Page 26 - Well Logging and Formation Evaluation

16 Well Logging and Formation Evaluation

slower. This is particularly true where nuclear sources have been lost in
the hole, and every effort should be made to ensure that they are recov-
ered undamaged at the surface. In the event of nuclear sources being lost
irretrievably in the hole, which occasionally happens (particularly with
LWD tools), there are usually special procedures to be followed involv-
ing notification of government bodies and steps taken to minimize the risk
of and monitor any potential nuclear contamination that may occur.

1.6 CORING

1.6.1 Core Acquisition

Particularly during the exploration phase of a field, coring presents an
important means to calibrate the petrophysical model and gain additional
information about the reservoir not obtainable by logs.
Usually the decision of when and where to core will be made in con-
junction with the geologist and operations department, taking into account
the costs and data requirements. Generally speaking, it is considered
essential to at least attempt to core a part of the main reservoir formation
during the exploration and appraisal phases of drilling.
A so-called conventional core will usually consist of multiples of 18m
3
and be 4 in. in diameter. The outer barrel has a diameter of 6 / 4 in. It is
acquired while drilling using a metal sleeve into which the core passes
during drilling. At the end of coring, the core barrel is retrieved at the
surface and the core recovered from the barrel and laid out in 3-ft
sections in core boxes for initial assessment on the wellsite and then
transportation to the designated core laboratory. Special techniques may
sometimes be proposed to improve the quality of the core and to preserve
the in-situ fluids. These include:

• Using a large-diameter core (5in.)
• Using a fiberglass or aluminium inner sleeve, which may be cut into
sections at the surface, thereby preserving the core intact within the
sleeve
• Sponge coring, whereby a polyurethane material surrounds the core in
the inner sleeve, thereby absorbing and retaining any formation fluids
• Resin coring, whereby a special resin is injected onto the surface of the
core to seal the fluids inside
• Freezing the core as soon as it reaches the surface in order to preserve
the fluids inside