Page 157 - Hydrocarbon Exploration and Production Second Edition

144 Data Gathering

changes in original core properties through alteration of formation clay mineralogy,
precipitation of minerals or evaporation of pore fluids will cause inaccuracies in
petrophysical measurements. Additionally, mechanical damage can render the whole
core useless for testing purposes. Significant technological and procedural advances
have been made in the past 10 years to reduce friction, minimize filtrate invasion,
retain fluids and maintain rock integrity.
The borehole section which has been cored will subsequently be logged using
wireline tools (see Section 6.3.4). Upon arrival in the laboratory a GR measurement
will be taken from the core itself, thus allowing calibration of wireline logs with core
data. Additionally, if the core is still in a sleeve a CT scan (X-ray scan) may be made of
the whole core to identify optimum locations to sample and slice the core.
In addition to a geological evaluation on a macroscopic and microscopic scale,
plugs (small cylinders of 3 cm diameter and 5 cm length) are cut from the whole
core, usually at about 30 cm intervals. Core analysis is carried out on these samples.
Routine core analysis of plugs will include determination of
porosity
horizontal air permeability
fluid saturation
grain density.
SCAL will include measurements of
electrical tests (cementation and saturation exponents)
relative permeability
capillary pressure
strength tests.

Finally, the core will be sectioned (one third:two thirds) along its entire length
(slabbed) and photographed under normal and ultraviolet light (UV light will reveal
hydrocarbons not visible under normal light, as shown in Figure 6.33).
The main cost factor of coring is usually the rig time spent on the total operation
rather than the follow-up investigations in the laboratory. Core analysis is complex
and may involve different laboratories. It may therefore take months before final
results are available. As a result of the relatively high costs and a long lead time of
some core evaluations, the technique is only used in selected intervals in a number
of wells drilled. It is therefore vital that efforts are made to maximize core recovery
and integrity so that all the core material cut can be used.
Mudlogging is another important direct data gathering technique. The returns to
surface (drill cuttings and gas levels) and ROP are continuously recorded and
analysed to establish the nature of the formation and fluid fill.

6.3.3. Sidewall sampling

The sidewall sampling tool (SWS) can be used to obtain small plugs (2 cm diameter,
5 cm length, often less) directly from the borehole wall. The tool is run on wireline
after the hole has been drilled and logged. Some 20–30 individual bullets are fired
from each gun (Figure 6.34) at different depths. The hollow bullet will penetrate the