Page 280 - Petrophysics 2E

FLOW UNITS FOR SHALY SANDSTONES 253

The new estimate of water saturation is:

Using Equation 4.93 for Vsh = 0.20

(c) Repeating the calculations in (b) for Vsh = 0.40, Equation 4.88 gives
S, = 22% and So = 78%. The Simandoux equation (4.93) gives: Sw =
31% and So = 69%. Thus, the higher the shale fraction, the lower
is the estimate of water saturation, and consequently the higher the
estimate of hydrocarbon in place.

FLOW UNITS FOR SHALY SANDSTONES

The concept of flow units for clastic rocks with low shale content was
discussed in Chapter 3. In formations with high shale content the flow
unit equation (Equation 3.47) becomes:

where RQI (pm) is the reservoir quality index for the formation, and is
defined as:

(4.101)

The permeability, k, and porosity, 0, are expressed in mD and fraction,
respectively. Hsh is the slope of a straight line obtained when plotting
RQI versus porosity on a log-log graph and FZI,h is the intercept of the
straight line at Q = 1, as shown in Figure 4.32. FZIsh is the hydraulic unit
factor which uniquely defines the flow unit. Hsh is influenced primarily
by the degree of cementation of the pay zone, whereas the flow zone
indicator FZIsh is a function of the distribution and type of shale and
resistivity.