Page 28 - Well Logging and Formation Evaluation

18 Well Logging and Formation Evaluation

permeability, and grain density. Additional plugs are cut in the axis of the
core to determine vertical permeability.
Usually a gamma ray detector or density-type device is run over the
whole length of the core in order to provide a reference log that may be
correlated to the wireline data. Since the “driller’s depths” to which a core
is referenced are typically different from “logger’s depths,” as measured
by wireline, it is necessary to make a shift before the core may be com-
pared to logs. The conventional-plug measurements are usually performed
at ambient conditions (or sometimes a few hundred psi confining pres-
sure) and therefore need to be corrected to in-situ conditions before they
may be compared to the logs. The correction factors to be used are deter-
mined through further special core analysis (SCAL).

1.6.3 Special Core Analysis

SCAL measurements are typically performed on a special set of larger-
diameter (1.5in.) plugs cut from the core. These may be cut at a regular
sampling increment, or the petrophysicist may specify certain depths
based on the results of the conventional analyses. The most important
criterion is obviously to obtain a broad spectrum of properties that fully
encompass the range of properties seen in the reservoir.
In order to ensure that the SCAL plugs are homogeneous, it is normal
procedure to subject the plugs to a CAT (computed axial tomography)
scan prior to using them for future measurements. It is hard to say how
many SCAL plugs are required for a typical program, since this depends
on the reservoir type, thickness, and homogeneity. In general a SCAL
program may use between about 5 and 50 plugs.
While many measurements are possible on core plugs, I will concen-
trate on the ones that are of direct relevance to the petrophysical model.
These are:

• Porosity and permeability at overburden conditions. Here it is
important to state the pressures at which the measurements should be
performed. In Chapter 7 the equations are given for calculating the
equivalent isostatic stress at which the measurements should be per-
formed to be equivalent to in-situ conditions. Typically measurements
are made at five pressures that will encompass the likely range of
pressures to be encountered during depletion of the reservoir.
• Cementation exponent (m). In this measurement, the resistivity of the
plugs is measured when they are 100% saturated with brine represen-