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early exception to these observations because, as we saw in Chapter 8, the
smaller weight density of oil and of gas compared to that of water means
that the pressure at the top of the reservoir is higher than the pressure that
would have been found if the reservoir had been full of water only. This is the
initial driving force of flowing wells, enlarged when the well itself is full of
oil or gas, or oil from which solution gas is liberated as it rises to lower pres-
sures.
That reservoir geometry can lead to higher pressures than the normal hydro-
static can be seen in Fig. 14-1, which, although schematic, is representative
of several Iranian fields. A thousand metres of oil column of mass density
850 kg m-3, as compared with water of mass density 1010 kg m-3, gives rise
to an excess pressure of (1010-850) g X 1000 = 1.57 MPa (= 227 psi). Such
a column of gas of mass density 200 kg m- would lead to an excess pressure
of 7.94 MPa (1150 psi) at the top of the reservoir. If these pressures were en-
countered at a depth of 1500 m, for example, they would lead to excess pres-
sure gradients from the surface of about 1 kPa/m (0.04 psi/ft) and 5.3
kPa/m (0.23 psi/ft). If mud is lost to the formation, and oil fills the hole
to the surface, the excess pressure there will be about 3.9 MPa (570 psi). For
gas, the excess pressure would be about 19.8 MPa (2900 psi). These pressures
are normal in the sense that they arise from normal physical causes in a water
environment that has normal hydrostatic pressures.

Fig. 14-1. Reservoir geometry can lead to “abnormal” pressures within an accumulation.