Page 565 - Petrophysics

532 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

(b’)
(4
Figure 8.23. An idealized schematic of elementary blocks [IPJ.

(2) two principal planes: defining matches, Figure 8.23(b), (b’); and
(3) one series of parallel planes: defining sheet, Figure 8.23(c).

FRACTURE STORAGE CAPACITY

In contrast to the matrix porosity, fracture porosity contributes only a
few percent to the total porosity. Fracture aperture is typically up to a few
millimeters in width, and typical fracture spacing is in the centimeter to
meter range. Because fracture apertures are generally significantly greater
than typical matrix pore-throat sizes, they contribute the major portion
of the total transmissivity of the petroleum rocks, and consequently are
an important factor in the movement of fluids. Fracture porosity initially
is very high, but, over time, fractures may become partially filled with
fines. This filling process considerably reduces the fracture porosity to
less than five percent. Since only fracture conductivity is necessary in
flow calculations, not much attention has been given to fracture porosity
or storage capacity. The overall fracture storage capacity, which indicates
how much fluid is held within the fracture network of a particular
reservoir, is best estimated from pressure buildup tests.