Page 167 - Petrophysics

140 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

area [6]. The general form of the correlation is as follows:

(3.69)

where:

k = permeability to air, mD
S,, = residual water saturation, %
SvP = specific surface area
$o = open porosity, %

al, az, a3, a4, and a5 are constants for a given formation, determined
empirically.
Practically all permeability-porosity correlations should be used only
for qualitative purposes. To obtain an accurate correlation between the
porosity and permeability, one must include a large number of physical
factors that characterize a porous medium, including irreducible fluid
saturation, specific surface area, grain size distribution, grainshape,
packing and layering, lithology and mineralogy, degree and type of
cementing etc. Although some formations may show a correlation
between permeability and porosity, a large number of physical factors
influencing these two parameters differ widely in different formations.

Estimating Permeability in Carbonate Rocks

Although the absolute porosity provided by natural fractures is
negligible (< 3%), the effective porosity is considerably enhanced
because fractures connect the available pore volume. Consequently, the
reservoir permeability and petroleum recovery are greatly enhanced. The
net impact of fracture connectivity may a decisive factor in exploiting a
particular reservoir. Many methods have been proposed for estimating
fracture permeability, including parallel plate models, electric analog
systems, core analysis, we11 logging, and pressure transient testing.
The equation for volumetric flow rate between the two smooth plates,
combined with Darcy’s law, provide the basic approach for estimating
fracture permeability and its influence on fluid flow in naturally fractured
rocks. Parsons used this approach to express the total permeability
of the fracture-matrix system in which vertical fractures occur in
sets of specified spacing and orientation relative to overall pressure
gradient [39]. Murray used a parallel-plates model and a geometric
approach applicable to folded rocks to demonstrate that, in folded beds
with extension fractures normal to the bedding and parallel to the fold
axis, the fracture porosity and permeability are functions of bed thickness
and curvatures [40]. He assumed that extension fractures form primarily