Page 231 - Petrophysics

204 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

1,000 ohm-m or more depending on the amount of shale inter-bedding.
In non-porous carbonate rocks, resistivity may be as high as a few
million ob-m. The resistivity of reservoir rocks is a function of salinity
of formation water, effective porosity, and quantity of hydrocarbons
trapped in the pore space [l]. Relationships among these quantities
indicate that the resistivity decreases with increasing porosity and
increases with increasing petroleum content. Resistivity measurements
are also dependent upon pore geometry, formation stress, composition
of rock, interstitial fluids, and temperature. Resistivity is, therefore,
a valuable tool for evaluating the producibility of a formation.

FORMATION RESISTIVITY FACTOR

A rock that contains oil and/or gas will have a higher resistivity than
the same rock completely saturated with formation water, and the
greater the connate water saturation, the lower the formation resistivity.
This relationship to saturation makes the formation resistivity factor an
excellent parameter for the detection of hydrocarbon zones.

RESISTIVITY MEASUREMENT

The resistance of brine in a container of length L and cross-sectional
area A to the flow of electricity is measured by applying a voltage E,
in volts, across the liquid and recording the amount of current I, in
amperes, that will flow, as shown in Figure 4.1. According to Ohm’s
law the resistance, r,, is equal to:

E
r, = - (4.1)
1,

The resistivity of the brine is:

A EA
Rw = rwL = -- (4.2)
I.
I,
Now consider a block of porous rock (clean sand) of the same
dimension A and L, and 100% saturated with the same brine (Figure 4.2).
On applying the same voltage E across the block of sand, a current Io will
flow. The resistivity of this porous rock sample, &, is:

A EA
& = ro- = -- (4.3)
I. I, I.