Page 68 - Fundamentals of Reservoir Engineering

SOME BASIC CONCEPTS IN RESERVOIR ENGINEERING 7

At the time of drilling an exploration well and discovering a new reservoir, one of the
main aims is to determine the position of the fluid contacts which, as described in the
previous section, will facilitate the calculation of the oil in place.

Consider the exploration well, shown in fig. 1.3, which penetrates the reservoir near the
top of the oil column. The gas-oil contact in the reservoir will be clearly "seen", at
5200 ft, on logs run in the well. The oil-water contact, however, will not be seen since it
is some 225 ft below the point at which the well penetrates the base of the reservoir.
The position of the contact can only be inferred as the result of a well test, such as a
4
5,6
drill stem or wireline formation test , in which the pressure and temperature are
measured and an oil sample recovered. Analysis of the sample permits the calculation
of the oil density at reservoir conditions and hence the oil pressure gradient (refer
exercise 1, Chapter 2). Together, the pressure measurement and pressure gradient are
sufficient to define the straight line which is the pressure depth relation in the oil
column. If such a test were conducted at a depth of 5250 ft, in the well in fig. 1.3, then
the measured pressure would be 2402 psia and the calculated oil gradient 0.35 psi/ft,
which are sufficient to specify the oil pressure line as

+
p = 0.35D 565 (psia ) (1.8)
o
and extrapolation of this line to meet the normal hydrostatic pressure line will locate the
oil-water contact at 5500 ft.

This type of analysis relies critically on a knowledge of the hydrostatic pressure regime.
If, for instance, the water is overpressured by a mere 20 psi then the oil-water contact
would be at 5300 ft instead of at 5500 ft. This fact can be checked by visual inspection
of fig. 1.3 or by expressing the equation of the overpressured water line, equ. (1.6) as

p = 0.45D 35 (psia )
+
w
and solving simultaneously with equ. (1.8) for the condition that p w = p o at the oil-water
contact. The difference of 200 ft in the position of the contact can make an enormous
difference to the calculated oil in place, especially if the areal extent of the reservoir is
large.

It is for this reason that reservoir engineers are prepared to spend a great deal of time
(and therefore, money) in defining the hydrostatic pressure regime in a new field. A
5,6
simple way of doing this is to run a series of wireline formation tests in the exploration
well, usually after logging and prior to setting casing, in which pressures are
deliberately measured in water bearing sands both above and beneath the
hydrocarbon reservoir or reservoirs. The series of pressure measurements at different
depths enables the hydrostatic pressure line, equ. (1.6), to be accurately defined in the
vicinity of the hydrocarbon accumulation, irrespective of whether the pressure regime is
normal or abnormal.

Such tests are repeated in the first few wells drilled in a new field or area until the
engineers are quite satisfied that there is an areal uniformity in the hydrostatic
pressure. Failure to do this can lead to a significant error in the estimation of the