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 Driller's depth is always recorded, and it constitutes the primary depth system, unless it is later superseded by a more accurate measurement such as the depth from an open- or cased-hole wireline log.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. Measurement errors due to bottom-hole assembly Another potential area for error is the Bottom Hole Assembly (BHA). The BHA consists of the drill bit, drill collars and stabilizers. Additionally, it can also include a downhole motor, MWD ( measurement while drilling ) and LWD ( logging while drilling ) tools . Errors come about if the total BHA is not correctly measured or recorded. Often BHA changes may be made during operations and if these changes are not recorded then the depths will be incorrect. Ideally the BHA is operated to minimize "sagging" within the borehole. Pipe stretch and compression will occur from time to time but are not corrected for during normal operations, even though they can introduce fairly significant cumulative errors on driller's depth, particularly in deep wells or in areas of hard rock. 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. An important aspect in this is identifying the required accuracy for the logged driller's depths. If drillers and geologists are "OK" with (for example, +/- 15 m) at these depths, but later the reservoir engineers trying to map fluid water contacts require a higher level of precision (for example, +/- 3 m), then by the time the drilling is finished the higher level of accuracy cannot be recreated. This leads to the concept of true along-hole (TAH) depth, where the measurements made are defined using the accuracy of the pipe length calibration method and (if any) the accuracy of the corrections applied the correction methodology. One methodology that has been introduced is called Driller Way-point Depth (patent applied)Bolt H., A Method for Determining Well Depth, Driller's Way-point Depth Ltd., International Patent Application No. PCT/GB2018/000030, February 2017 which results in TAH depth. The measurement uncertainty is then a combination of the drill pipe length measurement accuracy, how accurately the correction parameters are measured, and the fidelity of the correction model applied. Examples For some deep wells, e.g. 7000 m or 25000 ft deep, the drill pipe elongation due to its own weight and temperature must be taken into account. This can be on the order of 24 m (80 ft). Wireline does not behave this way: it tends to lengthen under tension but shorten with increasing temperature. One can only assume by how much this net effect varies. Wireline depth correction for temperature and tension has been around since before the days of computer data acquisition, and is generally seen as reliable. Based on experience, the impact on a geological model previously based on wireline depth, when drilling at greater than 7,000 m and using LWD (driller's) depths, can introduce differences in marker depths of up to 25 m (80 ft): the driller's depths are consistently higher than the more reliable wireline depths.
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 ), measurement while drilling , 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 (lowest astronomical tide) and mean sea level). This is most relevant for wells drilled from floating rigs, where the potential for error is greatest. Measurement errors due to bottom-hole assembly Another potential area for error is the bottom hole assembly. This consists of the drill bit, drill collars and stabilizers. It can also include a downhole motor, tools for measurement while drilling and logging while drilling . Errors come about if the total bottom hole assemblyis not correctly measured or recorded. Bottom hole assembly changes may be made during operations and if these changes are not recorded then the depths will be incorrect. Ideally the bottom hole assembly is operated to minimize "sagging" within the borehole. Pipe stretch and compression will occur from time to time but are not corrected for during normal operations, even though they can introduce fairly significant cumulative errors on driller's depth, particularly in deep wells or in areas of hard rock. Dealing with driller depth errors If exploration derived prognosticated 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. An important aspect in this is identifying the required accuracy for the logged driller's depths. If drillers and geologists are "OK" with (for example, +/- 15 m) at these depths, but later the reservoir engineers trying to map fluid water contacts require a higher level of precision (for example, +/- 3 m), then by the time the drilling is finished the higher level of accuracy cannot be recreated. This leads to the concept of true along-hole depth, where the measurements made are defined using the accuracy of the pipe length calibration method and (if any) the accuracy of the corrections applied the correction methodology. One methodology that has been introduced is called Driller Way-point Depth (patent applied)Bolt H., A Method for Determining Well Depth, Driller's Way-point Depth Ltd., International Patent Application No. PCT/GB2018/000030, February 2017 which results in true along-hole depth. The measurement uncertainty is then a combination of the drill pipe length measurement accuracy, how accurately the correction parameters are measured, and the fidelity of the correction model applied. Examples For some deep wells, e.g. 7000 m or 25000 ft deep, the drill pipe elongation due to its own weight and temperature must be taken into account. This can be on the order of 24 m (80 ft). Wireline does not behave this way: it tends to lengthen under tension but shorten with increasing temperature. One can only assume by how much this net effect varies. Wireline depth correction for temperature and tension has been around since before the days of computer data acquisition, and is generally seen as reliable. Based on experience, the impact on a geological model previously based on wireline depth, when drilling at greater than 7,000 m and using logging while drilling (driller's) depths, can introduce differences in marker depths of up to 25 m (80 ft): the driller's depths are consistently higher than the more reliable wireline depths.
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