Page 86 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

74 Reservoir Engineering

Klinkenberg plots were proportional to the product of viscosity and the square
root of absolute temperature.
As shown in Figure 5-51 permeability of reservoir rocks can decrease when
subjected to overburden pressure [131]. When cores are retrieved from a
reservoir, the confining forces are removed and the rock can expand in all
directions which can increase the dimensions of the available flow paths. In
reservoirs where this is significant, it is imperative that permeability measured
in the laboratory be conducted at the confining pressure that represents the
overburden pressure of the formation tested.
As a general trend, air permeability decreases with increasing connate water
saturation. Relationships between air permeability and connate water saturation
in Figure 5-52 show a linear decrease in the logarithm of permeability as a
function of water saturation that depends on the individual field [132].
The fluid system of an oil reservoir consists usually of gas, oil, and water. In
the case of such a heterogeneous system, flow of the different phases is a
function of fluid saturation in the reservoir by the different phases. The lower
the saturation of a certain liquid, as compared to other liquids, the lower the
permeability to that liquid. This type of permeability is termed effective
permeability and is defined as permeability of the rock to one liquid under con-
ditions of saturation when more than one liquid is present. Typical permeability-
saturation relations for oil and gas and for oil and water are shown in Figure 5-53.
From the practical point of view, permeability may be considered as a measure
of productivity of the producing horizon. Knowledge of permeability is useful
in a number of reservoir problems. The concept of effective permeability is of
particular importance since it emphasizes a need for production practices, which
tend to maintain good permeability of the reservoir to oil.

90

80

- PERM..md poR.%A
URVE SOURCE OF CORE
70
0 Basol Tuscaloosa. Mas. 249
@ Basol Turcolooso. Miss. 163
60 @ Southern Calif. Coort 315 25
@ Los Angers Basln,Calif. I IO 22
50
0 1000 Moo 3Ooo 4Ooo 5ooo
OVERBURDEN PRESSURE, psig
Figure 5-51. Air permeability at different overburden pressures [l31].