Well Planning - Reference Systems and Coordinates.- (1) Depth References

Depth References
During the course of a directional well, there are two kinds of depths:
• Measured Depth (MD) is the distance measured along the actual
course of the borehole from the surface reference point to the survey
point. This depth is always measured in some way, for example, pipe
tally, wireline depth counter, or mud loggers depth counter.
• True Vertical Depth (TVD) is the vertical distance from the depth
reference level to a point on the borehole course. This depth is always
calculated from the deviation survey data.
In most drilling operations the rotary table elevation is used as the working
depth reference. The abbreviation BRT (below rotary table) and RKB
(rotary kelly bushing) are used to indicate depths measured from the rotary
table. This can also be referred to as derrick floor elevation. For floating
drilling rigs the rotary table elevation is not fixed and hence a mean rotary
table elevation has to be used.
In order to compare individual wells within the same field, a common
depth reference must be defined and referred to (e.g. When drilling a relief
well into a blow-out well, the difference in elevation between the
wellheads has to be accurately known). Offshore, mean sea level (MSL) is
sometimes used. Variations in actual sea level from MSL can be read from
tide tables or can be measured.