Page 48 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

36 Reservoir Engineering

V, = 7758 V,,4 = 7,758 A h @ (5-35)

where Vb is the bulk volume in ac-ft. A is the area in ft2, h is the reservoir
thickness in ft. and @ is the porosity expressed as a fraction.

The permeability of a rock is a measure of the ease with which fluids flow
through the rock. It is denoted by the symbol k and commonly expressed in
units of darcies. Typical sandstones in the United States have permeabilities
ranging from 0.001 to a darcy or more, and for convenience the practical unit
of permeability is the millidarcy which equals 0.001 darcy. Some other useful
conversion factors are given in Table 5-6.
Absolute Permeabllity

If a porous system is completely saturated with a single fluid, the permeability
is a rock property and not a property of the flowing fluid (with the exception
of gases at low pressure). This permeability at 100% saturation of a single fluid
is termed the absolute permeability.
Darcy Equation

The Darcy equation relates the apparent velocity, v, of a homogeneous fluid
in a porous medium to the fluid viscosity, p, and the pressure gradient, Ap/L

(5-36)

This equation states that the f hid velocity is proportional to the pressure
gradient and inversely proportional to fluid viscosity. The negative sign indicates
that pressure decreases in the L direction when the flow is taken to be positive.
The flow rate, q, is understood to be positive during production and negative
during injection. As shown in Table 5-6, the Darcy equation can de written as:

(5-37)

Linear Flow. In the Darcy equation for linear displacement:

where q = fluid flow rate, cm3/sec
A = cross-sectional area of rock perpendicular to flow, cme
p = pressure difference (in atm) across the distance L parallel to flow
direction, cm
p = viscosity of fluid, cp
A rock has permeability of one darcy if it permits the flow of one cc per
second of a one-phase fluid having viscosity of one centipoise under the pressure