Page 83 - Geology of Carbonate Reservoirs

64 PETROPHYSICAL PROPERTIES OF CARBONATE RESERVOIRS

Interfacial contact angles.
Solid surface is quartz in A and calcite B.

θ = 158º

θ = 30º θ = 83º θ = 35º
H 2 O A
H 2 O

Organic Isooctane Isooctane + 5.7 % Isoquinoline Naphthenic acid
liquid isoquinoline

θ = 106º
θ = 30º θ = 48º θ = 54º
B
H 2 O
H 2 O H 2 O

Figure 3.6 This figure illustrates the different wetting characteristics of four organic liquids

on (a) silica and (b) calcite solid surfaces. Different liquids have different affinities for solid

surfaces; that is, some liquids preferentially wet (spread over and adhere to) certain solids.
Note that three of the four organic liquids have strong preferences to wet calcite but naph-
thenic acid does not. Some polar organic compounds in oils can transform water - wet carbon-
ate reservoirs to oil - wet ones. (Adapted from illustrations in Amyx, Bass, and Whiting
(1960) .)

from 90 ° between solid and liquid. For reservoir specialists, the main reason that
capillarity is important is because capillary attraction is the force behind capillary
pressure . This force depends on the properties of the liquids and solids in the porous
medium, on the size and distribution of pore throats, and on the reservoir saturation
history. Reservoirs consist of rocks with many different mineralogical compositions
and a variety of pore sizes and shapes, and they may contain more than one liquid.
The variety of minerals present in carbonate reservoirs is limited, however, so if the
fluid properties are known, then capillary pressure characteristics provide an excel-

lent guide to the size and distribution of pores and pore throats. These pressure
characteristics are usually expressed graphically as capillary pressure curves .

3.2 CAPILLARY PRESSURE AND RESERVOIR PERFORMANCE

Capillary pressure can be thought of as the force necessary to drive a nonwetting

fluid through pores already saturated with a wetting fluid. The magnitude of the
capillary pressure depends on interfacial tension and the radius of curvature of the

fluid interface. Following the example of Amyx et al. (1960) , consider a capillary
tube of radius r in a beaker of oil and water (Figure 3.7 ). The pressure in the oil at
points A and B is given by P oA and P oB , respectively. The pressure in the water at A

and B is given by P wA and P wB . Assume that the beaker is large enough for the