Page 88 - Geology of Carbonate Reservoirs

CAPILLARY PRESSURE AND RESERVOIR PERFORMANCE 69

throat size sorting. Nonreservoir curves exhibit characteristics similar to the mar-
ginal reservoir samples except that their pressure - saturation trajectories and dis-
placement pressures indicate even smaller pore throat radii, poorer size sorting, and
lower pore – pore throat connectivity. The rock represented by curve 8 is a good
candidate for a seal because it has such a high entry pressure and low nonwetting
phase saturation at maximum pressure.
Reservoirs and nonreservoirs can be distinguished based on their respective capil-
lary pressure characteristics. According to Sneider (1988) reservoirs generally have

• Capillary pressure ( P c ) less than about 300 psia at 1% bulk volume occupied

by mercury

• Mercury saturation greater than 3% bulk volume at P c = 1000 psia (pore throats

about 0.01 μ m in radius)
• Mercury saturation greater than 3% bulk volume with more than 50% of pore

throats larger than 0.05 μ m at P c = 2000 psia
• Displacement pressure generally less than 100 psia
Nonreservoirs have

• P c greater than about 500 psia at 1% bulk volume occupied by mercury

• Mercury saturation of about 2% bulk volume at P c = 1000 psia (pore throat
radius ∼ 0.1 μ m)

• Mercury saturation of less than about 3% and more than 50% of pore throat
radii smaller than 0.05 μ m at 2000 psia

• Displacement pressures greater than 100 psia when the rocks are not artifi cially

fractured
Sneider ’ s criteria are one of several methods that can be classified as “ rock typing ”

or flow unit identification and quality ranking. Selected references on other methods

of rock typing are included in the Suggested Reading section at the end of this
chapter.
MICP curves contain useful information about reservoir rock characteristics. In
addition to providing data on pore throat geometry, they can be used to estimate
recovery effi ciency and permeability. Studies on Smackover reservoirs in Alabama
have shown that median pore throat size is strongly correlated with permeability in
Smackover reservoir rocks (Kopaska - Merkel, 1991 ). “ Pore facies ” constructed from
petrographic and stratigraphic data, as described by Kopaska - Merkel and Mann
(1993) , can then be sorted on their capillary pressure curve characteristics in order

to obtain estimates of reservoir recovery efficiency and heterogeneity for the pur-

poses of selecting or grading candidate fields, sectors of fields, or reservoir zones for

enhanced and improved oil recovery.
3.2.2 Converting Air –Mercury Capillary Pressures to
Oil–Water Equivalents

Air – mercury capillary pressure data must be converted to brine – oil equivalents
before applying them to reservoirs. The conversion most commonly used is