Depth in a well

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, "depth" also refers to the location itself.

By extension, depth can refer to locations below, or distances from, a reference point or elevation, even when there is no well. In that sense, depth is a concept related to elevation, albeit in the opposite direction. Depth in a well is not necessarily measured vertically or along a straight line.

Because wells are not always drilled vertically, there may be two "depths" 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), rig table (RT), and kelly bushing (KB).

The specification of depth

Although it is an intuitive concept, depth in a well is the source of much confusion because it is frequently not specified correctly. Absolute depth should always be specified with three 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. Example: the top of a reservoir may be found at 1,500mMDRT in a particular well (1,500 m measured depth below the rotary table), which may be equal to 1,492mTVDMSL (1,492 m true-vertical-depth below mean sea level) after correction for deviations from vertical.

Depth in a well as used in the oil and gas industry

• Well depth values taken during the drilling operation are referred to as "drillers depth". The "total depth" for the well, core depths and all analysis of core / mud and other materials from the drilling hole are measured in "drillers depth".
• Well depth values from the wireline loggers operation are referred to as "loggers depth". The loggers depth are typically considered more reliable than the drillers depth.
• The difference between loggers and drillers depth are due to different stretch in the drilling string when drilling, and the wire line entered into the bore hole during wireline logging operations. This difference is estimated and referred to as "core shift". A core from a certain drillers depth is lined up with a wireline log (loggers depth) and structures in the core are compared with the log and matched.
• Depth increases in the "down" direction, so an elevation is a negative depth. There is no need to specify above or below the depth reference.

  Example: RT = -10mMDLAT

• The term "subsea" by itself should not be used, as it is ambiguous. It could mean: below sea floor or bottom, below mean sea level (MSL), below lowest astronomical tide (LAT), etc.
• Differential (or relative) depths or thicknesses should generally be specified with at least two components: 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).
• It is important to remember that depths, whether "absolute", "relative", "true", etc, have an intrinsic uncertainty and are never really true.
• 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 these depths are frequently different from the "drillers' depth", after various corrections, tie-ins, etc, have been applied.
• 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.

Depth in practice: examples from in the oil and gas industry 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).

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

Figures

Fig. 1: The specification of depths

Fig. 2: Differential depths: reservoir thickness, isochor, isopach

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 10 m, and its true vertical thickness (isochore), 14 m.

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 10 m, and its true vertical thickness (isochore), 14 m.

External references

• Lowest Astronomical Tide (LAT)
• Seas and Submerged Lands Act 1973 (Australia)
• Log Data Acquisition and Quality Control, Ph. Theys, 1991, Editions Technip