Page 210 - Geology of Carbonate Reservoirs

DETECTING FRACTURED RESERVOIRS 191

type I reservoirs. Examples of type II reservoirs include the Agha Jari Field in Iran
with 9500 - MMbbl reserves, the Haft Kel Field in Iran (2660 MMbbls), and the Spra-
berry trend in Texas (447 MMbbls) according to Nelson (2001) .

Type III reservoirs are confirmed to occur in some of the largest fields in the

world where both matrix and fractures are capable of signifi cant fl ow, although

fractures provide only a permeability assist. Among fields with type III reservoirs
are Kirkuk Field in Iran (15,000 MMbbls), Hassi Messaoud, Algeria (6000 MMbbls),
and Dukhan, Qatar (4570 MMbbls). Kirkuk Field had initial flow rates of up to

100,000 BOPD on one of the first wells drilled into the fractured Asmari Limestone.

Because the pressure differential on initial flow was very low, the reservoir was

recognized as a fractured system immediately and was managed as such from that
time onward (Nelson, 2001 ). It is not always easy to recognize fractured reservoirs
and many secondary recovery efforts fail because injected fluids follow unknown

fracture systems instead of sweeping the expected matrix porosity systems.
7.4 DETECTING FRACTURED RESERVOIRS

There are a variety of methods for finding fractures in the subsurface: (1) seismol-

ogy, (2) subsurface geological methods, (3) mapping fractures on outcrops and
extrapolating fracture patterns to the nearby subsurface, and (4) exploratory drilling
of structures that are likely to have tectonic fractures. Extensively fractured rock
can have a distinctive seismic signature that is sometimes recognizable directly on
seismic records or less directly with the aid of seismic attributes (seismic wave char-
acteristics such as amplitude, frequency, polarity, spatial extent, shear wave charac-
ter, and amplitude versus offset — AVO). AVO analysis is a data - processing technique
used on prestack seismic data and is widely used in exploring for gas sands. Although
much of the literature on AVO processing focuses on gas sand reservoirs, the
method should also be useful in carbonates under certain circumstances. Ideally,
both P and S wave velocities would be recorded along with offset and bulk density
of the target rocks. Differences in return velocities of shear and compressional
waves and P wave polarity, which changes as the wave passes from water - wet sands
to gas sands, can indicate the presence of gas. AVO methods are also useful for
detecting fractured reservoirs because the bulk density, returned P – S wave veloci-

ties, and possibly the fluid characteristics in fractured zones differ from those in
nonfractured zones. Most seismic records do not include S wave data, but P wave
data at different offsets are routinely recorded. That information can be processed
to record a component of S wave data. A detailed explanation of the AVO technique
is beyond the scope of this book. Suggestions for further reading are included at
the end of this chapter.
Subsurface geological methods utilize data from previous drilling, coring, logging,
completion, and production. Image logs and cores are direct proof that fractures
exist. Drilling time logs, mud circulation logs, caliper logs, and acoustic logs are
usually helpful indicators of fractures, but they have limited use in distinguishing
between cavernous dissolution porosity and fractures. More about borehole logs
and their application appears later in the discussion on indirect methods for detect-
ing fractures (Section 7.4.2 ).