Since there isn't a single reference or measurement system for calculating the depth in sub-surface environments, two engineers talking about a single drilling might give different answers when asked to give a measurement of depth.
The two main depth references used in the "downhole" (i.e. sub-surface) environment are Driller's Depths and Logger's Depths (also called Wireline Logger's Depths). These measurement systems are recorded quite differently and generally Logger's Depths are considered the more accurate of the two:
Driller's Depth Measurement:
Tied to drilling operations i.e. logging while drilling (LWD), measurement while drilling (MWD) and coring.
Driller's Depth is always recorded, and it constitutes the primary depth system, unless it is later superseded by a more accurate measurement such as the depth from an open- or cased-hole wireline log.
Driller's Depth should always have:
a unit of measurement e.g. meter/feet
a datum reference e.g. Kelly Bushing
Importance of accurate depth!
For any logging company Depth is the primary measurement. Without accurate depth the other readings are of no value because they would no longer matter.
How do logging companies measure depth?
Wireline
Depth wheel(s) measure the length of wireline as it is spooled in/out
Record depth using timestamps
TimeStamps:
Is a sequence of characters or encoded information identifying when a certain event occurred, usually giving date and time of day, sometimes accurate to a small fraction of a second.
How is the Depth combined with the tool readings?
Wireline
Tools are attached to wireline, each measure point has a known distance
Memory based horizontal logging systems
Record depth using timestamps
Sync the timestamps and you have the tool readings with depth
Pipe tally is the main reference data used to track depth
All drillers should know already exactly what is in the hole being drilled and lengths of each pipe recorded on the tour sheet. Remember not all pipes are created equal so accurate pipe strapping is important!
