Page 81 - Geology of Carbonate Reservoirs

62 PETROPHYSICAL PROPERTIES OF CARBONATE RESERVOIRS

measurements of n range from 1.2 to 3.0 (Sneider, 1988 ), and it is standard practice
to use a value of 2.0 if n is not known. More recently, it has been recognized that,
in carbonates with multiple pore types such as fractures, vugs, interparticle porosity,
and intraparticle microporosity, n can change when each of the different pore types
is subjected to lowering of its original saturation. In other words, different values of
n could be used for different ranges of S w . But in every case, n increases in oil - wet
reservoirs. Values of n greater than 8.0 have been reported for oil - wet reservoirs
(Tiab and Donaldson, 2004 ).
Careful choices of m, a , and n values for use in the Archie equation are critical
in the determination of S w and, ultimately, in estimating hydrocarbon reserves. In
addition, carbonate reservoir rocks commonly exhibit bimodal porosity systems that
may include combinations of intergranular mesopores and megapores with intra-
granular micropores. Microporosity commonly results when certain forms of dia-
genesis alter the internal structure of carbonate grains to produce micropores within

each affected grain. Micropores are water - filled; therefore averaged values of Archie
water saturation for combined intergranular and intragranular porosity may range
from 55% to 87% (Asquith and Jacka, 1992 ). This represents total water saturation ,
or S wt . To determine the actual productive potential in such bimodal porosity systems
requires a determination of the effective water saturation , or S we . Effective water
saturation pertains to the intergranular pores that determine productivity potential.
Although estimates of effective and total porosity may be obtained from log data,
the actual percentages of microporosity and macroporosity must be determined by
examining rock samples. According to Asquith and Jacka (1992) , the uncorrected
average S wt in an oolitic grainstone reservoir with bimodal porosity was 69.4%. After
corrections were made, the average S we was computed to be 34.9%. Clearly, accurate
values of S w are vitally important in reservoir characterization. Values for parame-
ters in the Archie equation must be chosen carefully based on information about
rock properties and average saturations ( S wt ) calculated for bimodal pore systems
can lead to miscalculations of reservoir productivity unless corrected, effective satu-
rations ( S we ) are determined.

3.1.2 Wettability

In qualitative terms, wettability is the tendency for one fluid rather than another to
be preferentially attracted to a solid surface. For example, if in an air - fi lled container
a drop of water falls on a glass plate, it will readily spread across the surface and
wet the plate. On the other hand, if a drop of mercury falls on the glass plate in
identical surroundings, it remains in a spherical, bead - like form because air is the
wetting fl uid.
In hydrocarbon reservoirs, one must deal with the interface between gas and
liquid, between two immiscible liquids, and between liquids and solids. Adhesion
tension is a function of interfacial tension and determines which fluid will wet a

solid. For a system consisting of oil and water in contact with a solid (Figure 3.5 ),
there is a contact angle ( θ ) which, by convention, is measured through the denser
fl uid and varies between 0 ° and 180 ° . The interfacial tension between the solid and
the lighter liquid is represented by σ so , between the solid and the denser liquid by
σ sw , and between the two liquids by σ wo . Adhesion tension is defi ned as