Page 89 - Introduction to Petroleum Engineering

PERMEABILITY 73
TablE 4.2 Examples of Permeability
Porous Medium Permeability

Coal 0.1–200 md
Shale <0.005 md
Loose sand (well sorted) 1–500 md
Partially consolidated sandstone 0.2–2 d
Consolidated sandstone 0.1–200 md
Tight gas sandstone <0.01 md
Limestone 0.1–200 md
Diatomite 1–10 md

500 md, while fine‐grain sandstone might have a permeability of just a few
millidarcies. Rough estimates of permeabilities for a variety of media are listed
in Table 4.2.

Example 4.4 Flow Rate From Darcy’s Law
Assume the permeability of a cylindrical rock sample (often called a core)
is 150 md, the length of the core is 6 in., the diameter of the core is 1 in., the
pressure drop across the core is 20 psi, and the viscosity of brine passing
through the core is 1.03 cp. Use Darcy’s law to calculate the magnitude of
volumetric flow rate in bbl/day.

answer
1 1  1  2
2
Cross‐sectional area of the core: A = π d = π  ft  = 0 0054ft. 2
4 4  12 
Volumetric flow rate through the core:
(150md )( 0 0054. ft 2 )
.
q = 0 001127 (20psi ) = 0 036. bbllday
/
(103. cp )(0 5. ft )

4.2.1 Pressure Dependence of Permeability
Rock above a formation is known as the overburden. Pore pressure decreases in the
formation as fluid is withdrawn from the formation during production. The weight of
the overburden compresses formation rock as pore pressure decreases. The decrease
in pore space due to compression leads to reduction in formation porosity and per-
meability. The rate of permeability decrease can be quite variable, depending on
the strength of the rock and the structure of pores. Figure 4.3 shows the change in
permeability for a reservoir with an initial pore pressure of 2500 psi. As pressure
declines, the permeability decreases from the initial value of about 33 md to about
26 md at pore pressure of 1000 psi.