Flow, deformation, and transport - Physical Processes in Earth and Environmental Sciences - page 103

are porous to the extent that all adjacent pores communi-
cate, as is commonly the case in sands or gravels. Severe
lateral and vertical gradients arise when pores are closed by
compaction, as in clayey rock; the hydrostatic condition
now changes to the geostatic condition when pore pres-
sures are greater due to the increased weight of overlying
rock compared to a column of pore water (Fig. 4.3).

Interlayering of porous and nonporous rock then leads to
high local pressure gradients down which subsurface fluids
may move. In passage down an oil or gas exploration well,
pressure may jump quickly from a hydrostatic trend toward

p 0 = Atmospheric p 0 = Atmospheric

In the hydrostatic condition
all liquid levels are equal

p 0 = Atmospheric
h 0

Escape from a reservoir at a rate
determined by the local difference
in pressure between hydrostatic and
atmospheric

There is no change to this
principle when the fluid
occupies void space that
has continuous connection
to the surface

h 1

u 1

h 2

u 2

Fig. 4.2 The hydrostatic condition is equally valid for liquid in reser-
voirs or porous rock.

√

2 gh

Exit velocity = u =

h 3

Pressure, kg m -2

u 3

100 . 10 5 500 . 10 5

During flow from a reservoir along a
pipe or channel there is energy loss
downstream due to friction (drag)

Calculated for:
r water = 1,000 kg m -3
r rock = 2,380 kg m -3

Flow in

1

Slope of line gives
hydraulic gradient

Geostatic
gradient

2

Hydrostatic
gradient

Flow out

3

Fig. 4.1 Flows induced by hydrostatic pressure.

Fig. 4.3 Hydrostatic and geostatic pressure gradients in the
Earth's crust.

Low
atmospheric
pressure

High
atmospheric
pressure

Strong wind causing wind shear

and water “set-up” on lee-shore

b

Low High

Subsurface flow down horizontal
hydrostatic pressure gradient
(modified by Coriolis force in 3D)

Sloping isobars

A B

Fig. 4.4 Barotropic flow due to a horizontal gradient in hydrostatic pressure caused and maintained by atmospheric dynamics. The spatial
gradients in atmospheric pressure and wind shear may act together or separately. In both cases hydrostatic pressures above B are greater than
hydrostatic pressures at all equivalent heights above A , by a constant gradient given by the water surface slope.