Page 85 - Introduction to Petroleum Engineering

POROSITY 69
TablE 4.1 Porosities of Media of geologic Origin
Media Porosity (%)

Coal 2–3
Shale 5–10
Sandstones <25
Limestones <25
Dolomites <30
Loose sand (well sorted) 32–42
Diatomites 40–60

Example 4.1 Core Porosity
A sandstone core sample is cleanly cut and carefully measured in a laboratory.
The cylindrical core has a length of 3 in. and a diameter of 0.75 in. The core is
dried and weighed. The dried core weighs 125 g. The core is then saturated
with freshwater. The water‐saturated core weighs 127.95 g. Determine the
porosity of the sandstone core. Neglect the weight of air in the dried core and
assume the density of water is 1 g/cc.
answer
Bulk volume:
.
 075 in  . 2   254 cm  3
.
3
V = π rh = 3 14159 ×   (3 . . in ×   = 21 7.cc
2
in ) =1 325 .
.
B
 2   1 in. 
Pore volume = mass of water in core divided by density of freshwater:
m (127 95. g −125 g)
V = w = = 295 cc
.
P
ρ
w 1 g/cc
Porosity = pore volume divided by bulk volume:
.
V 295
φ = P = = 0 136
.
.
V 21 7
B
4.1.1 Compressibility of Pore Volume
The city of Long Beach, California, sank in the 1940s, 1950s, and 1960s as oil
and water were pumped from the underlying formations, most significantly the
Wilmington Oil Field. The extent of sinking, or subsidence, ranged from inches up
to 29 ft. Considerable problems arose with damage to roads, buildings, and wells.
Extensive flooding occurred in the area. The cause of the subsidence was compres-
sion of the pore volume in the subsurface formations. Pore volume compression
coincided with extensive oil and water production and corresponding pressure