Page 58 - Basic Well Log Analysis for Geologist

RESISTIVITY LOGS

The deep induction log (Ry gq) does not always record an gamma ray log is run in track #1 as a lithology and
accurate value for deep resistivity in thin, resistive (where correlation curve (Fig. 21). A Microlaterolog* ts sometimes
R, >> 100 chm/meters) zones. Therefore, an alternate recorded in track #3 (Fig. 21).
method to determine true resistivity (R,) should be used.
The technique is called R, minimum (R, min) and is Dual Laterolog-Microspherically Focused Log*
calculated by the following formula:
The Dual Laterolog* (Fig. 22) consists of a deep reading
Ry min = (LL-8* or SEL*) X Ry/Ring (R,) resistivity device (Ry, and a shallow reading (Rj)
4)
resisitivity device (Ry, s). Both are displayed in tracks #2
Where:
and #3 of the log on a four cycle logarithmic scale. A
Rimin = true resistivity (also called R, minimum)
natural gamma ray log is often displayed in track #1 (Fig.
Rar = resistivity of mud filtrate at formation
22).
temperature
The Microspherically Focused Log* is a pad type.
Ry, — = resistivity of formation water at formation
focused electrode log (a pad type focused electrode log has
temperature
¢lectrodes mounted in a pad that is forced against the
LL-8*= shallow resistivity Laterolog-8*
borehole wall) that has a very shallow depth of
SFL* = shallow resistivity Spherically Focused Log*
Investigation, and measures resistivity of the flushed zone
The rule for applying R, min is to determine R, from both the (R,,). When a Microspherically Focused Log (MSFL*) is
Dual [Induction Focused Log tornado chart (Fig. 19) and run with the Dual Laterolog* (Fig. 22), the resulting three
from the R, 3, formula, and use whichever value of R, is the curves (i.e. deep, shallow, and MSFL*) are used to correct
greater. In addition to the R, »j, method for determining R, (for invasion) the deep resistivity (Ryyg) to true formation
in thin resistive Zones, correction curves (Schlumberger. resistivity (Suau ct al, 1972). A tornado chart (Fig. 23) ts
1979, p, S4-55) are available to correct the deep induction necessary to correct Ry yg to R, and to determine the
log resistivity (Ry_g) to R,. diameter of invasion (d;) and the ratio of R/R,,. The
procedure is illustrated in Figure 23.
Laterolog*
Microlog (ML*)
The Laterolog* is designed to measure true formation
resistivity (R,) in boreholes filled with saltwater muds The Microlog* (Fig. 24) is a pad type resistivity device
(where Ry = Ry). A current from the surveying electrode that primarily detects mudeake (Hilchie, 1978). The pad is
is forced into the formation by focusing electrodes. The in contact with the borehole and consists of three clectrodes
focusing electrodes emit current of the same polarity as the spaced one inch apart. From the pad, two resistivity
surveying electrode but are located above and below it. The measurements are made; one ts called the micro normal and
focusing. or guard electrodes, prevent the surveying current the other is the micro inverse (Fig. 24). The micro normal
from flowing up the borehole filled with saltwater mud device investigates three to four inches into the formation
(Fig. 20). The effective depth of Laterolog* investigation is (measuring R,,,) and the micro inverse investigates
controlled by the extent to which the surveying current Is approximately one to two inches and measures the
focused. Deep reading Laterologs* are therefore more resistivity of the mudcake (R,,.). The detection of mudcake
strongly focused than shallow reading Laterologs*. by the Microlog* indicates that invasion has occurred and
Invasion can influence the Laterolog*. However, because the formation is permeable. Permeable zones show up on
resistivity of the mud filtrate is approximately equal to the the Microlog* as positive separation when the micro normal
where Rip =
curves read higher resistivity than the micro inverse curves
resistivity of formation water (Ry = Ry) when a well is
drilled with saltwater-based muds, invasion does not
(Fig. 24). * Shale zones are indicated by no separation or
-
strongly affect R,; values derived from a Laterolog*. But,
“negative separation” (i.e. micro normal < micro inverse).
when a well is drilled with freshwater-based muds (where
Ray? 3 Ry, the Laterolog* can be strongly affected by
F Positive separation can only occur when Rane © Rin &* Rie. To verify these
invasion. Under these conditions, a Laterolog* should not
values if therc is any doubt, check the log heading for resistivity values of
be used (see Fig. 16). The borehole size and formation
the mudeake. drilling mud, and mud filtrate.
thickness affect the Laterolog*, but normally the effect is
Remember that even though the resistivity of the mud filtrate (Rit) ts
less than the resistivity of the mudeake (Rw). the micro normal curve will
small enough so that Laterolog* resistivity can be taken as
read a higher resistivity ina permeable zone than the shallower-reading
R,.
micro inverse curve. This is because the filtrate has invaded the formation,
The Laterolog* curve (Fig. 21) appears in track #2 of the
and part of the resistivity measured by the micro normal curve ts read from
log and has a linear scale. Because saltwater-based mud
the rock matrix, Whereas the micro inverse curve measures only the
Ry gives a very poor SP response. a natural
mudeake (Rime) which has a lower resistivity than rock.