Page 146 - Petroleum Geology

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Fig. 6-8. Invasion of homogeneous, permeable, water-saturated bed by mud filtrate (plan
and section). 1 = borehole filled with mud; 2 = mud cake; 3 = flushed zone; 3 and 4 = in-
vaded zone; 5 = uncontaminated zone. Invasion of petroleum-bearing permeable beds
differs mainly in that there is a greater tendency for gravity segregation, invasion tending
to be deeper at the bottom of the bed.

In spite of these difficulties, it will be clear that within the flushed zone
there will be a fluid the resistivity of which can be measured. If the tempera-
ture of the filtrate in the rock is known, then the Formation Factor can be
obtained, in theory, by applying eq. 6.8b:

where Rmf is the resistivity of the mud filtrate at the temperature of the
flushed zone, Rxo and sxo the resistivity and water saturation of the flushed
zone. In a water sand, s,, = 1. It must be remembered, however, that these
measurements are in the zone most affected mechanically by the drilling,
so the value of the Formation Factor obtained may not be representative
of the bed.
The Microlog, which is the most important resistivity device for the
geologist after the Normals, makes use of these disturbances close to the wall
of the borehole with electrodes of very short spacing. It consists of a two-
inch Normal (5 cm) and a one-inch by one-inch Inverse in an insulating pad