Geology Reference
In-Depth Information
2.2.3.2
Deviated Well
Many wells are purposely drilled to deviate from the vertical. This means that points in
the well are not directly below the surface location. The shape of the well is determined
by a deviation survey, the terminology of which is given in Fig. 2.6. The primary informa-
tion from a deviation survey is the azimuth and inclination of the borehole and the
downhole position of the measurement, for a number of points down the well. This in-
formation is converted by some form of smoothing calculation into the
xyz
coordinates
of selected points in well-log coordinates, known as true vertical depth (TVD, Fig. 2.6a)
and is given in the log of the survey. The TVD must be corrected for the elevation of the
Kelly bushing to give the locations of points with respect to the datum. In a deviated well
the log depths are converted to a sea-level datum with the following equation:
SD = KB - TVD ,
(2.2)
where SD = subsea depth, KB = elevation of Kelly bushing or other measurement of
surface elevation, TVD = true vertical depth from the deviation survey.
The calculated position of the points determined from a deviation survey depends on
the spacing between the measurement points and the particular smoothing calculation
used to give the TVD locations. If the measurement points are spaced tens of meters or
a hundred or more feet apart, the positions of points toward the bottom of a 3 000-m
(10 000-ft) well might be uncertain by tens of meters or a hundred feet or so. This is because
the absolute location of a point depends on the accuracy of the location of all the points
above it in the well. Small errors accumulate. The relative positions of points spaced a
small distance apart along the well should be fairly accurate. Points will be accurately
located if the deviation survey is based on points spaced only a meter or less apart.
Locations in a deviated well are commonly given as the
xyz
coordinates of points in
the well relative to the surface location, for example, P
1
and P
2
in Fig. 2.7a. Here
z
is the
vertical axis, positive upward, the subscript “1” will always denote the upper point and
the subscript “2” will denote the lower point. A unit boundary is likely to be located
between control points, making it necessary to calculate its location.
Fig. 2.7.
Calculation of position of a unit boundary in a deviated well.
a
Boundary located between two
control points.
b
Boundary located below the control points
