Page 300 - Petrophysics 2E

LAB-DERIVED EVALUATION OF SHALY 271

The equivalent conductance of clay-exchange cations B can be
estimated from Equation 4.138, with C,, = B:

(-">
B = 4.66(1 - 0.6exp -

B = 4.66(1 - O.6exp - 4.66
=
(io:)
Substituting these values into Equation 4.140a:

(4)s: + (4 x 0.04 x 4.66 x 0.636)S, - 12.43 x 0.04 = 0

(4) S: + (0.4742) S, - 0.4972 = 0

The positive solution of this quadratic equation is:

S, = 29.8%.

EXAMPLE

The Shannon sandstone of the Teapot Dome field is composed of fine-
to medium-sized sand particles containing dispersed clay. The formation
water is relatively fresh, and salinity varies from a low of 3,700 ppm in
the northern portion of the field to a high of 13,000 pprn in the southern
portion. Reservoir temperature also varies across the Shannon Field, from
a high of 118'F in the northwest part to 70"-95°F in the southern and
eastern parts of the reservoir [32].
The adsorbed water technique was used to measure the cation
exchange capacities (CEC) of a large number of cores obtained from
several wells [33]. CEC values ranged from a low of 1.58 meq/100g
in samples containing no visible clays, to a maximum of 8.65 in cores
described as highly shaly. Similar CEC values were found in all wells.
A variety of correlations of the measured CEC values with the Shannon
sandstone porosities were attempted Wigure 4.43), and though none
were found acceptable, it was observed that CEC values increase with
decreasing porosity. This is because the loss of porosity in the Shannon
reservoir is caused by an infilling of clays with an associated increase
in CEC. The salinity of the brine used to saturate core samples for the
laboratory electrical property tests is 15,000 ppm NaCl and the resistivity
of this brine at 25°C is 0.386 ohm-m.
Table 4.10 shows the laboratory-derived values of 0, FRL, and CEC,
expressed in meq/g of rock sample, for 15,000 ppm NaCl at 25°C. The
measured resistivity index values and the corresponding S, values are
presented in Table 4.1 1.