Page 66 - Reservoir Formation Damage

50 Reservoir Formation Damage

Fabric and Texture

Lucia (1995) emphasizes that "Pore space must be defined and clas-
sified in terms of rock fabrics and petrophysical properties to integrate
geological and engineering information." Fabric is the particle orientation in
sedimentary rock (O'Brien et al., 1994). Defarge et al. (1996) defined:
"Texture, i.e., the size, shape, and mutual arrangement of the constitutient
elements at the smaller scale of ... sedimentary bodies, is a petrological
feature that may serve to characterize and compare" them. Petrophysical
classification of rock fabrics, such as shown in Figure 3-1 by Lucia (1995),
distinguishes between depositional and diagenetic textures. Lucia (1995)
points out that: "The pore-size distribution is controlled by the grain size in
grain-dominated packstones and by the mud size in mud-dominated pack-
stones." Lucia (1995) explains that: "Touching-vug pore systems are defined
as pore space that is (1) significantly larger than the particle size, and
(2) forms an interconnected pore system of significant extent" (Figure 3-2).

Porosity
Porosity, (j), is a scalar measure of the pore volume defined as the
volume fraction of the pore space in the bulk of porous media. The porous
structure of naturally occurring porous media is quite complicated. The
simplest of the pore geometry is formed by packing of near-spherical
grains. When the formation contains different types of grains and fractured
by stress and deformation, pore structure is highly complicated. For
convenience in analytical modeling, the porous structure of a formation
can be subdivided into a number of regions. Frequently, a gross classifica-
tion as micropores and macropores regions according to Whitaker (1999)
and Bai et al. (1993) can be used for simplification. However, in some
cases, a more detailed composite description with multiple regions may
be required (Cinco-Ley, 1996; Guo and Evans, 1995). Such descriptions
may accommodate for natural fractures and grain packed regions of
different characteristics. The various regions are considered to interact
with each other (Bai et al., 1995).

Spherical Pore Space Approximation
For simplification and convenience, the shapes of the pore space and
grains of porous media are approximated and idealized as spheres. The
pore volume can be approximated in terms of the mean pore diameter,
D, as:

3
V P = nD /6 (3-D