Definition and Significance
Measured Depth (MD), often abbreviated as MD or simply “depth,” refers to the length of the drilled borehole in the oil industry. It represents the total length of the wellbore measured along the actual well path. In conventional vertical wells, MD coincides with the true vertical depth. However, in directional or horizontal wells, particularly those employing extended reach drilling techniques, the MD and true vertical depth can deviate significantly.
Key Facts
- Definition: Measured Depth (MD) is the length of the drilled borehole. It represents the total length of the wellbore measured along the actual well path.
- Deviation in Directional or Horizontal Wells: In conventional vertical wells, MD coincides with the true vertical depth. However, in directional or horizontal wells, especially those using extended reach drilling, the MD and true vertical depth can deviate greatly.
- Calculation: The MD is determined by measuring the lengths of individual joints of drillpipe, drill collars, and other drillstring elements with a steel tape measure. These lengths are then added together to obtain the total MD.
- Stretching of Drillstring: When the drillstring is screwed together and put into the wellbore, it stretches under its own weight and that of the bottomhole assembly. This stretching is not taken into account when reporting the well depth, so the actual wellbore is slightly deeper than the reported MD.
- Importance in Calculations: MD is used in various calculations in the drilling field, including annular volume, drill string and tubular volume, well control, hydrostatic pressure calculation, formation temperature estimation, and more.
Calculation and Considerations
The MD is determined by meticulously measuring the lengths of individual joints of drillpipe, drill collars, and other drillstring elements using a steel tape measure. These measurements are then diligently added together to obtain the total MD. It is crucial to note that when the drillstring is assembled and inserted into the wellbore, it undergoes stretching due to its own weight and the weight of the bottomhole assembly. This stretching phenomenon is not factored into the reported well depth, resulting in the actual wellbore being slightly deeper than the reported MD.
Applications and Importance
MD plays a pivotal role in various calculations performed during drilling operations. These calculations include:
- Annular volume: MD is utilized to determine the volume of the annular space between the drillstring and the wellbore.
- Drill string and tubular volume: MD is employed to calculate the volume of the drill string and tubular components within the wellbore.
- Well control: MD is crucial for well control operations, such as determining the volume of fluid required to fill the wellbore and calculating the pressure required to control the well.
- Hydrostatic pressure calculation: MD is used to calculate the hydrostatic pressure exerted by the drilling fluid within the wellbore.
- Formation temperature estimation: MD is utilized to estimate the formation temperature at a specific depth.
Conclusion
Measured Depth (MD) is a fundamental parameter in drilling operations. It is the length of the drilled borehole measured along the actual well path. MD is particularly significant in directional or horizontal wells, where it can deviate substantially from the true vertical depth. The calculation of MD involves measuring the lengths of individual drillstring components and adding them together. MD finds application in various drilling calculations, including annular volume, drill string and tubular volume, well control, hydrostatic pressure calculation, and formation temperature estimation.
References
- Measured depth – Wikipedia
- MD | Energy Glossary
- Difference between True Vertical Depth (TVD) and Measured Depth (MD)
FAQs
What is Measured Depth (MD) in drilling?
- Answer: Measured Depth (MD) is the length of the drilled borehole measured along the actual well path. It represents the total length of the wellbore.
How is MD calculated?
- Answer: MD is calculated by measuring the lengths of individual joints of drillpipe, drill collars, and other drillstring elements using a steel tape measure. These measurements are then added together to obtain the total MD.
Why is MD important in drilling?
- Answer: MD is crucial for various drilling calculations, including annular volume, drill string and tubular volume, well control, hydrostatic pressure calculation, and formation temperature estimation.
How does MD differ from True Vertical Depth (TVD) in directional or horizontal wells?
- Answer: In conventional vertical wells, MD and TVD coincide. However, in directional or horizontal wells, MD can deviate significantly from TVD due to the wellbore’s inclination and azimuth.
What is the significance of stretching in drillstring components?
- Answer: When the drillstring is assembled and inserted into the wellbore, it undergoes stretching due to its own weight and the weight of the bottomhole assembly. This stretching is not accounted for when reporting the well depth, resulting in the actual wellbore being slightly deeper than the reported MD.
How is MD used in well control operations?
- Answer: MD is used in well control operations to determine the volume of fluid required to fill the wellbore and calculate the pressure needed to control the well.
Can MD be used to estimate formation temperature?
- Answer: Yes, MD can be used to estimate the formation temperature at a specific depth. This estimation is based on the geothermal gradient, which is the rate at which temperature increases with depth.
What are some common applications of MD in drilling calculations?
- Answer: MD is commonly used in calculations related to annular volume, drill string and tubular volume, well control, hydrostatic pressure calculation, and formation temperature estimation.