Page 530 - Petrophysics

498 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

signal processing. Vertical fractures appear as straight lines whereas
dipping fractures tend to appear as sinusoidal traces because
the televiewer shows the wellbore sandface as if it were split
vertically and laid flat. Another useful televiewer tool for detecting
natural fractures is the formation microscanner (FMS) device.
This tool can detect fractures that range from few millimeters to
several centimeters long, distinguishes two fractures as close as
1 cm apart (see Figure 8.7), and distinguishes between open and
closed fractures. Only fractures that are at least partially open
contribute to production.
(7) Very high productivity index. A productivity index of 500 STl3/D/psi
or higher is typical of naturally fractured wells produced under
laminar flow. Some wells in Iranian oilfields reported a productivity
index of 10,000 STl3/D/psi. In these wells 95% of flow is through
fractures [ 221.
(8) A considerable increase in productivity of the well flowing after an
artificial stimulation by acidizing is a strong indication of a naturally
fractured formation. Acidizing is done essentially to increase the
width of fractures and channels.
(9) Because of the high permeability of the fractures, the horizontal
pressure gradient is typically small near the wellbore as well
throughout the reservoir [22]. This is primarily true in Type-1 and
to a lesser degree in Type-2 fractured reservoirs.

Other indicators of the existence of the natural fractures in the
reservoir are:

(a) local history of naturally occurring fractures;
(b) lack of precision in seismic recordings;
(c) extrapolation from observations on outcrops; and
(d) pressure test results that are incompatible with porosity and
permeability values obtained from core analysis and/or well logging.

As can be deduced from the preceding discussion, no method used
alone provides a definite proof of the presence of fractures. FMS logs
and borehole televiewers often give a reliable indication of the presence
of major features; however, they do not resolve the full complexity
of many of the smaller-scale fracture systems. Fracture detection is
most certain when several independent methods confirm their presence.
Different naturally fractured reservoirs require different combinations of
methods of analysis. A combination of core analysis, pressure transient
test analysis, and various fracture-finding logs is strongly recommended
for detecting and locating fractures. Table 8.1 summarizes the many
techniques available for detecting natural fractures.