Page 193 - Petroleum Geology

170

We shall consider only the simplest aspects of oil-well production that will
enable us to reach the point of understanding the nature of the process. We
shall first consider an oil well producing without gas or water from an inclined
reservoir over a small interval at a depth of 2000 m, with the oil/water con-
tact at 2050 m (Fig. 8.13).
At the oil/water contact, the pressure in the oil is equal to that in the water;
and, if it is normal hydrostatic, it will be:
p = p,gz = 1020 X 9.8 X 2050 = 20.5 MPa (8.10)
= 2973psi.
The total head will be nominally zero, and pressures in the water outside the
reservoir will decrease upwards according to the relationship, Apw/Az = p,g.
This is a stable pressure gradient in the water, and the water will be at rest.
Within the oil reservoir, the pressure in the oil will also be 20.5 MPa (2973
psi) at the oil/water contact, and the pressures in the oil will decrease upwards
according to the relationship, Apo/Az = pog. This is a stable pressure gradient
in the oil, and the oil will be at rest. If the mass density of the oil is 780 kg
m-3, the pressure at the bottom of the interval to be produced at 2000 m

OIL POTENTIOMETRIC SURFACE
~~

?- 3 -
‘FLUID PRESSURES

\

Fig. 8-13. Static pressures in an oil well that is closed in (diagrammatic).