Page 56 - Basic Well Log Analysis for Geologist

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CHAPTER HI

RESISTIVITY LOGS

General
Resistivity logs are electric logs which are used to: (1) nearby formations. The two types of induction devices are
determine hydrocarbon versus water-bearing zones, (2) the Induction Electric Log and the Dual Induction Focused
indicate permeable zones, and (3) determine resistivity Log.
porosity. By far the most important use of resistivity logs is A second type of resistivity measuring device ts the
the determination of hydrocarbon versus water-bearing electrode log. Electrodes in the borehole are connected to a
zones. Because the rock's matrix or grains are power source (generator), and the current flows from the
non-conductive, the ability of the rock to transmit a current electrodes through the borehole fTuid into the formation,
is almost entirely a function of water in the pores. and then to a remote reference electrode. Examples of
Hydrocarbons, like the rock’s matrix, are non-conductive; electrode resistivity tools include: (1) normal, (2) Lateral.
therefore, as the hydrocarbon saturation of the pores (3) Laterolog*, (4) Microlaterolog* , (5) Microlog*, (6)
increases, the rock’s resistivity also increases. A geologist. Proximity Log*, and (7) spherically focused logs.
by knowing a formation’s water resistivity (R,,). its porosity Induction logs should be used in non-salt-saturated
(), and a value for the cementation exponent (m) (Table 1), drilling muds (1.0. Ryy = 3 Ry) to obtain a more accurate
can determine a formation’s water saturation (S,,) from the value of true resistivity (R,). Borcholes filled with
Archie equation salr-saturated drilling muds (Ray = Ry) require electrode
logs, suchas the Laterolog* or Dual Laterolog* with or
_fFXKR,y \ra
w R, without a Microspherically Focused Log*, to determine
accurate R, values. Figure 16 ts achart which assists in
determining when use of an induction log is preferred over
Sy = water saturation an electrode log such as the Laterolog*.
F = formation factor (a/h™)
a = tortuosity factor Induction Electric Log
m = cementation exponent
The Induction Electric Log (Fig. 17) is composed of three
R,. = resistivity of formation water
curves: (1) short normal, (2) induction, and (3) spontaneous
R, = true formation resistivity as measured by a deep
potential or SP. These curves are obtained simultaneously
reading resistivity log
during the logging of the well.
nm = saturation exponent (most commonly 2.0)
Short Normal—The short normal tool measures
The two basic types of logs in use todav which measure resistivity ata shallow depth of investigation which is the
ormation resistivity are induction and electrode logs (Table resistivity of the invaded zone (R,). When the resistivity of
). The most common type of logging device is the the short normal is compared with the resistivity of the
induction tool (Dresser Atlas, 1975). deeper measuring induction tool (R,). invasion 1s detected
An induction tool consists of one or more transmitting by the separation between the short normal and induction
curves (Fig. 17). The presence of invasion ts important
dils that emit a high-frequency alternating current of because it indicates a formation is permeahle.
constant intensity. The alternating magnetic field which is
Feonductivity = |000rresistivity. Conductivity in millimhos/meter; bed thickness of four feet. The short normal curve is usually
The short normal tool has an electrode spacing of 16
created induces secondary currents in the formation. These
inches and can record a reliable value for resistivity from a
scondary currents flow as ground loop currents
, 2rpendicular to the axis of the borehole (Fig. 15), and
recorded in track #2 (Fig. {7). Because the short normal
create magnetic fields that induce signals to the receiver
tool works best in conductive, high resistivity muds (where
vis. The receiver signals are essentially proportional to
Ria o> 3 Ry). Salt muds (where R,y = Ry) are nota good
mnductivity?. which ts the reciprocal of resistivity
environment for its use. In addition to providing a value for
(Schlumberger. 1972). The multiple coils are used to focus
R;, the short normal curve can be used to calculate a value
the resistivity measurement to minimize the effect of
for resistivity porosity if a correction ts made for unflushed
aterials in the borehole. the invaded zone, and other

R; from the short normal curve. an amplified short normal
rsislivily in ohnt-melers. oil in the invaded zone. To obtain a more accurate value of
4t