Page 313 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 289

2.8.2
Formation Evaluation [ ]

at high angle, though not recommended for horizontal cores. An ori-
ented core can be cut. Usually available in 30 ft sections, which can be
made up to 180 ft. Special core catchers can be used in the standard
barrel to allow recovery of soft, unconsolidated cores.
Rubber sleeve coring. A rubber sleeve is stretch-wrapped around the
core, supporting its weight and protecting it against fluid washing. This
is able to recover cores from unconsolidated or badly fractured rock
that will not support its own weight in a conventional barrel.
Operation of the system is complicated and it is recommended that an
experienced operator be available on the rig site to supervise the cor-
ing. The application is restricted to near-vertical wells and formations
of soft to medium compressive strength.
Horizontal coring. Systems are available to cover medium (286-715
ft) radius and short (20-40 ft) radius wells. Outer barrel stabilization
and roller bearings supporting the inner barrel are required. The sys-
tem can be run on rotary, or can be driven by a positive displacement
mud motor.
Applications include all formations except extremely hard and
abrasive, high angle, or horizontal. Oriented cores can also be taken.
Slim hole coring. It is possible in slim holes to drill using a contin-
uous coring method. The core bit has a drill plug that can be wireline
removed and an inner barrel set on wireline. After coring the length of
the inner barrel, the barrel and core are recovered on wireline. Another
core can then be cut or the drill plug can be replaced so that full-hole
drilling is resumed. This is only applicable in near vertical wells.
Low-invasion coring. This option is available where the core needs
to be contaminated by the mud as little possible. The corehead has no
inner gauge protection cutters. This leaves the initial filter cake that
forms on the core OD undisturbed. The core moves up into a special
inner barrel shoe positioned immediately above the last inner cutter,
protecting it from washing by the mud coming through the bit.
The mud has to be designed to be low fluid loss with the cored for-
mation, incorporating solid particles sized correctly for efficient bridg-
ing. It can be run in wells up to horizontal, oriented if required.
Pressure coring. This system allows the recovery of cores in near-
ly in-situ conditions. Advantages of this over conventional coring
methods include better measurements of fluid saturation, permeabil-
ity, gas content, and mechanical property data. The core barrel is suit-

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