-off test procedure."
  • Filter your search results by file type (PDF, articles, etc.) or by website to narrow down your search.
  • Use advanced search operators like "site:spe.org" to restrict your search to a specific website.

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