Page 96 - Fundamentals of Reservoir Engineering

SOME BASIC CONCEPTS IN RESERVOIR ENGINEERING 35

10
×
×
×
1.776x10 × 0.19 0.8 35.37 4290 9
= = 699.70 10 scf
×
0.887 660
×
2) The overall production schedule can be divided into three parts, the build-up,
plateau production and decline periods, as shown in fig. 1.13.
Rate G G G p
Q p 1 p 2 3
(MMscf /d)
Q = 100 (MMscf/d)
o

50 (MMscf /d)
20 (MMscf /d)

t 1 t 2 t 3

Fig. 1.13 Gas field development rate− −− −time schedule (Exercise 1.2)

It is first required, to determine G , that is, the cumulative production when the
2 P
reservoir pressure has fallen to 1200 psia and the plateau rate can no longer be
maintained. When p = 1200 psia, Z = 0.832 (fig. 1.8) and using the depletion
material balance, equ. (1.35),

p 1200
Z
9
G = G(1− p 1200 ) = 699.70 10 (1= 0.832 )
×
p
4290
2
i 0.887
Z i
G = 491.04 10 9 scf
×
P
2
Since the cumulative production during the two years build-up period is
6
9
G = Q avg × 2 × 365 = 50 × 10 × 2 × 365 = 36.5 × 10 scf
1 P
the gas production at the plateau rate of 100 MMscf/d is
9
9
G − G = (491.04 – 36.50) × 10 = 454.54 × 10 scf
p1
p
2
and the time for which this rate can be maintained is
G − G 454.54 10 9
×
p
t = 2 1 p = = 12.45 yrs
2
6
Q o 100 10 × 365
×
Therefore the time for which the plateau rate can be sustained will fall short of the
requirement by some 2.5 years.
3) During the exponential decline period the rate at any time after the start of the
decline can be calculated as