Page 275 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

244 Reservoir Engineering

Table 5-32
Approximation of Residual Oil Saturation
Reservoir oil viscoslty, Resldual oil saturation,
CP % PV
0.2 30
0.5 32
1 .o 34.5
2.0 37
5.0 40.5
10.0 43.5
20.0 46.5
Deviation of resldual oll
Average reservoir saturation from
permeablllty, viscosity trend,
md % PV
50 +12
100 +9
200 +6
500 +2
1,000 -1
2,000 - 4.5
5,000 - 8.5
~
From Reference 245.

DECLINE CURVE ANALYSIS
The conventional analysis of production decline curves for oil or gas pro-
duction consists of plotting the log of flow rate versus time on semilog paper.
In cases for a decline in rate of production, the data are extrapolated into the
future to provide an estimate of expected production and reserves.
The empirical relationships for the analysis of production decline curves were
first proposed by Arps [255] in which a decline rate, a, was defined as the
fractional change in the flow rate, q, with time, t:

(5-181)

If time is in days, flow rate in this equation is expressed in terms of stock tank
barrels per day in the case of oil and scf per day for gas. Other consistent units
of flow rate and time can be used. As shown in Figure 5-145, three types of
decline can occur: a constant percentage or exponential decline, a hyperbolic
decline, and a harmonic decline [245]. For the semilog plot, the exponential
decline is a straight line whereas the slopes of the hyperbolic and harmonic
decline curves decrease with time. For the exponential decline, the drop in
production per unit time is a constant fraction of the produttion rate. For a
hyperbolic decline, the decrease in production per unit time as a fraction of
the production rate is proportional to a fractional power (between 0 and 1) of