Page 129 - Petrophysics

102 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

OF
FACTORS AFFECTING THE MAGNITUDE PERMEABILITY
Permeability of petroleum reservoir rocks may range from 0.1 to
1,000 or more millidarcies, as shown in Table 3.1. The quality of a
reservoir as determined by permeability, in mD, may be judged as:
poor if k < 1, fair if 1 < k < 10, moderate if 10 < k 50, good if
50 < k < 250, and very good if k > 250 mD. In East Texas fields,
permeability may as high as 4,600 mD. Reservoirs having permeability
below 1 mD are considered “tight”. Such low permeability values
are found generally in limestone matrices and in tight gas sands of
the western United States. Stimulation techniques such as hydraulic
fracturing and acidizing increase the permeability of such rocks and allow
the exploitation of such low permeability reservoirs, which were once
considered uneconomical. Only 50 years ago rocks with permeability of
50 mD or less were considered tight.

TABLE 3.1
PERMEABILITY AND POROSITY OF SELECTED OIL SANDS
Name of Sand Porosity % Permeability (mD)

“Second Wilcox” (Ordovician) 12.0 100.0
Oklahoma Co., OK
Clinch (Silirian) 9.6 0.9
Lee Co., VA
Strawn (Pennsylvanian) 22.0 81.5
Cook Co., TX
Bartlesville (Pennsylvanian) 17.5 25
Anderson Co., KS
Olympic (Pennsylvanian) 20.5 35.0
Hughes Co., OK
Nugget (Jurassic) 24.9 147.5
Fremont Co., WY
Cut Bank (Cretaceous) 15.4 111.5
Glacier Co., MT
Woodbine (Cretaceous) 22.1 3,390.0
Tyler Co., TX
Eutaw (Cretaceous) 30.0 100.0
Choctaw Co., AL
O’Hern (Eocene) 28.4 130.0
Dual Co., TX