Page 40 - Well Logging and Formation Evaluation

30 Well Logging and Formation Evaluation

4. Check that the caliper is reading correctly inside the casing (find out
the casing ID) and that it is reading the borehole size in nonpermeable
zones that are not washed out.
5. Check the density borehole correction curve. It should not generally
exceed 0.02g/cc, except in clearly washed out sections (>18in.), for
which the density curve is likely to be unusable.
6. Inspect the resistivity curves. If oil-based mud (OBM) is being used,
the shallow curves will usually read higher than the deep curves
(except in highly gas or oil saturated zones). Likewise, with water-
based mud (WBM) the shallow curves will read less than the deep
curves, providing R mf < R w , or in hydrocarbon-bearing zones. In theory,
the curves should overlie each other in nonpermeable zones such as
shales. However, in practice this is often not the case, due to either
anisotropy or shoulder-bed effects.
7. Check the sonic log by observing the transit time in the casing, which
should read 47ms/ft.
8. Look out for any cycling-type behavior on any of the curves, such as
a wave pattern. This may be due to corkscrewing while drilling,
causing an irregular borehole shape. However, it is necessary to elim-
inate any possible tool malfunction.
9. Check that the presentation scales on the log print are consistent
with other wells or generally accepted industry norms. These are
generally:
GR: 0–50 API
Caliper: 8–18≤
Resistivity: 0.2–2000ohmm on log scale
Density: 1.95–2.95g/cc (solid line)
Neutron: -0.15 ± 0.45 (porosity fraction) (dashed line)
Sonic: 140–40ms/ft

2.2 IDENTIFYING THE RESERVOIR

For the next section of this chapter, it will be assumed that one is dealing
with clastic reservoirs. Carbonates and complex lithologies will be dis-
cussed later in the book. The most reliable indicator of reservoir rock will
be from the behavior of the density/neutron logs, with the density moving
to the left (lower density) and touching or crossing the neutron curve. In
clastic reservoirs in nearly all cases this will correspond to a fall in the
gamma ray (GR) log. In a few reservoirs, the GR is not a reliable indica-
tor of sand, due to the presence in sands of radioactive minerals. Shales