Page 127 - Petrophysics

100 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

PERMEABILITY

In addition to being porous, a reservoir rock must have the ability
to allow petroleum fluids to flow through its interconnected pores.
The rock’s ability to conduct fluids is termed as permeability. This
indicates that non-porous rocks have no permeability. The permeability
of a rock depends on its effective porosity, consequently, it is affected
by the rock grain size, grain shape, grain size distribution (sorting),
grain packing, and the degree of consolidation and cementation.
The type of clay or cementing material between sand grains also
affects permeability, especially where fresh water is present. Some
clays, particularly smectites (bentonites) and montmorillonites swell in
fresh water and have tendency to partially or completely block the pore
spaces.
French engineer Henry Darcy developed a fluid flow equation that
since has become one of the standard mathematical tools of the
petroleum engineer [ 121. This equation is expressed in differential form
as follows:

where:

u = fluid velocity, cm/s.
q = flow rate cm3/s.
k = permeability of the porous rock, Darcy (0.986923 pm’).
& = cross-sectional area of the rock, cm’.
p = viscosity of the fluid, centipoises (cP).
1 = length of the rock sample, cm.
3 = pressure gradient in the direction of the flow, atm/cm.
dl

One Darcy is relatively high permeability. The permeability of most
petroleum reservoir rocks is less than one Darcy. Thus a smaller unit
of permeability, the millidarcy (mD), is widely used in the oil and
gas industry. In SI units, the square micrometer (pm2) is used instead
of m2.
The permeability, k, in Equation 3.9 is termed the “absolute”
permeability if the rock is 100% saturated with a single fluid (or phase),
such as oil, gas, or water. In presence of more than one fluid, permeability
is called the “effective” permeability (b, k,, or k, being oil, gas, or water
effective permeability respectively). Reservoir fluids interface with each
other during their movement through the porous channels of the rock;