Page 217 - Geology of Carbonate Reservoirs

198 FRACTURED RESERVOIRS

5. Expand the scale of investigation to field size by adjusting the geological

concept (fracture model) to assume that fractures behave as ideal fracture
types (standard models in the literature). Using those ideal fracture models,
predict the spatial distribution of fracture types, intensity, and spacing at fi eld
scale using mathematical and geometrical manipulation of subsurface data as
outlined in Nelson (2001) .

6. Refine the model for fl uid flow in the types of fractures and matrix rock known
to exist in the reservoir based on well test or production data. Estimate pro-
portions of matrix and fracture porosity and permeability to develop a reser-
voir model for optimum and sustained recovery.
Full - diameter cores offer the best source of information about fracture and
matrix properties. Use that information to develop a more accurate model for total
reservoir characteristics. Keep in mind that fracture porosity is calculated differently
than matrix porosity and that crossfl ow (matrix fl ow interacting with fracture fl ow)
can occur in bimodal porosity systems. Pressure testing is especially useful to detect
crossfl ow.

SUGGESTIONS FOR FURTHER READING

The most comprehensive discussions on fractured reservoirs are in the fi rst and
second editions of Geological Analysis of Naturally Fractured Reservoirs by Nelson
(1985, 2001) . Examples of fractured reservoirs in mildly to severely deformed strata
are described and illustrated in Lorenz et al. (1997) , in a chapter in Carbonate Seis-
mology . Reservoir engineering aspects of fractured reservoirs are discussed in detail
in Chapter 8 of Tiab and Donaldson ’ s (2004) text, Petrophysics , 2nd edition. A
concise and clear discussion of AVO analysis in seismic exploration for hydrocarbon
reservoirs is given by Davies et al. (2003) .

REVIEW QUESTIONS

7.1. Define stress and strain.

7.2. What are three types of stress and how would structures produced by them
be different?
7.3. What is the typical material behavior associated with faults? With folds?

7.4. According to the work by Stearns and Friedman (1972) , what types of frac-

tures are typically found on limbs of anticlinal folds? What effect does the
orientation of maximum principal stress have on those fractures?

7.5. What types of fractures would you expect to find on the crests of anticlines or

synclines?
7.6. Fracture porosity is described by Nelson (2001) as “ scale - dependent. ” What
does that mean?

7.7. How might horizontal boreholes (directional drilling) be more effective than
vertical ones in exploiting fractured reservoirs along normal faults?