Page 274 - Petrophysics 2E

RESISTIVITY OF SHALY (CLAYEY) RESERVOIR ROCKS 247

The value of Rshd is difficult to evaluate. But because, in most shaly
sands, it is much greater than R,, its exact value is not too critical
and Equation 4.77 simplifies to (for Rshd >> Rw):

(4.78)

The total porosity &, is measured by the sonic log, while fshd is
determined from the sonic and density logs. IfRshd is not much larger
than R,, as is the case in the Rocky Mountain area where Rw/Rshd sz
0.25, Equation 4.78 overestimates S,.
(3) Structural shale exists as grain of clay forming part of the solid
matrix along with sand grains. This type of clay distribution is a rare
occurrence. They are considered to have properties similar to those
of laminar shale, as they are both of depositional origin. They have
been subjected to the same overburden pressure as the adjacent thick
shale bodies and. thus are considered to have the same water content.

Different clay distributions will affect the effective porosity and
permeability in a drastically different manner. A porositydependent
cutoff for reservoir permeability depends greatly on the distribution
mode and type of clay minerals present [24]. Figure 4.31 illustrates
the porosity-permeability relationship in fine-grained, well-sorted
sandstones as a function of clay minerals present in the reservoir rock.
For laminated shaly sands, in which sand and shale layers alternate, the
effective porosity or the fractional volume occupied by the fluid in the
sand-shale mixture, @m, is:

where ($cs is the maximum or total clean sand porosity and (1 - Vsh)
is the fractional volume occupied by the porous clean sand layer. The
fraction of the clayey rock matrix occupied by sand grains is:

For the case of dispersed shales, the effective porosity of the rock
matrix depends on the dispersed shale content. Inasmuch as shale fills
the intergranular pores up to Vsh = @cs, and for higher clay content the
sand grains are in suspension, the effective porosity is [25]:

(4.81)