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The permeability of a rock is a measure of the ease with
which fluids can flow through a rock. This depends on how well
the pore spaces within that rock are interconnected (Figure 3.6). If
none of the pore spaces were connected to other pore spaces, then
no oil could flow through the rock. The only oil that would come
into the well would be from the pores that were actually in contact
with the well bore. This would obviously not make for a very
profitable oil well. On the other hand if the rock has good
permeability oil can flow to the well from pores that are thousands
of metres from the well bore.

Figure 3.6 – Permeability

In sketch A above, the pore spaces are well connected
allowing fluids to flow quite easily. This rock has good
permeability. In sketch B, the pore spaces are largely isolated from
one another giving this rock poor permeability. Sketch A may be
somewhat confusing as it appears that the grains are being held in
free suspension with nothing to support them. Remember,
however, that this is just a simplified two dimensional sketch. The
grains make contact with other grains somewhere out of the plane
of the sketch.

Control questions

1. What two properties are needed for a rock to be a reservoir
rock?

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