Page 69 - Fundamentals of Reservoir Engineering

SOME BASIC CONCEPTS IN RESERVOIR ENGINEERING 8

hydrocarbons in place which in turn can result in the formulation of woefully inaccurate
field development plans.

Pressure (psia)
2250 2375 2500 Exploration
well

5000

GDT GAS
Depth 5150’
(feet) DPGWC
5281’
OIL COLUMN
Fig 1.3
TEST RESULTS
5500
at 5100 ft
pg = 2377 psia
DPOWC
5640’ dp g = .08 psi / ft
dD

Fig. 1.4 Illustrating the uncertainty in estimating the possible extent of an oil column,
resulting from well testing in the gas cap

Figure 1.4 illustrates another type of uncertainty associated with the determination of
fluid contacts from pressure measurements. The reservoir is the same as depicted in
fig. 1.3 but in this case the exploration well has only penetrated the gascap. A well test
is conducted at a depth of 5100 ft from which it is determined that the gas pressure is
2377 psia and, from the analysis of a collected sample (refer exercise 1.1), that the gas
gradient in the reservoir is 0.08 psi/ft. From these data the equation of the gas pressure
line can be defined as

+
p = 0.08D 1969 (psia ) (1.9)
o
Having seen no oil in the well the engineer may suspect that he has penetrated a gas
reservoir alone, and extrapolate equ. (1.9) to meet the normal hydrostatic pressure line

+
p = 0.45D 15 (psia ) (1.7)
w
at a depth of 5281 ft, at which p w = p g. This level is marked in fig. 1.4 as the deepest
possible gas water contact (DPGWC), assuming there is no oil.

Alternatively, since the deepest point at which gas has been observed in the well is
5150 ft (GDT − gas down to), there is no physical reason why an oil column should not
extend from immediately beneath this point. The oil pressure at the top of such a
column would be equal to the gas pressure, which can be calculated using equ. (1.9)
as 2381 psia. Hence the equation of the oil pressure line, assuming the oil gradient
used previously of 0.35 psi/ft, would be

p = 0.35D 579 (psia )
+
o