Page 69 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 45

1.4.4
Casing Design [ ]

expected. The pressures can be so high that increasing the mud density
to control it and then drilling ahead may not be a viable option. However,
these are normally of relatively low volume and if it is possible to allow
the formation to flow, the pressure can deplete fairly quickly.

Shale
Normal pressure gradient

Overpressure at top of gas sand
Impermeable Boundary

Gas
bearing Gas gradient
Sand
Normally pressured from aquifer

Shale Normal pressure gradient

Fig. 1-6 Pressure-Depth Graph Showing Abnormal Pore Pressure Due to Light
Fluid Column

Salt beds can also cause very high pressures underneath. Salt is a
low shear strength material that is nonporous (i.e., no “pore pressure”
to help support the overburden), impermeable (forms a seal—one of
the conditions necessary for overpressure), and flows plastically under
pressure. As such it can transmit hydraulically the full overburden pres-
sure from above the salt to formation fluids below it.
Mechanisms of subnormal pore pressure generation. Subnormal
pore pressures will occur as a reservoir is produced. The reservoir rock
has to compress since the overburden (which stays constant) has less
support from the pore fluids; therefore, as the pore pressure lessens,
the rock vertical stresses increase. The formation does not have to be
produced by a well; it could be that a fault could allow migration to a
higher zone (overcharging that zone) or to surface, leading to less than
normal pressure.

45