Driller's depth

The original depth recorded while drilling an oil or gas well is known as the Driller's Depth.

The Problem

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 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• Driller's Depth is always recorded, and it constitutes the primary depth system, unless it is later superseded by a more accurate measurement such as the depth from an open- or cased-hole wireline log.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's 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.

Measuring drill string length

Lack of uniformity of drill pipe

The bulk of the drill string is drill pipe which has a nominal length of 9.6 meter per pipe section, however, in reality, not all pipes are the same length. Why is this? Steel pipe has a "male" connection at one end (called the pin) and a "female" connection at the other end (called the box) and as each section of pipe is lowered into the hole it is connected to the pipe preceding it by threading together the male and female components.

Drill pipe connections (or drill pipe/collar, or collar/bit and any other connection) must have a very good sealing surface because high pressure mud will be traveling through the pipe and any pitted or galled areas could be quickly eroded out. This is usually referred to as a wash-out, or words to that effect, and can occur in any part of the drill string or bottom hole assembly. Because of this pipe is routinely inspected before and after use. Any imperfections are eliminated one of two actions:

1. A re-cut, where a thin section is shaved off the end of the pipe, and the box, or pin, is re-threaded resulting in pipe length reduction
2. chase and face, where the pipe threads are effectively moved further down, again resulting in reduced overall length

Galling and pitting problems are particularly evident after drilling hard formations as encountered for example in the Browse Basin.

Tracking of drill pipe length used

Tracking and recording of drill pipe at the rig site starts when individual joints are picked up. Joint numbers are manually marked on the side of the pipe. Typically three sections of pipe are joined together into a stand (of about 27–29 m in length) and stacked in rows of 10, with their base resting on the drill floor. Prior to running in the hole each stand is manually measured with a steel tape measure and the measurement recorded in a computer spreadsheet (previously a pipe tally book was used) alongside the stand number. To confirm at any stage what depth the drill bit is operating at, the driller consults the pipe tally records, and measures the length of the current stand of drill pipe below the rig floor.

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 prognosed depths are significantly different than the Driller's Depths, for example by 10-30m, 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.

Examples

For some deep wells, e.g. ±7000m or 25000 ft deep, the drill pipe elongation due to its own weight and temperature must be taken account of. This can be in the order of 24 m (80 ft). Wireline doesn't behave this way: it tends to lengthen under tension but shorten with increasing temperature. We can only assume by how much this net effect varies. Wireline depth correction for temperature and tension has been around since before days of computer data acquisition, and is generally seen as reliable. In our experience, the impact on a geological model previously based on wireline depth, when drilling at greater than 7000m 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 driller's survey do not call this elongation an uncertainty but rather call it bias or error. For the example above, in addition to the 25m (+80 ft) bias, there would be about ± 3m/12 ft to 10m/30 ft residual uncertainty depending on hole inclination. There are a few in the industry who know how to correct for this real time and some service companies have developed conceptual or prototype tools/processes to account for this elongation effect. In future, these corrections should become standard practice for the industry, but they are not at this time. The determination of an accurate depth has not traditionally been a popular area of research. At present, they are not. The impact of errors in depth is most critical when integrating data from more than one well, e.g. to build a reservoir model.

See also

• Specification of depth.
• Well logging

Glossary of terms

• Absolute depth: distance along a path (along hole, vertical, etc.) between a reference point (rotary table, ground level, mean sea level, etc.) and a given point downhole (after Ref. 3, §13.2).