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Measuring Depth, Directionally Drilled Wells, and Directional Surveys

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Accurate subsurface maps require that the data used to construct those maps be placed in the correct X (latitude) and Y (longitude) position. For many maps, especially depth structure maps but also net pay, isotherm, and isobar maps, it is important that data also be properly located in Z (depth) as well. There are significant uncertainties in all of these measurements (X, Y, and Z). It is important that geoscientists understand the sources and magnitudes of these uncertainties so that maps can be made as accurately as possible and important business decisions can be made on the basis of those maps with the explicit recognition of the uncertainty contained within the maps.*

X and Y uncertainty derives from any uncertainty associated with the surface location along with the added uncertainty related to the directional path of the well. Depth uncertainty similarly is associated with uncertainties in the measured depth of the well along with the added uncertainty of converting measured depth to true vertical depth (TVD). The goal of this chapter is not to instruct the reader in how to recognize and minimize these uncertainties. The primary goal is to make the reader aware that these uncertainties exist and that they must be considered in any subsurface map.

There are two distinct types of depth uncertainty to keep in mind. The first is relative depth uncertainty. This is the depth uncertainty of different log markers within a single well. The second depth uncertainty is absolute depth uncertainty. This is the uncertainty relative to a specific reference, usually mean sea level. Absolute uncertainty is most important when comparing depths from different wells.

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