Page 203 - Well Logging and Formation Evaluation

CHAPTER 13

WELL DEVIATION,

SURVEYING, AND

GEOSTEERING

13.1 WELL DEVIATION

The trajectory of a deviated well may be described in terms of its incli-
nation, depth, and azimuth. The inclination of a well at a given depth is
the angle (in degrees) between the local vertical and the tangent to the
wellbore axis at that depth (Figure 13.1.1). The convention is that 0
degrees is vertical and 90 degrees is horizontal. Parts of a degree are given
in decimals, rather than minutes and seconds. Gravity varies with latitude,
and its direction may be influenced by local features such as mineral
deposits and mountains, as well as the Earth’s rotation.
Depth in boreholes is measured either along the hole itself, in which
case it is referred to as measured or alonghole depths, with reference to a
fixed point, or as true vertical depth (TVD) with reference to a datum.
Depth references that are commonly used are as follows:

• Derrick floor. This is the elevated deck on which the rig crew work,
typically 10m or so above ground level on a land rig and 20–30m on
an offshore rig. Also sometimes referred to as a rotary table.
• Kelly bushing. This is the top of the bushing, which rotates on the
derrick floor (although kellys are rarely used on modern drilling rigs
with topdrives) and is typically 1ft higher than the derrick floor.
• Mean sea level. This is the elevation of the sea, averaging out the effect
of tides or seasonal variations. Usually the topography department will
establish the elevation of a land location prior to drilling. For offshore
locations, the elevation of the seabed will be known. On floating rigs,
a correction using tide tables will be used.

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