Page 284 - Petrophysics 2E

JAB-DERIVED EVALUATION OF SWY 257

If the pay zone has significant shale content, i.e. Vsh > 20%, then csh
cannot be approximated with zero. Using the same approach as above,
new equations must be derived for Hsh and FZIsh. Assuming the total
shale model and Timur or Walter and Rose permeability-porosity relation-
ships are applicable, the procedure to identify and characterize flow
units is as follows:

(1) First obtain all necessary data from core analysis and well logs.
(2) Obtain m from the Pickett plot or some other source.
(3) Plot RQI versus porosity on a log-log graph.
(4) Substitute the value of m into Equation 4.112 and calculate Hsh.
(5) Draw a straight line of slope Hsh across a cluster or clusters. Clusters
that best fit this straight line constitute distinct flow units. Data or
clusters that do not fit this straight line may belong to a different flow
unit. If the shale type distribution is homogeneous and the pay zone
has several flow units, then the straight lines should be parallel with
the same value of Hsh but different FZI,h values (flow units 1 and 2,
Figure 4.32). If the shale type distribution is heterogeneous or if the
formation contains different types and volumes of shale, the straight
lines will not be parallel, as shown in Figure 4.32 (flow unit 3).
(6) Obtain the flow zone indicator FZI,h from the graph at $ = 1. This
value of FZI,h should be approximately the same in all wells where
the flow unit is present.

This process of segregating flow units works best in formations with
low shale content. The choice of the permeability-porosity relationship
and the shale model heavily influence this process.

LAB-DERIVED EVALUATION OF SHALY (CLAYEY)
RESERVOIR ROCKS

Not since Archie presented his classic empirical equation in 1942,
relating FR to porosity and cementation factor for clean sands, has
there been another equation of equivalent impact in petrophysics as
that of Waxman and Smits [28]. They used a simple physical model
to derive an equation that relates the electrical conductivity of the
water-saturated shaly (clayey) sand to the water conductivity and the
cation exchange capacity per unit pore volume of the rock. The model
consists of two resistance elements in parallel: one element consisting
of the free electrolyte contained in the pore volume of rock, C,l, and
another resulting from the conductance contribution of the exchange
cations associated with the clay C,. The conductance of a rock is simply