Page 257 - Applied Petroleum Geomechanics

250 Applied Petroleum Geomechanics

D D
e e
erosion
Fluid Pressure, p f
0 0 0 0
1000 p 1 f = ρ f gD 0
Vertical Depth (m) 2000 = = ρ ρ gD D D 1 1 D D e e = = ρ f f gD + ρ D gD e
(D +
ρ
g
)
3000
e
1
4000
f
1
5000
up
f
l
f
i
D D
6000 0 0 p f p f 0 0 f gD 0 0 uplift t Hydrostatic Overpressure
pressure
7000
Figure 7.12 Formation uplift causing pore pressure increase in a perfectly sealed
condition.
calculation considered both fluid dynamics (Darcy flow and diffusion) and
thermodynamics (pressureevolumeetemperature relationships), and their
results showed that uplift with an ideal sealing condition may enhance
overpressures of gas reservoirs and most volatile oil reservoirs with gas to oil
ratio (GOR) > 1000 scf/bbl and decrease overpressures of black oil reser-
voirs with GOR < 500 scf/bbl.
Owing to formation uplift or unloading, the relationship of the effective
stress and sonic or seismic velocity does not follow the loading curve, and
unloading happens, as demonstrated by laboratory-measured compaction
data in Fig. 7.13. The unloading defines the rebound curves in the original
compaction/loading curve (Bowers, 2001). The unloading occurs along a
flatter effective stress path than the initial compaction curve in the velocity
and density curves, as shown in Fig. 7.13. In the unloading case, a higher
pore pressure exists for the same velocity value than the one without
unloading because it has a smaller effective stress in the unloading case.
Fig. 7.13 also indicates that the sediment density typically changes very little
in the unloading condition.
Zhang and Wieseneck (2011) studied the relationship of pore pressure
(calculated from the fluid kicks), vertical effective stress, and corresponding
sonic velocities and densities in the Bossier and Haynesville shale gas for-
mations (Fig. 7.14). Unloading may exist in Fig. 7.14 compared to Bowers’
loading curve. Even in the unloading case a power relation can be used to