Page 205 - Well Logging and Formation Evaluation

Well Deviation, Surveying, and Geosteering 195

west of true north in the Northern Hemisphere and east of true north in
the Southern Hemisphere. Likewise, vertical grid lines to the east of the
central meridian will point east of true north in the Northern Hemi-
sphere and west of true north in the Southern Hemisphere. The so-called
grid correction (G) is positive when TN is east of grid north, and nega-
tive when it is west. Typical values are -1.5 to +1.5 degrees.
• True north. This is the direction of the geographic north pole as defined
by the axis of rotation of the Earth. The meridians, or lines of longi-
tude, on maps point toward true north.

13.2 SURVEYING

Borehole position uncertainty defines the range of actual possible posi-
tions of a particular point in a well in terms of eastings, northings, and
TVD. Factors that affect borehole position uncertainty are:

1. Accuracy of measured depth determination in the well. Both
drillpipe and wireline cable suffer from stretch and inaccuracies in the
methods used to measure how much pipe or cable has been run into
the hole. This uncertainty becomes greater with increasing depth and
well deviation. For a vertical well drilled to 3,500m, one would expect
the measured depth at total depth (TD) to be known to an accuracy of
roughly 2m. For a deviated well drilled to 3,500m TVD with a devia-
tion at bottom of 50 degrees, this inaccuracy might rise to 5m.
2. Frequency of survey stations. When surveying a well, it is normal to
acquire “stations” at discrete depth intervals along the well. At each of
these stations, the hole’s inclination and azimuth will be measured.
Between stations, it is necessary to use an algorithm to determine
values to be interpolated. The accuracy of the final survey will depend
on the frequency of these stations and algorithm used.
3. Survey tool accuracy. Let us consider these different types of tools
separately:
(a) Magnetic survey tools. The accuracy of magnetic devices is limited
by their intrinsic accuracy and the extent to which they are affected
by magnetic interference. The strength and direction of the Earth’s
field is also a factor, since in a worst-case scenario of drilling in
the same direction as the Earth’s lines of flux, no azimuthal mea-
surement would be possible. Near the poles, the Earth’s field is
nearly vertical and this would be a factor. Magnetic interference
may come from any metal in proximity to the tool (such as the