Page 57 - Basic Well Log Analysis for Geologist

Vail Ts ViTyY LOGS

Table 4. Classification of Resistivity Logs.

INDUCTION LOGS (measure conductivity)

ELECTRODE LOGS (measure resistivity)
A. Normal logs Microlaterolog (MLL)*
B. Lateral Logt Microlog (ML)*
C. Laterologs* rams Proximity Log (PL)*
D. Spherically Focused Log (SFL)* Microspherically Focused Log (MSFL)*
DEPTH OF RESISTIVITY LOG INVESTIGATION

(Ry)
Zone
Flushed Zone (Ryo) Invaded Zone (Rj) Uninvaded

Microlog* Short Normal ** Long Normal
Microlaterolog* Laterolog -8*7* Lateral Log
Proximity* Log Spherically Focused Log*** Deep Induction Log
Microspherically Focused Log* Medium Induction Log Deep Laterolog*
Shallow Laterolog* Laterolog -3*
Laterolog -7*
Induction Log 6FF40

*Fora review of how to use Lateral logs see: Hilchie (1979).
When R,, is much greater than R,, the Laterolog — 8* and Spherically Focused Log* will have a shallower depth of investigation (closer to R,,)
than the medium induction, shallow Laterolog*, and the short normal.

curve is sometimes displayed in track #2 along with the Dual Induction Focused Log
short normal curve.
The modern induction log is called the Dual Induction
Induction—The induction device (Fig. 17) measures
Focused Log (Tixier et al, 1963). This log (Fig. 18) consists
electrical conductivity® using current generated by coils.
of a deep-reading induction device (Ry_g which measures
The transmitting coils produce an electromagnetic signal
R,), and is similar to an Induction Electric Log. The Dual
which induces currents in the formation. These induced
Induction Focused Log (Fig. 18) also has a medium-reading
currents are recorded as conductivity by receiver coils.
induction device (Rg Which measures R;) and a shallow
Modern induction devices have additional coils which focus
reading (Rx) focused Laterolog* which is similar to the
the current so that signals are minimized from adjacent
short normal. The shallow reading Laterolog* may be either
formations, the borehole. and the invaded zone. By
a Laterolog-8 (LL-8)* or a Spherically Focused Log
focusing the current and eliminating unwanted signals, a
(SFL)*.
deeper reading of conductivity is taken, and more aceurate
The Dual Induction Focused Log is used in formations
values of true formation resistivity (R,) are determined from
that are deeply invaded by mud filtrate. Because of deep
the induction log. The induction log has a transmitter/re-
invasion, a deep reading induction log (Ry.g) May not
ceiver spacing of 40 inches and can measure a reliable value
accurately measure the true resistivity of the formation (R,).
for resistivity down to a bed thickness of five feet.
Resistivity values obtained from the three curves on a Dual
The induction curve on the Induction Electric Log
Induction Focused Log are used to correct deep resistivity
appears in track #2 (Fig. 17). Because the induction device
(Riyg) to true resistivity (R,) from a tornado chart (Fig. 19).
is a conductivity measuring tool, an induction derived
This tornado chart (Fig. 19) can also help determine the
conductivity curve is presented in track #3 (Fig. 17). The
diameter of invasion (d;) and the ratio of R,,/R;. An
track #3 conductivity curve is necessary to more accurately
ranging from 0.2 to 2000 ohm/meters (Fig. 18) and
determine the R, value of low resistivity formations, and to example of the procedure is presented in Figure 19.
climinate possible errors when calculating true resistivity The three resistivity curves on the Dual Induction
from conductivity. Because the induction log does not Focused Log are recorded on a four cycle logarithmic scale
require the transmission of electricity through drilling fluid,
it can be run in air-, oil-, or foam-filled boreholes. correspond to tracks #2 and #3 on the Induction Electric
Log. Normally, a spontaneous potential (SP) curve is placed
* C, = LlOOO/R, where C, = conducuvity in millimhos/meter, and R, = true in track #1 (Fig. 18).
formation resistivity in ohm- meters.

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