Page 308 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 284

[ ] Well Programming
2.8.2

2.8.2. Coring

Coring attempts to remove a sample of formation from the well-
bore in an undamaged state. There are two basic classifications of cor-
ing operations: sidewall coring and full-hole coring.
Sidewall coring is done using a wireline tool to either blow a recov-
erable sample chamber sideways into the formation with an explosive
charge (e.g., Schlumberger CST), or to use a small rotating corehead to
cut a sample (e.g., Schlumberger MSCT). These were mentioned in
Section 2.8.1, “Electric Logging and Sampling.”
Full-hole coring uses special tools to cut and recover the core.
There is a variety of tools for various conditions, which are described
below. The single most important factor for a successful coring job is
proper planning, which should involve the drilling engineer, drilling
supervisor and/or toolpusher, geologist, mud engineer, coring compa-
ny specialist, and core analysis specialist.
Cores can be taken in any formation, at any hole angle up to hori-
zontal, oriented, in any circulating medium, using rotary or mud
motor, and kept under downhole pressure. Coring is very expensive
and selection of the best system to do the job is vital, as it ensures that
the objectives of the job are fully defined and justified.
Cores have been taken up to 600 ft in length. If a long or very long
coring run is planned, then the core barrel will require external stabi-
lization and internal bearings to support the inner liner. Long core
runs reduce rig time and therefore cost, but long core barrels should
not be used if:

1. The formation contains gas of sufficient pressure to cause the well
to kick as the gas expands when the core is pulled (except pressure
coring)
2. The bit life will be short due to the hard or abrasive nature of the
formation
3. The deviation profile of the well makes it difficult to run or pull the
core barrel
4. The likelihood of differential sticking is high since the core barrel
creates a large contact area with the formation

In general, it is best to cut and recover the largest diameter of core
possible while still allowing the barrel to be fished. This reduces the

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