The specification of depth

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Absolute depth is specified with 3 components:

• a unit (e.g. m for metre),
• a path (e.g. MD for measured depth),
• and the reference or datum they refer to (e.g. RT for rotary table),

and none of these 3 components should ever be left implicit.

• However, well depths are implicitly "loggers' depths", unless specified explicitly otherwise (e.g. "drillers' depth", "core depth", etc).
• Depth increases in the "down" direction, so an elevation is a negative depth. (e.g. RT = -10mMDLAT) and there is no need to specify above or below the depth reference.
• Do not use the term "subsea" by itself as it is ambiguous. It could mean: below sea floor, below mean sea level, below lowest astronomical tide, etc.
• Differential (or relative) depths or thicknesses should generally be specified with two components at least: a unit and a path, plus any eventual specifiers to remove any possible ambiguity. No specifier should ever be left "implicit" or "understood". There are cases where a path is not needed and in fact should not be specified, because it is defined by the specifier, e.g. isochore (true stratigraphic thickness, independent of well path or inclination).
• Remember that depths, whether "absolute", "relative", "true", etc, have an intrinsic uncertainty and are never really true.

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Details

Petrophysicists and drilling operations tend to express depths with reference to the rotary table or the original drill floor; geologists tend to use a common datum such as the mean sea level; geophysicists use the mean sea level. This can introduce much confusion when a unit is not specified with all 3 components: unit, path, and reference.

As an example, in Australia:

• Unit: the usual unit of depth is the metre (m).
• Path: common expressions of path are measured depth (MD) - elsewhere often known as along hole depth (AHD) - and true vertical depth (TVD). Note that using TV for true vertical depth is not consistent with the use of MD for measured depth, hence the recommended TVD.
• Reference:
  • the legal datum offshore Australia is Lowest Astronomical Tide (LAT) - (Ref. 1 & 2). Note that this requirement in itself can cause difficulties as it is difficult to measure offshore and can vary greatly between locations and even with time. There is, however, an advantage to this convention: tidal corrections should always be of the same sign (negative depth), i.e. the sea level is always higher than or equal to LAT.
  • A commonly used alternative is Mean Sea Level (MSL).
  • A datum used in the past was Mean Indian Spring Low Water (MILSW). It is comparable to, but not exactly the same as, LAT.
  • Common references used in operations include: Rotary Table (RT), Drill Floor (DF), Kelly Bushing (KB), Sea Bottom (SB), Ground Level (GL).

Use whatever is appropriate, as long as your depths are unambiguous.

Note that the distinction between "loggers' depth" and "drillers' depth" is becoming blurred due to the increasing use of logs acquired while drilling (LWD). At the time of writing, the common practice remains that the petrophysicists or geologists define the "official depths" in a well, and that depth is frequently different from the "drillers' depth", after various corrections, tie-ins, etc, have been applied.

Figures (click on the figures to see them full-size)

Examples

Specification of an absolute depth: in Figure 1 above, point P1 might be at 3207mMDRT and 2370mTVDMSL, while point P2 might be at 2530mMDRT and 2502mTVDLAT. Specification of a differential depth or a thickness: in Figure 2 above, the thickness of the reservoir penetrated by the well might be 57mMD or 42mTVD, even though the reservoir true stratigraphic thickness in that area (or isopach) might be only 10m, and its true vertical thickness (isochore), 14m.

References

1. Lowest Astronomical Tide (LAT)

2. Seas and Submerged Lands Act 1973 (Australia)

3. Log Data Acquisition and Quality Control, Ph. Theys, 1991, Editions Technip

Glossary of terms

• Absolute depth: distance along a path (along hole, vertical, etc) between a reference point (rotary table, ground level, mean sea level, etc) and a given point downhole (after Ref. 3, §13.2).

Credits

The original of this article has been reviewed by two referees of the | FESWA Data Standards and Best Practices Group.