Page 374 - Petrophysics

342 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

TABLE 5.1A
CALCULATION OF CAPILLARY PRESSURE FOR AIR DISPLACING WATER FROM A
BEREA SANDSTONE CORE
S(ave)
~~ ~~ ~~ ~
1,300 0.30 0.827 4.135
1,410 0.40 0.769 4.865
1,550 0.50 0.71 1 5.879
1,700 0.60 0.653 7.071
1,840 0.70 0.595 8.284
2,010 0.75 0.566 9.885
2,200 0.80 0.538 11.843
2,500 0.90 0.480 15.293
2,740 1 .oo 0.422 18.370
3,120 1.05 0.393 23.818
3,810 1.10 0.364 35.518
4,510 1.20 0.306 49.769
5,690 1.25 0.277 79.219

Units of pounds per square inch (psi) were used to determine the three
constants (A, B, and C) for Equation 5.44 by the least-squares regression
analysis of the experimental data in order to avoid very large numbers
that would result from the use of kPa in these calculations.
The values of the constants for Pc(hy) versus S are: A = -25.5296, B =
17.6118, and C = -4.5064. The regression analysis was used once more
to obtain the constants for Pc(hy) versus Si, which differ only with respect
to the first constant (A); the second set of constants is: A2 = -24.5296,
B2 = 17.6118, and C2 = -4.5064.

PORE SIZE DISTRIBUTION

An approximate pore size distribution of rocks can be obtained from
capillary pressure curves if one of the fluid phases is non-wetting. If one
phase is non-wetting, cos 8 in Equation 5.11 is assumed to be equal to
1.0 at all saturations. The capillary pressure is then a function of only the
interfacial tension and the radius of the pore. Equation 5.1 1 is based on
uniform capillary tubes; however, a rock is composed of interconnected
capillaries with varying pore throat sizes and pore volumes. The capillary
pressure required to invade a given pore is a function of the size of pore
throat. Although determination of the pore throat size distribution of
rocks based on capillary pressure curves is only an approximation, the
distribution is an important parameter for analysis of many fluid transport
properties of porous media [23].