Page 85 - Geology of Carbonate Reservoirs

66 PETROPHYSICAL PROPERTIES OF CARBONATE RESERVOIRS

depends on interfacial tension between wetting and nonwetting phases and between

fluid and solid surfaces. In a reservoir with fi xed fluid properties, capillary pressure

values reflect pore characteristics, and the radii of the capillary tubes are assumed
equivalent to the porethroat radii. That assumption means that pore throats are
modeled as cylindrical tubes. In most reservoirs, pore throats are not cylindrical, but
may be sheet - like, especially the intercrystalline pore systems characteristic of dolo-
mites. For sheet - like pore throats, the capillary pressure equation, according to
Wardlaw (1976) , is more realistically written

(σ wo cosθ wo )
P c =
r

3.2.1 Capillary Pressure, Pores, and Pore Throats

Capillary pressure is inversely related to pore throat radius, but remember that
capillary pressure calculations are based on a model in which pore throats are
cylindrical tubes. Pore throats have complex geometries so that computed pore
throat radius represents the effective pore throat radius . Rearranging the expression
for P c provides the equation to compute effective pore throat radius:
2σ wo cos θ
r eff =
P c

In this expression, σ is the interfacial tension of the air – mercury system (480 dynes/

cm), θ is the air – mercury – solid contact angle (140 ° ), and P c is capillary pressure in

2
2
dynes/cm (1 psi = 69,035 dynes/cm ).
In an air – mercury system, the nonwetting mercury displaces air at low pressures
in large pore throats. If the large pore throats are uniform in size (well sorted) and
are well connected within the rock (pore – pore throat systems with high accessibility
and high coordination numbers), saturation by the nonwetting phase will proceed
at low pressure along a flat trajectory until all accessible pores and pore throats

have been filled to a limit for that particular rock. A plot of mercury injection capil-
lary pressure and fl uid saturation takes on a characteristic shape in response to the
manner in which the pores and pore throats are saturated (Figure 3.8 ). The initial
part of the curve reflects the pressure exerted by a nonwetting fluid against a wetting

fluid just until the latter is displaced. This initial pressure is known as entry pressure ,

and the pressure at which the nonwetting fluid begins to move wetting fl uid from
its position in a pore is the displacement pressure . It is common to see the terms
entry pressure and displacement pressure used synonymously. The vertical axis of

the capillary pressure curve reflects pore throat radius in micrometers ( μ m), with

largest values at the origin. The vertical axis also represents the height of a given
water saturation above the free - water level (where capillary pressure is zero). Capil-

lary pressure curve trajectories, as they trace fluid saturation at increasing pressure,
indicate pore throat size, sorting , and accessibility . Pressures at points of infl ection
on the curve represent threshold pressures , and a range of threshold pressures on a
single curve indicates the presence of several pore throat size clusters in the sample.
The terminal saturation value of the wetting phase is referred to as the minimum
unsaturated pore volume .