Depth Calculations

In the oil and gas industry, depth in a well is the measurement, for any point in that well, of the distance between a reference point or elevation, and that point. It is the most common method of reference for locations in the well, and therefore, in oil industry speech, ā€œdepthā€ also refers to the location itself.

Because wells are not always drilled vertically, there may be two ā€œdepthsā€ for every given point in a wellbore: the measured depth (MD) measured along the path of the borehole, and the true vertical depth (TVD), the absolute vertical distance between the datum and the point in the wellbore. In perfectly vertical wells, the TVD equals the MD; otherwise, the TVD is less than the MD measured from the same datum. Common datums used are ground level (GL), drilling rig floor (DF), rotary table (RT), kelly bushing (KB) and mean sea level (MSL). [1]

Terms and Abbreviations:

The common references used in operations include:

  • Kelly Bushing Height (KB):Ā The height of the drilling floor above the ground level. Many wellbore depth measurements areĀ taken from the Kelly Bushing. The Kelly bushing elevation is calculated by adding the ground levelĀ to the Kelly bushing height.
  • Rotary Table (RT):Ā e.g. MDBRT or TVDBRT. MDBRT stands for Measured Depth Below Rotary Table (MDBRT),
  • Rig Floor (RF),
  • Drillerā€™s Depth below rotary table (DDbrt):Ā The depth of a well or features within the wellbore as measured while drilling. The measured length of eachĀ jointĀ ofĀ drillpipeĀ or tubing is added to provide a total depth or measurement to the point of interest. Drillers depth is the first depth measurement of a wellbore and is taken from theĀ rotary tableĀ level on theĀ rigĀ floor. In most cases, subsequent depth measurements, such as those made during the wellĀ completionĀ phase, are corrected to theĀ wellheadĀ datumĀ that is based on drillers depth (reference: Schlumberger Oilfield Glossary).
  • Ground Level (GL),
  • Casing Bowl Flange (CBF),
  • Tubing Hanger Flange (THF),
  • Mean Sea Level (MSL),
  • Subsea Level (SS),
  • Sea Bottom (SB),
  • Measured Depth (MD),
  • True Vertical Depth (TVD).

Depth Specifications:

Although depth calculation is an intuitive concept, it is the source of much confusion because it is frequently not specified correctly. Absolute depth should always be specified with three components:

  • a unitĀ (e.g. ft for feet, m for meter,ā€¦),
  • a pathĀ (e.g. TVD for True Vertical Depth, MD for measured depth)
  • andĀ the reference or datumĀ they refer to (e.g. SS for Subsea Level, RT for Rotary Tableā€¦),

Any combination of unit, path, and reference can be used, as long as they result in fully specified, unambiguous depths.

How to determine the true vertical depth?

True vertical depth is obtained from a record of theĀ deviation survey report. These surveys are generally run on deviated wellbores. It recordsĀ measured depth (MD), inclination (deviation angle), azimuth angle, true vertical depth, and dogleg severityĀ at various increments. To obtain a TVD, simply obtain a measured depth, go to the survey, and read off the TVD. If the desired measured depth is not in the survey, then extrapolate between the two closest points.

NB:Ā InclinationĀ is taken to be the angle of the well course from the vertical.Ā AzimuthĀ is taken clockwise from geographic north.Ā In other words, the inclination angle measures the vertical direction and the azimuthal angle examines the horizontal direction.

Reference:

[1]: Depth in a well

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