Page 131 - Petrophysics

104 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

Very Small Irregular Grains

Horizontal Permeability, kH = 150 mD
Vertical Permeability, kv = 15mD
Figure 3.9. Effects of small, irregular grains on permeability [U].

this category. Reservoirs with directional permeability are called
anisotropic. Anisotropy greatly affects fluid flow characteristics of the
rock. The difference in permeability measured parallel and vertical to
the bedding plane is the consequence of the origin of the sediment,
because grains settle in the water with their longest and flattest sides in
a horizontal position. Subsequent compaction of the sediment increases
the ordering of the sand grains so that they generally lie in the same
direction [ 131.

(b) Lamination: Platy minerals such as muscovite, and shale
laminations, act as barriers to vertical permeability. In this case the k&v
ratio generally ranges from 1.5 to 3 and may exceed 10 for some reservoir
rocks. Sometimes, however, kv is higher than kH due to fractures or
vertical jointing and vertical solution channels. Joints act as barriers to
horizontal permeability only if they are fdled with clay or other minerals.
The importance of the clay minerals as a determinant of permeability is
often related not only to their abundance but also to their mineralogy
and composition of the pore fluids. Should the clay minerals, which coat
the grain surfaces, expand and/or become dislodged due to changes in
the chemistry of the pore fluids or mud filtrate invasion, as explained in
Chapter 10, the permeability will be considerably reduced.

(c) Cementation: Figure 3.6 shows that both permeability and
porosity of sedimentary rocks are influenced by the extent of the
cementation and the location of the cementing material within the pore
space.