Page 233 - Petrophysics

206 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

Brlm Reai8tlvity: Ohm Me- 63 75. F

Figure 4.3. General relationship between formation factor F and brine resistivity Rw
factor (courtesy of Core Luboratories).

is an important relation in well log interpretation for locating potential
zones of hydrocarbons.
Of all the rock parameters measured by modem well logging tools,
resistivity is essential because it is used to determine water saturation,
which is then used to calculate the volume of oil- and/or gas-in-place.
The producibility of the formation also can be estimated by comparing
the resistivity close to the wellbore (i.e., flushed-zone resistivity), where
mud filtrate has invaded the formation, and resistivity of the virgin portion
of the formation @e., true resistivity, Rt.)

DETERMlNATlON OF FORMATION WATER RESISTIVITY

The value of Rw can vary widely from well to well in some
reservoirs because parameters that affect it include salinity, temperature,
freshwater invasion, and changing depositional environments. However,
several methods for determining the reservoir water resistivity have
been developed, including: chemical analysis of produced water sample,
direct measurement in resistivity cell, water catalogs, spontaneous poten-
tial (SP) curve, resistivity-porosity logs, and various empirical methods.

Chemical Analysis
Although direct measurement of Rw in a resistivity cell is aIways
preferred, chemical analysis of water samples is still performed.