Page 287 - Petrophysics 2E

260 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

Combining Equations 4.122-4.126, one can obtain the Waxman-Smits
general equation for water-saturated shaly (clayey) sands:

(4.127)

with C,,, the equivalent conductance of clay exchange cations,
expressed in mho/cm2/meq-' and given by:

c,, = 10-~h~, (4.128)

Equation 4.127 can be written as follows:

(4.129)

or:

(4.130)

where FR is the Archie formation resistivity factor for clean sand, Cw/Co.
Hoyer and Spann used Equation 4.129 to quantify the effects of shaliness
on electric log response [29]. They showed that ifCeqQ,/Cw < 0.10, the
shaliness effect is negligible, and conventional clean sand relationships
can be used to interpret well logs. Shaliness effect is significant when
Ceq&/C, > 0.10, and shaly sand relationships must be used to interpret
logs. Several methods are available for measuring the cation exchange
capacity of a rock, Qv. Mortland and Mellow described a procedure
for measuring CEC that gives results with an accuracy equivalent to
that using the ammonium acetate method [30]. This procedure requires
repeated equilibration of the crushed rock sample with concentrated
barium chloride (BaC12) solution and washing to remove excess barium
ions, followed by conductometric titration with standard magnesium
sulfide (MgSO4) solution. Table 4.8 gives porosity, @; permeability, k;
formation resistivity factor, F* ; cation exchange capacity, Qv; water
conductivity, Cw; and rock conductivity at 100% water saturation, Co,
of some of representative sandstone samples.
Waxman and Smits presented a lengthy but rigorous procedure
to determine CEC [28]. Because the method used was destructive,
conductivity measurements were made on rock samples adjacent to the
locations where the cores originated. The rock samples were equilibrated
by repeated flushing and storing in a desiccator filled with the appropriate
salt solution. The CEC and conductivity measurements were made at
25OC on the shaly sands equilibrated with NaCl solutions at 10 different