Page 27 - Well Logging and Formation Evaluation

Basics 17

• Cutting plugs from the core at the wellsite, which may be sealed and
used to measure the formation fluids
• Using tracers in the mud to attempt to quantify the extent of invasion
of drilling fluid

If samples have been obtained and preserved so that it is expected that
the in-situ fluids are representative of the formation, the following
techniques may be applied:

• Centrifuging of samples to produce formation water, which can be
analyzed for chemical composition and electrical properties
• Applying Dean-Stark analysis to determine the relative amounts of

water and hydrocarbons, thereby producing a measurement of S w

1.6.2 Conventional Core Analysis

As soon as possible after drilling, sections of the core (typically 0.5m
every 10min) are sealed and kept as preserved samples. The remaining
whole core is typically cleaned, slabbed, and laid out so that the geolo-
gist and petrophysicist can visually inspect the core and examine any
sedimentary features. Important information the petrophysicist can learn
from such an inspection include:

• The homogeneity of the reservoir and any variations that are likely to
be below the resolution of logging tools
• The type of cementation and distribution of porosity and permeability
• The presence of hydrocarbons from smell and appearance under
ultraviolet (UV) light. Oil/water contacts (OWCs) can sometimes be
established in this way
• The types of minerals present
• Presence of fractures (either cemented, natural, or drilling induced) and
their orientation
• Dip features that may influence logging tools’ response

After slabbing, the usual procedure is for conventional plugs (typically
0.5in. diameter) to be cut at regular intervals. The plugs are then cleaned
by refluxing with a solvent for 24 hours and dried at a temperature that
will remove any water (including clay-bound water). These plugs are
then measured for porosity (using a helium porosimeter), horizontal