Page 149 - Geothermal Energy Systems Exploration, Development, and Utilization

3.3 Drilling Mud 125
with a total length of 12 290 m (40 320 ft) with a departure (horizontal distance of
final point from start point) of 10 900 m (35 770 ft)! The entire horizontal section
was drilled within a reservoir target which is only 6 m (20 ft) thick.

3.2.4
Coring

Coring – or core drilling – is a technique where not all of the rock is destroyed but
only a kerfs area while the remaining ‘‘core’’ is recovered to the surface, depending
on the applied technique either by pulling the complete drillstring with the core
barrel or only the inner core barrel (which contains the core) by wireline. Normally,
all core barrels consist of an outer core barrel which is connected to the core bit
at the lower end and to the drillstring at its top end, and an inner core barrel
which takes the core. In order to protect the core against destruction by the drilling
process the inner barrel does not rotate with the drillstring due to a rotational
bearing between both barrels.
The advantages of coring are that cores give an exact picture of the formation
and allow direct measurements of rock permeability/porosity, and so on. In high
enthalpy reservoirs, coring sometimes represents the only possible way to obtain
reservoir information due to temperature limitations of logging tools.
The disadvantages are the high additional costs (due to a general time delay
compared to drilling) and sometimes a higher risk for the borehole. However,
the additional cost should be compared to geophysical logging runs which may
be needed if no cores are taken. Sometimes coring may represent the only
opportunity to obtain detailed reservoir information.
Two different techniques are to be mentioned:
• Continuous coring (wireline coring): The wireline technique is mainly used if a
long section of the borehole is to be cored. It reduces the time to retrieve the core
to the surface remarkably because there is a need to pull the whole drillstring only
if the core bit has to be changed; otherwise the inner core barrel which contains
the drilled core is pulled out of the hole by wireline. Of course this principle
only can work if the drillstring has an inside diameter larger than the outer
diameter of the inner core barrel. Therefore, normally special drillstrings – and
sometimes even special drillrigs – are used in conjunction with wireline coring.
• Spot coring: In most of the cases ‘‘spot coring’’ will be done, which allows
recovering cores between 9 and 27 m length per run. It allows using conventional
drillpipe as it is also used for normal drilling operations.

3.3
Drilling Mud

The drill mud has a lot of different functions in a borehole. To have the proper drill
mud in the hole is of high importance for its success. Hence, sometimes the mud
is called to be the ‘‘blood’’ of a borehole!