reservoir geomecanics

372 Reservoir geomechanics
Schematic geologic
cross-section
PRESSURE
Pore pressure
profile
Maximum
column
height
Impermeable caprock
Permeable reservoir
Oil
Water
Gas
Fault
DEPTH
Hydrostatic gradient
GP
OP
Large hydrocarbon column
WP
Small hydrocarbon column
OP
WP
Critical pore
Low WP
pressure
High WP
pressure
Figure 11.18. Illustration of how the accumulation of hydrocarbons in a footwall reservoir induces
buoyancy pressure on the seal (in this case a reservoir bounding fault). For dynamic leakage
mechanisms, when the water phase pore pressure is relatively high, the quantity of hydrocarbons
that can accumulate in a reservoir before the dynamic limit is reached is smaller. After Wiprut and
Zoback (2002). Reprinted with permission of Elsevier.
sealing capacity and the less the total accumulation of hydrocarbons is likely to be
(Finkbeiner, Zoback et al. 2001).
In many ways, the case where buoyancy pressure induces fault slip is analogous to the
case described for the A-central fault (and other potentially leaking faults) at Visund.
However, there is an important difference between that case and what may happen in
areas of young, uncemented and ductile formations such as those found in the Gulf of
Mexico. In a brittle rock (such as encountered in Visund) fault slip is expected to cause
brecciation and a permanent loss of fault seal capacity. In an uncemented and ductile
shale in the Gulf of Mexico, fluid flow along a fault may only occur while the fault
is slipping. Hence, following a slip event, the fault surfaces in the ductile shale may
easily deform plastically and seal up after slip has ended such that the fluid flow up the
fault plane is episodic, something akin to the fault valve model of Sibson (1992). An
example of this will be discussed below.
In Figure 11.19 (from Finkbeiner, Zoback et al. 2001) werevisit fluid pressures in
a dipping sand reservoir surrounded by relatively impermeable shale in the context
of the centroid concept introduced in Figure 2.12. Wedosotofurther examine the
dynamic controls on column heights illustrated in Figure 11.19. InFigure 11.19a we
show the conditions under which hydraulic fracturing of the caprock will occur. Above
the centroid, the pore pressure in the reservoir is higher than that in the shale caprock and
eventually reaches the least principal stress in the shale. Note that the Mohr diagrams in