Page 230 - Petrophysics 2E

CHAPTER 4

FORMATION

RES STIV ITY
I

AND WATER

a
SATU RAT1 0 N

Sedimentary formations are capable of transmitting an electric current
only by means of the interstitial and adsorbed water they contain. They
would be nonconductive if they were entirely dry. The interstitial
or connate water containing dissolved salts constitutes an electrolyte
capable of conducting current, as these salts dissociate into positively
charged cations, such as Na+ and Ca2+, and negatively charged anions,
such as C1- and SO,. These ions move under the influence of an electrical
field and carry an electrical current through the solution. The greater
the salt concentration, the greater the conductivity of connate water.
Freshwater, for example, has only a small amount of dissolved salts and
is, therefore, a poor conductor of an electric current. Oil and gas are
nonconductors.
The electrical resistivity (reciprocal of conductivity) of a fluid-saturated
rock is its ability to impede the flow of electric current through that
rock. Dry rocks exhibit intinite resistivity. In electrical logging practice,
resistivity is expressed in ohm-m2/m or simply ohm-m. The resistivity
of most sedimentary formation ranges from 0.2 to 2OOOohm-m. The
resistivity of poorly consolidated sand ranges from 0.20ohm-m for
sands containing primarily saltwater, to several ohm-m for oil-bearing
sands. For wellconsolidated sandstones, the resistivity ranges from 1 to

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