Page 245 - Applied Petroleum Geomechanics

Abnormal pore pressure mechanisms 239

Pressure (MPa)
0 20 40 60 80 100
0

500
1000 Overburden stress
Depth TVD (m) 1500 Pore

2000
2500
pressure
3000 Effective stress
Hydrostatic
3500 pressure
Overpressure
4000
Figure 7.4 Hydrostatic pressure, pore pressure, overburden stress, and effective stress
in a borehole. TVD is the true vertical depth.

Pore pressure gradient is more practically used in drilling engineering
because it is more convenient to be used for determining mud weight (mud
density), as shown in Fig. 7.5. Pore pressure gradient at a given depth is the
pore pressure divided by the true vertical depth. The mud weight should be
appropriately selected based on pore pressure gradient, wellbore stability,
and fracture gradient before setting and cementing a casing. The drilling
fluid (mud) is applied in the form of mud pressure to support the wellbore
walls for preventing influx and wellbore collapse during drilling. To avoid
fluid influx and wellbore instability in an open hole section, a heavier mud
pressure than the pore pressure is needed. However, when mud weight is
higher than the fracture gradient of the drilling section, it may break the
formation, causing mud losses or even lost circulation. To prevent a
wellbore from unintentional hydraulic fracturing by the high mud weight,
as needed where there is overpressure, a casing needs to be set to protect the
overlying formations from fracturing, as illustrated in Fig. 7.5.
Pressure gradients and mud weight are expressed in the metric unit, SG
3
or g/cm (i.e., specific gravity) in Fig. 7.5. However, pressure gradients and
mud weight are often reported in the English or the US unit system in the
oil and gas industry. The pressure gradient conversions between the US
and metric units can be found in Table 7.2. In the drilling industry, the