Page 93 - Geology of Carbonate Reservoirs

74 PETROPHYSICAL PROPERTIES OF CARBONATE RESERVOIRS

and low recovery effi ciency. Whether reservoir porosity is depositional, diagenetic,
or fracture formed, pore/pore throat size ratio, homogeneity in distribution of pore
geometry, and coordination number of pore – pore throat systems must be investi-
gated in order to understand the extent to which rock properties infl uence reservoir
fl uid behavior.

SUGGESTIONS FOR FURTHER READING
Fundamentals of capillarity and capillary pressure are clearly described in the stan-
dard reservoir engineering text by J. W. Amyx et al. (1960) , Petroleum Reservoir
Engineering . R. Coss é ’ s (1993) Basics of Reservoir Engineering , is an English transla-
tion of a French text that is helpful in presenting a different “ slant ” on some material
covered by Amyx et al. (1960) , although the book is rather uneven in its treatment
of subject matter. D. Tiab and E. C. Donaldson ’ s (2004) Petrophysics , 2nd edition, is
a voluminous textbook on principles of reservoir petrophysics, including some geo-
logical topics. It is arguably more useful as an encyclopedic reference than a text.
“ Rock typing ” is a way of identifying pore and pore throat characteristics that
correspond to highest storage capacity combined with highest capacity to fl ow: in
short, a way to identify rock samples and group them into categories with best,

intermediate, and poorest flow unit quality. The paper by C. Genty et al. (2007) in
which NMR measurements are used to identify genetic pore types in carbonate
reservoirs is one of a variety of these methods. F. J. Lucia ’ s (1995) paper includes
his method for identifying three petrophysical rock types based on porosity, perme-
ability, particle size, and MICP characteristics. G. W. Gunter et al. (1997) describe a
method that revolves largely around the Winland R35 criterion, as does the method
presented in Martin et al. (1997) .

REVIEW QUESTIONS

3.1. Write an expression that defines water saturation, or S w , and defi ne S w in
words.
3.2. What do the exponents a, m , and n represent in the Archie equation and what
are the ranges of representative values for each?
3.3. Calculations of S w in carbonate reservoirs with 100% moldic porosity require
carefully chosen values of m that differ greatly from those in ordinary detrital
carbonates or in terrigenous sandstones. What is a representative m value for
reservoirs with 100% moldic porosity?

3.4. What is meant by the “ transition zone ” in a reservoir?

3.5. All rocks were either deposited in water or passed through the water table

during burial so that all original pores were filled with water. How is it possible

that some reservoirs can be oil - wet?
3.6. In a naphthenic acid and water system, naphthenic acid forms a contact angle

of 35 ° on quartz surfaces and forms a contact angle of 106 ° on calcite surfaces.