Page 186 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

Formation Evaluation 155

8-in. f203-mm) Hole
20

15

i mm
10
9
a
7
6
-
R1x1
Rmc 5
4

3

2

1.5

1
1 1.5 2 3 4 5 6 7 a 910 15 20
-
R2
Rmc

Figure 5-86. Chart for finding R, from microlog readings [199].

(text continued from page 151)
be estimated. The microlog gives a reasonable value of Rxo when mudcake
thickness is known. The main disadvantage of this tool is that it cannot be
combined with an R, device, thus a separate logging run is required. The
microlog is primarily a fresh mud device and does not work very well in salt-
based muds [206]. Generally the mudcake is not thick enough in salt-based muds
to give a positive separation opposite permeable zones. The backup arm on the
microlog tool, which provides a caliper reading, is also equipped with either
a proximity log or a Microlaterolog. The proximity log is designed for fresh
muds where thick mudcakes develop opposite permeable formations. There is
essentially no correction necessary for mudcakes less than in. If invasion is
shallow, the R, measurement may be affected by R, because the proximity log
reads deeper into the formation than the other microresistivity logs. When the