BAKER HUGHES - Drilling Fluids Reference Manual

RESERVOIR APPLICATION FLUIDS
Permeability is a measure of the ability of a rock to permit fluid flow.
K
Q × M × L
=
P × A
where:
K = Permeability in darcies
L = Length in cm of rock sample
Q = Fluid flow rate in cm 3 /sec
M = Viscosity (in centipoises) of test fluid
A = Cross-sectional area of rock sample in cm 2
P = Pressure applied in atmospheres
Most oil and gas is produced from sandstones. Both porosity and permeability are needed for
production. Porosity creates the spaces to hold the oil or gas. Permeability allows the oil and gas to
flow out of the rock.
Classification of Porosity
Primary porosity is either inter-granular, between grains, inter-particle, or inside fossil shells.
Secondary porosity can be inter-crystalline, as for recrystallized dolomites; fenestral, normal gaps,
larger than grain supported interstices; moldic, formed by solution and vuggy or grading into
cavernous. Fractured porosity is more difficult to observe. It ranges from microscopically small to
cavernous in size. It can be formed either by tectonic activity or some non-tectonic processes, such
as immediately beneath unconformities. This would include weathered fractures, especially in
limestone.
Theoretically, porosity is
independent of grain size, but
in practice, porosity generally
decreases with increases in
grain size. Also generally,
porosity and permeability
increase with increased
sorting. The more spherical
the grains, the less the
porosity, due to the tendency
of closer packing.
Figure 6 - 3
Grain Packing and Porosity
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 6-5