Page 19 - Basic Well Log Analysis for Geologist

BASIC RELATIONSHIPS OF WELL LOG INTERPRETATION

zone are uncontaminated by mud filtrate: instead. they are hydrocarbons. As the mud filtrate invades the
saturated with formation water (Ry), ot]. or gas. hydrocarbon-bearing zone, hydrocarbons move out first.
Even in hydrocarbon-bearing reservoirs, there is always a Next. formation water is pushed out in front of the mud
of
layer formation water on grain surfaces. Water saturation filtrate forming an annular (circular) ring at the edge of the
(S,: Fig. 1) of the uninvaded zone is an important factor in invaded zone (Fig. 3). The annulus effect is detected by a
reservoir evaluation because, by using water saturation higher resistivity reading on a deep induction log than by
data. a geologist can determine a reservoirs hydrocarbon one on a medium induction log.
saturation, The formula tor calculating hydrocarbon Log resistivity profiles illustrate the resistivity values of
saturation 18: the invaded and uninvaded zones in the formation being
investigated. They are of particular interest because. by
Sp 1.0
Sy
=
—
using them, a geologist can quickly scan a log and look for
S, = hydrocarbon saturation (i.e. the fraction of pore potential zones of interest such as hydrocarbon zones.
volume filled with hydrocarbons). Because of their importance, resistivity profiles for both
Sy = water saturation uninvaded zone (i.e. fraction of water-bearing and hydrocarbon-bearing zones are discussed
pore volume filled with water) here. These profiles vary, depending on the relative
resistivity values of R,, and R,,-¢. All the variations and their
The ratio between the uninvaded zone’s water saturation
associated profiles are illustrated in Figures 4 and 5.
(Sy) and the flushed zone’s water saturation (S,,) is an
Water-Bearing Zones—Figure 4 illustrates the borehole
index of Avdrocarbon moveabilin.
and resistivity profiles for water-bearing zones where the
resistivity of the mud filtrate (Rj) is much greater than the
Invasion and Resistivity Profiles
resistivity of the formation water (R,,) in freshwater muds.
Invasion and resistivity profiles are diagrammatic.
and where resistivity of the mud filtrate (Rap) is
theoretical, cross sectional views moving away from the
approximately equal to the resistivity of the formation water
borehole and into a formation. They illustrate the horizontal
distributions of the invaded and uninvuded zones and their (Ry) in saltwater muds. A freshwater mud (i.e. Ray > 3 Ry)
corresponding relative resistivities. There are three results ina “wet” log profile where the shallow (R,,,).
medium (R;), and deep (R,) resistivity tools separate and
commonly recognized invasion profiles: (1) step, (2)
record high (R,,). intermediate (Rj). and low (R,)
transition, and (3) annulus. These three invasion profiles are
resistivities (Fig. 4). A saltwater mud (i.e. Ry = Ryyp)
illustrated in Figure 3.
results in a wet profile where the shallow (R,,), medium
The step profile has a cylindrical geometry with an
(R;), and deep (R,) resistivity tools all read low resistivity
invasion diameter equal to d. Shallow reading, resistivity
(Fig. 4). Figures 6a and 6b illustrate the resistivity curves
logging tools read the resistivity of the invaded zone (Rj).
for wet zones invaded with both freshwater and saltwater
while deeper reading, resistivity logging tools read true
muds.
resistivity of the uninvaded zone (R,).
Aydrocarbon-Bearing Zones Figure 5 illustrates the
The transition profile also has acylindrical geometry with
borehole and resistivity profiles for hydrocarbon-bearing
two invasion diameters: d; (flushed zone) and d, (transition
zones where the resistivity of the mud filtrate (R,y¢) is much
zone). It is probably a more realistic model for true borehole
greater than the resistivity of the formation water (Ry) for
conditions than the step profile. Three resistivity devices are
freshwater muds, and where Ry- is approximately equal to
necded to measure a transitional profile: these three devices
R,, for saltwater muds. A hydrocarbon zone invaded with
measure resistivities of the flushed, transition, and
freshwater mud results in a resistivity profile where the
uninvaded zones R,,. Rj, and R,; (see Fig. 3). By using
shallow (R,,), medium (R;). and deep (R,) resistivity tools
these three resistivity measurements, the deep reading
all record high resistivities (Fig. 5). In some instances. the
resistivily tool can be corrected to a more accurate value of deep resistivity will be higher than the medium resistivity.
drilled. However, it is very important to a geologist because hydrocarbon zone invaded with saltwater mud results in a
true resistivity (R,), and the depth of invasion can be
When this happens. it is called the annulus effect. A
determined. Two modern resistivity devices which use these
three resistivity curves are: the Dual Induction Log with a
resistivity profile where the shallow (R,,), medium (Rj).
Laterolog-8* or Spherically Focused Log (SFL)* and the
and deep (R,) resistivity tools separate and record low (R,,,).
Dual Laterolog* with a Microspherically Focused Log
intermediate (R;) and high (R,) resistivities (Fig. 5). Figures
(MSFL)*.
7a and 7b illustrate the resistivity curves for hydrocarbon
An annulus profile is only sometimes recorded on a log
zones invaded with both freshwater and saltwater muds.
because it rapidly dissipates in a well. The annulus profile is
detected only by an induction log run svon after a well is
Lithology—tIn quantitative log analysis, there are several
the profile can only occur in zones which bear Basic Information Needed in Log Interpretation
reasons why it is important to know the lithology of a zone