Page 193 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

164 Reservoir Engineering

vertical line from 1.9 to 2.9 g/cc. The ribs are the lighter curved lines trending
left to right. The experimental data for constructing the ‘ribs” are shown in
the corners of the plot. Long-spacing count-rates are on the abscissa and short-
spacing count-rates are on the ordinate axis. The computer receives data from
the sonde and plots it on the chart. If the point falls off the “spine,” it is brought
back along one of the ribs. Bringing the point back along the “rib” will change
the intersection point on the ‘spine.” The correction that is produced is called
Ap and may be either positive or negative depending on the mud properties.
Negative Ap values occur in heavy (barite or iron), weighted muds. Positive values
occur in light muds and when the density pad is not flush against the borehole
wall (as occurs in rough or “rugose” boreholes. The Ap curve is useful for
evaluating the quality of the pb reading. Excursions from 0 that are more than
f0.20 gm/cc on the Ap curve indicate a poor quality reading.

hferpretation. An equation similar to the Wyllie equation is used to calculate
porosity values from bulk density.

(5-106)

where p, = bulk density of matrix at 0% porosity, g/cc
pb = bulk density from log, g/cc
pf = bulk density of fluid, g/cc

Table 5-27 lists commonly used values for p, and pp and, along with Fig-
ure 5-91 shows how pr changes with temperature and pressure [58]. As with
the sonic tool, an incorrect choice of matrix composition may give negative
porosity values.
If a zone is hydrocarbon saturated but not invaded by mud filtrates, the low
density of the hydrocarbons will increase the porosity reading to a value that is
too high. In this case, Hilchie [ZOO] suggests using the following equation:

(5-107)

where pw = density of formation water, g/cc (estimated from Figure 5-91) [58]
p,, = density of hydrocarbons, g/cc (from Figure 5-92) [199]

Neutron Log. Neutron tools measure the amount of hydrogen in the formation
and relate it to porosity. High hydrogen content indicates water (H,O) or liquid
hydrocarbons (CxHz) in the pore space. Except for shale, sedimentary rocks do
not contain hydrogen in their compositions.

Neutrons
73~~. are electrically neutral particles with mass approximately equal
to that of a hydrogen atom. Highenergy neutrons are emitted from a chemical
source (usually AmBe or PuBe). The neutrons collide with nuclei of the forma-
tion minerals in elastic-type collisions. Neutrons will lose the most energy when
they hit something with equal mass, such as a hydrogen atom. A few micro-
seconds after being released, the neutrons have lost significant energy and enter