Changes

file:conversion-of-well-log-data-to-subsurface-stratigraphic-and-structural-information_fig2.png|{{figure number|2}}Well course of two types of deviated wells. True stratigraphic thickness and true vertical thickness of a dipping stratigraphic unit are shown in relation to the measured interval in a well penetrating the unit.

file:conversion-of-well-log-data-to-subsurface-stratigraphic-and-structural-information_fig2.png|{{figure number|2}}Well course of two types of deviated wells. True stratigraphic thickness and true vertical thickness of a dipping stratigraphic unit are shown in relation to the measured interval in a well penetrating the unit.

file:conversion-of-well-log-data-to-subsurface-stratigraphic-and-structural-information_fig3.png|{{figure number|3}}Segment of a curved well path showing angular and dimensional relationships between the top and bottom of the interval.

file:conversion-of-well-log-data-to-subsurface-stratigraphic-and-structural-information_fig3.png|{{figure number|3}}Segment of a curved well path showing angular and dimensional relationships between the top and bottom of the interval.

file:conversion-of-well-log-data-to-subsurface-stratigraphic-and-structural-information_fig4.png|{{figure number|4}}Linear approximations of a curved well course by the (a) tangential method,<ref name=pt06r20 /><ref name=pt06r23>Dailey, P. 1977, A guide to accurate wellbore survey calculations: Drilling-DCW, May, p. 52–59 and 118–119.</ref> (b) angle averaging method,<ref name=pt06r20 /> and (c) balanced tangential method.<ref name=pt06r20 />

file:conversion-of-well-log-data-to-subsurface-stratigraphic-and-structural-information_fig4.png|{{figure number|4}}Linear approximations of a curved well course by the (a) tangential method,<ref name=pt06r20 /><ref name=pt06r23>Dailey, P., 1977, A guide to accurate wellbore survey calculations: Drilling-DCW, May, p. 52–59 and 118–119.</ref> (b) angle averaging method,<ref name=pt06r20 /> and (c) balanced tangential method.<ref name=pt06r20 />

file:conversion-of-well-log-data-to-subsurface-stratigraphic-and-structural-information_fig5.png|{{figure number|5}}Circular approximations of a curved well course showing angles used for the approximations. (a) Radius of curvature method showing chords of horizontal and vertical circles. This method assumes a constant radius of curvature (constant increase or decrease in deviation between survey points). (b) Minimum curvature method showing chord of single circle and the angle ϕ, which describes the chord.

file:conversion-of-well-log-data-to-subsurface-stratigraphic-and-structural-information_fig5.png|{{figure number|5}}Circular approximations of a curved well course showing angles used for the approximations. (a) Radius of curvature method showing chords of horizontal and vertical circles. This method assumes a constant radius of curvature (constant increase or decrease in deviation between survey points). (b) Minimum curvature method showing chord of single circle and the angle ϕ, which describes the chord.

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* ''i'' = survey point number (i = 0 at surface)

* ''i'' = survey point number (i = 0 at surface)

The intervals can be defined in several ways depending on the accuracy and simplicity of calculation required. The ''tangential'' or ''terminal angle method'' ([[:file:conversion-of-well-log-data-to-subsurface-stratigraphic-and-structural-information_fig4.png|Figure 4a]]) assumes a constant deviation for the entire interval from one survey point to the next. Thus, the measured depths (MD_''i'', MD_''i''–1) for each interval coincide with the depth at the survey points, and the angle used would be for the lower survey point. Although easy to calculate, this method is likely to be substantially in error and is generally not recommended.<ref name=pt06r20>Craig, J. T. Jr., Randall, B. V., 1976, Directional survey calculation: Petroleum Engineer, March, p. 38–54.</ref><ref name=pt06r57>Inglis, T. A., 1987, Directional Drilling, Petroleum Engineering and Development Studies, Volume 2: London, Graham and Trorman, chap. 9, p. 155–171.</ref> It is mentioned here for historical reasons, as it has been widely used. Alternatively, the ''angle averaging method'' ([[:file:conversion-of-well-log-data-to-subsurface-stratigraphic-and-structural-information_fig4.png|Figure 4b]]) uses the average for the two survey points at either end of the segment.

The intervals can be defined in several ways depending on the accuracy and simplicity of calculation required. The ''tangential'' or ''terminal angle method'' ([[:file:conversion-of-well-log-data-to-subsurface-stratigraphic-and-structural-information_fig4.png|Figure 4a]]) assumes a constant deviation for the entire interval from one survey point to the next. Thus, the measured depths (MD_''i'', MD_''i''–1) for each interval coincide with the depth at the survey points, and the angle used would be for the lower survey point. Although easy to calculate, this method is likely to be substantially in error and is generally not recommended.<ref name=pt06r20>Craig, J. T., Jr., and B. V. Randall, 1976, Directional survey calculation: Petroleum Engineer, March, p. 38–54.</ref><ref name=pt06r57>Inglis, T. A., 1987, Directional Drilling, Petroleum Engineering and Development Studies, Volume 2: London, Graham and Trorman, chap. 9, p. 155–171.</ref> It is mentioned here for historical reasons, as it has been widely used. Alternatively, the ''angle averaging method'' ([[:file:conversion-of-well-log-data-to-subsurface-stratigraphic-and-structural-information_fig4.png|Figure 4b]]) uses the average for the two survey points at either end of the segment.

A better approximation, the ''balanced tangential method'' ([[:file:conversion-of-well-log-data-to-subsurface-stratigraphic-and-structural-information_fig4.png|Figure 4c]]), is derived by placing the interval depths (MD_i, MD_''i''–1) half way between the individual survey points, thus assuming that the deviation is constant in an interval around the measured point.

A better approximation, the ''balanced tangential method'' ([[:file:conversion-of-well-log-data-to-subsurface-stratigraphic-and-structural-information_fig4.png|Figure 4c]]), is derived by placing the interval depths (MD_i, MD_''i''–1) half way between the individual survey points, thus assuming that the deviation is constant in an interval around the measured point.