Page 321 - Petrophysics 2E

292 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

The bulk density, Pb in g/cm3, is read from the density log. The density
of fluid in pores, generally mud filtrate, is 1 .O when fresh muds are used
and 1.1 for salty drilling muds. If the formation is saturated with gas in the
vicinity of the borehole, Le., little or no mud invasion, = 0.7 g/cm3
[25]. In shaly (clayey) formations Equation 4.157 becomes [43]
:

(4.158)

Determining porosity, especially in carbonate rocks, is one of the most
important applications of neutron logs. These rocks generally contain
smaller amounts of clay minerals than the sandstones. Neutron logs also
can be used to define bed boundaries and, when used in conjunction
with other logs, as an indicator of lithology of gas-bearing zones. Modern
neutron log data is recorded directly in apparent porosity units with only
a minor correction, required to account for salinity, temperature, and
tool positioning. Porosity can be determined from the combination of
neutron and density logs using Equation 4.96. The presence of shale (clay)
in reservoir rocks (sandstone, limestone, or dolomite) will influence to
some degree the measured response of all three porosity logs.
As previously stated, the standard practice at present for estimating
the reservoir permeability distribution is to combine permeability values
obtained from laboratory measurements on cores with logderived param-
eters, such as porosity and water saturation. The following summary of
a field case is a typical example of such practice [50].

EXAMPLE

The Howard-Glasscock field is located south of Big Spring, Texas, and
has produced oil from the lower Grayburg and San Andres carbonate
formations since 1929. The 80-well field was unitized in 1972 and, during
1973,40 additional wells were drilled to expand the ongoing waterflood,
which was initiated in 1964. Ten wells were cored and 38 wells were
logged. The objective is essentially to [50]:

(a) calculate average values of permeability, porosity and water
saturation,
(b) prepare contour maps of porosity and water saturations, and
(c) estimate oil reserves.

Core Interpretation of Data
The core recovery efficiency was 98.2% or an average of 404.5ft
of core recovered per 411.8ft attempted per well. This recovery is