Page 43 - Reservoir Geomechanics

2 Pore pressure at depth in sedimentary basins

Pore pressure at depth is of central importance in reservoir geomechanics. In Chapter 1,
I referred to the fact that pore pressure and stress magnitudes are closely coupled (Figure
1.4). The importance of pore fluids and pore fluid pressure on the physical properties
of reservoirs is discussed in Chapter 3 in the context of effective stress (the difference
between external stresses acting on the rock matrix and pore pressure) and poroelasti-
city. In Chapter 4, pore pressure is shown to have an effect on the strength of both intact
and faulted rock. Elevated pore pressures pose a severe risk during drilling when hydro-
carbons are present and place important constraints on the density of drilling mud (i.e.
mud weights) used during drilling (Chapter 10). Elevated pore pressure also influences
maximum hydrocarbon column height in some reservoirs as well as the leakage poten-
tial of reservoir-bounding faults (Chapter 11). Reductions in reservoir pore pressure
with production (depletion) can cause significant deformation in a reservoir including
compaction and permeability loss (especially in poorly consolidated and weak forma-
tions) and, perhaps counter-intuitively, induce faulting in some reservoirs in normal
faulting regimes or the surrounding region (Chapter 12).
Ireview several fundamental principles about pore pressure in this chapter. First,
I define pore pressure and discuss variations of pore pressure with depth. Second,
I discuss the way in which a reservoir can be hydrologically subdivided (compart-
mentalized) into distinct pressure and flow units. Third, I briefly discuss some of the
mechanisms of overpressure generation that have been proposed. Finally, I discuss the
relationship between pore pressure, effective stress and porosity. The ways in which
porosity decreases with depth can be used to estimate pore pressure from either seismic
data (before drilling) or in relatively impermeable formations (such as shales) in wells
already drilled using geophysical well logs. There are a number of compilations of
papers on pore pressure in sedimentary basins (Law, Ulmishek et al. 1998; Huffman
and Bowers 2002; Mitchell and Grauls 1998) that discuss the subjects addressed in this
chapter in more detail.

Basic definitions

As illustrated in Figure 2.1, pore pressure is defined as a scalar hydraulic potential acting
within an interconnected pore space at depth. The value of pore pressure at depth is
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