Page 211 - Geology of Carbonate Reservoirs

192 FRACTURED RESERVOIRS

Exploratory drilling of structures is based on the concept that folds and faults

have specific fracture patterns that correspond to structural geometry. As we saw
earlier, fractures associated with faults are generally parallel to fault slip planes and
fracture density is higher on hanging walls of normal faults than on footwalls (Fried-

man and Wiltschko, 1992 ). In addition, specific fracture types and geometrical
arrangements occur on anticlines. It is riskier to drill structural geometry in search
of fractures than it is to drill prospects based on seismological data or zones where
subsurface geology indicates fractures. In the best situations, drilling should be based
on information from all methods: seismology, subsurface geology, and structural
geometry. Once fractures have been detected in the subsurface, the next step is to
identify or predict the location of zones with highest fracture intensity (closest

spacing) to ensure the most economical and effi cient field development. That can
be accomplished by either direct or indirect observation.

7.4.1 Direct Observation of Fractures in the Borehole

Direct observations include core examination, viewing borehole walls with down-
hole TV cameras, and making borehole impressions with inflatable packers. Core

examination is the best method for direct determination of fracture dip, fracture
intensity, rock strength, rock fabric, and what type of fractured reservoir (of the four
categories discussed above) exists. Knowing the extent to which fracture and matrix

porosity influence reservoir performance will greatly improve the odds for improved
management strategies and development outcomes. Downhole cameras can only be

used in dry, gas - filled, or clear, water - filled boreholes, but they can provide informa-
tion about the presence of fractures and their orientation, and some information

about fracture intensity. Inflatable impression packers have a coating that enables
an impression of the borehole wall to be made when the packers are set in the
borehole and infl ated. They are defl ated and removed to examine the impressions.
The method is not sensitive enough to detect small natural fractures, it is ineffective
in mud - caked holes, and inflated packers commonly fail in large, jagged washouts,

a common characteristic of some extensively fractured zones.

7.4.2 Indirect Methods to Detect Fractures in the Borehole

The main methods for indirect detection of fractures in the borehole are (1) well

log evaluation, (2) flow test evaluation, and (3) mathematical and graphical manipu-
lation of reservoir rock data. Nelson (2001) discusses nine different logging tools
and their use in indirect detection of fractures. He notes that log responses have
been used with varying degrees of success to detect fractures and fracture intensity,
but not fracture spacing. And because the log responses are not unique to fractures,
the log analyst must have detailed knowledge of each tool and its response to a wide
variety of rock properties, some of which could cause responses in unfractured rock
that resemble those in fractured reservoirs.
One of the logging tools commonly used to determine the presence of fractures
is the sonic amplitude log . Compressional wave amplitudes are reduced dramati-
cally when they encounter fluid - filled fractures and shear wave amplitudes virtu-

®
ally disappear. The acoustic log (Sonic Log Schlumberger registered trademark)
can be used as a qualitative check for possible fractured zones, especially when it