Page 243 - Petrophysics 2E

2 16 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

resistivity value of Rw25 to a concentration value:

0.03207
Csm = 1.54681 (4.21)
(Rw25)

Equation 4.21 is, for practical purposes, an acceptable representation
of the data used to plot Figure 4.1. However, if a higher degree of accuracy
is required for concentration, then the following iteration scheme will
yield the desired value of Csm for a given value of Rw25:

Calculate an initial value of CSm by substituting the value of Rw25 into
Equation 4.2 1.
Substitute this value of Csm into Equation 4.13 and calculate the
concentration correlation function &.
Calculate a new value of Rw25 by substituting the value of X, into
Equation 4.12.
Calculate the resistivity value and the calculated ARw25, between the
given resistivity value and the calculated Rw25 value. If ARw25 is less
than ohm-m, or some other required degree of accuracy, then
the calculated value of Csm in Step a is accurate. If not, the value
of Csm is increased (for ARw25 > 0 or decreased for ARw25 < 0)
and Steps b, c, and d are repeated until the criterion of accuracy is
satisfied.

If concentration in ppm is required, substitute the calculated
concentration value, Csm, into Equation 4.17 and calculate C,. A more
accurate value of C,, can be obtained by substituting the value of C,,
into the density Equation 4.16 and then calculating the value of C,, from
Equation 4.15.

Shale-Properties Method
The shale-properties method estimates Rw from the shale beds
associated with the formation of interest [5]. Assuming the water of the
shale completely occupies the total pore space of the shale, the shale
formation factor Fsh is obtained from:

(4.22)

where $ash is the apparent porosity of the shale, and msh is the
cementation exponent for shales. The water resistivity of the shale zone
is calculated from:

(4.23)