Page 33 - Drilling Technology in Nontechnical Language

24 Drilling Technology in Nontechnical Language Second Edition

Vital Rock Properties

Petroleum is generated within small voids, or pore spaces, of a source
rock. Many rocks have pore spaces within them. Sandstone is comprised
of grains of predominantly quartz, chemically cemented together with
minerals that accumulate where the grains touch.

The mineral cement holding these grains together may be very strong,
in which case the rock is described as being highly consolidated or well
consolidated. However, in some cases, this bonding is not very strong, and
such a rock would be described as poorly consolidated or unconsolidated.
Materials that have spaces within them, like a sponge, are described
as porous. The extent of this porosity is measured as the fraction of the
total rock volume occupied by the pore spaces. Porosity is expressed as
a percentage; if 20% of the total volume of a rock was made up of pore
spaces, the porosity would be 20%. Rock porosity is very important,
for without porosity, oil cannot be generated, migrate, or accumulate in
a reservoir.
Within a porous rock, it is possible for the pore spaces to be connected.

Fluids (gas, oil, or water) can flow between the pores, moving through the
rock. However, there are also rocks that are porous, but the rock spaces are
not connected. Fluid in the pore spaces cannot move through the rock. As

mentioned in the first chapter, the ability of a rock to allow fluid to flow

through it is called permeability. A rock can be porous but impermeable
(cement is an example of a porous, impermeable solid), but a permeable rock
must have porosity (there must exist pore spaces that can be connected).
Permeability is extremely important, for without it, oil generated in the
source rock cannot migrate to a reservoir and cannot be exploited.
Permeability is measured in darcies. A rock cube of 1 cm × 1 cm × 1

cm sides that transmits fluid with a viscosity of 1 centipoise at a rate of 1
cc per second with a pressure differential of 1 bar has a permeability of
1 darcy. In layman’s terms, a rock with a permeability of 1 darcy is very
permeable. Most reservoir rocks are measured in millidarcies (1/1,000th of
a darcy) rather than darcies.
Shale has very low porosity and very low permeability. Shale minerals
form flat crystals that stack up like plates on a shelf. When clays are

originally deposited, they are comprised of 70%–80% water. As water
is squeezed out of the clay during diagenesis, these flat crystals become

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