Page 156 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

Formation Evaluation 125

the interface between fresh mud filtrate and salty formation water. This interface
is usually a few inches to a few feet away from the borehole. Only two ions are
assumed to be in solution in the mud and formation water: Na' and C1-. Chloride
ions are concentrated in the formation water, and being more mobile than Na+
ions, move toward lower concentrations in the borehole (Figure 5-70). This
creates a net negative charge near the borehole and a current flows toward the
undisturbed formation. The liquidjunction potential accounts for about 20% of
the electrochemical component.
The membrane (E,) potential is created at the bed boundary between a
permeable bed (sand) and an impermeable bed (shale). The shale acts as an
ion-selective membrane, allowing only the smaller Na' ion to move through the
clay crystal structure from the salty formation water toward the fresh drilling
fluid in the bore. This creates a net positive charge along the shale. It also
creates a large concentration of negative charges associated with the C1- ion in
the permeable bed. This phenomena is also shown in Figure 5-70. The mem-
brane potential accounts for about 80% of the electrochemical potential. The
total effect of these two potentials is a net negative charge within the permeable
zone when the connate water is saltier than the mud filtrate.

lntepretation. The total electrochemical component of the total potential is what
the SP records. It can be calculated with the following equation:

(5-94)

FRESH MUD
(Few No' B CI- Ionsf
63 ALTY FORMATION WATER

MUD CAKE-*

BOREHOLE
PERMEABLE FORMATION

Figure 5-70. Ionic movement that contributes to the development of an
SP curve.