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74 PROPERTIES OF RESERVOIR ROCK
50
40
Permeability (md) 30
20
10
0
500 1000 1500 2000 2500
Pore pressure (psi)
FIguRE 4.3 Pressure dependence of permeability.
4.2.2 Superficial Velocity and Interstitial Velocity
Two types of velocities often appear in reservoir engineering discussions. Sometimes
called Darcy velocity, superficial velocity is the quotient of flow rate and cross‐
sectional area:
q
u = (4.13)
A
The term “superficial” is a good descriptor for this velocity because it ignores
the effect of porosity. On the other hand, interstitial velocity does account for
porosity:
u q
v = = (4.14)
φ φ A
Interstitial velocity is the average velocity of fluid moving through a porous rock.
Superficial velocity is much less than interstitial velocity.
4.2.3 Radial Flow of liquids
Equations 4.11 and 4.12 describe flow rate for a linear geometry. For flow of a liquid,
such as oil, in radial geometry, Darcy’s law becomes
kh
q = 0 00708 p ( e − p ) (4.15)
.
w
o
o ( /
µ o B ln r r )
w
e
