Page 175 - Petrophysics

148 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

the horizontal direction is considerably different than the permeability
in the y and z directions. The net impact of such changing permeability
in different directions on the natural recovery of a reservoir and the
efficiency of a waterflood project can be of significant importance.
Horizontal well test analysis and selective zonal well testing techniques
provide the estimates of directional permeability. Discussion of
horizontal well test analysis is beyond the scope of this book. Core
samples are also analyzed for directional permeability in the laboratory.
Usually core plugs used for permeability measurement in the laboratory
are cut perpendicularly (at !IO"), i.e. parallel to the bedding plane, from
the main large core taken from the wellbore. However, to measure the
vertical permeability, a core plug has to be cut in the direction of the main
core taken from the wellbore, i.e. perpendicular to the bedding plane.
The latest technological developments in well logging also provide the
estimates of directional permeability.

Anisotropy

Directional permeability is frequently used to express the degree of
heterogeneity in the formation. From the engineering point of view,
the net effect of anisotropy is the loss or gain in effective permeability
of a reservoir rock. Such loss or gain in effective permeability may be
due to increased permeability in one direction and reduced permeability
in other direction; thereby the resulting average permeability is always
less than the highest permeability in any direction in the reservoir.
For example, reservoirs with vertical fractures have higher fracture
permeability in the vertical direction and low matrix permeability in the
horizontal direction. Such variation in permeability is termed anisotropy.

(3.96)

Horizontal (k~) and vertical (kv) permeability are determined from core
analysis on a regular basis. kH and kv can more accurately be determined
from interference testing. Selective zonal well test analysis in the same
wellbore is typically used to estimate vertical permeability. Partidly
penetrating wells, for instance, may develop a spherical flow regime,
which can be analyzed to estimate vertical and horizontal permeability
as shown in Figure 3.33a [56].
The relationship between different petrophysical proprieties and
fluid saturation is well established for clean sandstone rocks. Several
empirical models have been developed to calculate water saturation,
and all the required parameters for the evaluation of clean reservoirs.