Page 28 - Basic Well Log Analysis for Geologist

BASIC RELATIONSHIPS OF WELL LOG INTERPRETATION

Figure 4. Horizontal section through a permeable water-bearing formation and the concomitant resistivity profiles which occur
when there is invasion by either freshwater- or saltwater-based drilling muds (see Fig. 5 for resistivity profiles in a
hvdrocarbon-bearing formation).

Note: These examples are shown because freshwater muds and saltwater muds are used in different geographic regions,
usually exclusively. The geologist needs to be aware that a difference exists. To find out which mud is used in your area. ask
your drilling engineer. The type of mud used affects the log package selected. as we will see later
Freshwater Muds—tThe resistivity of the mud filtrate (R,,;) is greater than the resistivity of the formation water (R,)
because of the varying salt content (remember. saltwater is conductive). A general rule when freshwater muds are
used is: Ry > 3Ry. The flushed zone (R,,). which has a greater amount of mud filtrate, will have higher
resistivities. Away from the borehole, the resistivity of the invaded zone (Rj) will decrease due to the decreasing
amount of mud filtrate (R,,s) and the increasing amount of formation water (Ry).
With a water-bearing formation, the resistivity of the uninvaded zone will be low because the pores are filled with
formation water (R,,). In the uninvaded zone, true resistivity (R,) will be equal to wet resistivity (R,) because the
formation is [00% saturated with formation water (R, = R, where the formation is 100% saturation with formation
water).
To summarize: in a water-bearing zone. the resistivity of the flushed zone (R,,) is greater than the resistivity of the
invaded zone (R,) which in tum has a greater resistivity than the uninvaded zone (R,). Therefore: Ry, > Ry o-> Ry
in water-bearing zones.
Saltwater Muds—Because the resistivity of mud filtrate (R,,-) is approximately equal to the resistivity of formation water
(Rar = Ry), there is no appreciable difference in the resistivity from the flushed (R,,) to the invaded zone (Rj) to
= Rj = R,); all have low resistivities.
the uninvaded zone (R,, XO
Both the above examples assume that the water saturation of the uninvaded zone is much greater than 60%.