Page 65 - Geology of Carbonate Reservoirs

46 CARBONATE RESERVOIR ROCK PROPERTIES

Qualitative Description Permeability (md)
Poor to fair < 1.0 − 15
Moderate 15 − 50
Good 50 − 250
Very good 250 − 1000
Excellent > 1000

Research by Russian geoscientists indicates that 80% of common sedimentary
− 3
rocks have permeabilities in the range of 0 − 10 md, 13% are in the range of

− 3

10 − 1.0 md, 5% are in the range of 1.0 − 1000 md, and only 2% have permeabilities
of more than 1000 md (North, 1985 ). If the work of the Russian scientists is generally
applicable, it indicates that over 90% of all sedimentary rocks are either seals or
“ tight gas sands ” that produce no more than 5 bbls of oil per day (North, 1985 ).
Evaporites are the least permeable rocks, being impermeable to water. Shales are
permeable to water but not generally permeable to oil. Very high permeability
through connected vugs and fractures is relatively common in carbonate rocks,
notably in limestones rather than dolostones. The El Abra Formation of the Poza
Rica and Golden Lane trends in Mexico (Cretaceous) and some of the Permian
carbonates in West Texas and New Mexico are known for high permeability,
dissolution - diagenetic porosity. Individual wells in vuggy - fractured Asmari
Limestone (Oligocene − Miocene) of Iran have produced over 100,000,000 barrels
of oil (North, 1985 ).
Permeability is expressed as (1) specifi c permeability , (2) effective permeability ,
and (3) relative permeability . Specific permeability, described in the previous para-

graphs, is the permeability of a reservoir rock to a single fluid. It is measured on

core samples, commonly by commercial laboratories. Effective permeability is a

measure of the permeability to another fluid when the reservoir is already saturated,
that is, the effective permeability to oil of a reservoir rock already saturated with

water. The presence of a wetting fluid impedes the entry of a nonwetting fl uid;
therefore effective permeability is lower than specific or absolute permeability. In

other words, the sum of k o + k w + k g is less than absolute permeability because the
mutually interfering presence of oil, water, or gas retards flow. Relative permeability

is the ratio of effective permeability at a given saturation to absolute permeability
at 100% saturation, or k r = k e / k . Next to basic lithology, effective porosity and spe-
cific permeability are the most important variables used to describe reservoir rocks.

Absolute permeability, or simply permeability, may vary directly with interparticle
porosity in detrital reservoir rocks such that ϕ = a + b log k .
Permeability, like porosity, is sensitive to variations in texture and fabric in res-
ervoir rocks, but for each 1% change of porosity, at least in siliciclastic sandstone
reservoirs, the change in permeability is greater by a factor of 7 − 10 (North, 1985 ).
Unlike porosity, permeability varies with grain size, as well as packing, sorting, and
fabric. Fine - grained detrital rocks with comparatively high intergranular porosity
have low permeability. In ideal reservoirs with intergranular porosity and uniform
grain size, permeability varies approximately as the fourth power of the average
pore radius, or approximately as the square of the grain diameter (North, 1985 ).
Most reservoirs, especially carbonate reservoirs, are not represented by this ideal
model. The wide variety of genetic pore types and attendant varieties of pore − pore