Page 84 - Introduction to Petroleum Engineering

68 PROPERTIES OF RESERVOIR ROCK

Area

H
Pore

Grain

Berea sandstone
microscope zoom 25×

FIguRE 4.1 Porous medium.

of sandstone. Some rocks have pore spaces that are not connected. Our interest
here is confined to connected pores that contribute to commercial storage and
flow capacity.
Bulk volume is the volume of the block shown in Figure 4.1. It includes both grain
volume and the volume of space between grains in the block. The bulk volume V of
B
the block is the product of area A in the horizontal plane and gross thickness H:
V = AH (4.1)
B
The volume of space between grains is called pore volume. Porosity ϕ is the ratio
of pore volume V to bulk volume V :
p B
V
φ = p (4.2)
V B
Typically, 30–40% of the bulk volume of sand is open volume. Porosities of commer-
cially viable reservoir systems range from a few percent for shales and coals to about
50% for diatomaceous formations in California. The porosities of most conventional
oil and gas formations range from 15 to 25%. Porosities for a variety of media are
listed in Table 4.1.
Consider another example. A beaker contains 500 ml of marbles. In terms of bulk
volume, the marbles occupy a bulk volume of 500 ml. How much water must be
added to bring the water level to the top of the marbles? If 200 ml must be added,
then the pore volume is 200 ml and the porosity is 40%. What is the actual volume
of the marbles? That volume, typically referred to as grain volume for rocks, would
be 300 ml. In summary, bulk volume V is the sum of grain volume V and pore
G
B
volume V :
p
V = V + V p (4.3)
B
G