Driller's Depth - Examples

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.