Page 44 - Reservoir Geomechanics

28 Reservoir geomechanics

Pore pressure at
depth is equivalent
to a hydraulic
potential measured Pore pressure is
with respect to assumed to be uniform
Earth’s surface in a small volume of
interconnected pores

Figure 2.1. Pore pressure at depth can be considered in terms of a hydraulic potential defined with
reference to earth’s surface. Conceptually, the upper bound for pore pressure is the overburden
stress, S v .

usually described in relation to hydrostatic (or normal) pressure, the pressure associated
with a column of water from the surface to the depth of interest. Hydrostatic pore
hydro
pressure (P p ) increases with depth at the rate of 10 MPa/km or 0.44 psi/ft (depending
hydro
on salinity). Hydrostatic pore pressure, P p , implies an open and interconnected pore
and fracture network from the earth’s surface to the depth of measurement:
z

P hydro ≡ ρ w (z)gdz ≈ ρ w gz (2.1)
p w
0
Pore pressure can exceed hydrostatic values in a confined pore volume at depth.
Conceptually, the upper bound for pore pressure is the overburden stress, S v , and it is
sometimes convenient to express pore pressure in terms of λ p , where λ p = P p /S v , the
ratio of pore pressure to the vertical stress. Lithostatic pore pressure means that the
pressure in the pores of the rock is equivalent to the weight of the overburden stress S v .
Because of the negligibly small tensile strength of rock (Chapter 4), pore pressure will
always be less than the least principal stress, S 3 .
In general, I will consider most issues involving pore pressure in quasi-static terms.
That is, I will generally disregard pressure gradients that might be associated with fluid
flow. With the exception of the problem of how drawdown (the difference between the