Page 100 - Drilling Technology in Nontechnical Language

Chapter 4 – PLANNING AND DRILLING A DEVELOPMENT WELL OFFSHORE 91

Where a 5″ production tubing connects to a 5″ liner, the inside of the
conduit from the reservoir to the surface will all be the same diameter and
is all therefore accessible from the surface with wireline and other tools.
This type of design is called a monobore completion.
In a horizontal well, the target location is not directly underneath the
rig, so the well must be drilled along an accurate path to the target. Once in
the reservoir, the well must remain a certain distance above the oil-water
contact but not so far above it that it approaches the gas-oil contact.

The BHA navigates through the reservoir by measuring the
characteristics of the reservoir while drilling, using logging tools that are
constructed inside a drill collar. These techniques are called, logically,
logging while drilling (LWD). The logging tool for this job measures
electrical resistivity—the closer it approaches water, the lower the
resistivity measured. As the reservoir was repeatedly logged during the
exploration and appraisal drilling, the engineers have a good picture of
how the resistivity varies with depth. (Appraisal drilling refers to drilling
done after a discovery is made by an exploration well to further appraise
the discovery.)
Initially, the well is drilled vertically. At the kickoff point, the
rotary drilling assembly is pulled out. (A rotary drilling assembly is a

configuration of drill collars and other downhole tools that drills by rotating
the drillstring from the surface, as opposed to an assembly that powers the
bit with a downhole motor.) Next, a special directional drilling assembly
is run in the well (see fig. 4–6). This is designed to exert a side force

at the bit, so that the bit starts to drill away from vertical. The direction
that the well drills towards is determined by aligning the side force in the
appropriate direction.
The direction of the well relative to true north is called the azimuth,
and it is usually measured in degrees clockwise. True east will be 090°.
The angle between the wellbore center and vertical is called the
inclination. The horizontal section of the well, if it is exactly horizontal,
will have an inclination of 90°. A vertical well has an inclination of 0°.
There are various tools and techniques used to deviate the wellbore.
Directional drilling techniques are covered in more detail in chapter 8,
“Directional and Horizontal Drilling.”

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