Page 115 - Drilling Technology in Nontechnical Language

106 Drilling Technology in Nontechnical Language Second Edition

reservoir engineers. At this point, casing can be run to protect the well into
the reservoir and to isolate the reservoir from the formations above.
The reason for drilling horizontal wells is to increase the production
potential of the well. If a beautiful horizontal well is drilled in the reservoir,
it would not be good to run steel pipe inside it and pump cement around
it. Cement has a nasty habit of plugging the reservoir and restricting
production. Instead, the well will be left to produce as drilled, if the
rock is strong enough to produce without collapsing or producing solids.
Otherwise, some kind of screen can be run that is not cemented in place
but that holds back any produced solids and supports the hole if it starts
to collapse.

Drilling the horizontal section

Production casing is run and cemented with the casing shoe in the
reservoir and horizontal. A steerable motor assembly with MWD and
LWD is run in to drill the final horizontal hole through the reservoir.

The LWD tool in this well has two sensors, a resistivity sensor and a
gamma ray sensor. If the drilling assembly drops towards the oil-water
contact, the resistivity reading will decrease. If the drilling assembly
builds up towards the shale on top of the reservoir, the gamma ray reading
will increase.
Shale minerals are naturally radioactive, and thus they give off gamma
rays that can be detected. The level of radioactivity is not enough to harm
humans, which is just as well, considering that most of the upper earth’s
crust consists of shales! It is also possible to tell from the gamma radiation
spectrum (the frequencies present in the radiation) what type of mineral is
responsible for the radiation. Thus, a particular shale can be recognized by
its gamma radiation spectrum.
Using knowledge of the reservoir structure, the LWD and MWD
readings allow accurate navigation in the reservoir. It is possible to drill
with a high degree of accuracy; within three or four feet either way.
It is very important to minimize damage to the reservoir as it is drilled
through. Normally a well is drilled so as to remain overbalanced on the
pore pressures with the mud hydrostatic pressure, but this pressure will
tend to force mud and mud solids into the formation. It is possible to drill

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