The original depth recorded while drilling an oil or gas well is known as the Driller's 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 is tied to drilling operations and other closely associated activities such as 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 Driller's Depth is always recorded, and it constitutes the primary depth system, unless it is later superseded by a more accurate measurement such as the depth from an open- or cased-hole wireline log.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 1) a unit of measurement e.g. meter or feet, 2) a datum reference e.g. rig floor. Measurement of Driller's Depth There are several parts of the drilling site to be considered while measuring: The assembly of rotating parts that goes down the hole, which is a series of drill pipe connections and drill collars, ending in a rotating bit. There may optionally be tools for logging while drilling as well. The length of all these is measured, and this gives us the Driller's Depth at a given point in time. As more pipe is added, and the drilling deepens, this measurement will be updated. On land, the floor of the rig provides us with a starting altitude, and all depth measurements are calculated relative to it. Since it is unlikely for the height of the ground to change significantly during drilling, this is assumed to be absolute. Offshore, this absolute datum must be independent of the tide. Common absolute datum include LAT (Lowest astronomical tide) and MSL (Mean sea level). This is most relevant for wells drilled from floating rigs, where the potential for error is greatest. In practice, Driller's Depth measurement is a manual operation, not changed significantly over the years and there are many facets of the system with potential to introduce errors and inconsistencies. This process was first described by Reistle and Sikes in 1938,Reistle, C. E., Jr., and Sikes, S. T., Jr., Well-depth measurements, Am. Petroleum Inst. Drilling and Production Practice, 1938, p. 80–95 and has not changed significantly. Dealing with Driller Depth errors If exploration derived prognosed depths are significantly different from the Driller's Depths , for example by 10−30 m, then warning bells come up – it is possible a pipe section or stand has been left off the calculations. If this is suspected, then the drill string should be measured (in tension) when the string is next pulled out of the hole, and the results checked with the tally. Mudloggers should be vigilant, as they provide the opportunity to cross check with the drilling company.
The original depth recorded while drilling an oil or gas well is known as the driller's 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 logger's depths are generally considered the more accurate of the two: Driller's depth measurement is tied to drilling operations and other closely associated activities such as 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 1) a unit of measurement e.g. meter or feet, 2) a datum reference e.g. rig floor. Measurement of Driller's Depth There are several parts of the drilling site to be considered while measuring: The assembly of rotating parts that goes down the hole, which is a series of drill pipe connections and drill collars, ending in a rotating bit. There may optionally be tools for logging while drilling as well. The length of all these is measured, and this gives the driller's depth at a given point in time. As more pipe is added, and the drilling deepens, this measurement will be updated. On land, the floor of the rig provides us with a starting altitude, and all depth measurements are calculated relative to it. Since it is unlikely for the height of the ground to change significantly during drilling, this is assumed to be absolute. Offshore, this absolute datum must be independent of the tide. Common absolute datum include LAT (Lowest astronomical tide) and MSL (Mean sea level). This is most relevant for wells drilled from floating rigs, where the potential for error is greatest. In practice, driller's depth measurement is a manual operation, not changed significantly over the years and there are many facets of the system with potential to introduce errors and inconsistencies. This process was first described by Reistle and Sikes in 1938,Reistle, C. E., Jr., and Sikes, S. T., Jr., Well-depth measurements, Am. Petroleum Inst. Drilling and Production Practice, 1938, p. 80–95 and has not changed significantly. Dealing with driller depth errors If exploration derived prognosicated depths are significantly different from the driller's depths , for example by 10−30 m, then warning bells come up – it is possible a pipe section or stand has been left off the calculations. If this is suspected, then the drill string should be measured (in tension) when the string is next pulled out of the hole, and the results checked with the tally. Mudloggers should be vigilant, as they provide the opportunity to cross check with the drilling company.
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