Changes

* Wells are not straight; even nominally vertical holes commonly show substantial horizontal displacement even if the deviation is too small to produce a large change in true vertical depth ([[:file:conversion-of-well-log-data-to-subsurface-stratigraphic-and-structural-information_fig1.png|Figure 1]]).

* Wells are not straight; even nominally vertical holes commonly show substantial horizontal displacement even if the deviation is too small to produce a large change in true vertical depth ([[:file:conversion-of-well-log-data-to-subsurface-stratigraphic-and-structural-information_fig1.png|Figure 1]]).

* Wells are not commonly deviated from the surface, but rather are drilled approximately vertical to a kick-off point, where deviation is built up to a planned degree (see “[[Wellbore trajectory]]”). The rest of the well can be drilled at a constant angle, or the well can be returned toward the vertical to penetrate the horizon of interest ([[:file:conversion-of-well-log-data-to-subsurface-stratigraphic-and-structural-information_fig2.png|Figure 2]]).

* Wells are not commonly deviated from the surface, but rather are drilled approximately vertical to a kick-off point, where deviation is built up to a planned degree (see [[Wellbore trajectory]]). The rest of the well can be drilled at a constant angle, or the well can be returned toward the vertical to penetrate the horizon of interest ([[:file:conversion-of-well-log-data-to-subsurface-stratigraphic-and-structural-information_fig2.png|Figure 2]]).

* Deviation is rarely constant in a well, even when that is the objective of drilling.

* Deviation is rarely constant in a well, even when that is the objective of drilling.

* ''t'' = top of interval of interest

* ''t'' = top of interval of interest

The ''minimum curvature method'' approximates the well path as a single circular arc. The equations in this method (from <ref name=pt06r20 /><ref name=pt06r57 />) are as follows:

The ''minimum curvature method'' approximates the well path as a single circular arc. The equations in this method (from Dailey<ref name=pt06r20 /> and Inglis<ref name=pt06r57 />) are as follows:

:<math>\phi  = \cos^{-1}[\cos \alpha_{i-1} \cos \alpha_{\rm i} + \sin \alpha_{\rm i} \sin \alpha_{i-1} \cos (\beta_{i} - \beta_{i-1})]</math>

:<math>\phi  = \cos^{-1}[\cos \alpha_{i-1} \cos \alpha_{\rm i} + \sin \alpha_{\rm i} \sin \alpha_{i-1} \cos (\beta_{i} - \beta_{i-1})]</math>