Page 110 - Drilling Technology in Nontechnical Language

Chapter 4 – PLANNING AND DRILLING A DEVELOPMENT WELL OFFSHORE 101

In a critical well like this one, where the wellbore has to be drilled
accurately, an MWD tool can be run as part of the rotary drilling assembly.
The MWD transmits inclination and azimuth information to the surface.

To determine the exact position of an object in space (e.g., an aircraft),
it is necessary to know its height above the ground and its geographical
coordinates (north-south and east-west). Position within the earth is
expressed in the same way: the vertical depth below a particular reference

(e.g., mean sea level or the drill floor), called true vertical depth (TVD), and
the geographical coordinates.
It is simple to calculate the position of the wellbore within the earth
from the inclination and azimuth at known depths. The measured depth
(MD) along the hole from the surface to the MWD tool is always known.
For each survey, there are three items of information: the MD along the
wellbore, the inclination, and the azimuth. While there are several ways of
calculating the position, the most commonly used is called the minimum
curvature method because it assumes a perfect arc (segment of a circle)
between two survey points.
Knowing the current position, inclination, and azimuth of one survey
point, the distance along the hole to the next survey point, and the inclination
and azimuth of the second survey point, the position of the second point
can be calculated. These calculations can be done by hand but they are
somewhat tedious, and computer programs are used nowadays for this. If
two surveys had exactly the same azimuth and inclination, determining
the new TVD and coordinates would be simple trigonometry, but even in
a tangent section, the wellbore is almost never perfectly straight, and so
perfect circular arcs are still assumed.
In a deviated well, the measured depth is either equal to TVD (in the
initial vertical part of the well) or is greater (as the well starts to deviate).
Both depth measurements are important; hydrostatic pressures and
formation fracture pressures are calculated using TVD, but the lengths
of objects placed in the wellbore, such as casing, are calculated using MD
(fig. 4–13).

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